Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 170238
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
938 PM CDT Sun Jul 16 2017

.UPDATE...
Issued at 937 PM CDT Sun Jul 16 2017

Updated to include 06Z aviation discussion below.

&&

.SHORT TERM...(This evening through Monday)
Issued at 416 PM CDT Sun Jul 16 2017

Main concern in the short term is the potential for triple digit
temperatures across far western Minnesota on Monday, followed by
the chance for showers and thunderstorms late Monday across the
northwest part of the CWA.

For temperatures, an H850 thermal ridge will track eastward from
the Dakotas and lean over western MN. Forecast soundings in
western Minnesota show deep mixing with south/southwest winds
which would be favorable for hot temperatures. Areas to the east
have much more shallow mixing, an easterly component to the winds,
and low level saturation which means could see some
scattered/broken stratus which would limit the heating across
eastern MN and western WI. Given the recent trend with
temperatures meeting or exceeding guidance, decided to go on the
high side of the forecast guidance for Monday afternoon.

For precipitation, at this point it appears the main area of
precipitation will be to the west of the forecast area Monday
afternoon closer to the better low level convergence. Forecast
soundings mix very deeply and erode the CAP, but HiRes models
keep much of the area dry, and it appears that the precip it does
develop will be more like alto-cumulus/virga than measurable
precipitation. Did carry the slight/chance pops across the
northwest part of the forecast area, but the better chance for
storms in across eastern SD and northwest MN.

.LONG TERM...(Monday night through Sunday)
Issued at 416 PM CDT Sun Jul 16 2017

Thunderstorm activity that develops over the Dakotas and northern
MN late Monday afternoon may continue maturing through Monday
evening as it progresses into western and central MN, but is
expected to weaken overnight as it encounters strong capping
further southeast. Maintained medium range PoPs along the cold
front during the evening, and tapered them off to nothing from
southern MN into western WI.

The front will sag south into central MN/northwest WI Tuesday
morning and southwest MN to west central WI by Tuesday afternoon.
Building instability with a moistening boundary layer
characterized by dew points in the mid 70s and weak capping
should foster fairly widespread thunderstorms by afternoon along
and ahead of the front. MBE velocities less than 20 kt, deep warm
cloud depth of about 11,000 ft, pwats around 2 inches, and 850 mb
dew points exceeding +15 to +17C all favor locally torrential
rainfall. It wouldn`t take long for at least few inches to fall
in spots. The front will focus thunderstorms across southern MN
Tuesday evening and push it out for the overnight into Wednesday
morning.

The ridge will build back north into South Dakota Wednesday
morning, which will force the front to lift back north into
northwest Iowa and southeast South Dakota by Wednesday afternoon.
Models show another complex of storms firing over South Dakota by
afternoon, and spreading east or southeast rapidly during the
evening. There is some concern for a high end MCS given
potentially extremely unstable conditions near and south of the
front with dew points in the mid 70s to lower 80s, 300 mb winds on
the cool side of the front exceeding 100 kts, and westerly mid
level winds of 50 to 60 kts. A modest low level jet of 30 kts
could help limit the potential for a derecho. Models have been all
over the place with where this thing will track, but generally
they seem too far south into the 590+ dm heights for this type of
pattern, with the exception of the ECMWF which fits nicely with
the conceptual model. Ultimately it will likely depend on how far
south the Tuesday night convection forces the surface front, and
how quickly it can lift back north. If the front is slow to
return, it is possible any complex would remain elevated and/or
have issues developing at all. One thing that does grab my
attention is how quickly said complex reaches southeast lower
Michigan on the ECMWF. The complex develops over eastern South
Dakota late Wednesday afternoon and accelerates to the Detroit
area by mid morning Thursday - less than 18 hours after it began
which would be moving about 40 kts.

The pattern repeats itself Thursday and Thursday night with
another potential MCS developing over the Plains north of the
returning boundary and sliding eastward across MN/WI/IA late
Thursday night.

The pattern changes a bit for the weekend with a weak surface low
tracking along the Canadian border. Still expecting showers and
thunderstorms along the warm front/cold front as it passes
through. PoPs remain relatively low for now with significant
timing differences amongst guidance.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 937 PM| CDT Sun Jul 16 2017

Made no significant changes from earlier forecast. We`ll see winds
veer around to the south by mid-morning as high pressure continues
to work eastward. It still looks like any precipitation associated
with return flow should stay north/west of the TAF sites until
after the forecast period.

KMSP...No specific concerns at this time.

/OUTLOOK FOR KMSP/
Monday overnight...VFR. South wind 10 to 15 kt becoming southwest.
Tuesday...MVFR possible with a chance of showers and
thunderstorms, especially in the afternoon. Mainly southwest wind
around 10 kt.
Tuesday night...MVFR possible with a chance of showers and
thunderstorms. South wind 10 kt or less.
Wednesday...VFR, but brief MVFR possible with slight chance of
showers and thunderstorms. South wind less than 10 kt becoming
variable.
Wednesday night...VFR, but brief MVFR possible with a slight
chance of showers and thunderstorms. Mainly northeast wind 5 kt or
less.
Thursday...VFR. Northeast wind 5 to 10 kt.

&&

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

$$

UPDATE...
SHORT TERM...JRB/AMK
LONG TERM...BORGHOFF
AVIATION...



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