Severe Thunderstorm Warning 83, NWS Milwaukee, WI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI
327 PM CDT TUE SEP 21 2010

.SHORT TERM...TONIGHT THROUGH FRIDAY.
FORECAST EMPHASIS ON PRECIPITATION TONIGHT...RETURNING WED NIGHT
INTO FRI.

A COLD FRONT CROSSING SOUTHERN AND EASTERN WISCONSIN THIS EVENING
WILL BRING THUNDERSTORMS TO THE REGION. SPC MESOSCALE RUC ANALYSIS
SHOWED SBCAPE VALUES UP TO 2500 J/KG WITH NO SBCIN FOR PARTS OF
SOUTH CENTRAL WISCONSIN AT 3 PM THIS AFTERNOON. SFC CONVERGENCE
ALONG THE COLD FRONT...AS WELL AS FORCING FROM A 500MB SHORTWAVE
CROSSING EASTERN IOWA INTO NORTHERN IL LATE THIS AFTERNOON...WILL
HELP TO ORGANIZE ONGOING CONVECTION IN FAR EASTERN IA/SW WI/NW IL
INTO MORE OF A NORTH TO SOUTH LINE WITH POSSIBLE BOWING SEGMENTS.

WITH A WET BULB ZERO HEIGHT OF 10K AND LOWER...WITH THE GIVEN LATE
AFTERNOON CAPE...AND EFFECTIVE BULK SHEAR UP TO 40KT...ISOLATED
LARGE HAIL IS POSSIBLE. WINDS ARE FAIRLY UNIDIRECTIONAL ALONG THE
FRONT...SO STRAIGHT LINE WINDS WITHIN BOWING SEGMENTS WILL BECOME
MORE LIKELY AS CONVECTION CONTINUES. A SEVERE THUNDERSTORM WATCH IS
IN EFFECT UNTIL 9 PM FOR SOUTHERN TIER AND JEFFERSON/WAUKESHA/MKE
COUNTIES.

PRECIP WILL TAPER OFF FROM NW TO SOUTH CENTRAL AROUND MIDNIGHT
BEHIND THE FRONT. THE FRONT WILL HANG UP OVER NORTHERN IL
OVERNIGHT...SO LINGERING SHOWERS WITH ISOLATED THUNDERSTORMS ARE
POSSIBLE ACROSS FAR SOUTHERN WI THROUGH THE EARLY MORNING HOURS.

SURFACE BOUNDARY SAGS A LITTLE FARTHER SOUTH INTO CENTRAL IL FOR WED
MRNG SO WL REMOVE POPS IN THE SOUTH...AT LEAST DURING THE MORNING.
BENIGN CONDITIONS SHORT-LIVED AS SHORT WAVE TROUGH ALREADY PUSHING
INTO WEST COAST MOVES RAPIDLY EWD AND BEGINS TO AMPLIFY ACROSS
NORTHERN PLAINS WED NGT. IN RESPONSE...NOSE OF STRONG 50KT LOW LEVEL
JET TAKES AIM ON CENTRAL WI WITH STRONG SURGE OF THETA-E ADVECTION
THU NGT.  ELEVATED INSTABILITY INCREASES RAPIDLY WITH MID LEVEL
LAPSE RATES INCREASING TO 7 C.  SURGE OF WARM AIR ADVECTION WL PULL
WMFNT RAPIDLY NWD ACRS SRN WI LATE WED NGT INTO THU MRNG. CONFINED
LIKELY WORDING FOR CONVECTION OVER NRN TWO-THIRDS OF CWA CLOSER TO
BEST LOW LEVEL CONVERGENCE.

WMFNT SHIFTS RAPIDLY NWD INTO NRN WI ON THU WITH BULK OF CWA IN WARM
SECTOR.  KEPT LIKELY WORDING IN THE NORTH IN THE MORNING AS WMFNT
SHIFTS NWD AND IN THE FAR NORTHWEST IN THE AFTN AHEAD OF APPROACHING
CDFNT.  3 HR PRES FALLS OF 2 TO 3 MB WITH 925MB WINDS INCREASING TO
40 TO 45 KTS IN THE AFTN.  WL LIKELY RESULT IN SUSTAINED 20 TO 25KT
WINDS FOR SEVERAL HOURS IN ERN CWA...WITH DRY ADIABATIC LAPSE RATES
ALLOWING WIND GUSTS TO POSSIBLY REACH 35KTS. AT THIS
POINT...THINKING WINDS WL REMAIN BLO WIND ADVISORY CRITERIA.

STRONG SHORT WAVE TROUGH AND CDFNT SWEEPS THROUGH SRN WI THU NGT SO
WL CONT LIKELY WORDING WITH LINGERING -SHRA DIMINISHING FRIDAY
MORNING.

