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COMET Cooperative Research Project on Severe MCSs and Heavy Precipitation
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Overview:

The COMET Cooperative Research Project between the Department of Earth and Atmospheric Sciences of Saint Louis University (SLU) in St. Louis, Missouri; NWS Louisville, KY; NWS St. Louis, MO; and NWS Paducah, KY was completed in Spring 2000.  This research began in 1994 originally between SLU and NWS St. Louis to study severe mesoscale convective systems (MCSs) and heavy precipitation producing systems.  

Many significant findings resulted from the COMET research based on project objectives. Project members focused their efforts on two primary problem areas that are of vital interest to the meteorological community and that threaten life and property. The first area concerned detailed evaluations of linear (MCSs that produce damaging straight-line surface winds and tornadoes across the middle Mississippi and Ohio River Valleys, a geographical region vulnerable to such systems. The second research problem concerned heavy precipitation, including 1) investigation of various processes associated with warm and cool season heavy precipitation producing systems, 2) evaluation of the ability to produce more accurate and scientifically-based quantitative precipitation forecasts (QPFs) of warm and cool season mesoscale systems, and 3) development of regional climatologies of heavy precipitation producing weather systems.

Specific goals concerning severe MCSs included:

Specific goals concerning QPF issues included:

Specific educational goals included:

Research results from NWS Louisville:

Severe weather

Several severe MCSs across Kentucky and southern Indiana were studied in detail during the project, including those on April 15, 1994; May 14, 1995; May 18, 1995; April 20, 1996; and March 28, 1997. Other cases were archived for possible future research. In each of these studied events, severe squall lines contained embedded bowing line segments which were highly correlated to straight-line/downburst surface wind damage. In addition, several of the bow echoes contained transient tornadoes along and just north/poleward of the apexes of the bowing segments.

April 15, 1994:  An extensive squall line on April 15, 1994 produced 7 bowing segments across central Kentucky and southern Indiana resulting in widespread wind damage, numerous mesocyclones and cyclonic circulations, and several tornadoes. We evaluated the pre-storm and "near-storm" environments to assess stability, vertical wind shear, and forcing fields. NWS Louisville-Ft. Knox (KLVX) WSR-88D data also were analyzed exhaustively to study the detailed evolution of multicellular convective development; rear inflow jets; multiple circulations/mesocyclones, some of which produced tornadoes; mesoscale airflow structure; bookend vortices aloft; mid-altitude radial convergence (MARC) signatures; and embedded high precipitation (HP) supercell structure. Numerous time-height cross-sections were constructed to study the typical evolution of bow echo mesocyclones and their relationship to damaging straight-line winds and tornadoes. This case was documented as a conference paper and also as a journal article in the December 1999 issue of Weather and Forecasting (see below).

May 14, 1995: This event featured an intense bow echo that passed just south of Louisville, KY.  It produced widespread winds over 60 mph with reported gusts up to 100 mph, hail, and a few tornadoes of F0-F2 intensity. Major damage occurred to many buildings, structures, trees, and power lines with damage in the millions of dollars. The large majority of damage resulted from the intense straight-line winds. We investigated the life cycle of this storm complex, including the intense straight-line winds that produced surface damage, the evolution of cyclonic circulations/mesocyclones that led to tornado development, and mid-altitude radial convergence (MARC) signatures that accentuated downburst severity. Results showed that a significant and persistent MARC signal (maximum values of about 37 m/s (75 kts) at an altitude of about 4-6 km) existed near the center of a non-severe MCS that quickly transitioned into an intense bow echo as a subsequent downburst apparently caused rapid new echo development and acceleration and bowing in the low-levels. The maximum MARC values were noted 10-20 minutes prior to the onset of damaging surface winds from the bow echo. Thus, the recognition of MARC in this case acted as a severe weather warning precursor to subsequent significant wind damage and tornado development. This case appeared as a conference paper at the 19th Conference on Severe Local Storms (see below). WSR-88D radar imagery is available from this case.

May 18, 1995: During the afternoon and evening of May 18, 1995, a squall line and embedded organized bowing segment moved rapidly across south-central Kentucky. This segment exhibited a persistent convective-scale frontal structure (i.e., a line echo wave pattern/LEWP), a strong rear inflow jet, multiple cyclonic circulations/mesocyclones near the leading convective line along and just north of the bow apex, and bookend vortices aloft. In addition, the bow echo eventually evolved into a hybrid-like storm complex with high precipitation (HP) supercell structure. Deep-layered mesocyclonic flow within the HP storm produced at least two tornadoes. Funnel clouds and at least one other tornado occurred along the bow apex prior to transformation to a multicellular-HP supercellular system. This case appeared as a conference paper at the 16th Conference on Weather Analysis and Forecasting (see below). WSR-88D radar imagery is available from this case.

