Alaskan Autumn Storm of 22-24 November 2010 By

Richard Grumm National Weather Service State College, PA 16803

and Rick Thoman and James A. Nelson

National Weather Service Alaska Region 1. INTRODUCTION A surge of tropical moisture (Figure 1) over the western flank of an anomalously strong and poleward shifted subtropical ridge (Fig. 2) produce rain over portions of Alaska on 22-24 November 2010. Liquid rainfall over inland regions of Alaska is rare during the cold season. Thus rainfall amounts approached record levels in many locations. Temperatures were much above normal, as much as 25F above normal. Barrow went above freezing. The cold ground caused freezing rain. In Fairbanks, the rain froze on the roads and exposed ground until the ground temperatures rose above freezing. During the event portions of central AK received relatively rare liquid rain which approached record values. Barrow's maximum temperatures on the 22 and 23 of November were about 25 degrees above normal. Anchorage and Fairbanks suffered from an extended period of freezing rain on the “deeply frozen ground”. The freezing rain continued until the surface ground temperatures warmed above freezing. Higher elevations received heavy snow from the weather system. Rain and freezing rain in interior Alaska are unique events. Fairbanks had over 0.60 inches of rainfall, one of the largest winter season rainfall events on record. The record event occurred on 20 January 1937 when 0.99 inches of rain fell on the city. Several other locations set daily precipitation records (Need table of amounts and previous records for Barrow etc). True freezing rain, with ice on trees and power lines is also a relatively rare event. The last true freezing rain events in Anchorage were observed in

1995 and 1980 (exact dates would be useful). The value of anomalies in identifying potential high impact weather events (HIWE) has been demonstrated by Hart and Grumm (2001), Grumm and Hart (2001), and Graham and Grumm (2010). The first two studies focused on the eastern United States and the latter on the western United States. Junker et al. (2008) showed the value of precipitable water anomalies to identify atmospheric rivers (AR: Ralph et al 2008) and to characterize the potential for heavy rainfall along the West Coast of the United States. These concepts were applied to the NCEP GEFS and it was shown that forecasts of anomalies may aid forecasters in better anticipating heavy rainfall events (Junker et al 2009. It will be shown that the Alaskan event of the 22-24 November 2010 was associated with a strong signal when examined using standardized anomalies. This paper will document the rare Alaskan warm rain event of 22-24 November 2010. The focus is on the anomalous pattern that produced the unique conditions and NCEP GEFS forecasts of the event. 2. METHODS The overall pattern was reconstructed using the 1.25x1.25 degree Japanese Re-analysis data (JRA25:Onagi 2007). The anomalies were derived using the JRA25 and comparing it to the 30-year mean and standard deviations computed from the NCEP/NCAR re-analysis data (Kalany et. al 1996). All anomalies herein are shown as standardized anomalies (Hart and Grumm 2001).

The NCEP GEFS was used to show forecasts of the event. Standardized anomalies were computed using the ensemble mean and the NCEP/NCAR re-analysis data. Precipitation forecasts are shown in milli-meter and in probabilistic categories of occurrence. 3. RESULTS i. large scale pattern The the precipitable water pattern and the 500 hPa pattern in 24-hour increments are shown in Figures 1 & 2 respectively. These data show a evolution of a sharp subtropical ridge over Alaska with 3 to 4 σ 500 hPa height anomalies by 18/0000 UTC with a closed 5760 m contour over southern Alaska at 19/0000 UTC. There was a surge of high PW air into the region with PW anomalies peaking over 6σ above normal at 23/0000 UTC. As the subtropical ridge developed, the 250 hPa jet over the ridge (Fig. 3) increased in strength. The total wind anomalies in this features were 2 to 4σ above normal and peaked at 5σ above at 19/0000 UTC. As this enhance jet shifted eastward, a strong jet entrance circulation move over much of central and southern Alaska (Fig. 3g-i). ii. Regional pattern The mean sea-level pressure (MSLP), PW, and 925 hPa temperatures over the region are shown in Figures 4-6 respectively. Two surface anticyclones were present, a cold arctic anticyclone was present over northwestern Canada (Fig. 4a-d) and a large northward displaced subtropical anticyclone was present over the north Pacific basin (Figs. 4-i). The flow about the latter feature was associated with the surge of high PW air into Alaska. These data show several periods of PW anomalies in excess of 6σ above (Fig.5b,c, and e) with many periods of 3 to 5σ above PW values.

