EASTERN REGION TECHNICAL ATTACHMENT NO. 98-1 FEBRUARY, 1998 HURRICANE BERTHA Carin Goodall-Gosnell, Dan Bartholf, John Elardo, Bob Frederick, Cory Gates, Jim Hudgins, and Richard Thacker NOAA/NEXRAD Weather Service Office Newport, North Carolina 1. INTRODUCTION Around 2000 UTC 12 July 1996, Hurricane Bertha, a category 2 hurricane on the Saffir- Simpson Hurricane Scale (Table 1), made landfall south of Topsail Island, North Carolina (Fig. 1). The eye of Bertha passed within 40 n mi of NWSO MHX, located in Newport, NC, approximately 9 n mi west- northwest of Morehead City. This was the first hurricane to move inland over east- central North Carolina since the NEXRAD Weather Service Office (NWSO) Newport (MHX) and the collocated Doppler radar (KMHX) were put into service in January 1994. A close call occurred in August 1995, when the eye of Hurricane Felix appeared on the radar for two volume scans. The last major hurricane to make direct landfall over North Carolina was Hurricane Donna when her eye crossed Topsail Island in 1960. Damage estimates from Bertha across the NWSO MHX County Warning Area (CWA) were around 500 million dollars. Most of the damage was a result of significant loss of crops away from the coast. Along the coast, major damage from storm surge flooding was reported from North Topsail Island north to around Emerald Isle. There was also significant flooding along the western sections of the Pamlico Sound from New Bern north to Washington and Belhaven. Numerous trees and power lines were knocked down across all of eastern North Carolina, leaving many residents without electricity for several days. Included here is a collection of studies made by the forecasters at NWSO MHX. A history of Bertha has been constructed using satellite and Doppler radar pictures. A non- standard Z-R relationship was used on the Weather Surveillance Radar - 1988 Doppler (WSR-88D), and its performance has been compared with ground truth from various rain gauges. The vulnerability of eastern North Carolina to flooding, the public’s response to the hurricane threat, and the performance of various numerical weather prediction and storm surge models are also discussed. Appendix I shows a comparison between Hurricane Bertha and some of the more memorable hurricanes in North Carolina history. Appendix II is a detailed description of the effects of Hurricane Bertha on eastern North Carolina. Appendix III shows the official positions and maximum sustained winds during Hurricane Bertha’s lifetime. Appendix IV lists rainfall reports for several locations within 120 n mi of the KMHX radar.
Transcript
EASTERN REGION TECHNICAL ATTACHMENTNO. 98-1
FEBRUARY, 1998
HURRICANE BERTHA
Carin Goodall-Gosnell, Dan Bartholf, John Elardo,Bob Frederick, Cory Gates, Jim Hudgins, and Richard Thacker
NOAA/NEXRAD Weather Service OfficeNewport, North Carolina
1. INTRODUCTION
Around 2000 UTC 12 July 1996, HurricaneBertha, a category 2 hurricane on the Saffir-Simpson Hurricane Scale (Table 1), madelandfall south of Topsail Island, NorthCarolina (Fig. 1). The eye of Bertha passedwithin 40 n mi of NWSO MHX, located inNewport, NC, approximately 9 n mi west-northwest of Morehead City. This was thefirst hurricane to move inland over east-central North Carolina since the NEXRADWeather Service Office (NWSO) Newport(MHX) and the collocated Doppler radar(KMHX) were put into service in January1994. A close call occurred in August 1995,when the eye of Hurricane Felix appearedon the radar for two volume scans. The lastmajor hurricane to make direct landfall overNorth Carolina was Hurricane Donna whenher eye crossed Topsail Island in 1960.
Damage estimates from Bertha across theNWSO MHX County Warning Area (CWA)were around 500 million dollars. Most ofthe damage was a result of significant lossof crops away from the coast. Along thecoast, major damage from storm surgeflooding was reported from North TopsailIsland north to around Emerald Isle. Therewas also significant flooding along thewestern sections of the Pamlico Sound fromNew Bern north to Washington and
Belhaven. Numerous trees and power lineswere knocked down across all of easternNorth Carolina, leaving many residentswithout electricity for several days.
Included here is a collection of studies madeby the forecasters at NWSO MHX. Ahistory of Bertha has been constructed usingsatellite and Doppler radar pictures. A non-standard Z-R relationship was used on theWeather Surveillance Radar - 1988 Doppler(WSR-88D), and its performance has beencompared with ground truth from variousrain gauges. The vulnerability of easternNorth Carolina to flooding, the public’sresponse to the hurricane threat, and theperformance of various numerical weatherprediction and storm surge models are alsodiscussed.
Appendix I shows a comparison betweenHurricane Bertha and some of the morememorable hurricanes in North Carolinahistory. Appendix II is a detaileddescription of the effects of HurricaneBertha on eastern North Carolina.Appendix III shows the official positionsand maximum sustained winds duringHurricane Bertha’s lifetime. Appendix IVlists rainfall reports for several locationswithin 120 n mi of the KMHX radar.
2Eastern Region Technical Attachment 98-1
2. HISTORY OF BERTHA
Hurricane Bertha was an unusual, earlyseason Cape Verde storm. She formed as adepression off the coast of Africa on 5 July1996, and moved on a west-northwest track(Fig. 2 and Appendix III) around the bottomside of a large anticyclone centered along30(W in the North Atlantic. Berthaintensified into a category 3 storm by 0900UTC 9 July 1996, with sustained winds of100 kt and a central pressure of 960 mb.
On 10 July, a 500-mb trough moved throughthe northeastern United States, leaving in itswake a progressive zonal flow from theGreat Lakes to New England. In response,Bertha shifted to a more northwest track.
