Severe Thunderstorm Warning 89, NWS Detroit, MI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
303 PM EDT THU JUL 27 2000

THIS FORECAST PROVIDES THE VERY DEFINITION OF UNSETTLED WEATHER.
THE UPPER LOW SPINNING AWAY NEAR IRON MOUNTAIN WILL DRIFT SOUTHEAST
TOWARD LAKE MI...THEN SOUTH. SURFACE LOW...STACKED WITH THE UPPER
LOW...WL FOLLOW. WARM...MUGGY AIRMASS WL REMAIN ACROSS SE LOWER MI
WITH LIGHT SOUTHERLY FLOW. EVALUATING CONVECTIVE POTENTIAL IS THE
MAIN FORECAST CHALLENGE.

MODELS INITIALIZED THE LOCATION OF THE UPPER LOW WELL...WHICH THEY
SHOULD HAVE...IT/S BEEN THERE LONG ENOUGH. THE ETA INITIALIZED THE
STRENGTH BETTER (DEEPER) THAN THE NGM/AVN. HOWEVER...AM LESS THAN
IMPRESSED WITH THE MODELS THERMAL INITIALIZATION...AS THE LOW STILL
HAS COOLER AIR ALOFT THAN THE MODELS THINK. GRB HAD A 500MB TEMP OF
-14C...2-3C COLDER THAN ETA/NGM/AVN INDICATED.

WV IMAGERY SHOWS A PRETTY WELL DEFINED DRY PUNCH ROLLING INTO WRN
LOWER MI THIS AFTERNOON. THE LEADING EDGE OF THIS IS THE BEST
SYNOPTIC FOCUS FOR CONVECTION EARLY TONIGHT. ON THE MESOSCALE...ONCE
AGAIN TODAY THE LAKE BREEZES HAVE BEEN THE BIG PLAYERS...WITH OUR
FIRST STORMS OF THE DAY ALONG THE THUMB SHORELINE. THE LAKE ST CLAIR
BREEZE JUST GOT ACTIVE IN MACOMB COUNTY. THE DRIER AIR ALOFT IS
PROGGED TO EDGE INTO THE SRN ZONES EARLY TONIGHT...ONLY TO HAVE
MOISTURE AND DPVA ROTATE BACK IN OVERNIGHT. THE COMBINATION OF
COOLER AIR ALOFT...AND NUMEROUS MESOSCALE BOUNDARIES FLOATING
AROUND...JUSTIFIES KEEPING A CHC OF PRECIP IN ALL ZONES ALL NIGHT.
BETTER CHANCE WL BE IN THE NORTH...WHERE DEEPER MOISTURE WL RESIDE
THE LONGEST...AND WHERE MORE OUTFLOW BOUNDARIES ARE LOCATED AT THE
MOMENT.

REALLY...VERY LITTLE CHANGES THROUGH SAT. ALTHOUGH THE POSITION OF
THE UPPER AND SURFACE LOWS SHIFTS THRU THE PERIOD...BOTH REMAIN TO
OUR WEST. SE LOWER MI WL REMAIN IN LIGHT SOUTHERLY LOW LEVEL
FLOW...WITH WEAK THETA-E ADVECTION IN THE LOW LEVELS. AIRMASS
CHANGES LITTLE AFTER TONIGHT...REMAINING SEASONABLY MOIST
(PRECIPITABLE H2O 1.3 TO 1.5 INCHES)...AND SEASONABLY UNSTABLE (LI/S
-1 AT NIGHT...-5 IN THE HEAT OF THE DAY). WEAK WINDS ALL THE WAY UP
THE CHUTE MEAN SLOW STORM MOTION TO THE NE...LITTLE SHEAR TO ALLOW
FOR STORM ORGANIZATION...AND WL ALLOW LAKE BREEZES TO FORCE
CONVECTION. NUMERICAL GUIDANCE IN AGREEMENT WITH THIS STEADY AS SHE
GOES PHILOSOPHY. SO EXPECT SCT SHRA/TSRA IN THE DAYTIME...MAINLY
AFTERNOON. CHC POPS IN THE OVERNIGHT PERIOD...WITH NO MENTION OF
TIMING...AS CONVECTION HAS TENDED TO NOT WIND DOWN UNTIL VERY LATE
AT NIGHT.

