Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 240901

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
401 AM CDT Mon Jul 24 2017

.SHORT TERM...(Today and tonight)
Issued at 401 AM CDT Mon Jul 24 2017

Early morning satellite imagery together with surface obs and RAP13
analysis showed an area of high pressure centered over Lake
Superior, with light clockwise flow in place across the Upper
Midwest. Skies were mostly clear except for a few pockets of low
stratus. There was an area of thunderstorms upstream across the
Black Hills of South Dakota.

This area of thunderstorms should dissipate today, but the MCV
associated with it could produce a few showers and thunderstorms
across the far northwest part of the CWA Monday night. The HiRes
models show a few areas of convection, but not too confident in
their existence based on the forecast soundings of the GFS and NAM,
which show a capped environment. Instead it appears that the HiRes
models may be latching onto more of an altocumulus field.
Meanwhile expect today to be mostly sunny and slightly warmer than
yesterday with southeasterly winds.

.LONG TERM...(Tuesday through Sunday)
Issued at 401 AM CDT Mon Jul 24 2017

A few elevated showers and thunderstorms, or possibly accas, may
be ongoing at the start of the period Tuesday morning on the
leading edge of the strongest moisture advection. The strength of
the capping due to a dry boundary layer should prevent widespread
activity from developing. Scattered to broken mid level clouds may
persist for much of the day east of the EML poking into west
central MN, and a dry airmass from the continental high pressure
ridge over the Great Lakes will keep temperatures mild east of
I-35. The degree of sunshine will determine how hot west central
MN will get. Guidance has moderated from the very hot readings
being advertised Saturday night to more reasonable levels in the
mid 90s.

Showers and thunderstorms will fire on a cold front situated from
west central to northeast Minnesota during the very late
afternoon or evening hours. A severe threat continues to be
possible, but should be limited to a lower end event due to
strongest mid level flow confined to the north. Still waiting to
get into more hi-res model windows, but global models seem to
indicate the potential for upscale growth into clusters or a
couple MCSs during the overnight hours.

Perhaps the bigger threat will be heavy rain. A slow moving front
Tuesday night will facilitate a concentrated corridor of heavy
totals. Pwats around 2 inches, MBE velocities less than 10 kts,
and the potential for renewed development all night along this
front could result in several inches locally. Sided a bit more
with the Canadian and the ECMWF for QPF because while I do believe
there will be localized extreme totals which the NAM and GFS
indicate, it is impossible to pinpoint where at this juncture
given complex mesoscale features that aren`t resolvable until
hours before the event. The foreign models portray a bit more of a
generalized picture. Looking at the totality of the guidance,
including ensembles, it appears central MN stands the best chance
for excessive rainfall. Storms will continue into Wednesday across
southern MN into central Wisconsin, albeit in a likely weaker
state and the front beginning to accelerate should provide more
of a conditional heavy rain threat.

Northwest flow will prevail beyond Wednesday with near normal
temperatures and little chance for rain. There may be a front that
sags south next weekend, but some models wash it out before
reaching the area.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 954 PM CDT Sun Jul 23 2017

Made no substantive changes to earlier forecast. Lingering
stratocu over the eastern portion of the area is taking a bit
longer to dissipate than anticipated than expected, so will need
to keep an eye on KEAU (and perhaps KRNH) at the start of the
period for ceilings near MVFR category. Otherwise, high pressure
will move east through the period, with winds veering around to
the southeast. Still some concern for a bit of fog later, but with
mixed signals in the guidance and potential development still
several hours away chose to leave things as is.

KMSP...Only concern would be some lower VFR ceilings toward the
end of the period as precipitation tries to work into western
Minnesota. It`s possible this evolution could be faster than
currently forecast, so that would be the one thing to keep an eye
on in updated guidance.

Tuesday...MVFR possible with a chance of showers and
thunderstorms. South wind 10 to 20 kt.
Tuesday night...MVFR expected with showers and thunderstorms
likely, brief IFR possible. South wind 10 to 20 kt.
Wednesday...MVFR possible with a chance of showers and
thunderstorms. Southwest wind 10 kt or less becoming west.
Wednesday night...VFR. West wind less than 10 kt becoming
Thursday...VFR. Northwest wind 5 to 10 kt becoming north.




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