Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 281750

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1250 PM CDT SUN AUG 28 2016

.UPDATE...For 18z Aviation discussion below


.SHORT TERM...(Today and tonight)
Issued at 420 AM CDT Sun Aug 28 2016

Problems at both ends of the CWA this morning with strong to severe
storms spreading into west central MN while dense fog continues
across portions of eastern MN and west central WI.

The storms in the west are in the gradient of strong low level
theta-e advection associated with the low level jet and developing
warm front. Deep layer shear is around 40 knots. The area of
instability and deep layer shear spread northeast today with the
tightest gradient in the best LI`s stretching from west central into
central MN. This is where likely pops are confined through the
morning. The low level jet never really goes away today and is
situated across central MN this afternoon along with the approaching
warm front. Although mid/upper heights are rising and becoming more
anticyclonic through the day, it`s likely that some shower and
thunderstorm activity will fester across our central counties in
MN with chance pops continued through the afternoon.

The other problem this morning is rather widespread dense fog across
central and east central MN and much of west central WI. Recently
expanded the dense fog advisory for a second time to include more of
west central WI and also to push the expiration forward an hour.

Tonight has it`s own set of concerns with more thunderstorms
possible, some of which could be severe as well as contain heavy
rainfall. The low level jet will strengthen once again early this
evening across central MN and expand into northwest and north
central MN by late in the evening. Our surface warm front will also
be moving northward while a cold front pushes into the eastern
Dakotas. As the night progress, a short wave will be moving eastward
across the Canadian/ND border area. Thunderstorms will likely
develop over northwest MN by late in the evening with a complex
heading ESE during the overnight hours. The track of this complex is
in question with different solutions between the deterministic runs
and the various CAMS. WPC felt the activity will be farther south
and closer to the Twin Cities by daybreak Monday while we felt the
complex will be between Cambridge and Duluth. The more northern
solution goes along better with the location of the differential
theta-e advection in the 06z-12z time frame. A majority of the CAMS
are also favoring this area with some more across the Arrowhead. For
now, we introduced some likely pops in the 09z-12z period in the
Onamia and Mora areas.

Erred on the low side of guidance today with respect to high
temperatures due to the extensive stratus and fog that will take
some time this morning to mix out. If sunshine develops quicker,
then highs will be about 3 degrees warmer than forecast. Lows
tonight will be very mild with upper 60s to around 70 across
central and southern MN with middle to upper 60s in west central

.LONG TERM...(Monday through Saturday)
Issued at 420 AM CDT Sun Aug 28 2016

Undoubtedly, the big weather story for the extended that will be
sweeping the weather community this morning is the fact that the
28.00 ECMWF is developing the open wave entering the Florida Straits
into a strong tropical system in the eastern Gulf before sending it
into western FL. Interestingly, the locally run 24 km HopWRF has
stuck with a solution like this the last few days when all other
global guidance backed away from developing any sort of tropical
system out of this wave. This may have an impact for our weather
locally at the end of the week/next weekend, but you`ll have to read
on to find out how!

Locally, models are in pretty good agreement through Friday, but
begin to diverge significantly by the weekend, which the handling of
the potential tropical system in the Gulf is playing a roll in.
There was a general slowing with the FROPA for Monday, with the
tropical dewpoints now not expected to fully clear the I-90 corridor
until Tuesday afternoon. We still look to have glorious weather
Wednesday through Friday with more active weather returning for the
holiday weekend.

For Monday, we will be starting out in the morning with likely a
decaying complex of storms in northwest WI from activity expected
more up in northern MN tonight. By the afternoon, a cold front will
be setup from northeast MN down into southeast SD. Based on current
forecast positioning of the front, we are anticipating thunderstorms
to develop Monday afternoon from central MN down toward SW MN.
Convergence along the front does not look overly impressive, with
any sort of upper wave going across northern MN, so scattered
coverage is expected. MLCAPE looks to increase to around 3000 j/kg,
with 20-30kts of deep shear, and DCAPE in excess of 1000 j/kg. So we
should see a few severe storms, with strong winds and marginally
severe hail being the main risks.

As the front moves east Monday night, we work chance pops east with
it, though this may be a bit overdone as LLJ with the front is
pretty weak and likely not strong enough to sustain convection
through the night once we loose daytime heating. For Tuesday, models
have slowed considerably how quickly the around 70 dewpoints clear
the area, with the GFS in particular hanging up enough moisture
along the I-90 corridor in the afternoon to bring a risk of
thunderstorms down along the IA border.

Tuesday night, our flow aloft finally turns northwesterly, with high
pressure coming out of the Canadian Prairies and heading for the
Great Lakes.  This will setup a couple of beautiful days Wednesday
and Thursday with sunny skies, dewpoints in the 50s, and highs in
the 70s. Friday looks dry as well, though as the western trough
continues east, a strong area of low pressure will start to emerge
over the Dakotas on Friday which will just mean we add a breezy
south wind to Friday to what we see Wednesday and Thursday.

For the weekend, both the GFS/ECMWF say we will have a front work
through at some point, but they vary significantly on details. The
GFS, which is less developed with the tropical system in the Gulf
works the front in Friday night and has it lingering across the
area through the weekend, painting a potentially dreary picture for
Labor Day weekend here. The ECMWF, which has a more developed
tropical system is also much slower with advancing systems across
the Plains, with the frontal passage holding off until Sunday. Given
the spread, we have chance pops spreading across the area Saturday,
but not much confidence in the precip forecast next weekend at this


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1250 PM CDT Sun Aug 28 2016

Now that showers and thunderstorms have ended across central MN,
expect dry conditions at sites until possibly overnight
(at KAXN/STC). The broken/overcast deck with sub-1000ft ceilings
will gradually scatter out this afternoon, but sct-bkn clouds
circa 2000-3000ft will persist. Should see enough clearing this
evening to bring a threat for another night with fog development,
although winds will be a tad stronger so widespread dense fog is
not expected. Will need to keep a close eye on WI sites (KRNH and
KEAU) where the potential for LIFR or even VLIFR vsbys is not out
of the question.

Thunderstorms are expected to develop over northwest MN tonight,
then drift east and possibly southeast overnight. There is still
uncertainty regarding how far south these will track. Have
included a Prob30 at KAXN and KSTC for the 08z-12z time frame for
now. A front will pass across the area on Monday, so another
round of showers and thunderstorms will be possible on Monday
afternoon/eve. South/southeast winds will prevail through the
period, with gusts possible during the afternoon time frames.

Sct-bkn borderline VFR clouds (2500-3000ft) will linger this
afternoon. Should see enough clearing tonight to bring the threat
for fog at least in the vicinity of the field. If that occurs,
will also have the potential for IFR CIGS/VSBYS. Shower and
thunderstorm threat with the frontal passage Monday should hold
off until closer to 00z Tuesday.

MON EVE...VFR. Chc -TSRA. Wind S 5-10 kts.
TUE...VFR. Wind N 5-10 kts.
WED...VFR. Wind NNE 5 kts.
THU...VFR. Wind SSE 5-10 kts.


.MPX Watches/Warnings/Advisories...


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