Tornado and Flash Flood in Minnesota, September 20, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
631 AM CDT THU SEP 20 2007

.DISCUSSION...
FORECAST AREA ENTERING THE PUSH/PULL REGION BETWEEN DEPARTING
SURFACE HIGH OVER NORTHEAST MINNESOTA AND NORTHERN WISCONSIN...AND
APPROACHING LOW PRESSURE SYSTEM IN THE WESTERN DAKOTAS. EASTERLY
SURFACE WINDS ARE HELPING TO KEEP DEW POINTS RELATIVELY LOW...WITH
MID 40S TO LOW 50S IN MOST LOCALES. REGION OF CONVECTION IN THE
EASTERN DAKOTAS AND NORTHWEST MINNESOTA IS ASSOCIATED WITH A
40-50KT LOW LEVEL JET...AND LOW/MID LEVEL THETA-E ADVECTION.

THE SHORT TERM CONCERNS MOST DEFINITELY REVOLVE AROUND THE SEVERE
CONVECTION POSSIBILITIES LATER TODAY. ITS A LITTLE BIT OF A
TRICKY FORECAST...BECAUSE WE WILL DEFINITELY BE FIGHTING A CAPPING
INVERSION OVER AT LEAST THE SOUTHERN PORTION OF THE FORECAST
AREA...WITH 700 MB TEMPERATURES FROM 10-12C PROGGED. ANOTHER ISSUE
IS THAT MOST OF THE UPPER SUPPORT...AND THE GREATEST SURFACE
PRESSURE FALLS MOVE THROUGH NORTHERN MINNESOTA /ALTHOUGH THE
SOUTHERN EDGE OF BOTH GRAZES THE NORTHERN PART OF THE FORECAST
AREA. THE NOSE OF THE LOW LEVEL JET IS ALSO SOLIDLY NEAR THE
INTERNATIONAL BORDER BY 00Z FRIDAY. NOW...MOST OF THIS THROWS THE
TORNADIC THREAT INTO QUESTION...BUT NOT SEVERE CONVECTION AS A
WHOLE. WE DO HAVE AN 80-90KT JETLET MOVING INTO WESTERN MINNESOTA
NEAR MIDDAY...WHICH COUPLES QUITE NICELY WITH A 100-120KT JET IN
ONTARIO. SHEAR PARAMETERS ARE QUITE STRONG...WHEN TAKING INTO
ACCOUNT THE MID/UPPER LEVEL WINDS AND THE PREVAILING SOUTHEASTERLY
LOW LEVEL WINDS. WITH THE STRENGTH OF THE LOW LEVEL WIND
FIELD...WE DO EXPECT THAT 60+ SURFACE DEW POINTS WILL MAKE A
RETURN TO AT LEAST WESTERN AND SOUTHERN MINNESOTA BY THE END OF
THE DAY...WHICH SHOULD LEAD TO AT LEAST 2000 J/KG SBCAPES. THE
THINKING HERE IS THAT A HAIL/WIND THREAT WILL BE A DEFINITE /SAY
NORTH OF THE HIGHWAY 7 CORRIDOR/...BUT THE TORNADIC THREAT IS MUCH
LESS SO /ALBEIT NOT ZERO...CONSIDERING HOW MUCH INSTABILITY AND
SHEAR AREA HANGING AROUND/.

OVERNIGHT...MCS SHOULD MOVE ACROSS THE NORTHERN PART OF THE
FORECAST AREA...BEFORE DROPPING SOUTHEASTWARD OVER EAST CENTRAL
MINNESOTA/WEST CENTRAL WISCONSIN AS A SECOND LOW LEVEL JET RACES
UP FROM EASTERN NEBRASKA AND WESTERN IOWA.

COLD FRONT MOVES THROUGH THE AREA FRIDAY...WHICH REALLY PLAYS
HAVOC WITH THE HIGH TEMPERATURES ON FRIDAY. HAVE GONE WITH A 15
DEGREE GRADIENT ACROSS THE FORECAST AREA...WITH WELL ABOVE NORMAL
TEMPERATURES LIKELY IN THE I-90 CORRIDOR AND IN WEST CENTRAL
WISCONSIN. BLAST OF COLD ADVECTION BEHIND THE FRONT WILL LEAD TO
GUSTY W TO NW WINDS FOR FRIDAY NIGHT.

SURFACE HIGH PASSES QUICKLY THROUGH THE REGION...WITH NEXT SYSTEM
SET TO IMPACT THE AREA SUNDAY NIGHT AND INTO EARLY NEXT WEEK.
CHANGED LITTLE WITH RESPECT TO STORM TIMING...DESPITE THE FACT
THAT THE 00Z GFS SPED THINGS UP A LITTLE. 00Z ECMWF CAME IN
SIMILAR TO THE 19/12Z RUN...WHICH IS A BIT SLOWER THAN THE 00Z
GFS. TEMPERATURES LOOK QUITE WARM ON SUNDAY...WITH STRONG
SOUTHERLY WINDS RETURNING AHEAD OF THIS SYSTEM.