During the afternoon and evening of 11July, Bertha weakened as she moved toaround 200 n mi east of the Georgia coast.Maximum sustained winds had decreased to70 kt and the central pressure had risen to980 mb. On the 2115 UTC 11 July visiblesatellite picture (Fig. 3), the low-levelcirculation was partially exposed and wellwest of the deep convection.
Examination of the 0000 UTC 12 July upperair data showed winds at the 300-mb levelwere from the west-northwest at between 20and 25 kt along the southeast coast of theUnited States (Fig. 4). This would help toexplain the shearing of the deep convectionaway from the low-level circulation. Berthacontinued to weaken into the early morninghours of 12 July with the pressure rising to991 mb by 0600 UTC as another weak shortwave approached the mid-Atlantic region.This weak, but noticeable, disturbance wasshort-lived, and on the morning of the 12July, Bertha had regained convection aroundher center, and was reorganizing. Theshort wave pulled Bertha's track to a more
north-northwest direction and it appeared asthough she would pull away from land. Butinstead of moving out to sea, landfall wasfurther north than the expected Savannah,GA, to Charleston, SC, corridor.
Infrared satellite imagery indicated anincrease in deep convection associated withBertha between 0600 UTC and 0900 UTC(Fig. 5) and reconnaissance flights showedthat the pressure had dropped to 985 mb by0900 UTC.
Examination of the 1200 UTC 12 July upperair data (Fig. 6) showed that the weak shortwave over the Tennessee Valley had movedeast toward the coast. This backed thewinds at 300 mb over the southeast NorthCarolina coast from west-northwest tosouth-southwest at 20 to 30 kt. As the upperlevel winds became more southerly, theshear that had been influencing Berthadecreased. Bertha also moved over the coreof the Gulf Stream, where satellite datashowed sea surface temperatures between83 and 86 (F. These factors helped Berthabecome better organized.
Reconnaissance flights reported that thepressure was down to 997 mb and themaximum sustained winds had increased to80 kt by 1200 UTC 12 July. At 1236 UTC,an ill-defined eye became visible east ofnorthern Georgia on the 1.1-n mi resolutionbase reflectivity product (Klazura and Imy1993) of the KMHX WSR-88D.
By 1345 UTC, WSR-88D reflectivity datashowed the eye becoming more organized,as the first spiral rain band moved ashorenear Wilmington (Fig. 7). A visible satellitepicture taken at 1400 UTC 12 July,indicated the low-level center was no longerexposed and was now under the deepconvection just south of the North Carolinacoast (Fig. 8).
3Eastern Region Technical Attachment 98-1
Bertha continued to deepen during themorning of 12 July as she moved toward thecoast at 10 to 15 kt, and was a strongcategory 2 storm by 1500 UTC. Sustainedwinds increased to 90 kt and the centralpressure dropped to 974 mb. The first rainband moved over NWSO MHX around1800 UTC as the eye approached Cape Fear(Fig. 9).
The Velocity Azimuth Display (VAD) WindProfile (VWP) indicated winds at 1000 fthad increased to around 60 kt with thepassage of this band (Fig. 10). The bandmoved over the office during the next hour,and as it moved away, winds dropped backto less than 50 kt. Bertha still had not madelandfall by this time.
Bertha remained a category 2 storm withsustained winds of 90 kt as she madelandfall around 2000 UTC betweenWrightsville Beach and North TopsailBeach, about 2 h before high tide. Berthahad a classic hurricane appearance atlandfall with distinct banding around the eyeas seen on both radar and satellite imagery(Figs. 11 and 12).
Another rainband moved over NWSO MHXshortly after landfall, increasing the 1000-ftwind to 55 to 60 kt (Fig. 13). After thepassage of this spiral band, 1000-ft windsdropped to a mere 15 kt. The eye ofHurricane Bertha passed about 34 n mi westof the KMHX radar a little less than 2 hafter landfall.
The final rain band moved into the areaaround 2253 UTC (Fig. 14). One thousandfoot winds increased to 65 kt from the south(Fig. 15) as the eye was almost due west ofthe office. The eye slowly lost definitionover the next hour, but the storm stillresembled a hurricane. Up until this time,rainfall over land was mostly stratiform.
However, by 2300 UTC, convection becameevident east of the eye with the strongestconvection occurring around 2330 UTC.
Around 0200 UTC, the final rain band wasstill in the area (Fig. 16), but the convectionhad diminished. One thousand foot windspeeds were still around 50 kt. At 0300UTC, Bertha's eye was completely filledwith precipitation; however, heavy rainscontinued over the northeastern NorthCarolina counties. One thousand foot windspeeds over NWSO MHX dropped toaround 45 kt and continued to dropthroughout the remainder of the night.
An exciting day at NWSO MHX had cometo an end!
3. FLOODING
North Carolina is unique in that it not onlyhas an expansive coastline with low, flattopography across the adjoining coastalplain, but it also has an extensive series ofsounds and tidal rivers. This makes much ofeastern North Carolina vulnerable tosignificant flooding, and particularlysusceptible to flooding from tropicalcyclones.
Hurricane Bertha demonstrated the impacton eastern North Carolina due to floodingfrom a moderate hurricane. The worstflooding in nearly 40 years was experiencedin this area causing significant propertydamage and devastating beach erosion.Emergency management officials reportedthat nearly every pier and dock located onthe Neuse and Pamlico Rivers was damagedor destroyed during the storm.
The approach of Hurricane Bertha from thesouth-southeast exposed eastern NorthCarolina to a prolonged period of east tosoutheast flow. The persistent winds began
4Eastern Region Technical Attachment 98-1
to pile up water on east-facing beaches andinlets along the Atlantic Ocean. Thedownwind areas of the Pamlico andAlbemarle Sounds, as well as the Neuse andPamlico Rivers, experienced increasingwater levels as much as 36 h in advance oflandfall, as the pressure gradient betweenBertha and an anticyclone to the northtightened.
