Area Forecast Discussion
Issued by NWS Twin Cities, MN

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815
FXUS63 KMPX 152138
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
438 PM CDT FRI MAY 15 2015

.SHORT TERM...(THIS EVENING THROUGH SATURDAY)
ISSUED AT 400 PM CDT FRI MAY 15 2015

EASIEST THING TO SAY IS CONFIDENCE IN POP FORECAST THIS PERIOD IS
LOW...THOUGH THE GENERAL THEME FOR POPS THROUGH SAT AFTERNOON COULD
BE SUMMED UP EASIEST AS LESS IS MORE.

WARM FRONT CURRENTLY TAKING SHAPE ACROSS NRN MO IS ASSOCIATED WITH A
BAND OF SHOWERS AND THUNDERSTORMS EXTENDING FROM SOUTH OF OMAHA
TOWARD ST. LOUIS. MODELS SHOW THIS FEATURE GETTING HERE CONSIDERABLY
SLOWER THAN WHAT GUIDANCE HAD BEEN SHOWING LAST NIGHT. IN
ADDITION...THE SREF ALONG WITH ALL OF THE CAMS SHOW ACTIVITY ACROSS
THE MPX AREA BEING VERY LIMITED IF OCCURRING AT ALL. THIS LACK OF
ACTIVITY DOES MAKE PHYSICAL SENSE AS 925-850 WINDS ARE DIVERGENT AS
THEY COME UP THROUGH HERE...WITH THIS LOW LEVEL DIVERGENCE ACTING AS
AN INHIBITOR TO THUNDERSTORM DEVELOPMENT. GIVEN TRENDS WITH THE CAMS
AND SREF MADE TWO CHANGES TO POPS THROUGH THE NIGHT. WE SLOWED DOWN
THEIR NE PUSH AND DECREASED THEM AS WELL...WITH CHANCE POPS ONLY
INDICATED OUT IN WEST CENTRAL MN LATE TONIGHT...WHEN THEY WILL BE
CLOSER TO THE STRONG MOISTURE TRANSPORT AND LOW LEVEL CONVERGENCE
GOING THROUGH THE DAKOTAS. THE BROAD BRUSHED ISOLATED THUNDER
MENTION OUTSIDE OF WRN MN IS INDICATIVE OF THE LOW CONFIDENCE IN
PRECIP DEVELOPMENT WITH THE WARM FRONT TONIGHT.

THIS LESS IS MORE THEME FOR PRECIP POTENTIAL CONTINUES INTO THE DAY
SATURDAY AS WELL. ALTHOUGH WE WILL SEE INSTABILITY INCREASE MARKEDLY
AS TEMPERATURES PUSH 80 WITH DEWPS IN THE LOW TO MID 60S...A FORCING
MECHANISM IS LACKING TO HELP GET ACTIVITY GOING SAT AFTERNOON. THINK
GREATEST THUNDERSTORM COVERAGE WILL BE WEST OF THE MPX AREA...CLOSER
TO THE SFC LOW/COLD FRONT AND THE UPPER TROUGH. FOR POPS ON
SATURDAY...REDUCED THEM CONSIDERABLY BEFORE 21Z. LEFT LIKELIES OUT
WEST AFTER 21Z...BUT REDUCED POPS TO AROUND 30 IN THE EAST...THOUGH
THIS IS LIKELY OVERDONE. AGAIN THE BROADBRUSHED NATURE OF THE POPS
TOMORROW AFTERNOON IS A GOOD INDICATION OF THE UNCERTAINTY THAT
EXISTS...BUT FEEL WE ARE STILL OVERDONE WITH POPS SATURDAY
AFTERNOON...IT IS REALLY SATURDAY NIGHT THAT LOOKS A LOT BETTER FOR
GETTING PRECIP INTO HERE AS THE MAIN UPPER WAVE ARRIVES.

FOR THE SEVERE WEATHER THREAT...DURING THE SHORT TERM...WHICH ENDS 7
PM SATURDAY...THE THREAT IS PRETTY MINIMAL...AS BETTER FORCING AND
SHEAR WILL BE OFF TO THE WEST THROUGH THIS PERIOD. THE SLIGHT RISK
CURRENTLY IN THE DAY 2 CONVECTIVE OUTLOOK FROM THE SPC IS WEIGHTED
MORE TOWARD WHAT MAY BE COMING AT US SATURDAY NIGHT.

