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?