Severe Thunderstorm Warning 110, NWS Grand Rapids, MI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
330 PM EDT FRI SEP 01 2000

WEAK SURFACE BOUNDARY STRETCHES ACROSS THE NORTHERN TIER OF COUNTIES
AT 18Z FROM NEAR LUDINGTON TO SAGINAW BAY. A THUNDERSTORM HAD FIRED
EARLIER ALONG THE BOUNDARY NR MBL...BUT HAS SINCE DISSIPATED.
FORECAST CONCERNS THIS EVENING AND OVERNIGHT PERTAIN TO CLOUDS AND
CHANCES FOR PRECIPITATION AS THE BOUNDARY SAGS SOUTH THROUGH THE
FORECAST AREA. FORECAST FROM TOMORROW ON...IS ONE OF LOW CONFIDENCE
AS THE SHORT TERM MODELS DIVERGE BY QUITE A BIT. BY SUNDAY
EVENING...THE ETA BRINGS A LOW BACK INTO THE NORTHERN GREAT
LAKES...WHILE THE AVN HAS EXPANSIVE RIDGING. CLOUDS AND PCPN ARE
OBVIOUSLY A PROBLEM THEN SATURDAY NIGHT AND SUNDAY DEPENDING ON
WHICH MODEL IS YOUR MODEL OF CHOICE. MORE ON THIS LATER.

FOR TONIGHT THOUGH...WILL NEED TO CONTINUE THE CHANCE POPS THAT ARE
ALREADY IN THE GOING FORECAST. AN UNSTABLE AIR MASS IS IN PLACE TO
THE SOUTH OF THE BOUNDARY AS INDICATED BY TEMPS IN THE UPPER 80S TO
AROUND 90 AND DEW POINTS AS HIGH AS 73 AT A NUMBER OF SITES. WITH
CONVERGENCE ALONG THE BOUNDARY EXPECTED TO CONTINUE INTO THE EVENING
AT LEAST...WILL MAINTAIN A 50 PCT POP FOR SHOWERS AND THUNDERSTORMS.
UPSTREAM SHORTWAVE ALSO APPEARS TO BE A PLAYER AS IT HAS ALREADY
TOUCHED OFF AN MCS OVER MN/WI. DEEP LAYER THICKNESS PROGS INDICATE
THAT CONVECTION OUT TO THE WEST WILL MAKE MORE OF AN EASTWARD TURN
AS THE EVENING HOURS WEAR ON. LOWS TONIGHT SHOULD MAINLY BE IN THE
MIDDLE TO UPPER 60S.

ON SATURDAY...COOLER AIR WILL BE FILTERING SOUTH BEHIND THE
BOUNDARY...BRINGING WITH IT SOME TRAPPED LOWER CLOUDS. WILL HAVE THE
CHANCE FOR A LINGERING SHOWER OR THUNDERSTORM DURING THE
MORNING...OTHERWISE MOSTLY CLOUDY NORTH AND PARTLY SUNNY SOUTH.
HIGHS SHOULD RANGE FROM THE UPPER 70S NORTH TO THE MIDDLE 80S IN THE
SOUTH.

SATURDAY NIGHT AND SUNDAY ARE A BIT TRICKY GIVEN THE MODEL
UNCERTAINTY. HAVE OPTED FOR THE WARMER ETA AT THIS POINT HOWEVER.
THE DIFFERENCE IN THE MODELS...ESPECIALLY WHEN COMPARING THE ETA
VERSUS THE AVN ARE IN REGARD TO THE HANDLING OF THE SHORTWAVE
CURRENTLY IN SOUTHERN CALIFORNIA. THE ETA HANGS ONTO A STRONGER
SHORTWAVE...WHEREAS THE AVN WEAKENS IT CONSIDERABLY AS IT MOVES INTO
THE NRN PLAINS. IT SEEMS REASONABLE THAT THE POTENT LOOKING
SHORTWAVE CURRENTLY IN THE WATER VAPOR IMAGERY WOULD HOLD STRONGER
AS IT EJECTS INTO THE PLAINS. AS LEE SIDE CYCLOGENESIS OCCURS IT
WOULD MAKE SENSE GIVEN THE HEAT ONGOING IN THE PLAINS THAT THIS
BOUNDARY WILL COME BACK NORTH.

SO...WILL PLAY SATURDAY NIGHT AS MOSTLY CLOUDY AS THE CLOUDS FILTER
IN ALONG WITH THE COOLER AIR. WILL STAY NEAR GUIDANCE VALUES IN THE
UPPER 50S TO THE LOWER 60S. ON SUNDAY...WILL KEEP THE MENTION OF
SHOWERS AND THUNDERSTORMS IN...GIVEN THE ETA SOLUTION OF BRINGING
THE WARM FRONT BACK INTO THE AREA. WILL USE A MIDDLE OF THE ROAD
CLOUD WORDING GIVEN THE UNCERTAINTY (CONSIDERABLE CLOUDS). WILL GO
SLIGHTLY ABOVE NGM MOS ON SUNDAY.

DUKESHERER

EXTENDED FORECAST DISCUSSION...
MEDIUM RANGE MODELS HAVE COME INTO SLIGHTLY BETTER AGREEMENT TODAY
WITH THE EXPECTED AMPLIFICATION OF THE ERN U.S. UPPER TROUGH...WITH
THE EXCEPTION OF THE CANADIAN WHICH CONTINUES MORE OF A ZONAL FLOW
PATTERN ON DAYS 3 THROUGH 5. DIFFERENCES EXIST WITH RESPECT TO SFC
FEATURES SUN NIGHT AND MON AS EVIDENCED BY THE ETA/AVN 60HR
COMPARISON. WILL FOLLOW EXTRAPOLATION OF 60HR ETA AND INCLUDE CHANCE
FOR CONVECTION ACROSS THE FORECAST AREA FOR SUN NIGHT. BELIEVE SFC
BOUNDARY WILL BECOME NEARLY STATIONARY/HUNG UP ACROSS SRN LOWER MI
ON SUN NIGHT/EARLY MON...BEFORE MOVING SE OF THE REGION MON EVENING
AS SHORTWAVE TROUGH DIGS IN FROM THE N. WITH AMPLIFICATION OF THE
ERN TROUGH AND SFC HIGH PRES BUILDING S FROM ONTARIO...COOLER AND
DRIER AIR WILL FILTER IN ON A NRLY/NERLY SFC FLOW LATE MON THROUGH
TUE.

GREENE

.GoRR...NONE.