In addition to the warm air, the subtropical anticyclone pushed abnormally warm air at 925 hPa (Fig. 6) and 850 hPa (not shown). The deep warm air produced conditions warm enough for rain. The arctic anticyclone was associated with cold air with -12 to -30C air over southeastern Alaska and northwestern Canada. This retreating cold air mass left the cold ground over central Alaska which produced the freezing rain at the ground during the event. The 850 hPa winds and wind anomalies are shown in Figure 7. These data show a narrow region of above normal 850 hPa winds which moved over western Alaska at 22/0000 UTC moving into interior sections by 22/1200 UTC. A second surge of modestly above normal winds moved into southwestern Alaska on from 22/1800 through 24/0000 UTC. The high terrain may have limited the deep penetration of these winds into interior sections. iii. GEFS forecasts For brevity 9 GEFS forecasts for each variable are shown. Only the ensemble mean and standardized anomalies are presented. More appropriate ensemble probability displays are limited to a few fields. Figure 8 shows the GEFS mean PW field and standardized anomalies from 9 runs all valid at 24/0000 UTC. These data show the surge of high PW air into AK at 24/0000 UTC. The consistent PW forecasts showed a surge of 10 to 20 mm PW values with a large area of 3 to 5σ anomalies over central AK with some areas of 6σ anomalies in some of these forecasts. These large anomalies imply low spread amongst ensemble members. The 500 hPa and 250 hPa heights and winds (Figs. 9-10) showed a consistent pattern over AK as did the 850 hPa temperatures (Fig. 11) which showed the surge of abnormally warm air into the region. These

forecasts, with large anomalies imply low spread and convergent forecasts. Based on the pattern and the low-level temperatures in the GEFS, not surprisingly, the GEFS predicted a wide swath of 16 to 32 mm (0.70 to about 1.5 inches) of liquid equivalent precipitation for the period ending at 1200 UTC 24 November 2010 (Fig. 12). Plume diagrams from the 22/0000 UTC GEFS for Anchorage and Fairbanks are shown in Figure 13. These data show the initial onset of freezing rain in Anchorage followed quickly by rain for the 23r and 24th of November. The GEFS showed around 1 inch (25mm) of rain in Anchorage with a range of 0.39 to 1.59 inches. Fairbanks showed a similar evolution with a higher mean rainfall (1.26 inches) and a higher maximum rainfall. The plumes, showing rain as the primary precipitation type in Fairbanks suggest very warm profiles which are shown in Figure 14. These data show the above freezing 2m and 850 hPa temperatures during the initial surge of precipitation. iv. impacts and significance During winter, defined at November through March, liquid precipitation is a rare event in Fairbanks (65N/148W), Alaska. Amounts in excess of 0.10 inches are considered to be extremely rare events. Climatologically, in the past 50 years there have been just 10 winter rain events with 0.10 inches or more of liquid precipitation. These events typically occur early and late in the season with 3 of 1 of all events observed during the first half of November. The total liquid rainfall of 0.95 inches on 22-24 November 2010 was the second greatest winter rain event at Fairbanks since 1904. The record rainfall, of 0.99 inches was set on 20 January 1937.

Since hourly observation began in March 1941 the 39 hours of rainfall on 22-24 November 2010 is a new record. There was a rainfall event of similar duration from 2-3 November 1935. In addition to the rainfall record, the temperature remained near 35F and remained above freezing for 49 consecutive hours. The low of 33F was a record high low and tied for the record high low for the month of November. Since the commencement of hourly observations this was the longest period of above freezing temperatures recorded at Fairbanks. It should be noted that from 6-9 December 1934 there was a period of 60 hours of above freezing temperatures. There may have been a long period of above freezing temperatures during the 2-3 November 1935 rain event. On 22 November 2010 the snow mixed with rain and ice pellets at Barrow, Alaska (71N/157W) then briefly changed to rain. While not unprecedented, rain this far north is an extremely rare in winter. At Nome, Alaska, (64N/165W), 1.27 inches of precipitation fell on the 20-21 November, nearly all of which fell as rain. This set the greatest rainfall amount record at Nome for November. At McGrath (63N/156W) 1.20 inches of precipitation on 22 November was the greatest calendar day precipitation in November, though it was not a 24 hour precipitation record. The 23/0000 UTC rawinsonde at Fairbanks recorded 0.74 inches precipitable water (PW), the second

