Area Forecast Discussion
Issued by NWS Twin Cities, MN

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000
FXUS63 KMPX 311312
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
812 AM CDT TUE MAR 31 2015

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 426 AM CDT TUE MAR 31 2015

THIS MORNING THE FORECAST AREA FINDS ITSELF UNDER WEAK LOW LEVEL
FLOW AND PREDOMINATELY CLEAR SKIES OUTSIDE OF A BATCH OF MID
CLOUDS SLIPPING SOUTHEAST ACROSS WESTERN WI. THE RESULT OF THE
LIGHT WINDS IN ADDITION TO THE LITTLE RAIN THAT FELL YESTERDAY HAS
LED TO AREAS OF DENSE FOG THIS MORNING IN WEST CENTRAL WI AND
EXTREME EASTERN MN. THIS FOG SHOULD LIFT SHORTLY AFTER SUNRISE.
COLD ADVECTION BEHIND THE CLIPPER SYSTEM THAT PASSED THROUGH
YESTERDAY WILL KEEP THE EASTERN FORECAST AREA COOL THROUGH THE
NOON HOUR...BUT WARMER 925-850MB TEMPS WILL BE PUSHED INTO MN THIS
AFTERNOON. OVERALL...TODAY WILL BE MILD AND DRY WITH LITTLE WIND
AFTER A COOL START. SOME OF THE MID LEVEL RH TRENDS SUGGEST A
SCATTERED TO BROKEN MID DECK THIS AFTERNOON. THE LEVEL FLOW WILL
INCREASE TONIGHT AS A SURFACE LOW SLIDES EAST ACROSS THE HIGH
PLAINS AND DEEPENS IN ND.

.LONG TERM...(WEDNESDAY THROUGH MONDAY)
ISSUED AT 426 AM CDT TUE MAR 31 2015

MAIN FOCUS IN THE EXTENDED FORECAST PERIOD CONTINUES TO BE THE
POTENTIAL FOR THE THUNDERSTORMS...AND PERHAPS THE FIRST SEVERE
WEATHER OF THE SEASON WEDNESDAY AFTERNOON/EVENING. THE THREAT HAS
INCREASED ACROSS SOUTHERN MN...WITH EVEN BETTER CHANCES TO THE SOUTH
FROM THE IA/MN BORDER DOWN TOWARD THE NE/IA BORDER.

THE THREAT FOR STORMS HINGES ON THE LOW LEVEL MOISTURE AND
CONVERGENCE SINCE BOTH ARE NEEDED TO HAVE ANY SHOT FOR SURFACE BASED
CONVECTION. FIRST OFF...THE LOW LEVEL MOISTURE. INITIALLY THIS
SEEMED OVERDONE BY THE MODELS WHEN SIMPLY LOOKING AT SURFACE
DEWPOINTS. THEY START OFF IN THE 30S WEDNESDAY MORNING...AND QUICKLY
BLOOM INTO THE LOW/MID 50S BY EARLY AFTERNOON. THIS SEEMS
UNREALISTIC GIVEN THE FACT THAT FORECAST SOUNDINGS SHOW MIXED LAYER
WINDS 30-35KTS...WITH 40-45KTS ATOP THE CHANNEL. ONE WOULD THINK
THAT THIS MIXING WOULD DRY OUT THE BOUNDARY LAYER AND QUICKLY ERODE
THE 50F SURFACE DEWPOINTS...BUT THAT CONCEPTUAL MODEL DOES NOT APPLY
TO THIS EVENT.

AS IS USUALLY THE CASE...THE FORECAST SOUNDINGS ARE THE KEY TO
DETERMINING THE ORIGINATION OF THE HIGHER DEWPOINTS. CAREFUL
ANALYSIS OF THE SOUTHWESTERN SITES...NAMELY KFSD/KRWF SHOW A SURGE
OF MOISTURE ADVECTION WITHIN THE H900-800MB LAYER WEDNESDAY MORNING.
PWAT INCREASES FROM AROUND 0.55 AT 7AM TO 0.85 BY 11AM. THIS
INCREASE IS BEFORE  THE MODEL GENERATES PRECIP...THUS MAKING A MORE
TRUE MEASURE OF THE ATMOSPHERIC MOISTURE. AS THIS ELEVATED MOISTURE
PLUME ADVECTS NORTH/EASTWARD...IT MIXES DOWN TO THE SURFACE.
THIS...COMBINED WITH DAYTIME HEATING WILL DESTABILIZED THE BOUNDARY
LAYER AND RESULT IN ML CAPE PROFILES OF AROUND 1000J/KG ACROSS
SOUTHERN MN BY WEDNESDAY AFTERNOON.

