Area Forecast Discussion
Issued by NWS Twin Cities, MN

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000
FXUS63 KMPX 181223
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
723 AM CDT MON AUG 18 2014

.UPDATE...
ISSUED AT 701 AM CDT MON AUG 18 2014

UPDATED TO INCLUDE 12Z AVIATION DISCUSSION BELOW.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 415 AM CDT MON AUG 18 2014

TODAY AND TONIGHT...INTERESTING PATTERN OVER THE NEXT 24-30 HOURS
LOOKS MORE LIKE AN OCTOBER-NOVEMBER SCENE RATHER THAN MID-AUGUST.
A SHORTWAVE OPEN TROUGH WITHIN THE PREVAILING WNW-ESE FLOW WILL
SLOWLY TRAVERSE ACRS THE REGION THRU TNGT...DRAGGING A SFC LOW
PRES CENTER WITH IT. NO REAL BONA FIDE FRONTS ARE ASSOCIATED WITH
THIS LOW...BUT THERE ARE SEVERAL SFC TROUGH SPOKES EMANATING FROM
THIS SFC LOW AND IT IS THESE DISTURBANCES WITHIN A HIGHLY MOIST
ENVIRONMENT THAT WILL PRODUCE EFFICIENT RAIN-MAKING STORMS THRU
THE NEXT DAY OR SO. ALREADY SEEING SLOW-MOVING STORMS OVER WRN WI
CONTINUE TO SHIFT E...AND THESE STORMS ARE WITHIN A PWAT
ENVIRONMENT OF OVER 1.6 INCHES. THERE IS ALSO ENOUGH MIDLVL INFLOW
TO PRODUCE TRAINING OF STORMS PLUS MUCAPES OF 1000-2000 J/KG. THIS
IS COMBINING TO PRODUCE POCKETS OF HEAVY RAIN OVER W-CENTRAL
WI...AND THE THINKING IS THAT A SIMILAR ENVIRONMENT FOR TDA INTO
TNGT WILL ALLOW FOR SIMILAR-TYPE STORMS TO FORM LATER TDA AND
TNGT. TIMING WILL BE TRICKY...BUT HAVE GONE THE ROUTE OF SHOWING A
PERIOD OF DRYNESS THRU THIS MRNG THEN GRADUALLY INCRG POPS THRU
THE DAY TDA INTO TNGT...BEFORE ENDING POPS FROM W TO E OVERNIGHT
INTO TUE MRNG AS THE SEMI-STACKED LOW PRES SYSTEM SHIFTS INTO THE
GREAT LAKES. GIVEN THAT THE SFC LOW DOES REMAIN TO THE E OF THE
PARENT UPR OPEN WAVE...THERE MAY WELL BE CONTINUED PRECIP
GENERATION THRU TNGT SO HAVE KEPT LIKELY POPS GOING FOR MUCH OF
THE CWFA LATE THIS AFTN THRU TNGT...ESP IN ERN PORTIONS. WITH THE
HIGH PWATS...SLOW STORM MOTION AND/OR TRAINING OF STORMS AND ADDED
LIFT...THERE IS STILL THE POTENTIAL FOR HEAVY RAIN AND FLOODING.
THE OTHER CONCERN...AND THIS IS FOR THIS MORNING...IS DENSE
FOG...PARTICULARLY FOR W-CENTRAL MN. HAD ALREADY SEEN A NUMBER OF
W-CENTRAL MN SITES DROP TO 1/4SM AND 1/2SM VSBY WITHIN AN
ENVIRONMENT THAT HAS NEARLY CALM WINDS AND DEWPOINT DEPRESSIONS OF
TWO OR LESS. THOUGH THERE IS A WIND SHIFT COMING IN FROM THE
DAKOTAS WITH ONE OF THE SFC TROUGHS...AM NOT CONVINCED IT WILL
ARRIVE IN ENOUGH OF A FORM TO MITIGATE DENSE FOG DEVELOPMENT SO
HAVE OPTED TO HOIST A DENSE FOG ADVY THRU DAYBREAK. THE MPX ARE
AND HOPWRF MODELS HIGHLIGHT THE REDUCED VSBY NICELY
BUT...SURPRISINGLY...THE SREF IS NOWHERE NEAR AS BULLISH SO HAVE
GONE WITH THE SHORT-TERM MODELS. THERE IS SOME CHANCE THAT THE
ADVY MAY NEED TO BE ADJUSTED GIVEN OB TRENDS BUT WILL MONITOR AND
ADJUST AS NEEDED.

.LONG TERM...(TUESDAY THROUGH SUNDAY)
ISSUED AT 415 AM CDT MON AUG 18 2014

THE SIMPLE TRANSITION FROM NORTHWEST FLOW TO SOUTHWEST FLOW WILL
BRING REPEATED CHANCES FOR SHOWERS AND THUNDERSTORMS THIS WEEK
WITH RAINFALL POTENTIAL FOR 1-2 INCHES FOR MOST LOCATIONS...AND
HIGHER AMOUNTS UNDER THUNDERSTORMS.

AT THE START OF THE PERIOD ON TUESDAY THE UPPER LEVEL WAVE
RESPONSIBLE FOR MONDAYS PRECIP WILL BE DRIFTING EASTWARD ACROSS
WISCONSIN...BUT FORECAST MODELS AGREE ON AN EMBEDDED SHORTWAVE
ROTATING AROUND THE BACKS SIDE ACROSS THE MN/WI BORDER...TO
EXPANDED POPS WESTWARD TUESDAY AFTERNOON. HIGH PRESSURE WILL BUILD
IN TUESDAY NIGHT...BUT BOUNDARY LAYER DEWPOINTS DONT REALLY DRY
OUT. THIS COMBINED WITH LIGHT WINDS AND CLEARING SKIES WILL
SUPPORT FOG DEVELOPMENT BY EARLY WEDNESDAY MORNING...SO WILL
CONTINUE THIS FROM THE PREVIOUS FORECAST.

