Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 180018
AFDMPX

Area Forecast Discussion National Weather Service Twin
Cities/Chanhassen MN 337 PM CDT Wed May 17 2017

.SHORT TERM...(This evening through Thursday)
Issued at 336 PM CDT Wed May 17 2017

A potent upper level wave will bring multiple rounds of storms
capable of producing severe weather. Damaging wind, tornadoes, and
large hail are all possible. These storms will be moving quickly
from the south at speeds near 50 mph.

Early afternoon water vapor imagery together with RAP13 heights and
winds showed a compact upper level PV anomaly across eastern
Nebraska. Forecast models continue to lift this low up across
southern and eastern Minnesota later today.  A surface low will
track along this same path. Since this is vertically stacked system,
the minimum pressure will be fairly steady and may even rise
slightly.

This system is dynamically driven meaning there is very strong
forcing in the presence of a marginal thermodynamic environment.
Storm should develop and move quickly this afternoon and evening. It
appears there will be two waves. The first is currently lifting
across southern MN, and the second is across southwest Iowa and will
rotate northeast later this evening. All modes of severe weather are
possible. Hail and tornadoes are more favored early on, with
damaging wind expected as the low level shear increases later this
evening. Heavy rain is also a threat this evening. Storm will be
moving quickly, but multiple rounds could lead to a few more inches
of rain by the time the system exits the region Thursday morning.
Run accumulation from the HiRes models show local maxima of 2 to 3
inches. Did not issue a flash flood watch due the progressive speed
of the storms, and the fact that any high-end potential flooding
event is very low.

.LONG TERM...(Thursday night through Wednesday)
Issued at 336 PM CDT Wed May 17 2017

By tomorrow evening, a large area of high pressure will be centered
across south central Canada, with dry northeasterly flow at the
surface making its way into Minnesota and cool 850H temps falling to
0 to +2 C.  Meanwhile, a closed upper low will be over the four
corners region and will be our next system of interest.  By Friday
morning the jet will intensify with the streak pointed from near the
TX panhandle pointed into Iowa.  POPs increase from south to north
on Friday as this system begins ejecting northeast and perturbations
in the flow are sent ahead of the main upper trough toward our area.

The main forcing with the large upper trough will arrive early
Saturday, so fully expect rain to be moving through early Saturday
morning from the south, and lasting through the day as the surface
low moves northward directly through our area.  Instability will be
limited to the warm sector so we could see thunderstorms in eastern
Minnesota and western Wisconsin as this lifts through.  As this
system continues on its northeasterly course this Saturday night
into Sunday, we will see northwesterly flow but with conditions
drying out, we should see highs a few degrees warmer than Saturday
(which with plentiful rain will be held in the low to mid 50s).

As we get into early next week, an upper shortwave trough will drop
southeast through Canada and bring more rain chances for early next
week.  This shortwave could support the formation of a deep longwave
trough, but the models diverge quite a bit at this point for our
sensible weather as the ECMWF differs in pushing a ridge in from
the west which would confine the trough to our northeast, and the
end result for us in this area would be different when compared to
some of the other medium range guidance such as the GFS.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday)
Issued at 704 PM CDT Wed May 17 2017

The frontal boundary will linger across the region for most of the
evening as low pressure tracks up the front. After 06z the area of
low pressure and associated boundary will be out of our area and
winds will be northerly at all sites. By then we will have
transitioned to a rain event with low ceilings. Expect IFR CIGS,
similar to a fall scenario. Low clouds will hang in here through
about 12z with a gradual rise in ceilings during the morning.
Precipitation should come to an end by 12z. Drizzle may be noted in
the western WI TAF sites and may continue longer than advertised in
the terminals. Winds will remain fairly strong and out of the north
until after 18z with a gradual decrease through the afternoon.

KMSP... Current thunderstorms will be out of the MSP area by 0130z,
but can`t rule out isolated development after that continuing
through about 04z. Winds will be more consistent after 03z. After
04z expect rain and lower ceilings. Current forecast of cigs at 010
should be good. Slight chance of dropping below for a short period
of time. May see some drizzle toward daybreak, but confidence is
limited. Ceilings rise by 12z to 015. winds remain strong from the
north and gusty to 20 kts until the 00z time frame.

/OUTLOOK

FOR KMSP/ Fri...VFR with MVFR/-TSRA possible in afternoon. Wind NE
at 10kt Sat...MVFR/RA with IFR possible. Wind E at 10G20kt Sun...VFR
with MVFR possible. Wind NW at 15G25kt

&&

$$

SHORT TERM...JRB
LONG TERM...SPD
AVIATION...DRL



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