Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 231739

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1239 PM CDT Sun Jul 23 2017

.SHORT TERM...(Today and tonight)
Issued at 431 AM CDT Sun Jul 23 2017

Today`s weather will be quiet with high pressure building in and
dry conditions expected. Skies will start out partly cloudy, but
should see clouds increase starting mid morning, with clearing
skies again toward the evening. Overnight expect clear skies and
light winds with some patchy fog.

Early morning water vapor imagery with RAP13 500mb heights and
winds showed a positively tilted upper level wave across the
arrowhead of Minnesota making its way toward the UP of Michigan.
This wave will drive a reinforcing shot of cool air as seen in the
H850 temperature advection. Forecast soundings show stratus
development atop the boundary layer in response to this cold air
advection, but at the same time there is subsidence aloft as seen
in the H500 height changes so that should limit any real chance of
seeing precip develop.

Upstream a few area of stratus have already formed, and this will
likely continue as the air mass advects across central Minnesota
into western Wisconsin. Eventually the subsidence will win out and
should see decreasing clouds by late afternoon and early evening.
Overnight light winds and clear skies should allow for patchy fog
to develop.

.LONG TERM...(Monday through Saturday)
Issued at 431 AM CDT Sun Jul 23 2017

The center of the high will drift over the Great Lakes Monday
with return flow beginning in earnest across western MN during the
day and spreading eastward Monday night. A narrow ribbon of deeper
moisture will lift northward ahead of an approaching trough with
pwats reaching about 1.75 inches Monday night across much of MN.
Showers and thunderstorms may develop during the evening over the
Dakotas and work their way into northwestern MN during the
overnight hours. With the LLJ focused more to the north, it`s
expected most of the activity will remain up there, but cannot
rule out some isolated convection on the leading edge of the EML
plume working into MN late, coincident with a sharp 850 mb
moisture gradient also working across. Strong capping will limit
the overall threat for convection, but models are hinting at some
activity across eastern MN toward morning.

Tuesday has the potential to be very hot and humid. Given the
expected lack of morning convection under the 8.5C/km EML and
accompanying thermal ridging working in, should see highs in the
90s across MN and nearing 100 to the lee of the Buffalo Ridge. It
will depend how deep we mix. NAM and GFS completely mix out the
EML inversion (which is hard to do locally), hence their 105
degree highs near Madison. Expect a similar circumstance to that
of last Monday where highs reached the upper 90s west with dew
points dropping into the 50s and 60s. A dry line of sorts develops
west of New Ulm and Hutchinson, with the annual maximum
evapotranspiration rates just to the east of that boundary
sending dew points into the mid or upper 70s. Heat indices west
of I-35 are likely to reach hazardous levels and if we do repeat
last Monday, could range from 100 to 108. Heat headlines would be
necessary, and may ponder an Excessive Heat Watch at some point in
the next 24 hours if this scenario is still being advertised with
threat for convection holding off until evening. High pressure
over the Great Lakes will still have influence over western
Wisconsin with trajectories coming from the cooler/drier airmass.

The mid level trough axis will pass through Tuesday and Tuesday
night. Moisture pooling in the vicinity of the surface front,
roughly bisecting MN from west central to northeast at 00z (slower
than 24 hours ago), should bring pwat values back toward 2 inches
with ThetaE also increasing accordingly. Shower and thunderstorm
chances increase along the front during the late afternoon when
when forcing and continued moistening of the boundary layer
should erode the cap. Areas away from this boundary should remain
largely capped with little chance for convection. Deep layer shear
will be supportive of organized severe convection with possible
upscale growth into line segments or a couple MCSs as the evening
wears on.

With the upper level forcing shifting eastward quickly Tuesday
night, the surface front should slow its southward progression
Wednesday, but its position will largely be dictated by
thunderstorm complexes. Right now it appears southern MN into
central WI could see redevelopment early Wednesday afternoon
before sagging south.

Beyond Wednesday, models are in better agreement maintaining
surface ridging. Near normal temperatures and dry weather will


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1238 PM CDT Sun Jul 23 2017

Deck of clouds continues to move south across MN/WI with primarily
low-end VFR ceilings. Expect ceilings to maintain these heights
and slowly rise some as the afternoon goes on. North winds of 10-15
kts will diminish towards evening, becoming calm & backing to the
east overnight. Areas of patchy fog may impact RNH & EAU towards
sunrise but otherwise expect VFR conditions Monday with
southeasterly winds

Ceilings will remain VFR through the afternoon with clouds
clearing out after 00Z. Northerly winds this afternoon
diminishing & becoming easterly overnight.

Mon...VFR. Wind SE 5 kts.
Tue...VFR. Chc TSRA. MVFR/IFR in +SHRA. Wind S-SW 10-15 kts.
Wed...VFR. Chc TSRA/SHRA early. MVFR/IFR in +TSRA. Wind NW 5-10



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