Severe Thunderstorm Warning 77, NWS Grand Rapids, MI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
400 AM EDT FRI SEP 7 2001

DEEP MOISTURE SURGING INTO THE AREA HAS CAUSED SCATTERED SHOWERS TO
BREAK OUT ACROSS THE AREA OVERNIGHT. HOWEVER THEY ARE HAVING A HARD
TIME MAINTAINING INTENSITY AS THEY ADVANCE EAST AND ENCOUNTER VERY
DRY/STABLE AIR MASS SEEN ON 00Z DTX SOUNDING. THIS TREND SHOULD NOT
LAST MUCH LONGER THOUGH WITH DEEP 30-40 KNOT SOUTHWEST FLOW AIMED
INTO THE AREA...AND MODELS AGREE THAT THE DEEP MOISTURE WILL REMAIN
OVER THE REGION THROUGH AT LEAST SATURDAY NIGHT. THIS SUPPORTS A
CONTINUATION OF SCATTERED CONVECTION BUT THE CHALLENGE IS RESOLVING
COVERAGE... TIMING...AND INTENSITY NEXT 48 TO 60 HOURS.

UPPER LONGWAVE TROUGH REMAINS OUT TO THE WEST THROUGH THE PERIOD...
WITH NUMEROUS SHORT WAVES EJECTING NORTHEAST TOWARD THE GREAT LAKES
REGION. WATER VAPOR IMAGERY SHOWS AN ELONGATED TROUGH/MOISTURE PLUME
CURRENTLY EXTENDING FROM WISCONSIN SOUTH TO TEXAS WITH SEVERAL
VORTICITY MAXIMA SITUATED ALONG IT. ALTHOUGH THE VORTICITY PATTERN
IN THE MODELS LOOKS QUITE NOISY... IT APPEARS LIKELY THAT THESE VORT
MAXIMA WILL BE SENDING NUMEROUS WAVES OF SHOWERS AND THUNDERSTORMS
THROUGH SOUTHWEST MICHIGAN TODAY. ONCE DAYTIME HEATING GETS UNDERWAY
AND INSTABILITY BUILDS... WE SHOULD SEE A DIURNAL INCREASE IN BOTH
COVERAGE AND INTENSITY. MODELS ACTUALLY SHOW FAIRLY IMPRESSIVE SHEAR
PROFILES FROM MID-AFTERNOON ON... ALONG WITH RESPECTABLE CAPE...SO
CAN/T RULE OUT SOME ISOLATED SEVERE TSTMS. MAY ACTUALLY BE SOME
LENGTHY PEEKS OF SUNSHINE THIS MORNING BASED ON CURRENT IR SATELLITE
LOOP BEFORE EXTENSIVE DIURNAL CU DEVELOPS. THIS MAKES THE HIGH TEMP
FORECAST VERY TRICKY...SINCE 850 MB TEMPS AND TRAJECTORY SCHEME
SUPPORT HIGHS IN THE UPPER 80S. WILL GO ABOVE MOS.

MODELS SUGGEST THAT THE AXIS OF AFOREMENTIONED SHORTWAVE TROUGH WILL
SHIFT EAST OF THE AREA LATER TONIGHT AND SATURDAY... AND THAT A BIT
OF A LULL IN CONVECTIVE ACTIVITY COULD SET UP BEFORE NEXT PIECE OF
SHORTWAVE ENERGY APPROACHES SATURDAY NIGHT. SINCE TIMING OF WAVES IN
SOUTHWEST FLOW PATTERN IS ALWAYS TOUGH... WILL STILL MAINTAIN POPS
ON SATURDAY BUT WILL HAVE THEM IN THE CHANCE CATEGORY.

HEAVY RAINFALL IS ALSO A VALID CONCERN GIVEN HIGH MOISTURE CONTENT
(PRECIPITABLE WATER VALUES AROUND 1.8 INCHES) AND DEEP UNIFORM FLOW
PROVIDING A POTENTIAL FOR TRAINING/REPEATING CELLS. HOWEVER...
INDIVIDUAL CELLS SHOULD BE QUICK MOVERS AND FLOODING POTENTIAL
APPEARS LOW AT THIS TIME.

MEADE

EXTENDED FORECAST DISCUSSION...
MEDIUM RANGE MODELS HAVE CHANGED ONCE AGAIN WITH THE 00Z RUNS...
LEAVING ANOTHER LOW CONFIDENCE EXTENDED FORECAST TO DEAL WITH.
LATEST AVN...ETA...AND CANADIAN MODELS DELAY SFC FRONTAL PASSAGE
UNTIL SUNDAY LATE MORNING/AFTERNOON IF IT EVEN HOLDS TOGETHER. IT
NOW APPEARS THE BEST DYNAMICS WILL BE WELL NORTH OF THE CWFA BY THE
TIME THE FRONT ARRIVES...SO IT MAY GO THROUGH WITH LITTLE FANFARE.
00Z AVN DEVELOPS A STRONG SECONDARY LOW PRESSURE WAVE TO OUR SW IN
RESPONSE TO A SHORTWAVE MOVING THROUGH THE MEAN TROUGH AND MOVES IT
NEWD ACROSS SRN LOWER MICHIGAN ON MONDAY...WITH SIGNIFICANT RAINFALL
INDICATED. THIS IS THE ONLY MODEL TO INDICATE SUCH FEATURE AND
ALTHOUGH THE AVN SEEMS OVERDONE WITH THE STRENGTH OF THE SFC LOW
...THE UPPER SHORTWAVE IS DEPICTED BY THE CANADIAN MODEL AS WELL.
ECMWF STILL INDICATES A MUCH COOLER SOLUTION BEHIND THE TROUGH
THAN THE OTHER MODELS. AT THIS POINT...WILL NOT CHANGE THE EXTENDED
FORECAST TOO MUCH AND FOLLOW A COMPROMISE BETWEEN THE AVN/CANADIAN
SOLUTIONS. WILL INCLUDE THE CHANCE FOR RAIN ON MONDAY ASSOCIATED
WITH THE SFC WAVE INDICATED BY THE AVN. WILL INDICATE A GRADUAL
COOLING TREND FOR SUN THROUGH TUES...THEN GRADUAL WARMING FOR
WEDNESDAY AND THURSDAY. WILL KEEP DAYS 6 AND 7 DRY AT THIS TIME.

GREENE

.GRR...NONE