The hurricane's landfall was about 2 hbefore high tide. The increasing waterlevels on top of the incoming tidecontributed to a high coastal storm surge,especially over Onslow County and westernCarteret Counties, between Topsail Beachand Emerald Isle.
a. Inland Storm Surge Flooding
Inland, the most serious flooding occurredin the communities of Belhaven,Washington, and New Bern. Belhaven,located adjacent to the Pamlico River inBeaufort County, had the record 6.6-ft stormsurge associated with the Hurricane of 1913eclipsed by Bertha's surge of around 7 ft(Fig. 17). Elsewhere, Washington on thePamlico River in Beaufort County and NewBern on the Neuse River in Craven Countyalso experienced significant flooding withstorm surges of around 7 ft, but remainedunder record levels.
b. Coastal Storm Surge Flooding
Hurricane Bertha's landfall over southeastNorth Carolina put the NWSO MHX CWAin the dangerous northeast quadrant of thestorm. Significant storm surges of 8 to 10 ftabove normal were experienced fromTopsail Beach north to Emerald Isle. On thebeaches, many sand dunes were seriouslyeroded or destroyed during the storm.Extensive property damage due to thecombination of strong winds and stormsurge was reported in these areas, especially
along the barrier islands and immediatecoast.
North of Emerald Isle, the storm surge wasgenerally 4 to 6 ft above normal with seriousbeach erosion, but much less flood and winddamage than was recorded in North TopsailBeach, Swansboro, and Emerald Isle.
Minor flooding occurred along theMorehead City waterfront due to BogueSound overwash. Eastern sections ofCarteret County experienced minor tomoderate flooding from the Pamlico Soundwith portions of Route 12 inundated nearCedar Island.
The Outer Banks experienced someoverwash on Route 12, but since Berthamoved well south and west of the area, thisoverwash was minor.
c. Seiching
Coastal flooding from a storm surgeassociated with a land-falling hurricane isalways a concern. However, with thecomplex sound and river system overeastern North Carolina, flooding due toseiching effects is also a major problem. Anexample of this phenomenon occurred whenthe sustained northerly winds fromHurricane Gloria emptied the PamlicoSound from the north, and water was notvisible as far as the naked eye could see.After the winds let up, the water returned,and there was massive flooding fromManteo to Elizabeth City.
The persistent, strong wind flow associatedwith slow moving Hurricane Bertha allowedwater to pile up rapidly on the downwindsides of the relatively shallow inlandwaterways. The onshore wind trajectorypushed high water levels onto the westernside of the Pamlico Sound, while easternportions near the central Outer Banks in the
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vicinity of Cape Hatteras were blown dry upto 200 yards from shore.
The high water levels on the westernPamlico Sound were pushed upstream onthe Neuse and Pamlico Rivers wheresignificant flooding ensued. Water levelshad risen 5 to 7 ft above normal by theevening of 12 July, causing extensivedamage in the communities of New Bern,Bath, Washington and Belhaven.
As the hurricane moved inland andcontinued north, the prevailing flow veeredto the south and southwest and allowedwater to accumulate on the northern portionof the Pamlico and Albemarle Sounds.Some minor flooding was reported onRoanoke Island, with more significantflooding on the northern reaches of theAlbemarle Sound, where water levelsreached up to 6 ft above normal by the earlymorning hours of 13 July.
Often, after a strong storm moves by, windsabruptly back to the opposite direction,allowing the water that was piled up on thenorthern and western sides of the sounds toquickly return, flooding the adjacentsouthern and eastern land areas. This typeof seiching happened most recently with theMarch 1993 “Storm of the Century.”However, with slow-moving HurricaneBertha, the winds turned around anddiminished more slowly, allowing the waterto return at a slower rate. Residents onHatteras Island were reportedly surprisedthat they didn't catch the backlash of water.
d. Storm Surge Models
The staff at NWSO MHX used output fromthree storm surge models to help themforecast oceanic and inland water levelsduring the storm. The primary model usedwas the well-known Sea, Lake, andOverland Surges from Hurricanes (SLOSH)
model (Jelesnianski et al. 1992) developedby the NWS Office of SystemsDevelopment Techniques DevelopmentLaboratory. The other two models wereproduced by local universities. TheCroatan-Albemarle-Pamlico Estuary System(CAPES) model was engineered by theNorth Carolina State University (Neuherz etal. 1997) while the Advanced Circulation(ADCIRC) model was developed at theUniversity of North Carolina (Luettich et al.1992).
SLOSH - For this event, the SLOSH modelindicated the best results (National WeatherService 1996). The model accuratelypredicted a 7 ft storm surge along the Neuseand Pamlico Rivers (Fig. 18) for a category2 hurricane moving slowly NNE.
CAPES - The CAPES model was run atNorth Carolina State University on the nightprior to the hurricane's landfall andproduced results comparable to SLOSH.The 2100 UTC advisory from the TPC wasused to input the predicted track, wind radiiand minimum pressure information. Adownward adjustment to the winds of 25%was made to account for the expectedweakening of the storm upon landfall.
The model handled the high water levels onthe western end of the Pamlico Soundextremely well (Table 2). The model notonly forecast the areas that would receivesignificant flooding, but it’s water levelforecasts of 7 to 8 ft over the westernPamlico Sound were very accurate. On anegative note, the model overforecastsound-side flooding along the Outer Bankson the east side of the hurricane'scirculation. This poor handling of theflooding on the eastern end of the basin wascaused by the model’s inability to deal withtime-dependent winds and the asymmetry ofBertha's wind field.
