Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 172356
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
656 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 rest 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
destabilization.

.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 00Z TAFS through 00Z Friday evening)
Issued at 649 PM CDT Thu Aug 17 2017

MVFR ceilings are in the process of departing the area, but will
continue to affect western WI this evening. The west to east
clearing will reach that area overnight. Clearing skies have
already reached the west metro, so VFR conditions are expected
the remainder of tonight across the MN TAF sites. The next wave is
already approaching from the Dakotas. Still looks like RWF and
MKT have the best shot at precipitation tomorrow morning, but
should be light. Other concern is the possibility of fog
developing during this overnight period while skies are clear
before the next wave moves in by morning. Think enough wind in
place will mitigate widespread fog concerns, but did mention it at
the always fog prone KEAU. Winds tomorrow will be much weaker
than they were today, commonly under 7 knots.

KMSP...Skies on the verge of clearing out at 00Z. VFR is expected
throughout the period.

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

&&

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

$$

SHORT TERM...MPG
LONG TERM...LS
AVIATION...SPD



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