Area Forecast Discussion
Issued by NWS Twin Cities, MN

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000
FXUS63 KMPX 240915
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
415 AM CDT SUN AUG 24 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 253 AM CDT SUN AUG 24 2014

1003 MB SFC LOW THIS MORNING WAS LOCATED NEAR BISMARK. ITS WARM
FRONT WAS SURGING NORTH INTO SRN MN...WITH A COLD FRONT BEGINNING
TO TAKE SHAPE AT 3AM ACROSS CENTRAL SODAK THROUGH CENTRAL NEB. AHEAD
OF THIS WARM FRONT...WE HAVE SEEN A CLUSTER OF TSRA COME OUT OF
SODAK AND INTO CENTRAL MN AT THE NOSE OF AN H85 WARM TONGUE...WHICH
IS CHARACTERIZED BY 20-22C TEMPS THAT HAVE ALREADY MOVED INTO SOUTH
CENTRAL MN. THIS ACTIVITY WAS HESITANT TO MOVE EAST EARLIER THIS
EVENING UNTIL IT FINALLY MANAGED TO GENERATE A STRONG ENOUGH COLD
POOL AROUND 6Z TO ADD AN ERLY COMPONENT TO ITS NRLY MOTION. HAD A
SEGMENT OF THE LINE THAT BOWED OUT MOVING ALONG AT 55 MPH...BUT THIS
LINE IS QUICKLY WEAKENING AS IT ENCOUNTERS COOLER/MORE STABLE AIR TO
ITS NORTHEAST. FOR THE REST OF THE MORNING...WE MAY CONTINUE TO SEE
A FEW ISOLATED SHOWERS/STORMS POP UP ON THE NRN EDGE OF THE WARM
TONGUE...BUT WITH MAIN UPPER FORCING GOING INTO NW MN...EXPECT ANY
ADDITIONAL ACTIVITY THIS MORNING TO BE ISOLD/SCT IN NATURE.

FOR THE REST OF TODAY...THERE WILL BE TWO CONCERNS...THUNDERSTORM
REDEVELOPMENT ON THE COLD FRONT AND HOW HIGH WILL HEAT INDICES GET.
ONLY MINOR DIFFERENCES EXIST WITH THE TIMING OF THE FRONT...BUT A
MODEL BLEND BRINGS IT JUST INTO WRN MN BY 18Z...A LITTLE WEST OF
I-35 BY 00Z...AND EXITING THE MPX CWA BY 12Z MON. BY THE
AFTERNOON...TEMPS NEAR 90 COMBINED WITH LOWER 70S DEWPS WILL ALLOW
MLCAPE VALUES TO BUILD TO BETWEEN 3K AND 4K J/KG. HOWEVER...FORECAST
SOUNDINGS ALSO SHOW THE WARM TONGUE AT H85 RESULTING IN A FAIRLY
STOUT CAPPING INVERSION FORMING AS WELL. GENERALLY LIKE THE IDEA OF
THE NMM/ARW/MPX WRFS...WITH STORMS DEVELOPING ALONG THE COLD FRONT
NEAR THE US-71 CORRIDOR AROUND 21Z...AND WORKING EAST WITH THE COLD
FRONT THROUGH THE EVENING. ALTHOUGH SHEAR WILL BE MARGINAL /BULK
SHEAR 30-35 KTS/...THE DEGREE OF INSTABILITY WILL MORE THAN MAKE UP
FOR THE WEAKER SHEAR. SHEAR/INSTABILITY PARAMETERS WOULD INDICATE WE
WILL LIKELY SEE SOME FORM OF SUPERCELLULAR STRUCTURE
INITIALLY...BEFORE STORMS CONGEAL INTO A LINE. HAIL AND WIND WILL
LIKELY BE THE MAIN THREATS...BUT A TORNADO OR TWO CERTAINLY CAN NOT
BE RULED OUT IN THE FIRST COUPLE OF HOURS AFTER STORM INITIATION.
CENTRAL MN CURRENTLY APPEARS TO BE THE MOST FAVORABLE AREA FOR
HAVING A TORNADO THREAT THIS AFTERNOON.

