National Weather Service United States Department of Commerce

September 22, 2005 Severe Thunderstorms

 

Synoptic conditions on September 22, 2005
A low amplitude mid level shortwave trough was moving ENE through the Great Lakes into southern Canada. The attendant surface cold front moved slowly southeast through the  CWA (County Warning Area) in the late afternoon and evening. A weak pre-frontal trough also moved southeast across the CWA just ahead of the cold front, but no significant convection ever developed on this feature due to strong capping over the warm sector. Morning debris clouds quickly gave way to sunshine and the atmosphere quickly destabilized with surface based CAPE (Convective Available Potential Energy) reaching around 3000 J/kg just ahead of the cold front. Deep layer wind shear was sufficient for storm organization with 0-6km environmental shear on the order of 40 to 45 knots. Model performance was good with NAM and GFS models in good agreement on the timing, placement, and amplitude of the large scale weather features.

The image below on the left shows 500mb heights and vorticity. The dashed red line shows the location of the mid level shortwave trough. Large scale lift in the atmosphere, which promotes storm development, occurs just ahead of these mid level shortwaves. The image at center is a surface map depicting the location of the surface cold front at 23Z, or 6pm EST. The image on the right is a map of Convective Available Potential Energy (CAPE). CAPE is a measure of the potential energy for storm growth if a thunderstorm updraft initiates.

 

Map of 500mb heights and vorticity. The dashed red line shows the location of the mid level shortwave. Surface map showing the location of the cold front. An image of CAPE, which is a measure of instability in the atmosphere.



Mesoscale/Stormscale conditions
A decaying MCS (Mesoscale Convective System) moved through southern lower Michigan during the morning hours. This along with ongoing convection over central lower Michigan lead to a differential heating boundary over far southern lower Michigan between rain cooled air to the north and warm and sunny conditions to the south. An Elevated Mixed Layer (EML) plume emanating off the high plains advected over the region and provided a strong cap over
the warm sector. The only location where this cap was overcome was immediately along the main surface cold front where mass convergence was maximized and the cap was sufficiently weakened. Storms initiated over Cass County Michigan and continued to build slowly southeast. There was a tendency for back-building of new storms over the same areas. Once the strong cap was overcome storm updrafts were able to take advantage of steep mid level lapse rates within the EML plume of 7.5C/km in the 700mb-500mb layer. This resulted in vigorous and very tall updrafts which were very efficient in producing large hail. Several storms took on HP (High Precipitation) supercell characteristics with well developed mid level mesocyclones. High LCL heights and unfavorable low level helicity profiles prevented these mesocyclones from descending.

Radar trends and observations
Many of the warned storms were very tall and had impressive elevated cores. Several impressive cross sections were taken including a 65-70dBz core to nearly 30 thousand feet on a storm over Fremont in Steuben County which produced golf ball size hail. A threshold of more than 4 pixels of 80+ DVIL (Digital Vertically Integrated Liquid) for 2 or more radar volume scans was used very successfully for warning decision making along with other tools, such as cross sections, legacy VIL, all tilts, and storm top divergence.

The image on the left below is Digital Vertically Integrated Liquid (DVIL) of the storm which produced golf ball size hail in Fremont, IN. This is a new radar product that NWS forecasters here at the Northern Indiana Forecast Office are evaluating and incorporating into the warning decision making process. The image on the right is a reflectivity cross section of the same storm. Radar operators at the NWS can cut cross sections of storms, which are produced by putting together images from the 14 elevation angles that the radar scans. This image shows several storm features including an elevated core, which marks the location of developing hail stones within the storm, and also a Bounded Weak Echo Region (BWER). The BWER marks the location of the intense thunderstorm updraft. In this image the BWER is the area of weaker reflectivity below the elevated core. The presence of the BWER is a strong indication of an intense storm with supercell characteristics.

National Weather Service Doppler radar produces over 70 radar products designed to assist in warning decision making. These products however are only tools, and the decision to issue warnings is made by National Weather Service meteorologists who are constantly interrogating radar, satellite, observational, and spotter data.
 

 

DVIL image of storm which was producing golf ball sized hail in Fremont, IN Reflectivity Cross Section of the storm which was producing golf ball sized hail in Fremont, IN     

 

 

Photo of a storm in southern lower Michigan...

The following are still captures taken from video shot by Kurt Hulst. The top picture is from near Kinderhook, Michigan in southern Branch County. The bottom picture is just over the border in Indiana about 6 miles east of Angola. The pictures show a nice lowering from beneath the thunderstorm which is called  a wall cloud. Also note on the right side of the second photo a feature sometimes referred to as a beavertail, which forms along the channel of air flowing into the base of the thunderstorm as it becomes ingested by the intense updraft. Hail size often increases when a storm forms a mesocyclone (rotation), since mesocyclones enhance the upward vertical motions within the thunderstorm and intensify the updraft. All else being equal, the stronger the updraft, the bigger the hail. This rotating storm was not able to produce a funnel cloud or tornado on this day, most likely due to the environmental conditions in the low levels of the atmosphere, which were not favorable for tornadoes.

 

 

photo of a wall cloud by Kurt Hulst

 

 

J.Hitchcock 9/29/05