Anticyclonic (Left-Moving) Supercell

Late Wednesday afternoon, April 7 2004, the airmass across most of central Alabama was very stable, with dewpoints in the 30s and 40s, not exactly what a meteorologist would consider a breeding ground for thunderstorms. An insignificant radar echo popped up over southwest Alabama around 4:40 pm. The cell developed in a more unstable airmass, with dewpoints around 60 degrees. It grew rather quickly and moved northeastward across Washington County, with the core reaching around 65 dBZ shortly after 5:00 pm.

As the storm moved into Clarke County, the first Severe Thunderstorm Warning was issued for this storm, and it produced wind damage (see map at bottom) and large hail in the town of Grove Hill. It continued across Wilcox County and quickly piqued the interest of the Birmingham Forecasters as it approached southwest Dallas County. At this point, it was definitely exhibiting supercell characteristics.

A supercell is a thunderstorm that possesses a deep, persistent rotation that maintains it's identity over several hours. It has a high probability of producing severe weather, including tornadoes. This supercell was isolated, which usually means it does not have to compete with other storms for needed energy to maintain itself. The storm was also picking up forward speed, and was moving around 40 mph as it moved through Dallas County.

The most interesting feature of this storm was that it was a left-moving supercell. Most supercells move to the right of the mean wind flow. This storm favored a left moving track and can be identified by a clockwise (anticyclonic) circulation in the mid levels (about 13,000 feet) of the thunderstorm in the following image.

1.5 Degrees Storm Relative Velocity image from the Birmingham WSR-88D (KBMX) at 0024 UTC. Note that the orientation of the green pixels (flow toward the radar) and red pixels (flow away from the radar) imply an anticyclonic circulation.

Another interesting feature of this supercell was the narrow band of reflectivity that extended along the radar radial out the back side of the storm. This is referred to as a "hail flare" or "3-body scatter spike." In brief, simple terms, this is caused by the radar beam hitting the wet hail, scattering to the ground below, then scattering back upward, and finally being scattered once again by the hail aloft. Due to the persistent core of high reflectivity values the storm exhibited, it likely produced large hail along its entire path, but it moved over mainly rural areas and most likely was not observed.

1.5 Degrees Base Reflectivity image from the Birmingham WSR-88D (KBMX) at 0024 UTC. Note the "flare" of echoes along the radar radial shooting out the back side of the storm. This signature is an indication of large hail.

The first report of severe weather associated with this storm in Birmingham's County Warning Area occurred in Dallas County at 8:05 pm. The largest hail report (golf ball) came from western Autauga County in the town of Jones. The supercell finally weakened as it moved into Chilton County, but still produced dime size hail near Clanton. By the time the supercell died in eastern Chilton County, it had a track length of 156 miles and persisted for 4 hours and 40 minutes.

Storm Total Precipitation image from the Birmingham WSR-88D (KBMX) at 0300 UTC. The supercell storm's path, from eastern Washington County to southeastern Chilton County, is clearly visible. Also notice the supercell's movement compared to the storms that moved across West Central Alabama.

Map of the path of the supercell storm (gray line), and locations of severe weather reports: 1 - 6:15-6:30 pm Wind damage and nickel-sized hail near Grove Hill 2 - 8:05 pm Wind damage and penny-sized hail near Selma 3 - 8:20 pm Golfball-sized hail in Jones 4 - 9:09 pm Dime-sized hail near Clanton

Additional Analysis

    1. INTRODUCTION  

    In the thunderstorm spectrum supercells are the most prolific severe weather producers with respect to hail and tornadoes. Most supercells rotate cyclonically (right-movers), which means they move to the right of the mean (0-6 km) wind flow. In rare circumstances supercells will rotate anticyclonically (left-movers), which means they move to the left of the mean wind flow. Typically, anticyclonic supercells evolve from the storm splitting process. This process has been well documented in radar data and numerical simulations.

    This paper will analyze an anticyclonic supercell (ACS) that formed over southwest Alabama on 7 April 2004. The ACS was unique in that it was distinct and did not evolve from storm splitting. The storm was longed lived, lasting 4-5 hours with a path length of approximately 156 miles. It also produced large hail (0.75-1.75 inches) and damaging (>50 kts) winds along a majority of its path length. A storm report from Grove Hill, Alabama, had penny to nickel size hail covering the ground up to six inches.

2. SYNOPTIC ENVIRONMENT

    On the afternoon of 7 April 2004, a short-wave trof was located over Texas with southwesterly 850-500mb flow over Alabama. A northward migrating warm front was located across southwest Alabama with surface dewpoints in the lower 60s south of this boundary. The first significant radar echo (>35 dBZ) of the developing ACS was located south of the warm front (Fig. 1).

