Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 180917
AFDMPX

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

.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 06Z TAFS THROUGH 06Z MONDAY NIGHT)
ISSUED AT 1118 PM CDT SUN AUG 17 2014

WIDELY SCATTERED SHOWERS LATE THIS EVENING WILL BE FOLLOWED BY A
MORE ORGANIZED AREA OF ACTIVITY OVERNIGHT. PREFER THE RAP 0.5KM
AGL CONDENSATIONAL PRESSURE DEFICITS AS A GUIDE FOR CIGS IN THE
NEAR-TERM. THINK IFR CIGS/VSBYS WILL PLAGUE SITES INTO MID MORNING
MONDAY. THEN OCCASIONAL SHOWERS/TS ARE EXPECTED AS THE TROUGH
PASSES ACROSS TOMORROW. EAST/SOUTHEAST WINDS AHEAD OF THE WARM
FRONT...THEN SHIFT TO WEST/SOUTHWEST FOR TOMORROW. SPEEDS AROUND
OR BELOW 10 KTS THROUGH THE PERIOD.

KMSP...
CIGS ARE EXPECTED TO DECREASE OVERNIGHT WITH BKN-OVC IFR LEVEL
CIGS PERSISTING INTO MONDAY MORNING. SCATTERED SHRA/TS ARE
EXPECTED ON MONDAY. WIND SPEEDS STAY AOB 10 KTS THROUGH THE
PERIOD BUT SHIFT FROM EAST/SOUTHEAST TO SOUTHWEST ON MONDAY
MORNING.

/OUTLOOK FOR KMSP/
TUE...VFR. CHC MVFR WITH AFTERNOON TSRA. WINDS NW 5-10 KTS.
WED...VFR. WIND VRB AT 5KTS.
THU...VFR. CHC MVFR WITH TSRA. WINDS SSE 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.
&&

$$

SHORT TERM...JPC
LONG TERM...JRB
AVIATION...LS






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