Severe Thunderstorm Warning 73, NWS Grand Rapids, MI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
258 PM EDT MON AUG 27 2001

MAIN FORECAST PROBLEM IN NEAR TERM IS SEVERE WEATHER POTENTIAL THIS
EVENING. STORMS HAVE YET TO FIRE UPSTREAM ACROSS WISCONSIN AS OF MID
AFTERNOON...DESPITE LAPS COMPUTED LIS OF MINUS 4 AND LOWER IN THIS
AREA. THIS MAY BE DUE TO COMBO OF RELATIVELY WEAK SFC CONVERGENCE
AND UPPER DIVERGENCE...AND DRY ATMOSPHERE AS SHOWN BY LAPS SOUNDINGS
ACROSS SOUTHERN WISCONSIN/NORTHERN ILLINOIS.

LATEST MESO-ETA BEGINS GENERATING PRECIP ACROSS GRR CWA BETWEEN 00Z
AND 03Z AS RIGHT ENTRANCE REGION OF UPPER JET AND ASSOCIATED
DIVERGENCE MOVES OVER  TONGUE OF HIGHER THETA-E AIR. LAPS COMPUTED
LIS ARE IN THE MINUS 6 TO MINUS 8 RANGE ATTM AS SFC HEATING HAS
RESULTED IN MAXES AROUND 80 DEGREES...WITH SFC DEW POINTS IN THE
UPPER 60S. THIS YIELDS SBCAPE VALUES IN THE 2500-3000 J/KG RANGE.
CONDITIONS SHOULD REMAIN FAVORABLE INTO THE EVENING HOURS AS THE
PREFRONTAL TROUGH MOVES THROUGH...FOCUSING LL CONVERGENCE. SOUNDINGS
INDICATE HAIL AND MICROBURST POTENTIAL WITH WET BULB ZEROS AROUND 10
KFT AND GOOD DRY WEDGE AT MID LEVELS ALONG WITH A DRY ADIABATIC SUB
CLOUD LAYER TO ENHANCE DOWNBURST INITIATION AND MAINTENANCE. WILL
INCLUDE WORDING IN THE ZONES FOR SMALL HAIL AND STRONG GUSTY WINDS
WITH SOME OF THE THUNDERSTORMS.

HIGHER POPS WILL BE ACROSS EASTERN ZONES...AS COVERAGE OF PRECIP
SHOULD BE ON THE INCREASE AS THE STORMS THAT FORM MOVE SOUTHEAST
THROUGH A FAVORABLE ENVIRONMENT. WILL USE SCATTERED WORDING IN ALL
BUT SE ZONE GROUP...WHERE LIKELY POPS WILL BE. DESPITE RATHER LOW
FFG IN THIS AREA...DO NOT EXPECT BIG HYDRO CONCERNS AS CORFIDI
VECTORS INDICATE STORM MOTION WILL BE SOUTHEAST AROUND 25 KTS. THE
PRESENCE OF A MID LEVEL DRY WEDGE MAY MEAN EVEN FASTER STORM MOTION
DUE TO GUST FRONT FORMATION.

STORMS SHOULD CLEAR EASTERN CWA BY 06Z WITH RAPIDLY IMPROVING
CONDITIONS AS SUBSIDENCE/DRYING BUILDS IN. REST OF FORECAST IS FOR
BENIGN WEATHER. WILL GO SLIGHTLY BELOW GUIDANCE FOR MINS ON TUE
NIGHT AS SFC RIDGE IS OVER THE CWA. OTHERWISE...TEMPS CLOSE TO
FWC/MAV COMPROMISE.

OSTUNO

EXTENDED FORECAST DISCUSSION...

THE 12Z AVN AND LATEST MRF/ECMWF/CANADIAN GUIDANCE CONTINUES
TO SUGGEST THAT ANOTHER COLD FRONT WILL SWING THROUGH FRIDAY
MORNING. PRECEDING IT/S ARRIVAL... CLOUDS WILL INCREASE THURSDAY
WITH A CHANCE OF SHOWERS AND THUNDERSTORMS FROM LATE THURSDAY
AFTERNOON THROUGH FRIDAY MORNING. 12Z AVN GUIDANCE PROGS THE FRONT
TO EXTEND FROM ROUGHLY LAN TO SBN BY 12Z FRIDAY. SO AFTER LINGERING
CLOUDS AND A FEW SHOWERS/TSTORMS FRIDAY MORNING... SKIES WILL BECOME
PARTLY SUNNY IN THE AFTERNOON AS WINDS BECOME NW-N AND ADVECT DRIER
AIR IN.

FOR LABOR DAY WEEKEND... AN H5 TROF WILL BE PARKED OVER THE REGION
SATURDAY THROUGH LABOR DAY. THAT...AND COOL AIR ALOFT WILL MEAN
QUITE A BIT OF CU FROM THE LATE MORNING THROUGH THE AFTERNOON HOURS
DAILY. FOR SATURDAY... WITH LIMITED SUNSHINE AND NORTH TO NORTHEAST
WINDS... HIGH TEMPS WILL STRUGGLE TO REACH THE LOWER 70S. SKIES
SHOULD CLEAR AND WINDS GO CALM SATURDAY NIGHT AS A SFC RIDGE OF HIGH
PRESSURE BUILDS IN... WHICH WILL ALLOW TEMPS TO FALL INTO THE 40S.
THEN SUNDAY... WE/LL BE COMING OFF A CHILLY START... AND QUITE A BIT
OF LATE MORNING/AFTERNOON CU WILL DEVELOP ONCE AGAIN. SO...HIGH
TEMPS BY THEN MAY STRUGGLE TO REACH THE 70 DEGREE MARK. IN FACT THE
ECMWF PROGS H8 TEMPS OF ONLY AROUND 8C THROUGH THE WEEKEND. IF THAT
WAS TO VERIFY... HIGH TEMPS WON/T GET OUT OF THE 60S. THE ECMWF MAY
BE SHOWING A COOL BIAS... AS NOTED THE PAST COUPLE OF WEEKS.

BY LABOR DAY... WE EXPECT MUCH OF THE SAME... FEATURING CLEAR TO
PARTLY CLOUDY SKIES AND SEASONABLY COOL TEMPS.

LAURENS

.GRR...NONE