THIS FRONTAL TIMING WILL ALSO PUT A WARM SECTOR WITH H925 TEMPS
INCREASING TO AROUND 25C OVER ERN MN DURING PEAK HEATING.
HOWEVER...DID CUT BACK ON FORECAST HIGHS TODAY BY A DEGREE OR TWO
FOR A COUPLE OF REASONS. ONE...WE WILL AGAIN SPEND MUCH OF THE
MORNING CLEANING LAYERED CLOUD COVER OUT OF THE AREA...AND TWO WITH
DEWPOINTS UP IN THE LOWER 70S...IT OFTEN MAKES IT DIFFICULT TO WARM
TEMPERATURES MUCH BEYOND 90. NOT TO MENTION THIS SUMMER...90 DEGREES
HAS BEEN A TOUGH NUT TO CRACK. GIVEN THE CLOUDY SKIES THIS
MORNING...A NARROW 4 HOUR WINDOW FOR GETTING HEAT INDICES AROUND 100
THIS AFTERNOON...FOLLOWED BY THE PASSAGE OF A COLD FRONT TONIGHT
SIGNALING THE ARRIVAL OF DRIER AIR...DECIDED TO STAY AWAY FROM
ISSUING A HEAT ADVISORY IN A MARGINAL SITUATION. WITH THAT
SAID...HEAT WILL BE MENTIONED IN THE HWO...GRAPHICAST...AND THE
FORECAST READS HIGHS NEAR 90...SO ITS NOT LIKE WE WILL NOT BE
GETTING THE WORD OUT ABOUT THE HEAT IN OTHER WAYS.

FOR TONIGHT...IT WILL BE FAIRLY UNEVENTFUL OUTSIDE OF THE
THUNDERSTORMS WORKING ACROSS WRN WI WITH THE COLD FRONT. THIS
ACTIVITY WILL BE PROGRESSIVE...BUT PWATS PUSHING 2" SHOULD BE ABLE
TO GET 0.5"-1.5" OF RAIN OUT OF MOST LOCATIONS IN WI. FOR WRN
MN...THE NATURAL AIR CONDITIONING WILL BE KICKING IN WITH THE WEST
WINDS BEHIND THE COLD FRONT...WITH DEWPS FALLING BACK INTO THE LOWER
50S AND TEMPS DROPPING INTO THE MID 50S.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 253 AM CDT SUN AUG 24 2014

THE LARGE SCALE PATTERN STILL LOOKS TO BECOME LESS AMPLIFIED FOR
THE LONGER TERM PORTION OF THE FORECAST WITH THE UPPER FLOW
BECOMING FAIRLY ZONAL ACROSS THE CONUS. THERE REMAINS SOME
DISAGREEMENT IN THE DETAILS WITH THE DETERMINISTIC SOLUTIONS OF
THE GFS... ECMWF... AND GEM... WITH THE RESPECTIVE ENSEMBLES
SHOWING AN UNDERSTANDABLE DEGREE OF SPREAD. SO... THERE REMAINS
SOME UNCERTAINTY ON PCPN CHANCES TIED TO SHORTWAVES MOVING THROUGH
THE ZONAL FLOW AS WELL AS CONVECTION FORCED NEAR THE BAROCLINIC
ZONE/FRONTAL BOUNDARY THAT WILL LINGER NEARBY FOR AT LEAST THE
FIRST PART OF THE WEEK. CONTINUED TO STICK CLOSE TO THE GEFS MEAN
IN TERMS OF THE LARGE SCALE MASS FIELDS... WHILE BLENDING IN THE
DETERMINISTIC ECMWF TO ACCOUNT FOR SOME OF THE UNCERTAINTY AND
FORECAST DETAILS.

