Elevated Mixed Layers and their Role in Significant Severe Thunderstorm Episodes in the Northeastern U.S.
Michael L. EksterNOAA/National Weather Service
Upton, NY
Peter C. Banacos NOAA/National Weather Service
Burlington, VT
Outline of Presentation• Study purpose and objectives
• EML fundamentals• structure
• formation
• movement
• Data and Methodology
• Study Results
• Case Studies
• Conclusions
• Future Work
Study Objectives• We know that EMLs play a large role in severe
weather episodes in the Great Plains
• Find out if EMLs play a part in high-end severe weather episodes in the Northeast U.S.
• How do they play a part?
• Examine synoptic scale patterns conducive to EML advection into the Northeast
• Increase forecaster awareness in the Northeast
A severe weather environment that includes an EML usually results in high-end convection. Why?• The EML prevents deep, moist convection
until high potential instability is achieved (bottom of EML acts as a lid or cap).
• In the absence of deep, moist convection, warm, moist low level air can flow poleward in an unimpeded manner (underrunning).
• Tendency to keep storms from becoming overly widespread (the exception is for severe MCSs).
• Prevention of deep vertical mixing. Generally does not allow SFC dewpoints to mix out.
• Very steep lapse rates in mid levels enhances CAPE, NCAPE = fast updraft accelerations.
• DCAPE enhancement
Elevated Heating = Steeper Lapse Rates
Warm
Cold
Cool
Cold
In the absence of widespread diabatic processes, EMLs are advected downstream without changing much character at all:
From Jon Finch, NWS DDCThis is a big key!
The largest contribution to time local rate of change of lapse rate is horizontal advection.
Elevated Mixed Layers in the Northeast U.S. ?
• Much research has been done on the Great Plains EML. Occurrence there is quite frequent – much closer to the source region.
• Very limited research has been conducted on the occurrence of EMLs in the Northeast.
Common in the plains, but quite rare here!
- Farrell and Carlson, 1989 (5/31/85 PA outbreak)
- Lanicci and Warner, 1991
- Bentley, 1995 (7/15/95 Derecho)
- Johns and Dorr, 1996
- LaPenta et al., 2002 (5/29/95 Great Barrington, MA)
Data and Methodology
• Used John Hart’s (SPC) SVRPLOT program to search for “higher-end” severe weather reports in the Northeast over the last 40 years or so.
• The term “high-end” is subjective.
• SPC considers individual events significant when the following criteria are met:
-Hail size > or = 2” in diameter
-Convective wind gust >64 kt (hurricane force)
-F2 or greater tornado
Data and MethodologyBreaking down the numbers for the Northeast…
• Of all severe weather reports since 1960…
- Less than 10% were high-end
- Approx. 15-20 reports per year
- Most were contained in only 1-2 episodes (or “severe weather days”)
- The high end cases accounted for
approximately ¾ of all fatalities
• Must be careful – spotter reports are sometimes unreliable!
Data and Methodology
• After determining what days to use in the study, sounding analyses were conducted for Northeast upper air locations in an attempt to find EMLs
• Plymouth State, UWY, NCDC
• Sounding analysis program (RAOB)
Data and Methodology• If a suspected EML was found in any of the
soundings, the following criteria had to be met:
-No upper low directly over region
-Lapse rates >=8.0 C/km over a depth >=200 mb
-RH values in the layer must INCREASE with height
-EML must be traceable back to the American/Canadian Rockies or the Mexican Plateau
(trace back EML manually then confirm with trajectories - ARL)
• Careful about sounding errors!
Confirming the sounding does contain an EML – trace back to source region
Killer tornado day Mass Pike
28 Aug 1973
Run a backward trajectory analysis
• NOAA ARL trajectory website
• Hysplit Model run using NCEP/NCAR reanalysis dataset (1940s – Present)
http://www.arl.noaa.gov/ready/open/traj.html
ELP 12z 25Aug73 LBF 12z 26Aug73 SSM 12z 27Aug73 ALB 12z 28Aug73
Soundings courtesy of Plymouth State
Northeast U.S. EML Source Regions – same source region as Great Plains EML
Early Season (April-June)
Mid-Late Season (July-Sept)
EML advection into the Northeast
• It’s quite amazing to know that EMLs can travel 2000-3000 km from their source region without changing much in character!
• Most were advected in on WNW or NW flow in the mid levels
• Majority of severe weather outbreaks in the Northeast are associated with NW flow
Mid level pattern conducive to EML advection into the Northeast U.S.
Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D-3
Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D-2
Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D-1
Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D0
Results• Severe weather days associated with EML
occurrences varied from year to year
• On severe weather days where EMLs were present, over 85% of those days contained numerous high-end severe weather reports
• Most EMLs advected in on WNW or NW flow aloft
• Recent EML severe weather events (88D era) were shown to contain numerous supercells
• 6 of the last 9 derechos that affected the Northeast were accompanied by EMLs
A few cases
From Johns et al., “About Derechos”
Johns, et al., “About Derechos””
PIT 00z 7/15/95
OKX 12z 7/15/95
8-9 June 1953
JFK, 15z RME, 15z
How do I look for an EML on shift?
• Use Volume Browser to load model 700-500 mb lapse rates. Look for anomalously high values upstream that seem to eject out of the Rockies
• Very high 700 mb temperatures – during the warm season, EMLs are most commonly associated with values greater than +12C which acts as the “lid” or “cap”
• Confirm using forecast soundings
• Upstream RAOBs
• SPC mesoanalysis graphics – mid level lapse rates
• ACARS
• EMLs almost always advect to the East Coast on mid level flow that is from the west or northwest – rarely from the southwest and never from the south
Conclusions
• EMLs are rare in the Northeast
• Given the right synoptic conditions, they do make it here
• If a forecaster is expecting a severe weather episode on a given day, and an EML is overhead or directly upstream, a high-end severe weather event should be expected!
• More confidence in using enhanced wording in the HWO, ZFP, SPS to highlight potential impact
Future Work
• Null Cases
• Closer look at possible EML augmentation during advection
• Relationship to severe weather mode
• In-depth climatological study – need some code
• Real-time case studies
Questions?Comments?