.LONG TERM...(SATURDAY NIGHT THROUGH FRIDAY)
ISSUED AT 400 PM CDT FRI MAY 15 2015

THE SEVERITY OF THE STORMS ON SUNDAY WILL LARGELY BE DETERMINED
BY WHAT THE THUNDERSTORMS COMING OUT OF THE NORTHERN PLAINS DO
SATURDAY NIGHT INTO EARLY SUNDAY MORNING...AND ALSO BY JUST HOW
QUICKLY THE SYSTEM TRANSLATES EAST SATURDAY NIGHT. THE MAIN
TROUGH WILL BE CROSSING THE ROCKIES AND MOVING INTO THE NORTHERN
PLAINS BY SATURDAY NIGHT. WIDESPREAD STRONG TO SEVERE STORMS
SHOULD BE ONGOING IN SD/NE BY SATURDAY EVENING. A STRENGTHENING
LLJ JET OUT AHEAD OF THE MAIN TROUGH SHOULD BE SUFFICIENT TO GET
THE STORMS ENERGIZED AS THEY TRAVEL EAST TOWARD WESTERN MN
/MAINLY AFTER MIDNIGHT/ SATURDAY NIGHT INTO SUNDAY MORNING. AS
MENTIONED ABOVE...THERE WILL BE A STRONG INSTABILITY AXIS DURING
THE DAY ON SATURDAY ACROSS MINNESOTA...BUT THE TIMING IS SUCH THAT
BY THE TIME CONVECTION ARRIVES IN MN...THE INSTABILITY WILL HAVE
WANED CONSIDERABLY. THE UPPER WAVE AND SHEAR PROFILE ARE STILL
TOO STRONG NOT TO SUPPORT WIDESPREAD THUNDER AND ISOLATED TO
SCATTERED STRONG STORMS LATE SATURDAY NIGHT.

THESE STORMS WILL LINGER AND MOVE EAST THROUGHOUT THE MORNING
SUNDAY JUST AS THE SURFACE LOW APPROACHES THE EASTERN
DAKOTAS...THIS IS A NOTABLY SLOWER EVOLUTION THAT WHAT HAS BEEN
SEEN OVER THE PAST COUPLE DAYS. THE MAIN WARM FRONT WILL BE WELL
NORTH OF THE FORECAST AREA...BUT A LOT OF THE GUIDANCE ARE ALSO
TRYING TO DEPICT A MORE SUBTLE EAST-WEST BOUNDARY CLOSER TO THE
LOCAL AREA BETWEEN 18Z-00Z. WE WILL HAVE TO SEE IF THIS IS A REAL
FEATURE OR SOME SORT OF MODEL-GENERATED ARTIFACT. THE 12Z NAM AND
SOME OF THE CAM OUTPUT FROM TODAY GENERATE 1 OR 2 BROKEN LINES OF
CONVECTION BEHIND THE INITIAL MORNING THUNDERSTORMS...WHICH ARE
FORECAST TO DEVELOP SOMETIME IN THE EARLY AFTERNOON. WHILE THE
MORNING STORMS POSE A SOMEWHAT MARGINAL RISK FOR SEVERE
WEATHER...ANYTHING ANY THAT DEVELOPS IN THE AFTERNOON /ESPECIALLY
IF IT IS AIDED BY SOLAR HEATING/ WILL NEED TO BE CLOSELY MONITORED
GIVEN THE FAVORABLE SHEAR PROFILE FOR SEVERE WEATHER...INCLUDING
TORNADOES. THE TORNADO THREAT WOULD BE ENHANCED BY THE LIKELIHOOD
OF BACKED FLOW AND LOW LCLS...BUT THEN AGAIN THAT IS ONLY IF WE
MANAGE TO TRIGGER THUNDERSTORMS SUNDAY AFTERNOON. THE BEST THREAT
AT THIS POINT FOR SEVERE WEATHER ON SUNDAY LOOKS LIKE FAR EASTERN
MN AND WEST CENTRAL WI. IN ADDITION TO THE SIMULATED REFLECTIVITY
THEY GENERATE...THE CAMS DO SHOW A FEW STREAKS OF >50 MS/S2
UPDRAFT HELICITY...WHICH HAS BEEN SHOWN TO BE A USEFUL STORM-SCALE
FIELD PROXY FOR SEVERE WEATHER. FURTHERMORE...THE 60-HR CIPS
ANALOG USING THE 15.12Z NAM DOES HIGHLIGHT NORTHEAST IA AND
WESTERN WI AS A GOOD LOCATION FOR SEVERE REPORTS. THIS HAPPENS TO
LINE UP WITH THE SREF CALIBRATED SEVERE PROBABILITIES FROM TODAY
AS WELL.