COLD ADVECTION AND SUBSIDENCE SHOULD BE IN PLACE ACROSS THE ENTIRE
AREA ON MONDAY AS LOW PRESSURE LIFTS TOWARD HUDSON BAY AND A
SURFACE RIDGE NOSES IN FROM THE NORTHERN HIGH PLAINS. CHANCES FOR
PCPN COULD RETURN TO THE SOUTHERN PORTION OF THE AREA MONDAY
NIGHT THEN LINGER INTO WEDNESDAY AS THE STALLED BOUNDARY NEAR THE
MINNESOTA/IOWA BORDER BECOMES MORE ACTIVE AS INSTABILITY POOLS
ALONG IT AND WE SEE SOME WEAK ISENTROPIC ASCENT RE-ESTABLISH
ITSELF OVER IT. THERE REMAINS QUITE A BIT OF UNCERTAINTY ON HOW
FAR NORTH PCPN MIGHT OCCUR... AND IF THERE HAS BEEN A TREND IT HAS
BEEN FOR A BIT OF A SOUTHWARD SHIFT IN THE PAST FEW MODEL CYCLES.
MEAN LAYER WIND WOULD GENERALLY TAKE ANY ACTIVITY EAST OR EAST-
NORTHEAST... SO WOULDN/T EXPECT THINGS TO MOVE TOO FAR NORTH FROM
WHERE ANYTHING INITIATES... AND WITH THE BEST COMBINATION OF
INSTABILITY AND ISENTROPIC ASCENT AND/OR CONVERGENCE LOOKING TO BE
OVER NORTHERN IOWA THE SOUTHWARD SHIFT IN GUIDANCE MAKES SENSE.
ADJUSTED POPS A BIT SOUTH... AND WOULDN/T BE SURPRISED IF
SUBSEQUENT SHIFTS ARE ABLE TO PUSH THINGS FARTHER SOUTH AND/OR
TIGHTEN THE POP GRADIENT ON THE NORTHERN EDGE. LOW CHANCE POPS
LOOK NECESSARY FOR MUCH OF THE AREA DURING THE THURSDAY-FRIDAY
TIME FRAME WITH AT LEAST SOME OF THE GUIDANCE... NAMELY THE
ECMWF... SUGGESTING AN INVERTED SURFACE TROUGH AND RENEWED WARM
ADVECTION WILL WORK INTO THE SOUTHERN PORTION OF THE AREA DURING
THAT TIME FRAME. HOWEVER... THE LACK OF MODEL AGREEMENT PRECLUDES
INCLUDING MUCH MORE THAN LOW CHANCE POPS AT THIS POINT IN TIME. BY
SATURDAY IT LOOKS LIKE WE SHOULD HAVE DECENT SURFACE RIDGING
ACROSS MUCH OF THE AREA... SO ALLOWED FOR A POP-FREE PERIOD GIVEN
THE LACK OF ANY STRONG SIGNAL IN ANY OF THE SOLUTIONS OTHER THAN
THE IDEA FOR SOME WIDELY SCATTERED DIURNAL SHRA.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SUNDAY NIGHT)
ISSUED AT 1054 PM CDT SAT AUG 23 2014

LOW STRATUS STARTING TO SPREAD ACROSS ALL TAF SITES LATE THIS
EVE...AND LIKELY TO REMAIN AS SUCH THROUGH THE OVERNIGHT HOURS...
ESP FOR THOSE SITES NOT RECEIVING RAINFALL. THE WI TAF SITES AND
KMSP LOOK TO HIT IFR CIGS AND STAY THERE FOR MUCH OF THE
OVERNIGHT HRS...BUT LITTLE TO NO VSBY RESTRICTION WITH WINDS AT
ARND 10 KT OVERNIGHT. WRN TAF SITES WILL BE SUSCEPTIBLE TO
SHRA/TSTMS APCHG FROM THE SW ALONG A WMFNT. KAXN-KSTC HAVE THE
GREATEST CHC OF OVERNIGHT PRECIP...SO HAVE BROUGHT THOSE CONDS
DOWN TO IFR VSBY IN TEMPO GROUPS WITH PREVAILING IFR CIGS. CONDS
GRADUALLY IMPROVE TMRW MRNG INTO TMRW AFTN...INCLUDING REACHING
VFR FOR ALL SITES. HOWEVER...ANOTHER ROUND OF CONVECTIVE
SHWRS/TSTMS LOOKS LIKELY TMRW AFTN...ESP FOR CENTRAL-ERN MN INTO
WRN WI. HAVE REFLECTED THIS WITH ANOTHER MENTION OF PRECIP LATE
TMRW AFTN INTO TMRW EVE...BUT AM HOLDING OF CB/TS MENTION ATTM.
WINDS WILL REMAIN CLOSE TO 10 KT THROUGH WHILE SWINGING FROM E-SE
OVERNIGHT TO S TMRW TO SW TMRW EVE.

KMSP...CIGS TO REMAIN SUB-1700 FT FOR AT LEAST THRU FIRST HALF OF
THE 24/06Z TAFS...BOUNCING BETWEEN LOW-END MVFR AND IFR. NOT
LOOKING FOR A VSBY RESTRICTION...BUT THE CIG RESTRICTION WILL BE
SOLID. CONDS UNDERGO A GRADUAL IMPROVEMENT TO VFR BY LATE
AFTN...BUT A ROUND OF SHWRS /POSSIBLY ISOLD TSTMS/ PUSH THRU SUN
EVE WHICH MAY BRING CONDS DOWN AGAIN. THIS PRECIP DOES NOT APPEAR
TO BE PROLONGED...WHICH SHOULD ALLOW FOR VFR CONDS TO PREVAIL BY
DAYBREAK MON MRNG.

/OUTLOOK FOR KMSP/
MON...VFR. WINDS W 5-10 KTS.
TUE-THU...MAINLY VFR. CHANCE OF MVFR IN OCNL SHRA/TSRA. WINDS N TO
NE 5-10 KTS.


&&

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

$$

SHORT TERM...MPG
LONG TERM...
AVIATION...JPC





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