&&

.AVIATION.../12Z TAFS/
CONCERN WITH THE TAFS IS LOCKING DOWN BEST WINDOWS OF TIME FOR
CONVECTION AND ESTIMATING SRLY WIND GUSTS. STRONG UPGLIDE AND
INCREASES IN ELEVATED INSTABILITY OVER THE ERN DAKOTAS WILL
CONTINUE TO DRIVE T-STORMS INTO WRN MN THROUGH MID MORNING. THESE
WILL SPREAD OVER AXN WITH SOME HAIL POSSIBLE GIVEN RADAR TRENDS.
CONVECTION SHOULD BEGIN DWINDLING LATE IN THE MORNING AND INTO
EARLY AFTN AS JET FADES. REDEVELOPMENT OF CONVECTION IS POSSIBLE
IN WRN MN NEAR AND SOUTH OF AXN LATER THIS AFTN...WITH CAP BEING
THE MAIN STIFLING POINT. FINALLY...AS MAJOR HEIGHT FALLS AND
RESPONDING LLJ BEGIN WORKING INTO MN AGAIN TONIGHT...BETTER
CHANCES OF T-STORMS CREEP UP AGAIN IN CTRL MN AND INTO WRN WI.

HAVE GENERALLY USED 200FT PROJECTED WINDS BY THE 06Z NAM MODEL
FOR SUSTAINED WIND SPEEDS AND UTILIZED THE TOP OF THE MIXED LAYER
FOR GUSTS DURING TODAY AND THIS EVE. HAVE TWEAKED DOWN THESE
SPEEDS IN AREAS WHERE MORE CLOUD COVER IS EXPECTED. GUSTS TO OVER
30 KTS AT RWF ARE VERY POSSIBLE THIS AFTN.

ALSO IN THE IMMEDIATE FIRST FEW HOURS...IFR STRATUS HAS GOTTEN
CAUGHT IN ERLY FLOW OVR CTRL WI AND IS ANTICIPATED TO SPREAD OVER
EAU BUT DISSIPATE SOON AFTER AS HEATING BEGINS.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...NONE.
WI...NONE.
&&

KAT/MTF

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
452 AM CDT THU SEP 20 2007

MNZ041>045-047>070-073>078-082>085-091>093-WIZ014>016-023>028-
211000-
DOUGLAS-TODD-MORRISON-MILLE LACS-KANABEC-STEVENS-POPE-STEARNS-
BENTON-SHERBURNE-ISANTI-CHISAGO-LAC QUI PARLE-SWIFT-CHIPPEWA-
KANDIYOHI-MEEKER-WRIGHT-HENNEPIN-ANOKA-RAMSEY-WASHINGTON-
YELLOW MEDICINE-RENVILLE-MCLEOD-SIBLEY-CARVER-SCOTT-DAKOTA-
REDWOOD-BROWN-NICOLLET-LE SUEUR-RICE-GOODHUE-WATONWAN-BLUE EARTH-
WASECA-STEELE-MARTIN-FARIBAULT-FREEBORN-POLK-BARRON-RUSK-
ST. CROIX-PIERCE-DUNN-PEPIN-EAU CLAIRE-
452 AM CDT THU SEP 20 2007

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL AND
SOUTH CENTRAL MINNESOTA...AND WEST CENTRAL WISCONSIN.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS WILL BE POSSIBLE THIS MORNING OVER PARTS OF WEST
CENTRAL MINNESOTA...SOME OF WHICH MAY BECOME CAPABLE OF PRODUCING
ISOLATED LARGE HAIL AND DAMAGING WIND GUSTS. THESE STORMS SHOULD
PUSH INTO NORTHERN MINNESOTA BY MIDDAY.

ADDITIONAL THUNDERSTORMS...CONTAINING A GREATER LIKELIHOOD OF
PRODUCING SEVERE WEATHER...ARE EXPECTED TO DEVELOP THIS AFTERNOON
AND EVENING OVER WEST CENTRAL MINNESOTA...MOVING ACROSS PARTS OF
CENTRAL AND SOUTHERN MINNESOTA...ESPECIALLY ALONG AND NORTH OF
THE HIGHWAY 7 CORRIDOR...THROUGH THE EVENING AND OVERNIGHT. LARGE
HAIL...DAMAGING WINDS...AND HEAVY DOWNPOURS WILL BE LIKELY WITH
THESE STORMS. ISOLATED TORNADOES MAY ALSO OCCUR...ESPECIALLY IN
WEST CENTRAL MINNESOTA TOWARD MID TO LATE AFTERNOON.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. THUNDERSTORMS ARE
EXPECTED TO BE ONGOING ON FRIDAY MORNING...CONTINUING ACROSS EAST
CENTRAL MINNESOTA AND WEST CENTRAL WISCONSIN THROUGH MUCH OF THE
DAY ON FRIDAY...ALTHOUGH SEVERE WEATHER IS NOT ANTICIPATED.

.SPOTTER INFORMATION STATEMENT...

SKYWARN SPOTTER ACTIVATION IS NOT ANTICIPATED THIS MORNING...BUT
MAY BE NEEDED THIS AFTERNOON AND EVENING.

KAT