Scientific Forecaster Discussion
fxus63 kmpx 212202
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
402 PM CST sun Jan 21 2018
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 European model (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 18z tafs through 18z Monday afternoon)
issued at 1230 PM CST sun Jan 21 2018
Low pressure is starting to consolidate over central/southern Colorado and
is still progged to head northeast to the Quad Cities Monday
afternoon. This system is slow to get here and really tried to slow
up the arrival of both low clouds and snow, with a timing close to
what we are seeing with the 12z GFS and nsslwrf. Mkt/eau still look
to take the brunt of this system (and after 18z Mon at that), with
msp/rnh riding the gradient. Models continue to trend dry for rwf,
so added them to axn/stc in the dry tafs group. Further changes are
still possible, especially for rwf/msp/rnh as slight variations in
where the band sets up will have large implications on how much/how
little snow they see.
Kmsp...right now, we are favoring a blend of the GFS and nsslwrf for
monday's snow. This would bring about 0.5" of quantitative precipitation forecast to msp, which
would translate to around 6" of snow. Heaviest snow still looks to
fall during the afternoon. Will likely have a period of 1/2sm or
less snow, but have time for more hi-res models to start coming in
before going lower than what we currently have for the afternoon.
/Outlook for kmsp/
Tue...MVFR thru mrng. Winds northwest 5-10 kts.
Wed...MVFR/IFR cigs. Winds 5 kts or less.
Thu...MVFR/IFR cigs. Winds southeast 5-10 kts.
WI...Winter Storm Warning from 6 am Monday to midnight CST Monday
night for wiz014>016-023>028.
Minnesota...Winter Storm Warning from 6 am Monday to midnight CST Monday
night for mnz060-062-063-067>070-077-078.
Winter Weather Advisory from 6 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.
Blizzard Warning from 3 am Monday to midnight CST Monday night