Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 172021

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
321 PM CDT Thu Aug 17 2017

.SHORT TERM...(This evening through Friday)
Issued at 322 PM CDT Thu Aug 17 2017

Strong cyclone that brought us the heavy rains yesterday is now
filling as it heads across northern WI. We are still in it`s moist
cyclonic flow, hence the expansive stratocu field across the area
this afternoon. However, it`s becoming quite cellular and we will
see it rapidly shrink across MN the resto of this afternoon and
dissipate in western WI before midnight. Beside the skies clearing
this evening, winds will also back down pretty quick as ridging for
the Dakotas moves in. This will create the main question for
tonight, the potential for fog. We are certainly plenty moist enough
and the cool highs of today with dewpoints still up in the low 60s
means we will have no problem cooling down to the cross-over
temperature. However, the next upper wave is already incoming over
eastern Montana and will be reaching western MN by Friday morning.
This will be spreading an expansive mid/upper cloud field across MN
tonight, which will really limit fog potential. It`s eastern MN into
western WI that looks to keep out of the clouds long enough to have
a fog threat, though here, winds just above the boundary layer look
to remain up around 20 kts and may be too strong to allow for the
formation of fog, so have just a patchy/area fog mention in east
central MN into western WI for tonight.

As this Montana wave moves across the Dakotas, models are all
similar in showing about a 30 kt LLJ coming up out of KS/Neb into
southwest MN. Most of the short term models show there will be a
batch of showers going across southern MN through the morning with
the isentropic lift on the nose of the LLJ. Only have chance pops
though as forecast soundings show plenty of moisture above 10k ft,
but are pretty dry below that. Once this morning activity moves by,
attention turns to western MN for the afternoon. With the upper
trough overhead, we will see steeper mid-level lapse rates (7-8
C/km), which will help promote the development of 1500-2000 j/kg of
MUCAPE. Surface winds are weak, with no strong boundary for storms
to latch on to, but given the instability and the deep shear
supplied by the 80-90kt northerly jet streak coming across the
eastern Dakotas, we like the expanded marginal risk across western
MN the SPC has in their Day 2 outlook update. Have only chance pops
again for tomorrow where thunderstorm potential looks greatest based
on where CAMs are generating the greatest amount of convection,
which is west of a Long Prairie to Owatonna line. This also lines up
with where the ECMWF/GFS/NAM have their highest degree of

.LONG TERM...(Friday night through Thursday)
Issued at 322 PM CDT Thu Aug 17 2017

The weather pattern remains active during the long term forecast
period, with shower and thunderstorm chances every couple of days.
The first will arrive late Friday, and the next in the Sunday eve
through Monday eve time frame.

The Friday chances are in response to a shortwave trough speeding
across Minnesota and Wisconsin from the Northern Plains. Only a weak
trough is progged at the surface/low levels, thus low level
convergence appears rather weak. That being said, steepening lapse
rates and 40+ knots of mid level flow coupled with a pocket of
healthy layer differential divergence nosing into far western and
south central MN could support an isolated severe threat (large
hail and strong winds).

Given the progressive nature of the shortwave trough, skies clear by
Saturday morning, with patchy fog development possible by
daybreak. Temperatures then moderate through the weekend as mid
level ridging prevails with southerly surface winds. Highs
Saturday look to range through the lower 80s, with mid 80s
becoming more common for Sunday highs.

By Sunday evening, model consensus places a frontal boundary on the
doorstep of western Minnesota. This front is forecast to sag across
the area Sunday night and Monday, with a prolonged chance for
showers and thunderstorms. Precipitable water values are forecast to
near 2 inches, so in addition to severe weather, heavy rainfall will
also be a hazard to monitor.

Tuesday through the end of the upcoming work week we see cooling
temperatures as a deep trough over northern Ontario sinks to the
Great Lakes. Highs in the 70s will be common for Wednesday
and Thursday under northwest flow aloft.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1254 PM CDT Thu Aug 17 2017

Main change to TAFs was to delay improvements to VFR an hour or
two at all terminals. Precip is pretty much done for today, save
for some occasional dz/-ra out in WI. Main question for tonight is
do we see fog form. Skies will be clearing early this evening,
with surface winds dropping to under 5kts. Those are the pluses in
the fogs court. However, cloud cover from the next system will be
rapidly moving in from the west tonight. How far east it gets
will determine where the fog potential resides, but WI terminals
look to have the greatest risk for fog, with mid/upper cloudiness
keeping the fog at bay in MN. Expect an area of showers at the
nose of the LLJ to be sliding across southwest/south central MN
Friday morning, which may graze RWF/MKT, so have VCSHs there
Friday morning.

KMSP...Confidence is pretty high in the TAF. Given the holes
showing up in the cloud field across MN, expect it to dissipate
quickly this afternoon, with 2z even looking a bit late to go VFR.
There is a threat for shra tomorrow, but best chances look to
slide south of MSP in the morning, with afternoon activity
confined to western MN.

Sat...VFR. Wind SW 5 kts.
Sun...VFR. Chance TSRA overnight. Wind SW 5 kts.
Mon...Chc MVFR/-TSRA. Wind S 5 kts.




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