6Eastern Region Technical Attachment 98-1
ADCIRC - This model accurately forecastthe areas that would receive major flooding(Fig. 19). However, the model significantlyunderforecasted water levels from Bath toBelhaven. This was a result of the modelusing an internally generated hurricane windfield based solely on the barometricpressure. The model assumed the hurricaneto be a symmetric, uniform storm while inactuality, the strongest winds were primarilyconfined to the northeast and east portionsof the storm.
Forecast surges along the beaches were lessthan half of the observed values (Fig. 20).This appeared to be a result of limitationsassociated with the projected track ofBertha. It is important to note that thismodel was specifically developed for non-tropical storm surges, and while it is capableof using wind fields, only pressure was inputin this instance since real-time windinformation was unavailable.
4. NCEP NUMERICAL MODEL SUITE
The ensemble of hurricane forecast modelsused by the Tropical Prediction Center(TPC; formerly the National HurricaneCenter) can be complemented by outputfrom the daily synoptic numerical models:the Eta model, the Nested Grid Model(NGM), and the Aviation (AVN) model.The performance of these three models willbe discussed. While the individual modelsolutions differed slightly from one another,their overall performance with respect toBertha was reasonably accurate.
Approximately 60 h prior to landfall, at1200 UTC 10 July, Hurricane Bertha waslocated at 25.4(N and 75.4(W, or about 55mi east of Eleuthera Island in the CentralBahamas. Bertha was moving to thenorthwest at 16 kt at this time. The 1200
UTC runs of the Eta, NGM, and AVN wereconsistent in showing that Bertha wouldtake a gradual turn to the north, bypassingFlorida and possibly making landfallsomewhere from Wilmington (ILM) to CapeHatteras, North Carolina (HAT).
A comparison of the three models showedvariable results from run to run. The Etamodel seemed to perform the best overall.It was the most consistent model from run torun, but occasionally overestimated thestorm's forward motion and was a bit to theleft of the actual track. In addition, the Etamodel initialized better than the othermodels at 0000 UTC 13 July, when thedecaying center of Bertha was over thecentral coastal plain of North Carolina. TheNGM had some problems at the 60- and 48-h projections, but performed well from 36 hto landfall. The AVN model solutions wereconsistently to the left of the actual trackand often overpredicted the storm's forwardmotion. The 1200 UTC 10 July (60-hforecast) run was an exception, as theforecast position was well to the right of thestorm's actual location at landfall.
a. Eta
At 1200 UTC 10 July, the Eta modelshowed remarkable geographical accuracyby placing Bertha near ILM at landfall.Unfortunately, it predicted the storm'sarrival about 12 h ahead of schedule. Thisrun of the model handled the 500-mbfeatures almost flawlessly. For the 0000UTC 11 July run, the Eta track shifted a bitto the left (Fig. 21), with landfall forecastedbetween Charleston (CHS) and MyrtleBeach (MYR), South Carolina. The timingwas once again about 12 h too fast, and thisrun seemed to downplay the strength of theshort wave (Fig. 22) approaching the midAtlantic area. The 1200 UTC 11 July Eta
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run was nearly identical to the previous run,and still 12 h too fast.
For the 0000 UTC 12 July model run, theEta forecast was very similar to the priortwo runs, but with a slight shift to the right,showing landfall along the northern SouthCarolina coast (Fig. 23). The timing of thisforecast was also improved with landfallpredicted closer to the time of the actuallandfall.
Only 8 h before landfall, the 1200 UTC 12July forecast track (Fig. 24) was 50 mi to theleft of the actual observed path of the storm.For the 0000 UTC 13 July run, the center ofBertha was initialized very close to theactual location of the hurricane at thesurface (Fig. 25).
b. NGM
The 1200 UTC 10 July NGM run (notshown) was inaccurate in its handling of the500-mb and surface features. Bertha wasreflected as an open weak short wave at 500mb, and the timing was almost 36 h too fast.The 0000 UTC 11 July run more accuratelypredicted the building ridge to the north ofBertha, but still underforecasted the strengthof the approaching short wave. There was alarge shift to the left with landfall predictednear CHS. However, for this projection, theNGM was the best of all three models withrespect to the timing of Bertha's landfall.
There was tremendous improvement in the1200 UTC 11 July NGM run. The surfaceforecast showed landfall near ILM, but 14 to16 h ahead of schedule. By the 0000 UTC12 July forecast, the NGM also showedimproved timing.
The 1200 UTC 12 July NGM run, less than12 h prior to the storm's landfall, wasexcellent. The surface features were nearly
perfect. However the timing was still just abit fast.
It is interesting to note that, for the 0000UTC 13 July NGM run, the model wasinitialized with Bertha centered about 50 misouth of the actual position 4 h afterlandfall.
c. AVN
The 1200 UTC 10 July AVN forecast wassimilar to the NGM for this run, 60 h priorto landfall. The timing was around 24 h toofast as the model put the center of thehurricane near HAT at 0000 UTC 12 July.The model overpredicted the speed of theapproaching short wave and failed toadequately depict the strength of the ridgingjust north of the system. For the 0000 UTC11 July AVN forecast, the model wassimilar to the NGM in depicting a large shiftto the left of its previous forecast track.Landfall was now projected to occur nearMYR and the timing was 12 h too fast. At500 mb, the model seemed to have a betterhandle on both the ridge to the north of thesystem and the short wave in the TennesseeValley.
At 1200 UTC 11 July, there was anotherlarge shift to the left in the forecast track ofBertha. The AVN now depicted landfallbetween CHS and Beaufort, South Carolina.The model now overpredicted the strengthof the 500-mb ridge to the north of Bertha.The effects of the short wave to the northwere minimal on this run. The AVNdeviated from both the Eta and the NGMsolutions and would have a hard timerecovering in the two subsequent runs.