THE HIGH PRESSURE WILL BE SHORTLIVED AS AN UPPER LEVEL LOW CARVES
OUT A HIGH AMPLITUDE TROUGH ALONG THE WEST COAST. THIS WILL LEAD
TO SOUTHWEST FLOW ACROSS THE CENTRAL CONUS...WITH A SHORTWAVE
COMING UP ACROSS THE DAKOTAS BY WEDNESDAY. THIS WILL SUPPORT
SOUTHERLY LOW LEVEL FLOW...AND H925 MOISTURE TRANSPORT SHOWS A
DIRECT CONNECTION TO THE GULF MOISTURE WEDNESDAY NIGHT. A WARM
FRONT IS FORECAST TO BE DRAWN OUT EAST/WEST ACROSS THE CWA...SO
EXPECT AN MCS TO MOVE ACROSS THE REGION. SPLIT THE 12HR PRECIP
GRID INTO TWO 6HR WINDOWS TO SHOW THE EASTWARD PROGRESSION OF THE
MCS WHICH SHOULD MIRROR THE VEERING LOW LEVEL JET.

MODELS SHOW THIS WARM FRONT LIFTING NORTHWARD...BUT REPEATED
ROUNDS OF CONVECTION MAY KEEP THE BOUNDARY FARTHER SOUTH THAN
CURRENTLY FORECAST. THIS WOULD HAVE A SIGNIFICANT IMPACT ON
TEMPERATURES. IF THE FRONT IS FARTHER NORTH...WILL SEE LESS CLOUD
COVER AND HIGHS CLOSER TO THE 90S. FOR NOW DID NOT STRAY FROM
GUIDANCE AND HAVE HIGHS IN THE LOWER 80S...BUT THE POTENTIAL IS
THERE. SEVERE WEATHER APPEARS POSSIBLE WITH ADEQUATE DEEP LAYER
SHEAR AND INSTABILITY ALONG AND SOUTH OF THE FRONT. HEAVY RAIN IS
ALSO A THREAT WITH TRAINING THUNDERSTORMS.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z TUESDAY MORNING)
ISSUED AT 701 AM CDT MON AUG 18 2014

MESSY FCST OVER THE NEXT 24-30 HRS. EARLY MRNG LIFR CONDS IN
COMBINATION OF BOTH FOG AND LOW STRATUS WILL GRADUALLY GIVE WAY TO
MIDLVL CLOUDS IN ADVANCE OF PRECIP LIKELY TO DEVELOP MID-TO-LATE
AFTN. SHOWERY-TYPE PRECIP WILL PRECEDE ANY CONVECTION...AND THE
CONVECTION WILL LIKELY REDUCE CONDS TO IFR. STILL SOME MINOR
UNCERTAINTY WITH THE TIMING BUT HAVE FOLLOWED A HOPWRF/HRRR
COMBINATION TO SLIGHTLY DELAY THE CONVECTION COMPARED TO THE
18/06Z TAFS. THE HEAVIEST SHWRS/TSTMS WILL CONTINUE OFF-AND-ON FOR
SEVERAL HOURS LATER TDA THEN GRADUALLY SHIFT OFF TO THE E
OVERNIGHT. GOING INTO DAYBREAK TMRW...ANOTHER ROUND OF LOW STRATUS
AND/OR FOG WILL LIKELY PRODUCE IFR-OR-WORSE CONDITIONS. WINDS TO
GRADUALLY SHIFT FROM SSW TO NW OVER THE NEXT 24 HRS WITH SPEEDS
AOB 10 KTS.

KMSP...IFR CONDS TO START OUT DUE TO CIGS /WITH MVFR VSBY/ WHICH
LOOKS TO LAST UNTIL ARND 16Z OR SO. AM CONCERNED WITH HOW MUCH
VSBY MAY DROP DUE TO FOGGY CONDS IN WRN MN THAT HAVE BEEN SLOWLY
ENCROACHING EWD...ESP WITH WINDS GOING LIGHT AND CLOUD COVER
INHIBITING MIXING. CONDS IMPROVE JUST ENOUGH BEFORE PRECIP IS
EXPECTED TO DEVELOP. HAVE PUSHED OFF THE START TIME OF TSTMS TO
BETTER MATCH SHORT-TERM MODELS AND CCFP SCENARIO...THEN WILL LOOK
FOR PRECIP TO END BY LATE EVENING. SOME MVFR VSBYS AGAIN EXPECTED
SHORTLY BEFORE AND LEADING UP TO SUNRISE TMRW BEFORE VFR CONDS
PREVAIL.

/OUTLOOK FOR KMSP/
WED...VFR. WIND E 5-10 KTS.
THU...VFR. CHC MVFR WITH TSRA. WINDS SE 5-10 KTS BECMG SW.
FRI...VFR. CHC MVFR WITH TSRA, WINDS NE 5-10 KTS.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...DENSE FOG ADVISORY UNTIL 9 AM CDT THIS MORNING FOR MNZ041>043-
     047>050-054>058-064-065-073.

WI...NONE.
&&

$$

UPDATE...JPC
SHORT TERM...JPC
LONG TERM...JRB
AVIATION...JPC






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