highest PW value on record since 1957 second only to exceeded 0.75" set on 2 November 2003. In Anchorage, there was 0.75 inches of precipitation during the event of which 0.06 inches fell as freezing rain. Due to the cold ground this caused serious problems to transportation despite temperatures near 34F during the event. There were over 30 vehicle accidents with 90 vehicles impacted. Icy roads kept schools closed for 3 days and cause the cancelation of many flights out of the Anchorage airport. Anchorage received 75% of its monthly precipitation during the 3 day event and set a daily precipitation record on 23 November 2010. In the Anchorage area this storm was comparable in impact to the event of 20-21 January 2008 (Fig. 15) when rain also fell in Anchorage. The PW values of 15 mm penetrated the region with PW anomalies on the order of +3 to +4σ above normal. The rawinsonde value of 0.81 inches on 23 November 2010 was a 99th

percentile value for the month.

4. CONCLUSIONS A surge of anomalously high PW air penetrated the State of Alaska on 22-24 November 2010. This surge of high PW was accompanied by above normal temperatures and an unseasonably strong subtropical ridge. The 500 hPa heights in southern Alaska exceed 5760 m in the early stages of the event. This warm moist air brought a rare cold season rain event to the State. This rare cold season rain event was relatively well predicted by the NCEP GEFS (Fig 12). The GEFS showed the potential for a significant QPF event and conditions warm enough to support the potential for

rainfall in lower elevations (Fig. 11). Overall, 9 different GEFS forecasts showed a high potential for the surge of above normal moisture, temperatures, and the strong 250 hPa jet over the massive subtropical ridge. These relatively consistent forecasts implied the potential for a significant cold season event to include rain over the region. The PW pattern and strong anomalies suggest and AR (Raph et al 2006) impacted Alaska. With a source of the air from the central Pacific, the air was warm and moist, each contributing factors to the high impact weather event (HIWE) in central Alaska. The strong subtropical ridge and anomalies 500 hPa heights with this system played a key role in the ability to transport the warm moist air into Alaska. The surge of high PW air and rain in November 2010 occurred with an MEI value of -1.606 for the Nov-Dec time period. The event of January 2008 also during the cold phase of ENSO with an MEI of -0.965. During November 2003, another rain event, the MEI as 0.361, a warm episode. The November 2003 event had PW anomalies over 5σ above normal (Fig.16). The large anomalies in both the JRA25 data and GEFS forecasts show that like many HIWE’s, standardized anomalies were of value in anticipating this event. Graham and Grumm (2010) and Hart and Grumm (2001) showed the value of standardized anomalies in identifying significant weather events. Clearly, a similar study could be conducted for Alaska. A study of this type might aid in identifying HIWE from the past to improve forecasting of similar events in the future. 5. Acknowlegements The AK Region of data and information on the event. PSU for data access in real-time. 6. References

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Graham, Randall A., Richard H. Grumm,

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Appendix-I AK-Region FTR excerpts.