Less than 24 h prior to landfall, at 0000UTC 12 July, the AVN was still depicting alandfall south of CHS. The surface low wasdisplaced too far west. The 1200 UTC 12July forecast from the AVN continued to
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1NM
N
1
Gi
Ri
1NM
N
1 (GiRi)/Gi �100%
project landfall well to the left of the actualtrack as the hurricane was forecast to makelandfall between CHS and MYR. Themodel had slowed the system down and wasnow about 6 h too slow. Once again, the500-mb features appeared reasonable, butthe surface low position was incorrectlyinitialized.
There was a questionable initialization onthe 0000 UTC 13 July AVN run as thecenter of the surface low was placed nearNewport/Morehead City, North Carolina, orabout 60 mi to the right of the actual centerof Bertha at that time.
5. WSR-88D PRECIPITATIONESTIMATES
Prior to Hurricane Bertha, the default Z-Rrelationship used by the KMHX WSR-88Dwas replaced by the tropical equation.Rainfall estimates from the radar wereobtained for the entire time that tropicalrains affected eastern North Carolina. Thetropical Z-R relationship was examined toassess the radar ’s per formance.Unfortunately, Archive Level II data areunavailable, so testing other relationshipsbased on data from this case was notpossible.
The Z-R relationship was changed fromZ=300R1.4 to Z=250R1.2 at 2148 UTC 11July. Only a few very light showers hadfallen up to this point so continuity betweenthe Storm Total Precipitation (STP) productand the rain gauge data was maintained. Atthis point, Hurricane Bertha was still well tothe south of North Carolina, but her largetropical circulation was beginning toimpinge on the region as more concentratedshowers were advancing onshore.
Fifty rainfall reports (Appendix IV) weregathered from various sources, such as localairports, NWS cooperative observers, NWSpersonnel, and trained spotters. Data fromthe 50 rain gauges were compared to STPtotals (Fig. 26) for the 30-h period from2148 UTC 11 July to 0332 UTC 13 July,using several different techniques (Gates1996). Mid-point precipitation estimatesbetween each STP threshold were used. Forexample, if the STP bin had an estimate of2.50 inches, it was recorded as 2.75 inches,the mid-point between 2.50 and 3.00 incheson the STP accumulation legend. For thisreport, only the bin closest to the gauge wasused, and only Mean Radar Bias (Eq. 1) andAverage Difference (Eq. 2) are discussed forthree geographic areas: the entire 124-n miradius KMHX radar umbrella, the MHXCWA, and longer ranges (75-124 n mi).
(1)
(2)
(Note: For both equations, G is the gaugemeasurement, R is the radar STP estimate,and N is the total number of gauge reports.)
Table 3 shows that the mean radar bias forthe entire 124-n mi range was 1.47 with anaverage difference of 29.4%. The WSR-88D precipitation processing algorithmsunderestimated rainfall amounts ascompared to gauge reports. (A bias of 1 andan average difference of 0% are consideredperfect.)
When the analysis was limited to the NWSOMHX CWA, the radar's performance withthe new Z-R relationship was quite
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respectable, with a mean radar bias of 1.38,and an average difference of 25.7%. TheMHX CWA is relatively small, with 90% ofthe area within 75 n mi of the radar. Theresults represent an objective view of howthe radar performed at the close and mediumranges.
Results for the longer ranges were not asfavorable with a bias of 1.61 and an averagedifference of 34.9%. This is reasonablegiven that the beam height increases from8,000 ft to 17,000 ft as range increases from75 n mi to 124 n mi when the radar isscanning at the 0.5( elevation angle. Theheight of the beam at longer ranges is likelyovershooting the heavy rain cores,especially in tropical situations where thehighest reflectivities are typically located atrelatively low altitudes.
An average difference of 29.4% does notproduce significant errors for lighter rainfallamounts of 1 to 2 inches. However, rainfallamounts between 4 and 6 inches wouldresult in estimates that would be off byabout 1.2 to 1.8 inches. With excessiverains of greater than 10 inches, estimateswould be off by 3 inches or more.
These figures are encouraging, especiallywhen compared to the radar's performanceduring Hurricane Gordon in November1994. A local study involving radar-gaugecomparisons during Hurricane Gordonshowed that WSR-88D rainfall estimateswere underestimated by 50% to 70%(Thacker 1996). Another study (Choy et al.1996) at the NWS office in Melbourne,Florida, indicated a 46% underestimation ofrainfall during tropical rains. During both ofthese events, the default Z-R relationshipwas used.
6. EVACUATION AND PUBLICAWARENESS
Hurricane Bertha came at the peak of thetourism season. The Coastal CountyEmergency Management coordinators havea copy of the HURREVAC computerprogram (Townsend 1984) that helps themmake evacuation decisions for hurricanes.They compare the output f romHURREVAC to the information containedin a comprehensive hurricane evacuationstudy (U.S. Army Corps of Engineers 1987).A strike probability of 30% is a criticalthreshold in the evacuation decision makingprocess. A tourist occupancy of MEDIUMwas used in HURREVAC, whichcorresponds to landfall during a summerwork week. An evacuee response ofMEDIUM was input because of the wetroads from rainfall prior to the storm. Withthese conditions, it was estimated that itwould take around 9 1/4 h to evacuate DareCounty (Hatteras Island) and around 20 h toevacuate the Outer Banks of Hyde County(Ocracoke Island). Given the sameconditions, Carteret County could beevacuated in 9 1/2 h, and Onslow County inabout 10 h.
