Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 050504

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1104 PM CST Sun Dec 4 2016

.UPDATE...For 06z Aviation discussion below


.SHORT TERM...(This evening through Monday)
Issued at 345 PM CST Sun Dec 4 2016

There are two concerns for the short term, one being potential for
freezing fog tonight and then high temperatures tomorrow ahead of
the arctic boundary.

The upper low/mid level trough is swinging through the CWA late
this afternoon. The steady snow has pushed east of the area and
only a few sprinkles remain with the upper low itself now pushing
into the metro. Following the passage of this low, skies will
clear this evening from west to east. In fact, the sun has been
out for a few hours across southwest Minnesota where temperatures
have warmed into the 40s.

Dew points are in the low to mid 30s this afternoon. As the clouds
clear and a bit of surface ridging passes across, winds will
likely decouple for a period of a few hours overnight.
Temperatures with the snow cover should drop into the 20s, or
several degrees below the crossover temperature so anticipating
some freezing fog to develop.

Mid level ridging will build in ahead of the arctic front Monday.
Gusty south winds across southern MN will push in mid morning with
925 mb temps of +2 to +4C. Mechanical mixing should reach that
level, so there is potential to reach the upper 40s to lower 50s
across east central and south central MN. 24 hour backward
trajectory forecast beginning 19Z Monday shows the source region
from northwest Oklahoma where temperatures are in the lower 50s
today. The snowpack has taken a hit today, so don`t believe it
will be a huge factor especially with some of the gusty winds but
it should temper back some of the potential. Raised highs again into
the mid 40s.

.LONG TERM...(Monday night through Sunday)
Issued at 345 PM CST Sun Dec 4 2016

Main items of interest in the longer term are the cold air that
will begin moving in after the passage of Monday evening`s frontal
passage, the gusty winds which will be around for a few days as
the colder temperatures get established, and the possibility of
some accumulating snow for Saturday. The model guidance is in
remarkably good agreement with respect to the large scale features
and overall synoptic pattern, so there is little doubt that we`ll
finally see a taste of winter temperatures in the area. However,
there are certainly some differences in the details that could
impact how cold it is able to get, and especially with respect to
any possible snowfall toward the end of the forecast period.
Given the overall good agreement, there is little reason to make
any substantive adjustments to a consensus approach at this point
in time. The only real exception to that would be to increase
winds a bit in the Tuesday to Wednesday time frame, with an
impressive pressure gradient across the area while we find
ourselves nestled between low pressure to the northeast and a
ridge building in from the northwest. This gradient will serve to
keep wind speeds up, which will help temperatures from bottoming
out as low as they might get at night for much of the week. Of
course, the winds will also serve to lower wind chill readings,
with some below zero readings expected Tuesday through Thursday

We will see some lingering light rain transition to light snow
Monday night into early Tuesday, but after that the only chance
for any snow during the week looks to be on Wednesday night into
Thursday as a shortwave trough rotating in from the northwest
brings some differential PVA ahead of a reinforcing short of cold
air. A better chance for snow will come on the weekend, although
the remains quite a bit of disagreement on things at this point in
time. But, for what it`s worth, we should see some warm advection
develop Friday night and continue Saturday as a shortwave trough
works east southeast into the region, and weak surface low
pressure emerges from the plains. The GFS is slower and weaker
with the upper wave in comparison to the ECMWF and Canadian
models. As a result it brings through an area of light snow Friday
night, and very little after that. The ECMWF and Canadian have a
more focused area of snow with the warm advection and differential
PVA, but are farther north (northern/central Minnesota/Wisconsin)
vs the GFS (Central/Southern Minnesota and Southern Wisconsin).
So, at this point, will just include a chance of snow moving from
west to east from late Friday night through Saturday night. But,
these Plains lows can be tricky, especially when moving east into
an area with a good cold airmass established, so will certainly be
worth keeping an eye on over the coming days.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1104 PM CST Sun Dec 4 2016

The stratus deck primarily affects west central WI sites
(KRNH/KEAU) at TAF issuance. Next concern will be fog development,
particularly at west central Wisconsin sites (KRNH and KEAU), and have
included VLIFR conditions there after 07z. Otherwise,
west/southwest winds will become southeasterly for Monday, and
become gusty by late morning when a mid level cloud deck will move
in ahead of the next system. Rain chances appear higher for west
central WI sites around/after 00z Tuesday.

Now that we`ve cleared out, will be watching for fog development
(along with the potential for a return to MVFR cigs). Have
currently included the most optimistic scenario in the TAF, which
only lowers vsbys to 4sm, and keeps cigs VFR.

Tue...MVFR. Chc -SN early. Wind W 15g25kts.
Wed...MVFR. Wind WNW 15g25kts.
Thu...MVFR. Chc -SN. Wind WNW 15g25kts.




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