• A WIDESPREAD WINTER WEATHER

EVENT CONTINUES TO BRING MIXED

PRECIPITATION AND TRANSPORTATION

ISSUES TO MUCH OF INTERIOR, NORTHERN AND SOUTHCENTRAL

ALASKA • BEGINNING MONDAY MORNING, MUCH

OF THE PRECIPITATION FELL AS EITHER

FREEZING RAIN OR RAIN THAT FROZE ON

“DEEPLY FROZEN,” SNOW-PACKED

ROADWAYS. WHILE UP TO 18 INCHES

OF SNOW WAS REPORTED NEAR THE

SOUTHERN SLOPES OF THE ALASKA

RANGE – THE RAIN/FREEZING RAIN IS OF

GREATEST IMPACT WITH ICE COVERING

NEARLY ALL OF ALASKA’S ROAD SYSTEM. • SUCH A WIDESPREAD EVENT OF THIS

MAGNITUDE IS EXTREMELY UNUSUAL. o FAIRBANKS HAS RECORDED

OVER 0.60”OF RAIN SINCE

MIDNIGHT. WITH ADDITIONAL

RAIN FORECAST, FAIRBANKS

WILL COME NEAR OR BREAK

THE JANUARY 20, 1937

RECORD OF 0.99” FOR

HIGHEST DAILY RAINFALL

DURING THE WINTER SEASON. o SEVERAL OTHER LOCATIONS

SET DAILY HIGH TEMPERATURE

AND PRECIPITATION RECORDS

THROUGHOUT THE STATE -

INCLUDING BARROW, ALASKA

(WHERE SNOW BRIEFLY

TURNED TO RAIN MONDAY). o ANCHORAGE LAST SAW A

“TRUE” FREEZING RAIN EVENT

IN 1995, PRECEDED BY 1980. o IN FAIRBANKS, THIS EVENT IS

SAID TO HAVE BROUGHT THE

CITY “TO A HALT” LOCATION: WIDESPREAD. FROM THE GULF

OF ALASKA TO BARROW AND NOME TO THE

CANADIAN BORDER. GENERALLY SPEAKING, AN

AREA THE SIZE OF TEXAS, NEW MEXICO AND

OKLAHOMA COMBINED • WFO FAIRBANKS ISSUED A WINTER

STORM WARNING FOR FREEZING RAIN

(A PROVISION IN ALASKA REGION’S

SUPPLEMENTS FOR RAINFALL ON COLD-SOAKED ROADS VS AN ICE STORM

WHICH ALSO IMPACTS TREES AND

POWER LINES). THIS WARNING WAS

ISSUED SUNDAY AFTERNOON WITH

APPROXIMATELY 14 HOURS LEAD TIME.

•

Figure 1. JRA precipitable water (mm) and precipitable water anomalies in 24 hour increments from a) 0000 UTC 16 November 2010 through i) 0000 UTC 24 November 2010. ANomlies in standard deviations from normal. Return to text.

Figure 2. As in Figure 1 except for 500 hPa heights (m) and 500 hPa height anomalies in standard deviations from normal. Return to text.

Figure 3. As in Figure 1 except for 250 hPa winds (ms-1) and total wind anomalies. Return to text.

Figure 4. As in Figure 1 except for JRA MSLP (hPa) and MSLP anomalies in 6-hour increments from a) 0000 UTC 22 November through i) 0000 UTC 24 November 2010. Return to text.

Figure 5. As in Figure 4 except for precipitable water (mm) and precipitable water anomalies. Return to text.

Figure 6. As in Figure 4 except for 925 hPa temperatures (C ) and temperature anomalies. Return to text.

Figure 7. As in Figure 4 except for 850 hPa winds and wind anomalies. Return to text.

Figure 8. GEFS ensemble mean forecasts of precipitable water and precipitable water anomalies valid at 0000 UTC 24 November 2010 from GEFS forecasts initialized at a) 0000 UTC 21 November, b) 1200 UTC 21 November, c) 0000 UTC 22 November, d) 0600 UTC 22 November, e) 1200 UTC 22 November, f) 1800 UTC 22 November, g) 0000 UTC 23 November, h) 0600 UTC 23 November and i) 1200 UTC 23 November 2010. Return to text.

Figure 9. As in Figure 8 except for GEFS 500 hPa heights and height anomalies. Return to text.

Figure 10. As in Figure 8 except for GEFS ensemble mean 250 hPa winds (ms-1) and wind anomalies. Return to text.

Figure 11. As in Figure 8 except GEFS 850 hPa temperatures ( C) and temperature anomalies. Return to text.

Figure 12. As in Figure 8 except for GEFS ensemble mean accumulated QPF (mm) valid for the period ending at 1200 UTC 24 November 2010. Return to text.

Figure 13. GEFS forecasts initialized at 0000 UTC 22 November 2010 showing precipitation by type at a point near Fairbanks and Anchorage, Alaska. Color coded data show the precipitation by type. The gray lines show the instantaneous 6-hour QPF for each member. . Return to text.

Figure 14. As in Figure 13 except for GEFS 2m temperatures, 850 hPa temperatures, and 700 hPa temperatures. Temperatures degrees C except for 2m temperatures degrees F. Return to text.

Figure 15. JRA25 precipitable water (mm) and standardized anomalies in 6-hour increments from a) 1200 UTC 19 January 2008 through i) 1200 UTC 21 January 2008. . Return to text.

Figure 16. As in Figure 15 except for the period of a) 0000 UTC 01 November 2003 through i) 0000 UTC 03 November 2003. Return to text.