SR SSD 98-43

12-1-98

Technical Attachment

Seminar Notes: The Challenge of Forecasting Damaging Wind Events

David Wally

NWSO LakeCharles, LA ,

On October 2, 1998, 1 had the opportunity to attend a seminar at the Texas A&M University Cooperative Institute for Applied Meteorological Studies (CIAMS). The seminar titled, "The Challenge in Forecasting Damaging Wind Events Across the Mid-Mississippi Valley Region," was presented by the SOO of NWSO St. Louis, Ron Przybylinski. The following summary details the abstract and notes taken at the seminar. The main emphasis will be placed on the circulation evolution of bowing Mesoscale Convective Systems (MCSS) and the Mid-Altitude Radial Convergence (MARC) velocity signature on Doppler radar, which has given significant lead times in detecting bow echoes/damaging wind events.

Meeting Abstract

Since July 1992, twelve MCSs producing swaths of damaging winds and/or non-supercell tornadoes across the Mid-Mississippi Valley region have been surveyed during the warm season (May-September). Storm reflectivity, Doppler velocity, and circulation evolutions from each of these cases have been studied. Preliminary findings of the environments from these events include moderate to high instability (CAPEs greater than 2600 J/kg), and moderate shear (I 3-19 m/s shear in the 0-3 kin layer). The presence of nearly east-west external surface frontal boundaries and surface dew point pooling south of these boundaries were found in nearly all cases. A typical convective system frequently traveled along and south of the frontal boundary.

During the intensifying sta(Te, each of these convective systems exhibited a linear multicell reflectivity structure containing several strong convective cells. The Mid-Altitude Radial Convergence (MARC) velocity signature was often identified along the leading edge of the linear MCS, and it became a good feature in the forecasting of the initial onset of damaging winds. The MARC velocity signature was found in over 80% of all cases studied. Once the MCS became a solid line of convection, the first circulation, often exhibiting mid-level origins, formed near the northern end of the line. In over 70% of the cases surveyed, this vortex frequently exhibited a relatively short life-span; often lasting no longer than 30 minutes.

As the MCS began to bow, i second vortex, originating southeast of the first circulation, often formed alone the cyclonic shear- side of the bow and became the primary circulation with the bowing MCS. This vortex originating within the lowest 3 or 4 km, rapidly deepens and intensifies during the following 10-15 minutes. In less than 30% of the cases studied, this vortex had a history of spawning brief FO or F I tornadoes. Subsequent 3rd and 4th vortices usually form near or just north of the apex of the bow. Similar to the second vortex, these vortices also form within the lowest 3 kin and then rapidly deepen and intensity during the following 10-15 minutes. Tornadoes (FO-F2 intensity) have been documented during this period of rapid deepening and intensification. This seminar will focus on a small subset of the twelve case studies and highlight the MARC velocity signature, vortex evolution, aiid overall reflectivity-velocity structure of bowing MCSS.

Seminar Notes:

The topics under study for these twelve MCSs included:

1. Survey of Storm Reflectivity

2. Doppler Velocity

3. Circulation Evolution

Radar Based Precursors for Damaging Winds with MCSs:

-Bowing of line echo.

-Rear Inflow Notches (RINS) and Weak Echo Channels (WECs) along the trailing edge.

-The highest echo top to be located over the overhang or the tight low-level reflectivity gradient.

- A strong reflectivity gradient along the leading edge of the concave shaped echo.

-Base Velocity.

-High VIL values. (A good indicator for hail, but has shown less of a direct relationship to downbursts. The magnitudes of VIL also fluctuate greatly with differing air masses and seasonal changes.)

Findings from these twelve cases on linear MCSs:

1. Mid-Altitude Radial Convergence (MARC) values greater than 22 m/s (approximately 45 knots) a good precursor for damaging winds.

2. A rapidly descending reflectivity core.

3. Suspended reflectivity core that initially begins at a higher height than most other storms.

(According to the Lake Charles WSR-88D PUP handbook, a large area of 55 dBZ between 25 and 30,000 ft in the cool season and 30 and 35 kft in the warm season are precursors for damaging winds).

Mid-Altitude Radial Convergence (MARC)

Similar to the "Deep Convergence Zone" (DCZ) in supercells (separates the updraft/downdraft).

Overall horizontal extent - 60 to 120 km (37 to 75 miles)

Depth - 6.2 km (from 3 to 9 km in height) or approximately 20,000 ft (from 10 to 30,000 ft in height).

Width - 2 to 6 km (1.2 to 3.7 miles).

Magnitude - typical velocity differences of 25 to 50 m/s (approximately 50 to 100 kt). Increasing VIL correlated well with a strengthening MARC.

Often shows before the bow echo.

Advantages Using the MARC

10-30 minute lead times.
Does not fluctuate with differing air masses and seasons (like VIL).

Disadvantage In Using the MARC

Underestimated when mid-level convergence is oriented orthogonal to the radar beam.

Circulation Evolution of Bowing MCSs

1 st Northern Vortex - Usually develops in the mid-levels and forms near the northern end of the line. In most cases it has a short life span of only 10-15 minutes.

2nd Northern Vortex - Forms to the southeast of the Ist northern vortex when the MCS begins to bow. This vortex originating within the lowest 3 or 4 km, rapidly deepens and intensifies during the flowing 10-15 minutes. Satisfies mesocyclone criteria (Vr >30 knots, core diameter < 3 nm, 3 km depth). Often evolves into a broad "book-end vortex." In less than 30 percent of the cases, this vortex has had a history of spawning brief FO or Fl tornadoes.

Subsequent vortices frequently form near the apex of the bow. These vortices also form within the lowest 3 or 4 km and then rapidly deepen and intensify during the following 10-15 minutes. Tornadoes of FO-F2 intensity have been documented during this period of rapid deepening and intensification.

Reference

Schmocker, G.K., R.W Przybylinski,and Y.J. Lin, 1996: Forecasting the Initial Onset of Damaging Downburst Winds with a Mesoscale Convective System (MCS) Using the Mid-Altitude Radial Convergene (MARC) Signature. Preprints, 15" Conf On Weather Analysis and Forecasting. Norfolk VA, Amer. Met. Soc., 306-311.