Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 190843

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
343 AM CDT Wed Jul 19 2017

.SHORT TERM...(Today and tonight)
Issued at 343 AM CDT Wed Jul 19 2017

The short term concern is timing of possible severe convection and
heavy rain threat.

Surface cold front dropped south of the state into northern Iowa
last evening. Temperatures cooled nicely with light winds and clear
sky. Some fog developed in northeast cwa and will linger until
around sunrise and should burn off quickly. Focus turns to possible
development of large scale high wind severe weather event for much
of the cwa late this afternoon and through the evening.

Deterministic models have been pointing to this possibility now for
last several days and the 00z runs continue this trend. Water vapor
imagery showing rather vigorous short wave moving through eastern
Montana this morning and will be the forcing and focusing mechanism
for the MCS development over western central MN by early this
evening. SPC Day 1 outlook with its enhanced risk paints the
scenario and the HIRES ARW confirms our earlier thoughts regarding
the event. We should see development by evening to the west as the
short wave arrives and the warm front retreats to the north into
southwest MN. Capping should limit development there for a time.
Very unstable airmass in place and once development occurs, it
should become predominately upscale and become a forward propagating
MCS and ride the instability/CAPE gradient over the southern third
of the cwa this evening. General movement of the QPF bullseye/RAP
vort maxima gives a general 45 to 50kt southeast forward movement.
Still could be some spatial and temporal issues but they should
resolve themselves later this morning. Damaging winds, large hail
and an isolated tornado or two is possible. The tornado threat
should be mainly early in any discrete cell development and also
possible in the mature MCS/Bow Echo/QLCS. The severe weather threat
will likely end before 06z if models are correct due to fast forward

Heavy rain will be a threat with the storms as well, with
high PWs around 2 or 3 standard deviations above normal. We still
believe 2 to 3 inch an hour rainfall rates are likely but because of
the fast movement and antecedent soil conditions a widespread flash
flood threat is not anticipated at this time. Some HIRES models did
show some indication of continued convective development to the
northwest across southern MN after this complex moves through. This
is certainly possible and the nw-se orientation of the scattered
convection could pose some backbuilding threat later tonight. This
will have to be monitored as it could certainly increase the
potential flash flood threat.

.LONG TERM...(Thursday through Tuesday)
Issued at 343 AM CDT Wed Jul 19 2017

In the longer term we`ll see mainly zonal flow (albeit with a low
amplitude ridge over the center of the CONUS) evolve to cooler
northwest flow for our area by the second half of the weekend.
This pattern looks to stay in place through the first part of next
week, but indications are that ridging will build across the
center of the country again by the middle of next week. However,
there is certainly some disagreement on the guidance at that
point, so won`t be going all in on any particular solution at this
point, with a consensus approach looking to be the safest bet.

A weak ridge of high pressure and drier air will work into the
area on Thursday, with dry weather expected across the area.
However, low pressure will work into the Plains by Friday, helping
to lift the baroclinic zone and better moisture back north into
our area. This will bring chances for showers and thunderstorms
into the area from southwest to northeast late Thursday night
into Friday night. The better forcing then looks to focus mainly north
of our area Saturday. Cold frontal boundary looks to drop south
through the area Saturday night and early Sunday, which won`t be
the ideal time for convection, but could still see some activity
Sunday, with the best chance looking to be over the southern and
eastern portion of the area where instability looks to be best as
forcing from the upper wave and low level boundary work through
the area.

High pressure will build over the region Sunday night and Monday,
with cooler and drier weather expected. We will then need to wait
for return flow to setup again, which looks to occur later Tuesday
into Wednesday as the 500 mb ridge builds to our west and the 850
mb theta-e ridge starts to work back into the area. The GFS and
ECMWF are in reasonable agreement that we`ll see some activity
develop in the return flow during that time frame, with chances
persisting into Wednesday as the warm frontal boundary sets up
shop nearby.


.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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