April 20, 1996: During the early morning hours on April 20,1996, a subtle bowing line segment within a squall line produced a tornado in Floyd county in south-central Indiana (just northwest of Louisville). Despite the subtlety, the presence and recognition of convective-scale frontal structure by Louisville forecasters allowed a timely tornado warning to be issued. The image at right reveals WSR-88D 0.5 degree base reflectivity contours (values over 40 and 50 dBZ shown by thin lines) associated with the squall line during the time of the tornado. The convective-scale low and frontal (LEWP) structure is denoted by bold lines, which can help forecasters identify convective attributes associated with enhanced wind damage ("W") along the bow apex (bulging cold/gust front) and tornadoes ("T") just north of the bow. The bold arrows indicate the overall storm-relative flow pattern associated with the bowing segment. Later in this event, another portion of the squall line evolved into a separate LEWP signature (not shown) which produced at least two tornadoes in east-central Kentucky (south and southwest of Lexington). Similar to other bow echo documentation, the tornadoes occurred within and just north of the maximum straight-line wind damage associated with the bow apex. This storm complex developed HP supercell characteristics, as the north-south oriented line interacted with an east-west oriented outflow boundary ahead of the line. The intersection likely intensified the low-level convergence and spin-up of the highly sheared environmental air into the storm complex. This promoted development of a deep-layered rotational circulation and a redistribution of the precipitation field resulting in the appearance of HP supercell characteristics.  Two tornadoes and nearby wind damage occurred during and just after this convective transformation.   This case will appear as a conference paper at the 20th Conference on Severe Local Storms to be held September 11-15, 2000 in Orlando, FL. WSR-88D radar imagery is available from this case. Conceptual Model of Convective-Scale Frontal Structure Associated with an Organized Bow Echo

March 28, 1997: On this day, a bow echo moved east across south-central Indiana resulting in wind damage along its path. Similar to other bowing line segments observed in Kentucky and southern Indiana, this convective system occurred within a moderately unstable and highly sheared environment with dynamic forcing. Several supercells occurred ahead of the line and produced large hail and a few tornadoes across central Kentucky on this day as well. WSR-88D radar imagery is available from this case.

Other severe weather events and signatures were researched at the NWS offices in St. Louis and Paducah to better understand the structure and evolution of linear MCSs and embedded bowing line segments.

Based on the events studied at NWS Louisville, a summary of common reflectivity, velocity, and mesocyclone characteristics associated with bowing line segments within squall lines is available in both HTML and PDF formats within the Scientific Training Documents and Exercises section of our Web site. A summary of environmental conditions and radar signatures also is available.

Heavy precipitation 

NWS Louisville and Saint Louis University (SLU) project members worked to develop seasonal heavy precipitation climatologies over the middle Mississippi and Ohio River Valleys. This goal was a large endeavor, requiring review of many past precipitation events in order to develop composite surface and upper-air charts for different types of heavy precipitation producing weather systems during different times of the year. However, this effort was valuable since such climatologies, in conjunction with scrutiny of individual weather systems, ultimately are quite useful to meteorologists in recognizing overall patterns, parameters, and processes that typically are conducive to heavy rainfall and snowfall production. Project participants at SLU produced various statistical analyses and composite patterns for numerous years of warm season heavy rainfall events across Kentucky and southern Indiana. A Web presentation shows research results in more detail. Additional information concerning heavy precipitation research at SLU is available.

Meanwhile, NWS Louisville members performed a similar task for heavy snowfall events, defined as 4 inches or more in a 24-hour period. We produced snow statistical analyses for Kentucky and southern Indiana from 1982-1996, and developed environmental composites of several individual events. From these composites, we noted at least two significantly different synoptic patterns associated with heavy snowfall across the Ohio Valley. The first pattern consists of relatively weak surface and 500 mb system development. This pattern features a southern stream shortwave within a broad southwest flow at 500 mb, a low-level jet, strong isentropic lift ahead of the shortwave, the presence of a jet streak across the Great Lakes with substantial along-stream wind variation within the entrance region of the jet (i.e., winds accelerate quickly into the jet), and frontogenesis across the Ohio Valley within the right entrance region. The northern jet streak also seems to play a significant role in holding cold air in place across the Ohio Valley despite warm advection from the south. The second pattern is associated with strong surface and upper-air development, including East Coast snowstorms that can bring heavy snowfall to eastern Kentucky, and slow moving closed off lows and deep easterly flow that can lead to significant snow across parts of Kentucky and Indiana if temperatures are cold enough. A Web presentation showing preliminary research results is available. This research is ongoing within a new joint research project with SLU, now designated a Cooperative Institute for Precipitation Systems (CIPS). 

Software from St. Louis University's Meteorology Department was used at NWS Louisville to generate various plotted and contoured surface, upper-air, sounding, and isentropic charts for the heavy snowfall events in our study.

Research also was conducted between SLU and NWS Louisville project members on two heavy precipitation events in central Kentucky, one a heavy snow case, the other a record convective rainfall event. A conference paper (see below) documented common mesoscale and synoptic scale weather parameters responsible for the January 17, 1994 snowstorm and the March 1, 1997 record rainfall events. Common traits included moderate-to-strong isentropic lift along and north of a low-level frontal boundary, a persistent moist south-southwesterly low-level jet, unseasonably high values of moisture and precipitable water, frontogenetical forcing associated with a tightening thermal gradient across the Ohio Valley, and upper-level divergence within and south of the entrance region of a distinct upper-level jet streak. In addition, low-level based instability was present upstream of the heavy precipitation zones in both cases with elevated convective and/or conditional symmetric instability (CSI) within the area of heavy precipitation. Diabatic heating apparently played a role in the snowstorm event as well. Finally, echo training occurred in both cases, resulting in a prolonged period of precipitation. More information on significant winter atmospheric processes is available.  