A mandatory evacuation of Ocracoke Islandbegan at 1230 UTC 10 July. Hatteras Islandwas ordered to begin evacuating at 1400UTC. Shelters were opened in Virginia, andin North Carolina at Greenville andWilliamston. The policy of the EmergencyManagers in this area is to open up thedistant shelters first and save the closer onesfor any last-minute evacuees.
The decision was made late on 10 July tobegin evacuation of the Bogue Banks(Carteret County's southern shore)beginning at 1000 UTC 11 July. Residentsin mobile homes and low lying areas in theremainder of the county were also urged to
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evacuate. Carteret County also opened upshelters.
Onslow County issued a voluntaryevacuation order for North Topsail Beach,Surf City, and all low-lying areas andmobile homes at 1300 UTC 11 July. CravenCounty recommended evacuation ofresidents living in flood prone areas near theNeuse and Trent Rivers and those in mobilehomes. A mandatory evacuation of theremainder of Dare and Hyde Counties,including the mainland areas, began in themorning of 12 July.
When the hurricane struck, NWSO MHXkept residents informed through liveinterviews with WKOO radio station inJacksonville, NOAA Weather Radiobroadcasts, statements to the WeatherChannel and other media outlets, and byamateur radio. The NWSO MHX BulletinBoard Service for emergency mangers,which contains all the latest hurricanewatches/warnings, advisory statements,tornado watches/warnings, etc., wasaccessed many times during the event.
During Hurricane Bertha, more than 20telephone conference calls were held withcounty emergency managers and the areacoordinator. The purpose of the calls wasto keep local officials apprised of thedeveloping situation and to supplementinformation provided by the TPC. Inaddition, the latest Hurricane Hotlineinformation was relayed. At times,conference calls were held every hour, butfor the most part took place at 3-h intervals.
Ironically, North Carolina's HurricaneAwareness Week was scheduled for theweek of 14-20 July. Needless to say, it wascanceled.
7. THE REMAINDER OF THE 1996HURRICANE SEASON
On 5 September 1996, Hurricane Franstruck eastern North Carolina at nearly thesame place that Hurricane Bertha did inJuly, but continued inland instead ofrecurving back out to sea. Personalobservations indicated that damage fromFran was less widespread than with Bertha,but the destruction that did occur was moresevere. Bertha had already cleared out theweak trees, power lines, roofs, houses andfoliage. Thus, although Fran was morepowerful than Bertha, the area was muchless susceptible to the extent of damage thatwas seen with Bertha.
Tropical Storm Josephine moved from theGulf of Mexico into eastern North Carolinaon 8 October 1996. This storm wasconsiderably weakened by her long trackover land. However, rainfall amounts fromJosephine were higher in some areas thanduring either Bertha or Fran. Road floodingfrom Bertha and Fran had just subsidedwhen Josephine moved over the area,causing water levels to rise once again.
8. CONCLUSION
Hurricane Bertha, a category 2 storm on theSaffir-Simpson scale, made landfall betweenNorth Topsail Beach and WrightsvilleBeach at 2000 UTC 12 July 1996. Thetropical system moved rapidly throughEastern North Carolina, becomingextratropical before exiting off the coast ofthe northeastern United States.
Several factors combined to create record tonear record flooding across eastern NorthCarolina: the location of Bertha’s landfall,which placed NWSO MHX, and our CWA,in the dangerous northeast quadrant of the
11Eastern Region Technical Attachment 98-1
storm; the relatively slow movement of thestorm; her arrival near high tide; high waterlevels from previous heavy rains; and, thelarge water volume, and shallow nature ofthe Albemarle and Pamlico Sound drainagebasins. In fact, the flooding associated withBertha was the most significant in east-central North Carolina in nearly 40 years,and in some locations reached record levelsof 7 to 10 ft.
A comparison of three numerical modelsshowed variable results from run to run.The AVN model was consistently to the leftof the actual track and greatly overestimatedthe forward speed of the storm. The NGMhad its problems as well at the 60- and 48-hprojections, but performed well from 36 h tolandfall. The Eta model seemed to be thebest overall performer. It was less variablefrom run to run, but occasionallyoverestimated the forward speed of thestorm and was a bit to the left of the actualtrack.
Of the three storm surge models available,the SLOSH model provided the bestinformation for forecasters, but the CAPESmodel also did well. The ADCIRC model, anon-tropical storm surge model, had thepoorest results.
The default WSR-88D Z-R equation waschanged to a more tropical relationship andresultant rainfall estimates were underdoneby about 30%. This is encouragingconsidering previous studies using thedefault Z-R relationship showed thatprecipitation amounts were underestimatedby between 50% and 70%.
Maximum reported sustained winds were 81mph with gusts reported as high as 108 mph.The highest rainfall amount reported was8.06 inches and storm tides reached as highas 10 ft. Many areas were flooded, while
sand dunes and piers along the beaches andmany inland crops were destroyed. Onlytwo people died in North Carolina duringHurricane Bertha, both as an indirectconsequence of the hurricane.
ACKNOWLEDGMENTS
The authors wish to thank Tim Obannon(OSF), Dan Berowitz (OSF), Central Wills(DAPM, NWSO MHX), Tom Addington(DAPM, NWSO ILM), DeborahMoneypenny (DAPM, NWSFO RAH) andFay Crossley (DAPM, NWSO AKQ) forsupplying numerous NWS cooperativeobserver rainfall reports and to JulieAdolphson (COMET) for providing someEta model output. Thanks also go to BonnieTerrizzi, James Ireland and Tom Kriehn fortheir help and suggestions.
REFERENCES
Choy, B. K., L. Mazarowski, and P. Glitto,1996: Tropical Storm Gordon: 72-hr rainfall totals over east centralFlorida and WSR-88D comparisons.NOAA Technical MemorandumNWS SR-174, National Oceanic andAtmospheric Administration, U.S.Department of Commerce, 99 pp.