    North of the warm front the air mass across Central Alabama was much more stable with surface dewpoints in the 30s and 40s, not an environment conducive for supercell longevity.

    Numerous studies have shown 0-6 km total shear values of 40 kts or more are necessary for supercell development. The Storm Prediction Center (SPC) outlook for Southwest Alabama was for a slight risk of severe thunderstorms with 0-6 km shear values 40-50 kts.

3. WIND PROFILE

    Vertical wind shear is a primary ingredient to maintain deep, persistent rotation in a convective storm. Studies have shown that right-moving storms are favored in environments characterized by clockwise turning hodographs and left-moving storms in counterclockwise or unidirectional hodographs. However, further studies (Bunkers 2002) showed a significant number of hodographs for left-moving storms exhibited clockwise curvature below 1 km.

    No upper-air soundings were available within 50 km of storm initiation, so a forecast sounding from the 1200 UTC Meso-Eta model was created for the location and time near supercell formation. Using the actual storm motion from BMX 88D Radar and shear vectors from the forecast sounding, a proximity hodograph (Fig. 2) was created. The hodograph showed clockwise shear below 1 km and unidirectional shear above 1 km. A study of 32 left-moving supercells (Edwards et al 2004) examined storm-relative helicity (SRH) as a method for determining the potential for large hail. The study noted that left-moving supercells that produced significant hail (= 2 inches) tended to occur with negative 1-3 km SRH but positive 0-1 km SRH.

    Numerical simulations of an ACS (Wilkelmson and Klemp 1981) showed that clockwise curvature in the lowest kilometer of the sounding was detrimental to the updraft of a left-moving storm because it produces an unfavorable vertical pressure field. They theorized that convergence along the gust front was necessary to compensate for the negative affects of veering shear vectors in the lowest kilometer. The hodograph in Figure 2 shows the surface storm relative inflow vector from the northeast at 15 ms-1 (30 kts). This would have provided a very favorable environment for strong low level inflow along the leading edge of the ACS.

4. RADAR IMAGERY

    Reflectivity and storm relative velocity radar images were impressive with this supercell. The supercell exhibited very high mid-level reflectivities (>65 dBZ) during its life cycle. One of the more interesting reflectivity aspects of this storm was the reflectivity spike (Fig. 3) extending along the radar beam on the back side of the storm. This is commonly referred to a “Three-Body Scatter Spike.” Large, wet hail scatters radar pulses to the ground, back up to the hail core, and then back to the radar. This generates weak reflectivity echoes that are false. The Three-Body Scatter Spike signature occurs in the mid levels of the storm, so there is normally enough lead time to issue a warning when this feature is observed.

    Storm relative motions within a storm are calculated by subtracting the storm motion from the ground relative wind fields. Storm Relative Velocity Map (SRM) products are generated automatically by NWS Doppler Radars after each radar elevation sweep.

    In Figure 4, the Doppler Radar is located at the top of the image, with green indicating motion towards the radar and red indicating motion away from the radar. This scan produced the highest rotational velocity couplet (Vr ~ 50 kts) at any elevation slice.

    Another interesting radar image was the Storm Total Precipitation (STP) Product (Fig. 5). This image clearly shows the track of the ACS was to the left of the northern storms. The track runs from southwest to northeast while the northern storms have a much more easterly component.

5. CONCLUSION

    Anticyclonic supercells are unique storms that certainly can look and feel like their more dominant cyclonic sisters. Due to the left moving motions of an ACS, the storm typically moves into a more stable airmass, especially if they are developing along a boundary. In this particular case the supercell was likely ingesting unstable air on the moisture rich side of the warm front. Surface dewpoints did indeed increase significantly across central Alabama as the ACS moved northeastward.

    It is unfortunate that the ACS moved through a surface data void region in Southwest Alabama. Any surface observing system could have provided quality storm inflow data. Many ACS’s may go undetected because NWS Doppler Radars do not detect anticyclonic mesocyclones. There has been extensive research done on ACS’s in recent years, I encourage everyone to read the articles referenced below.

6. REFERENCES

Bunkers, M.J., 2002: Vertical wind shear associated with left-moving supercells. Wea. Forecasting, 17, 845-855.

Edwards, R, R.L. Thompson, and C.M. Mead, 2004: Assessment of anticyclonic supercell environments using close proximity soundings from the ruc model. Preprints, 22 nd Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. Soc.

Grasso, L.D., and E.R. Hilgendorf, 2001: Observations of a left moving thunderstorm. Wea. Forecasting, 16, 500-511.

Lemon, L.R., 1998: The radar “three body scatter spike”: An operational large-hail indicator. Wea. Forecasting, 13, 327-340.

Wilhelmson, R. B., and Klemp J. B., 1981: A three-dimensional numerical simulation of splitting severe storms on 3 April 1964. J. Atmos. Sci.,38, 1581–1600.

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