THIS INSTABILITY WILL BE CAPPED OFF BY A WARM LAYER ALOFT.
HOWEVER...UPPER LEVEL HEIGHT FALLS WILL APPROACH FROM THE WEST LATE
WEDNESDAY AFTERNOON/EVENING...INDUCING ASCENT AND COOLING IN THE MID
LEVELS OF THE ATMOSPHERE. FORECAST SOUNDINGS SHOW COOLING
TEMPERATURES ALOFT...AND THEN INTITIATE CONVECTION SHORTLY
THEREAFTER.

IN SUMMARY...THE FACT THAT THE 50F SURFACE DEWPOINTS ARE MIXED DOWN
FROM AN ELEVATED MOISTURE PLUME RAISES CONFIDENCE THAT THEY COME TO
FRUITION. THE TIMING OF THE OMEGA/COOLING ASSOCIATED WITH THE HEIGHT
FALLS IS STILL SUSPECT. IF TOO SLOW...THE THREAT FOR SURFACE BASED
CONVECTION WILL DISAPPEAR TOWARD SUNSET. LASTLY...THE LOW LEVEL
CONVERGENCE IS ALSO SUSPECT...BUT CONFIDENCE IS INCREASING THAT IT
WILL BE SUFFICIENT TO INITIATE CONVECTION. SURFACE WINDS OFF THE NAM
BACK WITH TIME ACROSS SOUTHERN MN AHEAD OF THE FRONT...LIKELY A
RESULT OF THE SURFACE TROUGH THAT DEVELOPS IN EASTERN NE. THEREFORE
FEEL THAT THE THREAT FOR STRONG/SEVERE STORMS IS REAL WEDNESDAY
AFTERNOON EVENING AS FAR NORTH AS THE SOUTHERN HALF OF MN/WI. THE
MAIN THREAT WOULD LIKELY BE WIND GIVEN THE LOW LEVEL SPEED (NOT
DIRECTIONAL) SHEAR...WITH A SECONDARY THREAT OF HAIL. AT THIS TIME
THE TORNADO POTENTIAL IS LOW GIVEN THE EXPECTED LINEAR CONVECTIVE
MODE...AND UNCERTAINTY IN SURFACE BASED CONVECTION. THE HIRES 31.00
ARW/NMM SUPPORT THIS.

LOOKING AHEAD...THURSDAY WILL BE WINDY WITH LOW RH. NORTHWEST FLOW
WILL SET UP WITH SEASONABLE TEMPERATURES THROUGH THE END OF THE
WEEK. THERE IS POTENTIAL FOR WIDESPREAD PRECIP OF AROUND A HALF AN
INCH EARLY NEXT WEEK.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING)
ISSUED AT 759 AM CDT TUE MAR 31 2015

FOG IS HANGING TOUGH IN FAR EASTERN MN AND WESTERN WI EARLY THIS
MORNING...BUT CONDITIONS WILL IMPROVE RAPIDLY AND WE`LL END UP VFR
FOR THE REMAINDER OF THE TAF PERIOD.

KMSP...

HIGH CONFIDENCE IN VFR TODAY. THERE WILL BE A WIND SHIFT FROM
NORTHWEST TO SOUTHEAST LATE THIS EVENING. IN REALITY...THIS WILL
PROBABLY HAPPEN OVER 2-3 HOUR PERIOD...PERHAPS EVEN GOING CALM OR
LIGHT AND VARIABLE FOR A COUPLE HOURS LATE THIS AFTERNOON/EARLY
EVENING. THE WEATHER TURNS MUCH MORE ACTIVE TOMORROW.

/OUTLOOK FOR KMSP/
WED...VFR. SHRA LKLY/CHC MVFR TSRA IN THE AFTN/EVE. WINDS S 20G35KTS.
THU...VFR. WINDS W 25G40 KTS.
FRI...VFR. WINDS NW 10-15 KTS.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR MNZ045-053-
     063.

WI...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR WIZ014>016-
     023>028.

&&

$$

SHORT TERM...CLF
LONG TERM...JRB
AVIATION...CLF



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