Other heavy precipitation/QPF research was conducted at SLU, NWS St. Louis, NWS Paducah, and NWS New Orleans to meet other specific goals identified above. 

Science Sharing/Training Efforts

Throughout our COMET project, many severe weather seminars were given to meteorologists at NWS Louisville and other nearby NWS offices to enhance their knowledge and understanding of convective storm structure and to promote a high degree of severe weather warning proficiency and accuracy from squall lines and bow echoes. These seminars have had a direct positive influence on the warning program at NWS Louisville. Various Louisville, Lexington, Bowling Green, and Evansville TV meteorologists and weathercasters also have benefited from these seminars. In addition, research results from our heavy rainfall and snowfall study were presented periodically to NWS and media meteorologists to better understand mechanisms important to heavy precipitation production and for use in the forecast process.

COMET Cooperative Project Workshops were held at Saint Louis University on November 30-December 1, 1995; November 12-14, 1996; and November 16-18, 1999. These workshops were hosted by SLU and the NWS offices in St. Louis, Louisville, and Paducah. Many excellent oral presentations were given by project members and invited speakers to showcase research results concerning heavy precipitation and severe convective storm structure. In addition, several "hands-on" workshops were conducted to enhance interactive learning and science sharing. A severe weather training exercise given at the 1999 workshop by NWS Louisville is available in the Scientific Training Documents and Exercises section of our Web site.

Select Publications and Web Presentations associated with the COMET project:

Severe Weather Papers

Heavy Precipitation Papers

  • Preliminary Results of a Heavy Snow Climatology Across Kentucky and Southern Indiana (1982-1996).
    • -R.E. Cox, C.E. Swain, and T.W. Funk, NWS Louisville, KY. Web Presentation.
  • Creation of a Synoptic Climatology of Heavy Rainfall over Kentucky and Southern Indiana.
    • -S.J. Klaus, J.T. Moore, and C.E. Graves, Saint Louis University. Web Presentation.
  • Strategies for Estimating the Areal Coverage and Magnitude of a Precipitation Event.
    • -J.T. Moore, Saint Louis University. Web Presentation.
  • Evaluation of ETA (32 km) Model QPF During Rainfall Events Exceeding 2 Inches.
    • -S.A. Watson, J.T. Moore, C.E. Graves, Saint Louis University. Web Presentation.
  • A Climatology of Widespread Heavy Rainfall Events Across Missouri.
    • -F.H. Glass. Preprints,19th Conference on Severe Local Storms, Minneapolis, MN, Amer. Met. Soc., 14-18 September 1998, 540-543.
  • A Synoptic Climatology of Significant Rainfall Events over the Mid-Mississippi River Valley.
    • -S.R. Considine, F.H. Glass, and J.T. Moore. Preprints, 19th Conference on Severe Local Storms, Minneapolis, MN, Amer. Met. Soc., 14-18 September 1998, 544-547.
  • A Synoptic Comparison of Two Heavy Precipitation Events in the Ohio River Valley.
    • -P.S. Market, J.T. Moore, S.R. Considine, T.W. Funk, and S.M. Rochette. Preprints, 16th Conference on Weather Analysis and Forecasting, Phoenix, AZ, Amer. Meteor. Soc., 12-16 January 1998, 358-361.
  • A Comparison of Cool Season and Warm Season Heavy Rain Events in the Mid-Mississippi Valley.
    • - J.T. Moore, S.R. Considine, S.M. Rochette, F.H. Glass, and T.M. Tworek. Preprints, 16th Conference on Weather Analysis and Forecasting, Phoenix, AZ, Amer. Met. Soc., 12-16 January 1998, 267-269.
  • Overview of the 28 February - 2 March 1997 Kentucky Heavy Rainfall Event.
    • -J.T. Moore, T.W. Funk, P.S. Market, S.R. Considine, and S.M. Rochette. Conference on Flood Warning Systems, Technology, and Preparedness, St. Louis, MO, 29-31 October 1997.
  • The Synoptic and Mesoscale Features Accompanying the 1-2 March 1997 Kentucky Heavy Rain Event.
    • -J.T. Moore, P.S. Market, S.R. Considine, S.M. Rochette, T.W. Funk, M.C. Trexler, R.E. Cox, and L.J. Dattilo. 22nd Annual Meeting of the National Weather Association, Reno, NV, 20-24 October 1997.
  • Vertical Motion Forcing Mechanisms Responsible for the Production of a Mesoscale Very Heavy Snow Band Across Northern Kentucky.
    • -T.W. Funk and J.T. Moore. Postprints, 4th National Winter Weather Workshop, Kansas City, MO, NOAA Tech. Memo NWS CR-112, 19:1-11, September 1996.

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