Severe Thunderstorm Warning 25, NWS Grand Rapids, MI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
330 PM EDT SAT JUL 27 2002

...SCATTERED THUNDERSTORMS TONIGHT / THUNDERSTORM LIKELY MONDAY...

RATHER COMPLEX FORECAST THIS AFTERNOON.  THE WARM FRONT HAS BEEN
HELD STATIONARY OVER MO/IL/IN BY CONVECTION THAT MOSTLY HAS STAYED
SOUTH OF MICHIGAN TODAY. MEANWHILE... THE QUESTION STILL REMAINS...
WILL THERE BE THUNDERSTORMS?  IF THERE ARE THUNDERSTORMS... WHERE
WILL THE FORM... AND WHERE WILL THEY GO?  ALSO THE QUESTION OF WILL
THEY BE SEVERE COMES UP?

CURRENTLY THERE ARE TWO PRIMARY SHORTWAVES.  THE FIRST SHORTWAVE IS
MOVING THROUGH LOWER MICHIGAN AS I WRITE THIS.  THIS IS ALSO THE
LEADING EDGE OF THE DEEPER MOISTURE.  THERE IS A NARROW ARC OF
CONVECTION MOVING INTO SE LOWER MICHIGAN ASSOCIATED WITH THAT.
THERE IS A LARGE CLUSTER OF THUNDERSTORMS OVER MN ASSOCIATED WITH
THE PRIMARY NORTHERN STREAM WAVE.  THAT WAVE WILL STAY WELL NORTH OF
THE GRR CWA OVERNIGHT.  THERE IS A SEPARATE LOW LEVEL JET ASSOCIATE
WITH THAT CONVECTIVE SYSTEM AND IT WILL HELP TO KEEP THAT CONVECTION
TO OUR NORTH.  THE ONLY REAL THING TO GET THUNDERSTORMS OVER LOWER
MICHIGAN TONIGHT IS THE WARM FRONT THAT WILL LIKELY STILL GET PUSHED
NORTH TONIGHT AS THE PRIMARY NORTHERN STREAM WAVE PASSES WELL TO OUR
NORTH. CURRENTLY THERE IS LITTLE IF ANY CONVECTION IN SOUTHERN WI AT
THIS TIME.  SINCE THE MODELS (AVN/ETA/RUC) ALL SHOW THE MAX 1000/850
THTE CONVERGENCE... 850 AND 925 MB WARM ADVECTION OVER SOUTHWEST
MICHIGAN BETWEEN 21Z AND 03Z THIS EVENING... AND SINCE THERE IS
PROGGED TO BE A 35 KNOTS LOW LEVEL JET AIMED AT GRR AT 00Z... AND
THAT IS WHERE THE BEST WARM ADVECTION AND ISENTROPIC LIFT IS...
SEEMS THUNDERSTORMS WILL DEVELOP BY LATE AFTERNOON OR EARLY EVENING
OVER SRN WI AND THEN ROLL EAST AHEAD OF THE APPROACHING WARM
FRONT... ACROSS THE GRR CWA THIS EVENING.  THIS ALL SHOULD BE
PROGRESSIVE SO LITTLE THREAT OF MAJOR FLASH FLOODING EVEN WITH PWAT
VALUES NEAR 2 INCHES THIS EVENING.

SEVERE THREAT TONIGHT...
SINCE THERE IS SO MUCH CLOUDINESS AROUND SEEMS THE THREAT FOR SEVERE
IS LIMITED.  THE INSTABILITY IS ELEVATED ON THE SOUNDINGS FOR THE
EVENING HOURS... SO EVEN WITH 30 TO 40 KNOT MID LEVEL JET AND 30 TO
40 KNOT LOW LEVEL JET TO FEED THE SYSTEM... SEEMS A WET MICROBURST
SCENARIO IS MOST LIKELY... BUT EVEN THAT SHOULD BE LIMITED.  WITH
FREEZING LEVELS NEAR 15000 FT EARLY TONIGHT THE TREAT FOR HAIL WILL
BE LIMITED TOO.

SUNDAY...
WITH THE WARM FRONT THROUGH... THERE IS LITTLE FOCUS FOR CONVECTION.
THE WARM AIR AT MID TO HIGH LEVELS DOES NOT HELP THE CHANCES FOR
THUNDERSTORMS EITHER.  NOR DOES THE LOW LEVEL JET FOCUSED SOUTH OF
MICHIGAN AS SEEN ON THE AVN.  EVEN SO THE AIR IS RATHER UNSTABLE AND
WITH CAPES OF 1000 TO 2000 J/KG WITH PWAT VALUES 1.5 TO 2.0 INCHES.
SO I WILL HAVE A GENERIC 40 POP... BUT BELIEVE MOST OF THE ACTIVITY
WILL MISS SOUTHWEST LOWER MICHIGAN TO THE NORTH AND SOUTH.  THERE
SHOULD BE ENOUGH CONVECTIVE DEBRIS CLOUDS AROUND TO KEEP TEMPS FROM
GETTING AS WARM AS THE 1000/850 AND 1000/925 THICKNESS SUGGEST SO I
FORECAST HIGHS CLOSE TO MOS VALUES.

SUNDAY NIGHT... THAT IS THE TIME OF THE DRIEST AIR.  EVEN SO...
STILL INSTABILITY AROUND AND CAN NOT RULE OUT SOME SORT OF OUTFLOW
BOUNDARY INTERACTION SPREADING SOME CONVECTION OUR WAY... SO WHEN
WITH 30 POP.

MONDAY... MAIN COLD FRONT COMES THROUGH WITH GOOD FORCING. THERE IS
A 35 TO 40 KNOT LOW LEVEL JET AND GOOD SFC CONVERGENCE WITH THE
FRONT. ALSO THE FRONT WILL BE COMING THROUGH DURING THE AFTERNOON.
SO THAT IS WHEN THE BEST THREAT FOR SEVERE WILL BE.  WENT WITH
LIKELY POP THEN.

MONDAY NIGHT... QUICK DRYING BEHIND THE FRONT... SKIES SHOULD BECOME
MOSTLY CLEAR BY MIDNIGHT ALMOST EVERYWHERE IN THE CWA.

ZONES BY 345 PM.

WDM

EXTENDED FORECAST DISCUSSION...
THE PROBLEM THIS FORECAST CYCLE IS THE UNCERTAINTY IN TIMING AND
LOCATION OF SHORTWAVE ACTIVITY. A COOL FRONT SHOULD CLEAR THE
FORECAST AREA BY 00Z TUESDAY AS MODELS HAVE COME INTO GOOD AGREEMENT
WITH TIMING OF THIS FEATURE. FLOW TUESDAY WILL BE ZONAL BEHIND THE
FRONT. THE MODELS CONTINUE TO POINT TO A ZONAL FLOW PATTERN FOR THE
REMAINDER OF NEXT WEEK. WILL KEEP WEDNESDAY AND THURSDAY DRY BUT
WILL INCREASE CLOUD COVER WEDNESDAY. FRIDAY WILL AGAIN SEE A SYSTEM
QUICKLY MOVING THROUGH THE ZONAL FLOW BRINGING ONE DAY OF SHOWERS
AND THUNDERSTORMS. SATURDAY OUGHT TO BE PARTLY CLOUDY.

IOD

.GRR...NONE.