EXTENDED PERIOD BEGINS WITH UPPER LOW JUST TO OUR WEST. THIS BUNDLE
OF ENERGY SHEARS APART EARLY IN THE EXTENDED...WITH THE REMNANTS
DRIFTING SW INTO THE MID MS VALLEY BY EARLY NEXT WEEK. STRONG WRN
UPPER RIDGE WL FLOP OVER A BIT...WITH RIDGE AXIS FROM NRN PLAINS TO
WRN LAKE SUPERIOR. THE BERMUDA HIGH WL REMAIN DOMINANT TO OUR
EAST...LEAVING SE LOWER MI UNDER CONTINUED UPPER LEVEL TROFFING. THE
CANADIAN/OPERATIONAL MRF SEEM TO HAVE THE BEST HANDLE AND BEST
CONSISTENCY...WITH THE ETAXX EXTENDING THE ATLANTIC UPPER RIDGE TOO
FAR WEST.

PRESENCE OF UPPER LOW/TROFFING JUSTIFIES CHC POPS INTO MONDAY...
ALTHOUGH CHC OF RAIN WL BE HIGHER SUNDAY THAN MONDAY. BEYOND THAT IS
A TRICKY QUESTION. WITH SUCH A STAGNANT PATTERN SETTING UP THRU
WEDNESDAY...MESOSCALE FORCING/LAKE BREEZES COMBINED WITH CONTINUED
WEAK UPPER TROFFING SHOULD ALLOW POPCORN SHRA/TSRA IN THE HEAT OF
THE DAY. THESE PROBABLY WL NOT ACHIEVE 30 PERCENT AREAL COVERAGE ON
ANY PARTICULAR DAY...BUT STILL WARRANTS CHC POPS. PERHAPS A DRY DAY
THUR AS A DRIER WSW FLOW DEVELOPS AHEAD OF A NRN PLAINS COLD FRONT.

.DTX...NONE.

ZOLTOWSKI

MIZ047>049-053>055-060>063-068>070-075-076-082-083-281200-

SEVERE WEATHER OUTLOOK FOR SOUTHEAST MICHIGAN
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
355 PM EDT THU JUL 27 2000

...SEVERE WEATHER POTENTIAL STATEMENT FOR SOUTHEAST MICHIGAN...

DISPATCHERS...PLEASE NOTIFY YOUR EMERGENCY MANAGERS IMMEDIATELY.

SCATTERED SHOWERS AND THUNDERSTORMS WILL MOVE ACROSS ALL OF
SOUTHEAST LOWER MICHIGAN LATE THIS AFTERNOON AND EVENING...WITH
ISOLATED SEVERE THUNDERSTORMS EXPECTED ALONG THE LAKE HURON AND LAKE
ST CLAIR SHORELINE AREAS. SCATTERED THUNDERSTORMS WILL CONTINUE
THROUGH THE OVERNIGHT HOURS AS WELL...HOWEVER THE SEVERE
THUNDERSTORM THREAT WILL DIMINISH AFTER 7 PM. SPOTTER ACTIVATION
WILL BE NECESSARY EARLY THIS EVENING.

WARM AND HUMID AIR ALREADY IN PLACE OVER SOUTHEAST MICHIGAN IN
COMBINATION WITH A SLOW MOVING LOW PRESSURE SYSTEM WILL RESULT IN
SCATTERED THUNDERSTORMS LATE THIS AFTERNOON AND OVERNIGHT ACROSS ALL
OF SOUTHEAST LOWER MICHIGAN. THE BEST CHANCE OF STRONG TO SEVERE
THUNDERSTORMS WILL BE ALONG THE LAKE HURON AND LAKE ST CLAIR
SHORELINE AREAS THROUGH 7 PM.

THE STORMS THAT DEVELOP WILL MOVE NORTHEAST AT 15 MPH AND WILL BE
CAPABLE OF PRODUCING HAIL TO ONE INCH...WIND GUSTS TO 40 MPH AND
HEAVY RAINFALL. DUE TO THE SLOW MOVEMENT OF THE THUNDERSTORMS
PONDING OF WATER ON ROADS AND IN LOW LYING AREAS IS ALSO POSSIBLE.

STAY TUNED TO NOAA WEATHER RADIO...OR YOUR FAVORITE MEDIA OUTLET...
FOR LATER FORECAST UPDATES.

GSS