Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 152119

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
419 PM CDT Sat Jul 15 2017

.SHORT TERM...(This evening through Sunday)
Issued at 419 PM CDT Sat Jul 15 2017

A frontal boundary was moving across the region this afternoon,
with dewpoints pooling in the mid to upper 70s along it and
temperatures near 90. In the wake of the front across northern
Minnesota, temperatures were 70s with northerly winds.

The hot, humid conditions along and ahead of the boundary led to
afternoon MLCAPE values via the SPC mesoanalysis of 2000 to 3000
J/kg. Deep layer shear of 30 to 40 kts made for a marginal, yet
favorable environment for organize convection. The question for this
area is where exactly the storms will develop. There is considerable
agreement between the HiRes models which show a small window of
possibility in west central Wisconsin, say the Chippewa Falls/Eau
Claire area, Red Wing MN, and Albert Lea. At this point feel that
the slight risk for severe storms is spot on, and adjusted the
hourly pops to reflect that. Overnight the cold front will slowly
move through the region, and that will mean much cooler and drier
weather for Sunday. Forecast soundings show a few afternoon clouds
possible, so bumped up the sky cover for the time period centered
around the noon hour.

.LONG TERM...(Sunday night through Saturday)
Issued at 419 PM CDT Sat Jul 15 2017

The seemingly permanent western ridge this summer will flatten and
spread eastward across the Plains next week, setting up a rather
unsettled period for the Upper Midwest with excessive heat just
to the south.

The first in the series of waves will come Monday. Fast southwest
mid level flow across the Northern Rockies will largely take this
wave northeast across western and northern Minnesota along a cold
front. Very hot temperatures will lift north ahead of the front
from the Plains with 925 mb temps of +32C possibly reaching west
central MN by mid afternoon. Favorable south/southwest wind
direction and a dry flow from the Plains could send temperatures
up into the low 100s over Lac Qui Parle and Yellow Medicine
counties. There will be a tight thermal gradient to the east with
the influence of high pressure still intact across Wisconsin.
Despite the cooler temperatures in the 80s, dew points will be
seasonably high in the low or mid 70s across eastern MN where a
focused area of moisture advection from Iowa will surge in. Storms
are expected to fire along the cold front across northwestern MN
during the afternoon and spread southeast during the evening and
overnight. These storms may run out of gas Monday night with a
stout cap developing due to thermal ridging in place across
southern MN and western WI.

By Tuesday, the convection modified synoptic boundary should
stall somewhere across southern MN into central WI. Hot and very
humid conditions to the south with dew points in the mid 70s to
possibly near 80, and moderately steep mid level lapse rates,
would lead to a rather unstable atmosphere. Perturbations in the
mid levels could act to spark clusters of thunderstorms during the
afternoon along this boundary, posing a mainly heavy rain and
flood threat with pwats around 2 inches. Some severe weather is
also possible with favorable veering wind profiles.

Models differ quite a bit with the location of the front Tuesday
night and Wednesday, with the GFS to the north and the preferred
ECMWF/NAM/Canadian to the south. This should bring a period of
drier weather, but couldn`t remove PoPs given the model differences.
By Wednesday night another wave will approach from the west,
which should bring the front back northward and facilitate another
round of showers and thunderstorms. The front will drive south
into Iowa Thursday and lift north yet again for the weekend.

2 to 3 inches of rain will be possible this week across all of
the area, with significantly higher totals possible in localized
areas given the multiple rounds, training potential with each
round, and the very high pwat values.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1237 PM CDT Sat Jul 15 2017

A cold front has already progressed east of AXN and STC where
winds are already out of the northwest. Thunderstorms are
expected to occur along the front in Western Wisconsin this
afternoon and move south of EAU by 00Z. Thus, VCTS has been added
to the RNH TAF and -TSRA has been added to the EAU TAF.
Otherwise, all other TAF sites are expected to remain dry through
today and tomorrow.

KMSP...Not much change since the last TAF issuance with convection
likely remaining to the south and east of MSP this afternoon.
Winds are expected to remain near 10 kt today and become WNW
around 21Z.

Monday...VFR. Southeast wind 10 to 20 kt becoming south.
Monday night...MVFR possible late with a chance of TSRA. South
wind 10 to 20 kt.
Tuesday...MVFR possible with a chance of TSRA. Southwest wind 10
to 20 kt.
Wednesday...VFR with chance for TSRA late. NE wind near 5-10 kt.




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