Severe Thunderstorm Warning 65, NWS Detroit, MI

SOUTHEAST MICHIGAN AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
345 PM EDT SAT AUG 18 2001

IMPRESSIVE UPPER LOW ON SATELLITE IMAGES GRADUALLY MOVING SOUTHEAST
AND WILL BRING UNSETTLED WEATHER TO SOUTHEAST MICHIGAN FOR THE
UPCOMING 48 HOURS.  STRONG UPPER DIVERGENCE WILL BE IN PLACE OVER
SOUTHEAST MICHIGAN TONIGHT AHEAD OF THE UPPER LOW AS IT MOVES TO
NEAR THE NORTHWEST INDIANA/SOUTHWEST MICHIGAN BORDER BY 12Z SUNDAY.
MOISTURE WILL BE ON THE INCREASE AS WELL...WITH 850MB DEWPOINTS
RISING ABOVE 10C AND PRECIPITABLE WATER VALUES APPROACHING 1.5
INCHES.  ALREADY...IN THE INSTABILITY OF THE AFTERNOON AHEAD OF THE
UPPER WAVE...SCATTERED SHOWERS AND THUNDERSTORMS WERE OCCURRING FROM
ABOUT LANSING WEST AND NORTHWEST.

SHARS FLOOD CHECKLIST PROVIDED FOR NUMEROUS POSITIVE INDICATORS FOR
FLOODING OVERNIGHT.  THE ONLY POTENTIAL NEGATIVE FACTORS WERE THE
LOWERING HEIGHTS ALOFT AND MARGINAL 850MB DEW POINTS...WHICH SHOULD
BE ABOVE 11C BUT WILL LIKELY BE JUST AROUND THAT VALUE.  HOWEVER...
OVERNIGHT BOTH THE AVN AND ETA SUPPORT INCREASING 850MB WINDS NORTH
OF THE SURFACE WARM FRONT WHICH SHOULD LIE JUST SOUTH OF THE
SOUTHEAST MICHIGAN BORDER BY 12Z SUNDAY.  SURFACE DEW POINTS SOUTH
OF THE FRONT ARE IN THE MID AND UPPER 60S.  MOST FAVORABLE JET
DYNAMICS TEND TO STAY JUST SOUTH OF THE STATE OVERNIGHT...WITH THE
LEFT EXIT REGION OF A 50KT 500MB JET SWINGING ACROSS CENTRAL AND
NORTHERN OHIO.  WITH THE AVAILABLE MOISTURE AND TREMENDOUS LIFT
AHEAD OF THE APPROACHING UPPER LOW...AS WELL AS THE FRONT JUST TO
OUR SOUTH TONIGHT...THINK LOCALLY HEAVY RAIN IS STILL A GOOD BET.
IF WE HAD BEEN IN A REGIME WHERE RAIN WAS MORE FREQUENT OVER THE
LAST FEW WEEKS...SOME FLOOD HEADLINE MIGHT BE APPROPRIATE.  ON THE
OTHER HAND...RIVER LEVELS IN MOST PLACES ARE SEVERAL FEET BELOW
FLOOD STAGE...AND FLASH FLOOD GUIDANCE IS HIGH AS WELL.

SEVERE POTENTIAL APPEARS LIMITED OVERNIGHT AND SUNDAY...BUT HELICITY
VALUES WELL OVER 100 OVERNIGHT CANNOT BE IGNORED.  INSTABILITY ON
BUFKIT... AND ON PLAN VIEWS FROM BOTH THE AVN AND THE ETA SURFACE
THROUGH 800 MB...INDICATE LIFTED INDICES JUST BELOW ZERO TONIGHT
THROUGH SUNDAY EVENING...AND CAPE IS NOT VERY IMPRESSIVE EITHER.
MOST OF THE SIGNIFICANT WEATHER SHOULD BE WITH REGARD TO ANY HEAVY
RAINFALL...BUT COOLING ALOFT AND THE DEVELOPING DRY SLOT IN
SOUTHEAST MICHIGAN SUNDAY AFTERNOON COULD RESULT IN ENOUGH
INSTABILITY FOR AT LEAST SMALL HAIL THEN.  SURFACE LOW PROGGED TO
MOVE ACROSS SOUTHEAST MICHIGAN SHOULD PROVIDE FOR THE TIGHTEST
SURFACE GRADIENT OVER NORTHERN COUNTIES...AND WILL FORECAST GUSTY
WINDS SUNDAY ESPECIALLY IN THE THUMB THROUGHOUT THE DAY WITH WIND
BLOWING ACROSS THE LAKE.

