1 / 34

Elevated Mixed Layers and their Role in Significant Severe Thunderstorm Episodes in the Northeastern U.S.

Elevated Mixed Layers and their Role in Significant Severe Thunderstorm Episodes in the Northeastern U.S. Michael L. Ekster NOAA/National Weather Service Upton, NY

sahara
Download Presentation

Elevated Mixed Layers and their Role in Significant Severe Thunderstorm Episodes in the Northeastern U.S.

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. Outline of Presentation • Study purpose and objectives • EML fundamentals • structure • formation • movement • Data and Methodology • Study Results • Case Studies • Conclusions • Future Work

  3. 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

  4. 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

  5. Elevated Heating = Steeper Lapse Rates Cold Cold Warm Cool

  6. In the absence of widespread diabatic processes, EMLs are advected downstream without changing much character at all: The largest contribution to time local rate of change of lapse rate is horizontal advection. This is a big key! From Jon Finch, NWS DDC

  7. 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)

  8. 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

  9. 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!

  10. 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)

  11. 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!

  12. Confirming the sounding does contain an EML – trace back to source region Killer tornado day Mass Pike

  13. 28 Aug 1973

  14. NOAA ARL trajectory website Hysplit Model run using NCEP/NCAR reanalysis dataset (1940s – Present) Run a backward trajectory analysis http://www.arl.noaa.gov/ready/open/traj.html

  15. ELP 12z 25Aug73 LBF 12z 26Aug73 SSM 12z 27Aug73 ALB 12z 28Aug73 Soundings courtesy of Plymouth State

  16. Northeast U.S. EML Source Regions – same source region as Great Plains EML Mid-Late Season (July-Sept) Early Season (April-June)

  17. 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

  18. Mid level pattern conducive to EML advection into the Northeast U.S.

  19. Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D-3

  20. Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D-2

  21. Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D-1

  22. Composite 700 hpa anomalies for 6 randomly chosen Northeast EML cases D0

  23. 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

  24. A few cases From Johns et al., “About Derechos”

  25. Johns, et al., “About Derechos””

  26. PIT 00z 7/15/95 OKX 12z 7/15/95

  27. 8-9 June 1953 RME, 15z JFK, 15z

  28. 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

  29. 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

  30. 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

  31. Questions?Comments?

More Related