Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 250858
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
358 AM CDT Tue Jul 25 2017

.SHORT TERM...(Today and tonight)
Issued at 358 AM CDT Tue Jul 25 2017

Today will be interesting. The majority of the time was spend on
temperatures and dewpoint for today, followed by thunderstorms with
heavy rain and severe weather potential later this evening and
overnight.

A decaying MCS is slowly moving across Minnesota into Wisconsin.
Skies are clear on the back side of this so should see temperatures
warm quickly today across western Minnesota. Went above guidance for
dewpoints in this region given the overnight rainfall across west
central Minnesota, climatological peak of evapotranspiration, as
well as the expected pooling of dewpoints along the warm front.
Forecast dewpoints later today in the mid to upper 70s, together
with air temps near 90 yield heat indices of 100 to 105 for a few
hours during the late afternoon, so have a small heat advisory for
parts of west central Minnesota.

This hot, unstable air beneath a stout inversion should lead to late
afternoon MLCAPE values around 3000 to 4000 J/kg. Surface
convergence along the boundary should eventually break through the
CAP by early evening in eastern SD and western MN. Storms will
develop quickly and should become severe shortly after convective
initiation. Deep layer shear is only marginal for severe weather,
but an increasing jet aloft should help compensate for the
relatively weak mid-level flow. Hodographs along the warm front
would be favorable for a tornado or two, and SPC has a 5% contour in
this region if surface-based storms are able to develop by early
evening.

Widespread convection is expected closer to sunset when the low
level jet decouples from the boundary layer. These storms will pose
a flooding risk with rainfall rates easily in the 2 to 3 inch per
hour range. Fortunately there is not a strong surface low developing
upstream which would keep the low level jet anchored in one
location, so should see it gradually veer and become southwesterly
overnight, but there is still a small window of time in the late
evening where a legitimate training thunderstorm threat exists. Any
locations that does have an hour or two of training storms could
easily see 3-4 inches of rain or more. Eventually the cold pool
should develop and work in concert with the waning forcing for
ascent to drive the boundary southward and make for a more
progressive movement of the storms.

.LONG TERM...(Wednesday through Monday)
Issued at 358 AM CDT Tue Jul 25 2017

Thunderstorms will be ongoing early Wednesday morning ahead of the
cold front pushing into southern and eastern MN. Models are in
pretty good agreement in organizing an MCS over southwest MN and
driving it southeast into Iowa by afternoon. That will likely be
the main feature nearby for Wednesday. Forcing along the front to
the northeast of this area, particularly from west central WI
northeastward, will weaken overnight tonight, the core of the LLJ
will be shifting to the east, and the decaying upper jet will
leave little to support widespread convection into Wednesday
across Wisconsin. Kept 50-60 PoPs in place for now, but this may
be reduced further in future updates.

Surface high pressure builds in late week and into the weekend.
Mostly clear skies and near or slightly below normal temperatures
can be expected.

A front embedded in the northwest flow will push through Sunday.
It appears moisture will be limited with the expansive high
shutting off the Gulf, so not expecting widespread convective
development.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1050 PM CDT Mon Jul 24 2017

A band of TSRA/SHRA has developed along the nose of the
strengthening low level jet in northeast South Dakota late this
evening. A few showers will be possible along the leading edge of
elevated instability across Minnesota through 12z. However, the
main band of concentrated SHRA/TSRA should begin to affect
KAXN/KSTC between 7-9z with some SHRA/TSRA as far south as KRWF,
but this area will likely be more isolated. Between 9-12z the main
band of SHRA/TSRA will affect east central Minnesota, with the
bulk of the heavier SHRA between 12-15z. Timing is still not
certain, but as the concentrated area of SHRA/TSRA begins to move
into Minnesota, forecasters should have a better handle on timing
during the pre-dawn hours. Morning SHRA/TSRA should end between
15-18z in west central Wisconsin with a dry period before the next
chance of TSRA develops Tuesday evening.

KMSP...

Timing of the SHRA/TSRA might be a little later than previously
thought, especially considering the lack of instability (at the
surface and aloft), until 12z. There will be a few showers ahead
of the main batch of TSRA, but will not affect aviation as VFR
cigs/vsbys continue. There could be a brief period of MVFR vsby
in heavier storms between 12-15z. Otherwise, the next chance of
TSRA will occur Tuesday evening/overnight.

/OUTLOOK FOR KMSP/

Wed...VFR with TSRA possible early. Wind NW at 5 kts
Thu...VFR. Wind NE at 5-10 kts.
Fri...VFR. Wind E at 5kt.


&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Heat Advisory from 1 PM this afternoon to 7 PM CDT this evening
     for MNZ054>057-064-065-073.

&&

$$

SHORT TERM...JRB
LONG TERM...BORGHOFF
AVIATION...JLT


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