Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 082121

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
321 PM CST Fri Dec 8 2017

.SHORT TERM...(This evening through Saturday)
Issued at 320 PM CST Fri Dec 8 2017

Quick moving clipper moving into northeast MN this afternoon is
still on track to drop south along the MN/WI border this evening,
with maybe 1.5" of snow coming down for Ladysmith down to Eau
Claire. Models have remained fairly consistent with the handling of
this wave and only change made to the existing forecast for this
snow was to delay its arrival by a couple of hours.

Beside the snow, there also looks to be a 3 or 4 hour period behind
this systems cold front where winds could get a bit strong,
especially for western into south central MN. CAA in the wake of the
front will result in steep lapse rates with a mixed layer getting up
to between 850 and 800 mb. NAM shows top of the channel winds
immediately behind the front getting into the mid 40 kts, so did
boost winds above blended guidance, with some 40+ mph wind gusts
not out of the question. Beside the winds, forecast soundings also
show any post frontal stratus being located directly within the
dendritic growth zone, so it shouldn`t be tough developing
flurries/light snow through the night.

Big question for Saturday is how quickly do we loose the low clouds.
Even if we do lose the clouds though, we will be placed squarely
within the temperature gradient between the cold trough to the east
and warm ridge to the west, so any way you slice it, our Saturday
will have more clouds than sun. Not much spread in guidance for
highs tomorrow, so stuck pretty close to a mean of blended guidance.

.LONG TERM...(Saturday night through Friday)
Issued at 320 PM CST Fri Dec 8 2017

Biggest concern in the long term is the potential for a pretty
healthy clipper Sunday night into Monday morning, with the potential
for a good 3-4 inches north of I-94 in the MPX CWA. Otherwise the
highly amplified pattern we have in place does not look to begin to
break down and change until the 15th of December, or the last day of
the long term period. This means more NW flow with ample
opportunities for quick moving clippers. The coldest of the
temperatures looks to shift east of where it has been this week, so
we look to be a bit milder next week, with high temperatures near

The biggest change we have seen in the last 24 hours comes with the
clipper for Sunday night. With its 08.00 run, the ECMWF brought this
system quite a bit southwest, bringing snow into the MPX area. With
the 08.12 runs, we saw the GFS come southwest with the ECMWF, while
the ECMWF drifted a little north from it`s previous run. The result,
is the 08.12 ECMWF/GFS are in pretty good agreement on this low
drifting southeast along or just south of I-94, with a good burst of
snow coming down just to the north of the low track. QPF amounts are
forecast to be possibly as high as a quarter inch, which would work
out to over 4 inches of snow, which also brings in the chance of
needing some headlines, so this system bears some watching.

After Monday, model spread becomes quite large with exactly where
the through and ridge will setup, so stuck with the blended forecast
from Tuesday on. The one thing about the trough/ridge placement
though is we look to be under the influence more of the ridge than
the trough, hence highs next week largely near normal. There are
lots of small chances for snow as well that will be rifined with
time as models typically don`t hone in on forecast with these
systems until you get about 3 days out. After Sunday night,
the next weak clipper looks to come through the region Wednesday,
followed by another for Friday.

After Friday, the GEFS and EPS mean h5 height forecasts both show
the western ridge breaking down and shifting west out into the north
Pacific while the Hudson low heads back to the north of Hudson Bay.
This will keep us in near to above normal temperatures, though
the flow pattern looks to become more conducive to getting stronger
and more moist storm systems to traverse the upper MS valley as we
head into the second half of December.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1214 PM CST Fri Dec 8 2017

Our next system is up in Manitoba and quickly heading for the
MN/WI border. Warm advection out ahead of this wave has helped
clear out MVFR and lower cigs and looks like we will not go sub-
vfr until the cold front with the Manitoba low arrives. Still
looking good for about a 3 or 4 hour window of light snow in WI,
with the west edge of the snow shield swiping MSP. We may see on
and off light snow/flurries through the rest of the night as the
clouds expected in the wake of the cold front will be located
completely withing the dendritic snow growth zone, which is
usually a recipe for easy flurry generation. Right behind the
front there also looks to be a 2-4 hour window of very strong wind
gusts, possible gusting to over 40 kts at AXN/RWF/MKT so do have a
period where winds are quite a bit stronger than guidance. Lower
confidence on how long post frontal clouds stick around, but the
could be with us well into the afternoon Saturday.

KMSP...Confidence is high that CIGS will remain VFR until
basically the snow arrives. Still only expect a half inch, with
that 23z-02z window still looking good for when MSP sees snow. May
see NW winds gust into the 30s from about 3z-6z before slowly
quieting down the rest of the night. Confidence is low on when
MSP will lose the post frontal MVFR cigs. Could be as early as 9z
or wait until as late as 21z, given the time of year, we went the
longer route.

Sun...VFR. Chc IFR/-sn overnight. Wind NW 10G15 kts.
Mon...MFVR with chc IFR/-SN early. Wind NW 15G25 kts.
Tue...VFR. Wind NW 10 kts.




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