A STRONG COLD FRONT MOVES IN SUNDAY NIGHT AND WILL BRING MUCH
COOLER TEMPERATURES TO THE AREA ON MONDAY AND TUESDAY...WITH
READINGS CLOSE TO 20 DEGREES BELOW NORMAL. TEMPS RECOVER SLIGHTLY
BY THE MIDDLE OF NEXT WEEK AND THE PATTERN ALSO LOOKS DRIER.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON)
ISSUED AT 114 PM CDT FRI MAY 15 2015

MODELS DID NOT HANDLE THE PROLONGED RESIDENCE TIME OF THE MVFR
CIGS VERY WELL TODAY...AND WITH LIGHT WINDS LIMITING
MIXING...THESE CIGS WILL BE WITH US A BIT LONGER. CONFIDENCE IS
LOW IN IMPROVEMENTS TO VFR IN THE TAFS...AS IT COULD HAPPEN
EARLIER OR EVEN MUCH LATER THAN CURRENTLY INDICATED. WARM FRONT
RIGHT NOW IS STILL DOWN ALONG THE I-70 CORRIDOR...SO THE SLOWING
OF ITS ARRIVAL WITH GUIDANCE TONIGHT MAKES SENSE. TSRA COVERAGE
THROUGH 18Z SAT LOOKS SCT AT BEST...AND CONFIDENCE IN WHEN/WHERE
STORMS MAY BE IS TOO LOW AT THIS POINT TO INCLUDE ANY PRECIP
MENTION IN A POINT SPECIFIC FORECAST. ALONG WITH THE WARM
FRONT...SREF PROBS SHOW MVFR CIGS COMING UP AS WELL...THOUGH
THOSE PROBS ARE HIGHEST IN WRN MN...WHICH IS WHY AXN/RWF ARE THE
ONLY TWO TERMINALS WHERE MVFR CIGS WERE BROUGHT BACK IN SATURDAY
MORNING. AS FOR WINDS...THEY WILL BE LIGHT AND VRB TODAY AS THE
HIGH MOVES THROUGH...BUT SHOULD SEE SE WIND GUSTS UP OVER 20 KTS
BY 18Z SAT IN MN AS PRESSURE GRADIENT INCREASES IN RESPONSE TO A
SUB-1000MB LOW DEEPENING OVER WRN SODAK.

KMSP...CONFIDENCE ON GOING VFR WITH CIGS AT 21Z IS LOW. BASED ON
HOW EXPANSIVE THE MVFR CIGS ARE...WOULD SAY CHANCES OF THE CIGS
IMPROVING LATER THAN 21Z IS BETTER THAN IT HAPPENING BEFORE 21Z.
FOR TONIGHT...REMOVED ANY PRECIP MENTION...THOUGH WE COULD SEE
SOME ISO/SCT TSRA ACTTIVITY SAT MORNING AS THE WARM FRONT LIFTS
NORTH. CONFIDENCE IS JUST NOT HIGH ENOUGH IN IF/WHEN/WHERE
ACTIVITY WILL BE TO INCLUDE IN A TAF AT THIS POINT. SAT
AFTERNOON...PLENTY OF INSTABILITY WILL BE PRESENT...BUT WE LACK
MUCH IN THE WAY OF A FORCING MECHANISM TO GET STORMS GOING...THAT
LOOKS TO ARRIVE LATER SAT NIGHT...SO KEPT PRECIP MENTION OUT OF
THE TAF SAT AFTERNOON AS WELL.

/OUTLOOK FOR KMSP/
SUN...VFR WITH SCATTERED IFR/TSRA. WINDS S AT 15-25 KTS.
MON...VFR. WINDS NW AT 15-25 KTS.
TUE...VFR. WINDS N AT 10 KTS.


&&

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

$$

SHORT TERM...MPG
LONG TERM...CLF
AVIATION...MPG



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