Gates, C., 1997: Hurricane Bertha: radar-gauge comparisons and changing theZ-R relationship. Eastern RegionWSR-88D Operations Note, No. 10,National Weather Service, NOAA,U.S. Department of Commerce, 12pp.
Jelesnianski, C. P., J. Chen, and W. A.Shaffer, 1992: SLOSH-Sea, lakeand overland surges from hurricanes.
12Eastern Region Technical Attachment 98-1
NOAA Technical Report NWS-48,National Oceanic and AtmosphericAdministration, U.S. Department ofCommerce, 71 pp.
Klazura, G. E., and D. A. Imy, 1993: Adescription of the initial set ofanalysis products available from theNEXRAD WSR-88D system. Bull.Amer. Meteor. Soc., 74, 1293-1311.
Luettich, R. A. , J. J. Westerink, and N. W.Scheffner, 1992: ADCIRC: Anadvanced three dimensionalcirculation model for shelves, coastsand estuaries. Report 1: theory andmethodology of ADCIRC-2DDI andADCIRC-3DL. DRP TechnicalReport DRP-92-6, U.S. Army Corpof Engineers, 137 pp.
National Weather Service, 1996: A newstorm surge atlas for the PamlicoSound, North Carolina area.National Oceanic and AtmosphericAdministration, U.S. Dept. ofCommerce, 167 pp.
Neuherz, R. A., L. Xie, L. J. Pietrafesa, andK. K. Keeter, 1997: Forecastingstorm surge in the Albemarle andPamlico Sounds induced byhurricanes during the 1996 hurricaneseason. Preprints 22nd Conference onH u r r i c a n e s a n d T r o p i c a lMeteorology, Fort Collins, Amer.Meteor. Soc., 6 pp.
Thacker, R., 1996: A subjective comparisonof the WSR-88D storm totalprecipitation products and rain gaugemeasurements for hurricane Gordonin North Carolina on November 16-18, 1994. Unpublished local report, 9pp. Available from NationalW e a t h e r S e r v i c e O f f i c e ,Newport/Morehead City, NorthCarolina.
Townsend, J. F., 1984: A computercalculation and display system forSLOSH hurricane surge model data.NOAA Technical MemorandumNWS ER-67, National Oceanic andAtmospheric Administration, U.S.Department of Commerce, 19 pp.
U.S. Army Corps of Engineers, WilmingtonDistrict, 1987: Eastern NorthCarolina hurricane evacuation study.North Carolina Division ofEmergency Management. FederalEmergency Management Agency.
13Eastern Region Technical Attachment 98-1
Table 1. The Saffir-Simpson hurricane scale and the amount of damage that can be expected from storms within each category. Some recent storms (and their location of landfall) are also listed for each category.
Category Winds (mph) Damage Examples
1 74-95 minimal Florence 1988 (LA)
Charly 1988 (NC)
2 96-110 moderate Kate 1985 (FL)
Bob 1991 (RI)
3 111-130 extensive Alicia 1983 (N TX)
Emily 1993 (NC Outer Banks)
4 131-155 extreme Andrew 1992 (S FL)
Hugo 1989 (SC)
5 >156 catastrophic Camille 1969 (LA/MS)
Labor Day Hurricane 1935 (FL Keys)
Table 2. Storm surge forecasts from the CAPES model as compared to observed values during Hurricane Bertha.
Western Albemarle Sound Western Pamlico Sound
Forecast 5-6 ft 7-8 ft
Observed 4-5 ft 6-7 ft
Table 3. Mean radar bias and average difference for Hurricane Bertha using the tropical Z-R relationship, Z=250R1.2. Values are calculated for three geographic regions: the entire 124-n mi radius KMHX radar umbrella, the MHX CWA, and longer ranges (75-124 n mi).
0-124 n mi CWA 75-124 n mi
Mean Radar Bias (Eq. 1) 1.47 1.38 1.61
Average Difference (Eq. 2) 29.4% 25.7% 34.9%
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Figure 1. Map of eastern North Carolina with sites of interest indicated.
Figure 2. Track of Hurricane Bertha. See Appendix III for a list of official 3-h positions along this track.
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Figure 3. Visible satellite picture from 2115 UTC 11 July 1996.
Figure 4. 0000 UTC 12 July 1996, 300-mb heights (dm) and winds (kt). Contours are drawn every 60 dm.
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Figure 5. Infrared satellite picture from 0915 UTC 12 July 1996.
Figure 6. As in Figure 4, except for 1200 UTC 12 July 1996.
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Figure 7. Newport/Morehead City, North Carolina, WSR-88D (KMHX) base reflectivity product at 1345 UTC 12 July 1996. The color table on the right depicts the intervals of reflectivity in units of dBZ. The elevation angle is 0.5E, range of coverage is 248 n mi (460 km), and resolution is 1E x 1.1 n mi (2km). The maximum reflectivity is 55 dBZ.
Figure 8. Visible satellite picture from 1400 UTC 12 July 1996.
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Figure 9. As in Figure 7, except for 1812 UTC 12 July 1996. The range of coverage is 124 n mi (230 km) and the resolution is 1E x .54 n mi (1 km).
Figure 10. KMHX Velocity Azimuth Display Wind Profile product for 1714 UTC to 1812 UTC 12 July 1996. Vertical axis is altitude in thousands of feet. ND indicates no data. Wind barbs follow standard meteorological convention.
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Figure 11. As in Figure 9, except for 2002 UTC. The maximum reflectivity is 53 dBZ.
Figure 12. As in Figure 8, except for 2000 UTC.
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Figure 13. As in Figure 10, except for 1904 to 2002 UTC.