DRY MID LEVELS ON SUNDAY NIGHT ARE PROGGED BY BOTH THE AVN AND THE
ETA IN SOUTHEAST MICHIGAN...SO THINK MOST DIURNAL SHOWERS AND
THUNDERSTORMS SHOULD BEGIN TO WANE SUNDAY EVENING.  WRAPAROUND
MOISTURE...THOUGH...SHOULD BEGIN TO SWEEP BACK INTO THE SAGINAW
VALLEY LATE AT NIGHT...POSSIBLY CLIPPING THE THUMB AS WELL BEFORE
PUSHING ACROSS THE REST OF SOUTHEAST MICHIGAN DURING THE DAY.  WITH
THE UPPER-LEVEL LOW MOVING OFF TO THE EAST SLOWLY MONDAY...WILL NOT
FORECAST PRECIPITATION ANY MORE HIGHLY THAN SCATTERED...BUT AMPLE
MOISTURE REMAINS FOR AT LEAST A CHANCE WITH THE BETTER CHANCES
FARTHER NORTH IN THE HIGHER MOISTURE.  PLAN TO BE OPTIMISTIC AND
MENTION SKIES BECOMING PARTLY SUNNY LATE IN THE DAY IN THE SAGINAW
AND FLINT AREAS WHERE THE SETTING SUN BEHIND THE BACK EDGE OF THE
CLOUD SHIELD MAY YIELD A PERIOD OF SUN.  FORECAST SHORT-TERM
TEMPERATURES CLOSE TO GUIDANCE...AND TRENDED CLOSER TO THE WARMER
MAV ESPECIALLY FROM I-69 SOUTH DUE TO CLOUDS ALLOWING FOR LESS OF A
FALL AT NIGHT AND STILL A HIGH ENOUGH SUN ANGLE THAT TEMPERATURES
MAY BE ABLE TO RISE A BIT DURING BREAKS IN PRECIPITATION.

AS THE UPPER LOW FINALLY MOVES OFF TO THE NORTHEAST...THE EXTENDED
MODELS CONTINUE TO FORECAST AN UPPER RIDGE BUILDING OVER THE STATE.
THE ETAXX...MRF...AND CANADIAN GLOBAL ALL FORECAST A WEAKNESS IN THE
RIDGE DEVELOPING DURING MID-WEEK OVER MICHIGAN WITH A SHORTWAVE
MOVING OVER THE CENTER OF THE COUNTRY WHILE A LONG-WAVE TROUGH
EXTENDING FROM HUDSON BAY ROTATES EAST COINCIDENT WITH THE
SHORTWAVE.  TIMING DIFFERENCES ARE NOTABLE...WITH THE MRF BEING THE
FASTER OF THE THREE MODELS PUSHING A SURFACE LOW OVER FAR SOUTHERN
MICHIGAN AND PROVIDING FOR DEEP MOISTURE 12Z WEDNESDAY.  THE
CANADIAN GLOBAL FORECASTS A WEAK COLD FRONT TO MOVE SOUTH THROUGH
SOUTHEAST MICHIGAN THURSDAY...WHILE THE ETAXX ALSO PUSHES A WEAK
COLD FRONT THROUGH SOUTHEAST MICHIGAN MAINLY WEDNESDAY NIGHT.  BOTH
EUROPEAN MODELS SHOW THE RIDGE BEING DOMINANT WITH NOT MUCH
WEAKNESS...THOUGH THE UKMET FORECASTS A WEAK COLD FRONT BASICALLY
BISECTING LOWER MICHIGAN FROM WEST-TO-EAST 00Z FRIDAY.  SHOWING THE
BEST RUN-TO-RUN CONTINUITY AND ALSO BEING THE MODEL OF COMPROMISE IN
TIMING BETWEEN THE CANADIAN GLOBAL AND THE MRF...WILL TEND TOWARD
THE ETAXX AND INTRODUCE A CHANCE OF SHOWERS OR THUNDERSTORMS
WEDNESDAY.  THE CHANCE COULD LINGER INTO THE LATE NIGHT WEDNESDAY
BEFORE DIMINISHING...OR EVEN INTO THURSDAY IF THE SURFACE FRONT
SLOWS MORE THAN PROGGED...BUT WILL LEAVE THURSDAY DRY FOR NOW.

COORD WITH APX.  COMMENTS WELCOME.

.DTX...NONE.

DJF