Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 212358

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
558 PM CST Sun Jan 21 2018

Issued at 551 PM CST Sun Jan 21 2018

Updated to include 00z aviation discussion below.


.SHORT TERM...(This evening through Monday)
Issued at 402 PM CST Sun Jan 21 2018

Despite lingering uncertainties and some run to run model
differences with the placement of the snowfall gradient, we
decided to upgrade the watch to a Winter Storm Warning for much of
the metro. Feel pretty confident the southeastern sections of
those counties will reach warning criteria and with potentially
severe travel impacts driving through rapidly deteriorating
conditions, the warning seems justified.

The overall forecast reasoning remains the same. Precipitation
will break out tonight across Iowa and inch northward Monday
morning, eventually merging with the large deformation band to the
west from the mid level low. The area from Fairmont, St. James,
Mankato northeastward into west central Wisconsin still appears to
be the epicenter for heaviest snow. Even some thunder is possible
given the forcing and the mid level dry slot approaching from the
south. LI`s go negative across much of Iowa Monday morning. This
should result in 1 to 2 inch per hour snow rates within this
corridor, especially from late morning through late afternoon. The
mid level moisture cuts off abruptly northwest of the Twin
Cities which will cease snow production. As mentioned above, there
are still questions of where this will set up. The aggressively
approaching dry slot from the south usually favors a snow band
that sets up north of where originally expected. However, the
presence of fairly widespread convection to the south and
resultant latent heat release could focus the band a bit further
south. The biggest key is how the mid level low will evolve, and
unfortunately each model does this differently. The 12Z ECMWF and
its ensembles have made quite a noticeable shift to the west, with
the ensemble mean about 9 inches at MSP and its box and whiskers
ranging from 4 to 14 inches. The 50th percentile of snow from a
variety of models matches pretty closely with the storm total
grid. Until this event moves deeper into the hi-res model window,
we won`t have a clear idea how much convection or the dry slot
influences where the band sets up. There may be additional updates
to the headline configuration when clear trends present

Regardless, am expecting very difficult or impossible travel
conditions where the heavy snow develops. The wind will create
very large drifts. The Blizzard Warning still looks good as we`re
expecting 35 to 40 mph gusts and 1/4 mile visibility due to heavy
snow for at least 3 hours.

.LONG TERM...(Monday night through Sunday)
Issued at 402 PM CST Sun Jan 21 2018

The extended will be marked by a wavy progressive flow. This will
see the trough moving through now finish moving out on Tuesday,
ridging move back in for Wed-Thu, another trough for Friday
through the weekend, and ridging returning early next week as the
next trough for the middle/end of next week moves in out west.

With ridging moving in, we won`t cool down much in the wake of the
winter storm, with near normal highs (above normal lows) Tuesday
and Wednesday before the next ridge likely sends highs soaring
back into the 40s Friday, when GEFS h85 temps are near +12C above
normal. For the trough coming in this weekend, it will be coming
through much farther north than the current one, with its
associated surface low progged to come out of the Black Hills and
head for the Arrowhead. The front looks mainly dry, with the best
signal for precip in the ensembles east and north of our area.
That would be as mainly rain for the western Great Lakes, with
heavy snow in southern Canada.

Behind this low, we will get a slight cooler blast of air, with
h85 sinking back down to between -15C to -20C. This will give us
temps slightly below normal (highs in the teens/lower 20s) Sunday
into the start of the following week.

Looking way ahead to the weekend of Feb 3-4, GEFS mean h5 heights
and anomalies show ridging developing on either coast by then,
with a trough across the center of NOAM. This would send us toward
northwest flow and below normal temperatures as we head toward
that weekend, though we aren`t talking arctic air cold (highs in
the single digits or colder). This is also a drier pattern for us,
with clippers being the primary source of any precipitation.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 551 PM CST Sun Jan 21 2018

Conditions will steadily deteriorate over all TAF sites as the
evening and overnight hours progress, especially over
southern/eastern MN into western WI. MKT/EAU still look to be the
most impacted with MSP/RNH/RWF right on the gradient line.
AXN/STC look to receive little to no precipitation so have
maintained a dry forecast for those 2 sites yet IFR ceilings do
look likely. Frequent amendments do look likely for the southern
and eastern TAF sites depending on exactly where the tight snow
gradient sets up and how intense the snowfall is.

KMSP...Conditions will remain as VFR for much of the overnight
then ceilings will drop to near or just below MVFR thresholds at
the start of the Monday morning push. Precipitation may hold off
until right at the end of the push, but timing could be slightly
earlier than the 14z currently advertised. IFR conditions look
likely by late morning, including the potential for 1 in/hr rates
early-mid afternoon. Snowfall is expected to continue through the
evening push but with improving conditions such that VFR
conditions return by late Monday evening.

Tue...MVFR thru mrng. Winds NW 5-10 kts.
Wed...MVFR/IFR cigs. Winds 5 kts or less.
Thu...MVFR/IFR cigs. Winds SE 5-10 kts.


WI...Winter Storm Warning from 9 AM Monday to midnight CST Monday
     night for WIZ014.

     Winter Storm Warning from 6 AM Monday to midnight CST Monday
     night for WIZ015-016-023>028.

MN...Winter Storm Warning from 9 AM Monday to midnight CST Monday
     night for MNZ060-062-063-067>069.

     Winter Weather Advisory from 9 AM Monday to midnight CST Monday
     night for MNZ053-061-065-066-073.

     Winter Storm Warning from 3 AM Monday to midnight CST Monday
     night for MNZ075-076-084-085-093.

     Winter Storm Warning from 6 AM Monday to midnight CST Monday
     night for MNZ070-077-078.

     Blizzard Warning from 3 AM Monday to midnight CST Monday night
     for MNZ074-082-083-091-092.



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