Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 241255

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
755 AM CDT Mon Apr 24 2017

Issued at 653 AM CDT Mon Apr 24 2017

Updated to include 12z aviation discussion below.


.SHORT TERM...(Today and tonight)
Issued at 350 AM CDT Mon Apr 24 2017

Surface analysis early this morning shows the quasi-stationary
front still lingering over southern MN through far SW WI, with ENE
winds and temperatures in the 30s to the N of the front and S
winds with temperatures in the 50s S of the front. The front waves
back west to a low pressure center over western South Dakota and
western Nebraska. Aloft, flow continues to be generally zonal over
Minnesota and Wisconsin while a modest longwave trough axis sits
in the lee of the northern-central Rockies. With this trough set
to progress eastward, this will then nudge the developing low
pressure center eastward, through South Dakota today and through
central Minnesota tonight and into Lake Superior by daybreak
Tuesday morning. The surface front sitting over southern MN will
push northward today in response to slightly increasing H7-H5
heights, allowing for decent warming for much of the WFO MPX
coverage area. Warming will not be maximized due to the increased
cloud cover expected across the area today. In addition, some
scattered rain showers are expected over mainly the northern half
of the area through this evening in response to the northward
progression of the front and deeper moisture being advected into
the region in advance of the surface low. Not looking for much at
all in the way of QPF, generally a few hundredths north of the
I-94 corridor. The precipitation will gradually diminish and shift
off to the east late tonight into tomorrow morning then resume by
around daybreak on the backside of the surface low. As for
temperatures, the large temperature gradient this morning will
become tempered by this afternoon with the northward progression
of the front. This will resulting in a more uniform southerly flow
over the entire MPX coverage area. Highs will range from the
lower 60s near Lake Mille Lacs to the upper 60s by the Iowa
border. With the low pressure center crossing the area tonight,
cold air advection will commence with its passage, resulting in a
west-to-east temperature gradient instead of the north-to-south
gradient. Lows tonight will drop to the upper 30s in western MN
while lows in western MN hold in the upper 40s.

.LONG TERM...(Tuesday through Sunday)
Issued at 350 AM CDT Mon Apr 24 2017

The longer term concerns are overall development/movement of the
western conus trough with embedded short waves and timing of which
will determine overall p-type and QPF amounts Tue into Thu. Then
potential of a significant storm to affect the area into the
weekend. Overall...favoring a cool and wet period.

The front associated with the initial short wave should be exiting
into western Wisconsin Tuesday. This will lead to strong forcing
along the baroclinic zone producing strong F-gen into the eastern
cwa later Tuesday into Tuesday night. This should spread rain
across this region during the afternoon and have opted to mention
categorical PoPs. QPF amounts should be heaviest during this
period as well...with 0.50 to 0.80 inch amounts the
main trough lifts northeast. With the passage of the
trough the thermal structure of lower boundary begins to favor at
least a rain/snow mix developing Tuesday night into Wednesday
morning over the western cwa. We could see some light
accumulations over that region...less than one inch however.

Then the models diverge somewhat on overall movement of the trough
to the northeast Wednesday into Wednesday night. The GFS weakens
the wave as it lifts northeast and redevelops the front/wave
further to the east into Wednesday night. This splits the
precipitation to the northeast and farther southeast by Thursday.
The ECMWF is slower and a bit deeper with the trough and lifts a
wave farther north into Wisconsin Thursday. This would leave much
of the cwa vulnerable to some wet snow later Wed night into
Thursday morning. Best potential would be over west central
Wisconsin where we could see an inch or so of wet snow accumulate.

Following this we should see a general decrease in overall
precipitation as we are in between significant systems. There is
a small threat of showers affecting the area Friday/Friday as a
weak wave lifts northeast ahead of the deeper western conus
trough. This stronger system is forecast to eject north and east
over the weekend. This should provide an increasing threat of rain
over the weekend. The deterministic models diverge on when this
does take place...with the GFS holding off more widespread rain
until later Sunday into Monday due to the strength of the eastern
ridge. The ECMWF is faster lifting the system over the area Later
Saturday and Sunday. Still have time to resolve these model
differences. Also...dynamic lift associated with the trough could
cool the boundary layer enough to generate more wet snow for the
CWA. This would take us to the first of May.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 653 AM CDT Mon Apr 24 2017

Quasi-stationary front is still lingering over southern MN into
far southwestern WI this morning, associated with an area of low
pressure over the Dakotas. Though the better chances of rain with
the front will be in central/northern MN, there still may be some
scattered rain showers as far south as the I-94 corridor, which
would bring -RA to most of the TAF sites. Have gone conservative
at this point, restriction precipitation to just KAXN-KSTC this
afternoon with VCSH wording. However, it is not unreasonable to
see -SHRA added/expanded to additional TAF sites. Otherwise,
ceilings will continue to fill into the upper levels then
gradually lower late this afternoon through tomorrow morning with
the approach of the low pressure center. Winds will become
southerly at all sites today an increase to the 15-20kt range with
gusts 25-30kt for much of the late morning and afternoon periods.
Wind speeds will settle down this evening then directions will
shift around from S to W and NW approaching 12z, again due to the
movement of the surface low.

KMSP...Main concern will be the increasing potential for -SHRA
this afternoon, likely during the time of the evening rush. Not
expecting anything that would reduce flight categories away from
VFR but ceilings may be a little lower than the 080 currently
advertised and visibilities may be closer to 6sm than 10sm.

Tue...MVFR expected, IFR possible. P.M. -RA likely. Winds NW 5-15
kt becoming NE 15-25 kt.
Tue night...MVFR expected, IFR possible. -RA likely. Winds NE
10-20 kt.
Wed...MVFR expected in the morning. A.M. -RA likely. Winds NE
10-20 kt.
Wed night...MVFR possible. Chc -SHRA/-SHSN. Winds N 5-15 kt.
Thu...MVFR possible early. Chc -SHRA/-SHSN early. Winds N 5-15 kt
becoming variable less than 10 kt.




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