Severe Thunderstorm Warning 86, NWS Grand Rapids, MI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
1215 PM EDT SAT SEP 8 2001

...THREAT FOR SEVERE IS THE CONCERN FOR THIS AFTERNOON...

CURRENTLY THERE IS CONSIDERABLE LOW CLOUDINESS OVER MOST OF LOWER
MICHIGAN.  ALSO... THE WATER VAPOR LOOP SHOWS CONSIDERABLE MID LEVEL
DRYING GOING ON IN THE NVA AREA BETWEEN THE SUBTROPICAL SHORT WAVE
WHICH AT 15Z EXTENDED FORM NRN LK HURON TO EASTERN LOWER MICHIGAN
AND THE MUCH STRONGER SHORTWAVE ASSOCIATED WITH THE UPPER LEVEL
TROUGH AND SFC FRONTAL SYSTEM OVER THE PLAINS. VIS IMAGES AND SFC
OBS SHOW SOME CLEARING IN THE LOW CLOUDS OVER ERN IL WHERE THE UPPER
AIR ANAL SHOWS A DRY POCKET OF AIR BETWEEN 925 AND 850 MB.  THERE IS
A DEEP TROPICAL CONNECTION SEEN ON THE WATER VAPOR IMAGES FOR BOTH
THE LK HURON SHORT WAVE AND TO A LESS EXTENT THE FRONTAL SHORTWAVE.
MSAS SHOWS THE AXIS OF HIGHEST SFC DEW POINTS OVER IN AIMED AT
SOUTHWEST AND SOUTH CENTRAL LOWER MICHIGAN. THERE ALSO IS THE
ENTRANCE REGION TO A 80 KNOT 300 MB JET OVER WI TO HELP THE LIFT
FROM THE SHORT WAVE ASSOCIATED WITH THE FRONT OVER WI.

SO... WHAT WILL HAPPEN... MODIFIED DTX SOUNDINGS AND MODEL SOUNDING
SUGGEST 2000 TO 3000 J/KG WITH HIGHS IN THE MID 80S OVER SOUTHWEST
LOWER MICHIGAN THIS AFTERNOON. THERE IS 20 TO 30 KNOTS OF VERTICAL
SHEAR DUE TO THE 30 TO 40 KNOTS OF WIND ABOVE 2000 FT ALTHOUGH IT IS
UNI-DIRECTIONAL.  GIVEN CAPES NEAR 2500 J/KG... K INDEX VALUES IN
THE 35C RANGE... SFC BASED LIS NEAR 7C... ISOLATED SUPER CELL
THUNDERSTORMS SEEM A GOOD POSSIBILITY FOR SOUTHWEST MICHIGAN THIS
AFTERNOON. WITH THE WET BULB ZERO IS JUST OVER 11000 FT... HAIL
WOULD BE POSSIBLE WIND WOULD BE THE MOST LIKELY TYPE OF SEVERE
WEATHER FOR SOUTHWEST MICHIGAN THIS AFTERNOON.

MAIN QUESTION THEN IS WERE WILL THE THUNDERSTORMS BE? SINCE THERE IS
LITTLE GOING ON NOW.  BY THE TIME ANYTHING DID START... THERE IS
LIKELY TO BE A WEAK LAKE BREEZE BOUNDARY... LIKE THERE WAS
YESTERDAY. THAT WOULD FORCE THE CONVECTION INLAND.  ALSO THE RUC
SHOWS VERY POSITIVE 850 BASED LIS OVER WEST CENTRAL LOWER MICHIGAN
DURING THE AFTERNOON (DUE TO THE DRY AIR POCKET AT 850 MB). ALSO...
BY THE TIME ANYTHING COULD START... THE SHORTWAVE ENERGY AND UPPER
DIVERGENCE FIELDS WILL BE OVER THE CENTRAL PART OF LOWER MICHIGAN...
SO I DO NOT BELIEVE THERE IS MUCH OF A CHANCE OF THUNDERSTORMS THIS
AFTERNOON WEST OF US-131.  MOST LIKELY WHAT EVER DOES HAPPEN WILL BE
EAST OF A AZO TO GRR TO RQB LINE.  SO I DECREASED THE POP TO CHC
OVER WRN LWR... AND INCREASED THE CHANCE TO LIKELY SE PART OF OUR
CWA THIS AFTERNOON.  I ALSO PUT WORDING IN FOR THE THUNDERSTORMS TO
MOSTLY BE DURING THE MID TO LATE AFTERNOON SINCE NOTHING IS GOING ON
NOW OR IS EVEN CLOSE TO US.

THE HIGH TEMPERATURE FORECAST LOOKS FINE SINCE THAT DRY AIR TO THE
SOUTHWEST OF MICHIGAN SHOULD ALLOW THE EXTENSIVE LOW CLOUDS TO
DISSIPATE BY EARLY THIS AFTERNOON.

I INCREASED THE WIND SPEEDS BASED ON THE SOUNDING AND SFC WIND
REPORTS AROUND THE AREA AT 1100 AM.

.GRR...NONE

WDM