Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 221149

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
649 AM CDT Sat Jul 22 2017

.Updated for 12z Aviation Discussion...
Issued at 640 AM CDT Sat Jul 22 2017


.SHORT TERM...(Today and tonight)
Issued at 315 AM CDT Sat Jul 22 2017

A few showers and thunderstorms, that as of 3 am were in the
eastern Dakotas, are expected to track eastward this morning and
could impact a few locations across Minnesota before completely
falling apart. HiRes models show some redevelopment this afternoon
mainly along and east of I-35. The storm prediction center has a
slight risk across portions of eastern MN and western WI, but this
doesn`t appear to have high end potential so if there are any
changes made, would suspect to see the area trimmed to the east
and downgraded with subsequent updates. Cooler, drier air will
move in today and overnight with dewpoints falling into the upper
50s to lower 60s by Sunday morning.

Early morning satellite imagery together with RAP13 500mb heights
and winds showed a compact upper level PV anomaly across southern
Canada. This wave will move eastward today and drive a cold front
through the region. There will be a noticeable chance in dewpoint
temperatures as the front passes, with dewpoints falling from near
70 to the mid 50s. Forecast soundings show northwest wind increasing
today which will also help to dry out the boundary layer. For that
reason have a lower confidence in severe storms this afternoon
except for maybe east of I-35. Areas to the west will likely see too
much drying which will decrease the CAPE and hold off convection.

.LONG TERM...(Sunday through Friday)
Issued at 315 AM CDT Sat Jul 22 2017

Cooler and drier air will follow for Sunday into Monday with high
pressure passing overhead. Could be a bit chilly Sunday night
with lows possibly in the 40s in some areas of central MN and WI.

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 northern and western MN during
the overnight hours. With the LLJ focused more to the north, it`s
possible an MCS could develop over the Red River valley and spill
southeastward toward morning, likely weakening as it does so.

The mid level trough axis will pass through Tuesday and Tuesday
night. Moisture pooling in the vicinity of the surface front,
roughly bisecting MN northeast to southwest during the late
afternoon, could bring pwat values back toward 2 inches with
ThetaE also increasing accordingly. Shower and thunderstorm
chances increase during this time with moderately steep mid level
lapse rates in place. The best forcing will pass across northern
MN, and the strongest mid level flow looks to remain behind the
front. Therefore, the severe weather threat is uncertain but
most likely where the stronger mid level flow intersects the
richer low level moisture in a narrow corridor along the front.

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. It is usually the case boundaries and
subsequent thunderstorm complexes end up further south than
models indicate, so chances for storms Wednesday will be confined
to the south.

Beyond Wednesday, there is a question whether the ridge to the
southwest will bounce back up into the Dakotas or the trough will
be deep enough to keep it over the Rockies. Models have trended
much drier recently with the surface high/upper trough idea with
only the Canadian now bringing another wave in Wednesday night-
Friday. Way too early to have a feeling one way or the other,
especially lately with models performing so poorly even 12 to 24
hours out with convective trends.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 640 AM CDT Sat Jul 22 2017

VFR conditions out in western Minnesota, with some MVFR/IFR
clouds expected across western Wisconsin. A few showers and
thunderstorms are possible this morning, and again this afternoon.
Skies should clear out this evening with VFR conditions and
northwest winds overnight.

Mainly VFR conditions with perhaps some MVFR ceilings this
morning. Area of thunderstorms across central Minnesota could
reach MSP but it is weakening and should not produce any thunder
if it does manage to reach the TAF site. Additional development is
possible later this afternoon, but similarly confidence is too low
to include mention of thunder in the TAF.

Sun...VFR. Wind NW 10-15 kts.
Mon...VFR. Wind ENE - ESE 5 kts.
Tue...VFR. Chc TSRA late. Wind S/SSE 5-10 kts.




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