Figure 14. As in Figure 9, except for 2253 UTC.
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Figure 15. As in Figure 10, except for 2155 to 2253 UTC.
Figure 16. As in Figure 9, except for 0159 UTC 13 July 1996. The maximum reflectivity is 54 dBZ.
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Figure 17. Storm surge flooding from Hurricane Bertha at Belhaven, North Carolina, as compared to historical storms.
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Figure 18. Storm surge forecast from the SLOSH model for a category 2 hurricane moving north-northeast at 15 mph. Contours are drawn every 1 ft. Shaded areas indicate inundated land areas.
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Figure 19. Storm surge forecasts from the ADCIRC model for the Albemarle and Pamlico Sounds valid 0100 UTC 13 July 1996. Surge heights are contoured every 0.2 m.
Figure 20. As in Figure 19, except for the Atlantic Basin valid 2000 UTC 12 July 1996.
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Figure 21. Eta model 48-h surface pressure forecast valid 0000 UTC 13 July 1996. The contour interval is 4 mb.
Figure 22. Eta model 48-h forecast of 500-mb heights valid 0000 UTC 13 July 1996. The contour interval is 60 m.
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Figure 23. As in Figure 21, except for the 24-h forecast.
Figure 24. As in Figure 21, except for the 12-h forecast.
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Figure 25. As in Figure 21, except for the 00-h forecast.
Figure 26. KMHX Storm Total Precipitation product for the time interval from 2143 UTC 11 July 1996 to 0332 UTC 13 July 1996. Range of coverage is 124 n mi (230 km) and resolution is 1.1 n mi x 1.1 n mi (2 km x 2 km). The color table on the right depicts the intervals of estimated precipitation accumulation in inches. The maximum accumulation is 10.8 inches.
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APPENDIX I
Comparison Between Bertha and Other Memorable Hurricanes in North Carolina History
StormName
Date of Landfall Location ofLandfall
HighestWinds(mph)
Fatalities HighestStorm Surge
(ft)
8 August 1879 138
September 1883 53
16 September 1933 21 7
Hazel 15 October 1954 Little River 150 19
Ione 19 September 1955 Salter Path 125 5 11
Donna 1 September 1960 Topsail Island 100 8 8
Diana 14 September 1984 Brunswick City 92G100 6
Gloria 20 September 1985 Cape Hatteras 74G86 8
Bertha 12 July 1996 N. Topsail Island 108 2 10
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APPENDIX II
Damage and Effects From Hurricane Bertha
Highest Sustained Wind 81 mph New River (NCA), Jacksonville 1621E
Rainfall Amounts 6.50" Broad Creek5.44" Snow Hill5.09" Cherry Branch (5 mi east of Havelock)4.56"+ Havelock (top cap of rain gauge blew off and "a lot of
rain was sucked out")4.11" Greenville4.10" Williamston2.95" Newport8"+ estimated by radar over northern Craven and eastern
Carteret Counties
Storm Tides 8 ft Swansboro8 to 10 ft Emerald Isle7 ft Belhaven and Washington6 ft Bath4.5 ft Pungo RiverS. Pamlico River 3 ft above normal
Beach Erosion Sand dunes were breached in Emerald Isle and North Topsail Beach.
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Flooding Swansboro had 5-6 ft of water inside businesses on water front.Queens Creek Bridge just outside of town out. In Otway, WardsCreek bridge was washed out. Water flowed through Belhavenwith knee deep water in the town hall. Washington waterfrontunder water...water came within 1 ft of going over Route 17drawbridge. Ocean overwash on northern end of Ocracoke,Hatteras Island north of Buxton, and in Frisco.
Tornadoes One reported on highway 24 in Newport, another on highway 17, 6mi southeast of New Bern, and a third in the western portion of theAlbemarle Sound.
Damage Heavy damage to tobacco and corn crops in at least 12 eastern NC counties. The storm demolished fishing piers and stripped roofs from homes. In Onslow county, 33 single family homes, 87 mobile homes, 10 apartment buildings and 13 businesses were destroyed. Damage in Onslow County alone exceeded $100million. In New Bern, numerous boats were sunk and damaged at3 private marinas. Trees and power lines were downed all acrossarea. An estimated 186,000 customers were without power orphone service.
Deaths Two people died, one in a traffic accident in Kitty Hawk, the otherwhile working on a generator the day after the storm struck (electrocuted).
Evacuation The North Carolina Division of Emergency Managementestimated 250,000 people evacuated ahead of the hurricane.
Transportation Amtrak briefly curtailed service on a number of trains that travel through North Carolina. No alternate transportation was provided.
All operational Coast Guard aircraft normally stationed atElizabeth City, North Carolina were relocated pending the passageof the storm.
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APPENDIX III
Three-Hourly Positions Of Hurricane Bertha And Maximum Sustained Winds
Date/Time (UTC) Latitude Longitude Maximum Wind (kt)
Rainfall Reports (inches) From Hurricane Bertha And Distance (n mi) From KMHX RDA
Station Amount Distance
Broad Creek 6.50 6Cherry Point 5.39 8Lands End 4.62 9Morehead City 3.87 9Cherry Branch 5.09 105NW Havelock 6.25 11New Bern FAA 4.74 19Trent Woods 5.90 20Hoffman Forest 8.06 21Trenton 4.84 29Cedar Island 1.71 31Hobucken 2.80 32Aurora 4.24 375 SE Kinston 4.79 42Belhaven 4.0 471 E Washington 5.5 47Kinston Ag R 4.88 49New Holland 3.03 53Nr ILM AP 4.7 57LaGrange 4.75 56Snow Hill 5.72 57Snow Hill 5.44 57Greenville 4.86 57Greenville 4.36 575 SE Warsaw 3.45 58