.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY.
INTERESTING TO WATCH LAST SEVERAL DAYS HOW THE EXTENDED GUIDANCE HAS
HAD DIFFICULTIES HANDLING POTENTIAL SHORT WAVE TROFFING ACROSS THE
UPPER MIDWEST THIS COMING WEEKEND. NEED TO GIVE KUDOS TO HI RES
ECMWF WHICH WAS SHOWING THE POTENTIAL FOR SHORT WAVE TROFFING ON THE
00Z/20 MODEL RUN...WHILE OTHER EXTENDED GUIDANCE WAS A RUN OR TWO
LATER.  NORTHERN HEMISPHERE WATER VAPOR DOES SHOW SEVERAL UPSTREAM
LARGE WAVES OVER CENTRAL/NORTHEAST PAC SO THERE IS POTENTIAL FOR
SHORT WAVE ENERGY TO AFFECT UPPER MIDWEST THIS WEEKEND.

GFS/UKMT AND CANADIAN ALL IN AGREEMENT WITH ECMWF IN SHOWING
SHORT WAVE TROF SAGGING SOUTHEAST ACROSS UPPER MIDWEST OVER THE
WEEKEND.  ALTHOUGH SOME AGREEMENT EXISTS ON THIS SCENARIO...SPECIFIC
NUANCES IN THE FORECAST STILL QUITE A BIT OUT OF SYNC...SO
CONFIDENCE REMAINS LOW FOR THIS PERIOD. LATEST GFS FOLLOWING TRACK
OF EARLIER RUNS OF ECMWF WHILE LATEST ECMWF AND GEM FARTHER EAST
OVER CENTRAL GTLAKES WITH SHORT WAVE. MOST OF THE GUIDANCE IS
SHOWING MORE PROGRESSIVE SHORT WAVE SO FOR NOW...DUE TO LARGE
UNCERTAINTY...WL CONTINUE SMALL POPS FOR -SHRA ON SAT...AND ADD
SMALL POPS IN THE EAST FOR SAT NGT.

AT THIS POINT...EXPC DRY WEATHER TO RETURN FOR SUNDAY AND PERSIST
INTO MONDAY BUT WL NEED TO WATCH FOR SECONDARY WEAKER SHORT WAVE IN
WNW FLOW THAT MAY SWEEP ACROSS WRN GTLAKES LATER MON AND MON NGT.

WL BE INTERESTING TO SEE IF GFS 500MB 5 DAY ANOMOLY VALID AT 00Z/02
VERIFIES WHICH SHOWS LARGE POSITIVE 250M HEIGHT ANOMOLY OVER SOUTH
CENTRAL CAN.  WOULD BE QUIET AND WARM INTO THE START OF THE NEW
MONTH.

&&

.AVIATION...THE MAIN AREA OF THUNDERSTORMS WILL MISS THE MSN AIRPORT
THIS AFTERNOON. THERE ARE SOME SHOWERS IN THE VCTY AROUND 3 TO 4
THIS AFTERNOON...BUT THEY ARE DISSIPATING AS THE COLD FRONT MOVES
THROUGH. ANOTHER AREA OF SHOWERS IN FAR SW WI SHOULD GRAZE ACROSS
MSN LATER THIS AFTERNOON. HOWEVER...THE MAIN
THUNDERSTORMS...POSSIBLY SEVERE...WILL TRACK SOUTH OF A LINE FROM
MSN TO PORT WASHINGTON LATE THIS AFTERNOON INTO THIS EVENING AS THE
COLD FRONT MAKES ITS WAY ACROSS THE SOUTHEAST PART OF THE STATE.

EXPECT MAINLY MVFR CIGS/VSBYS WITH THE CONVECTION...THOUGH SOME IFR
POSSIBLE WITH STRONGER CELLS. LARGE HAIL AND DAMAGING WINDS ARE THE
MAIN SEVERE WEATHER THREAT.

PCPN CHANCES LINGER FOR SOUTHEAST TAF SITES THROUGH EARLY MORNING
WITH THE SURFACE TROUGH/FRONT STALLING ACROSS NORTHERN IL OVERNIGHT.

&&

.MARINE...SMALL CRAFT ADVISORY REMAINS IN EFFECT UNTIL 6 PM SOUTH OF
NORTH POINT LIGHT. NORTH OF THAT SITE...OFFSHORE WINDS HAVE
DIMINISHED WITH THE SFC TROUGH OVERHEAD...AND THEREFORE WAVES HAVE
DIMINISHED BELOW SMALL CRAFT ADVISORY CRITERIA AS WELL.

ANOTHER STRONG LOW PRESSURE SYSTEM MOVING THROUGH THE REGION
WEDNESDAY NIGHT THROUGH FRIDAY WILL BRING ANOTHER CHANCE OF GUSTY
WINDS AND HIGH WAVES. A SMALL CRAFT ADVISORY MAY BE NEEDED FOR THIS
PERIOD.

&&

.CONFIDENCE...
TONIGHT AND WED...MEDIUM TO HIGH.
THU AND FRI...MEDIUM.
FRI NIGHT THRU SUN...LOW.
SUN NGT THRU TUE...LOW TO MEDIUM.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 6 PM CDT TONIGHT LMZ645-646.

&&

TONIGHT...MRC
REST OF SHORT/LONG TERM...MBK
AVIATION/MARINE...MRC

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