Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 190417

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1117 PM CDT Tue Jul 18 2017

.UPDATE...For 06Z Aviation discussion below


.SHORT TERM...(This evening through Wednesday)
Issued at 352 PM CDT Tue Jul 18 2017

The surface map is rather muddled this afternoon, with several
outflow boundary joining a weak cold front across the upper MS
Valley. The cold front, characterized by dewpoints that drop back
into 50s at 3pm stretched from about Marshall to Little Falls. There
is a primary outflow boundary ahead of this that made it to about an
Albert Lea/Red Wing/Ladysmith line by 3pm. Along and southeast of
this boundary, they did get some sun today and sbcape around 2000
j/kg was noted on the 19z SPC meso analysis. This is where the
strongest/most widespread thunderstorms are expected today in the
form of multicell clusters with a marginal wind/hail risk, kind of
like what we saw Monday across central MN.

For the grids, it has been a day of change as the short term
guidance has really done a poor job all day in handling the
precipitation we have. Given the location of the primary instability
gradient on the southeast fringes of our CWA we did cut back chances
quite a bit to the west of where this boundary was at 2pm and also
sped up the departure of pops. Not expecting much precip tonight,
with the only possibility being some activity bubbling up along the
I-90 corridor to the north of the eastern wind shift/outflow, which
will be down in northern IA tonight.

Wednesday looks like a nice summer day, with reduced dewpoints (save
for south central MN where they will keep dewpoints in the low 70s)
and plenty of sunshine until the second half of the afternoon when
we should see upper clouds increase from the west as convective
debris moves in from the Dakotas. That boundary that will head into
northern Iowa tonight will come back north as a warm front Wednesday
and will lay the tracks for what looks like could be a significant
severe/excessive rain MCS at the start of the long term period.
However, expect storms Wednesday afternoon to remain west of our CWA
and initiate on the nose of the LLJ, which will be strengthening
across eastern SD.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 352 PM CDT Tue Jul 18 2017

A fairly strong Mesoscale Convective System should be ongoing at 7
pm Wednesday evening. This MCS will be capable of producing 4 to
6 inches of rain, and also pose a severe weather threat, with
damaging wind, large hail, and even a few tornadoes possible. As
of now, it is expected to track along the Minnesota River Valley
and southern Minnesota. The Storm Prediction Center has issued a
slight risk area for Wednesday, and WPC continues to highlight
the potential for heavy rain.

It remains unclear if the storms will become forward propagating and
turn into more of a severe wind threat, or if they will become more
back-building and transition into a heavy rain threat. HiRes models
show both scenarios, with the 18.12 WRF-ARW having storms develop
further south, which should be more surface based and develop into a
wind threat. Meanwhile the 18.12 WRF-NMM fires off convection north
of the boundary which would pose more of a heavy rain threat. It
should be noted that eventually this solution does transition into a
linear system. At this point, favor the severe weather threat based
on the stronger mid and upper level winds and veering low level jet.
Do still expect 2 to 3 inch per hour rainfall rates given the high
PWATS, but should see the storm eventually become cold pool dominant
with a few bowing segments.

The weather beyond Wednesday remains unsettled through Saturday when
an upper level trough drives a cold front through the Upper Midwest
and brings high pressure with drier, cooler air. Until that time
expect repeated chances for showers and thunderstorms. On Thursday
the boundary will shift and storms should be focused more across
northern Iowa and perhaps southern Minnesota. More widespread
precip is possible early Friday and Saturday. Temperatures will be
in the 80s, but should return back into the 70s for early next


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1118 PM CDT Tue Jul 18 2017

Main concern overnight is fog/BR development, given clearing
skies, light winds, and recent rainfall. Have included a period of
3-6sm reductions overnight, with 2-3sm at west central WI sites
(KRNH/KEAU). The next concern is the trend some hi-res models are
showing in bringing in a round of showers and thunderstorms
tomorrow morning from the Dakotas. Have not bit off on that yet,
but will need to monitor trends over the Dakotas overnight. Still
expect the main window for strong storms to be late Wednesday
afternoon and evening. Light northeast winds overnight shift to
the southeast on Wednesday and become gusty during the afternoon.

Could have some patchy BR development overnight with MVFR vsby
reductions. Still expect the storms to arrive after 00z Thursday,
but are monitoring trends of the hi-res models, with a couple
indicating convection arriving earlier to MSP (around 21z).

Thu...VFR. NE 5 kts.
Fri...Mainly VFR. -SHRA/TSRA likely. Winds SE 5-10 kts.
Sat...Mainly VFR. Chc MVFR in -SHRA/TSRA. Winds lgt and vrbl.




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