Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 152102

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
302 PM CST Fri Dec 15 2017

.SHORT TERM...(This evening through Saturday)
Issued at 301 PM CST Fri Dec 15 2017

Today and tonights forecast is pretty straight forward but there`s
some uncertainty on when exactly the snow will start and when it
will end. Regardless it does appear like most locations, especially
along and north of I-94, will get light snow this evening and
overnight so increased pops in this area. This system is pretty weak
and quick-moving so dont anticipate any areas receiving more than
about 2 to 3 inches. Saturday will be seasonably mild with cloudy
skies and light winds.

The main driver for the snow that is expected to move across the
region is the isentropic lift associated with the low level Theta_E
advection. The wind is pretty uniform across the isotherms, so there
is not much Fgen meaning this should be a more wide-spread light
snow event vs a concentrated narrow have band that accompanies
strong Fgen. Forecast soundings seem to echo this as well with a
broad area of weak omega that saturates the column and leads to
several hours of light snow. The 15.12 ECMWF with its broad area of
light QPF paints this picture nicely.

There is already plenty of low level moisture in place as evident by
the widespread stratus, and a few locations have been reporting
light snow all morning across western MN. It stands to reason that
light snow should develop rather quickly once the better forcing
with the isentropic lift moves across the region. For that reason,
sped up the timing of pops this afternoon and increased them later
this evening. Ended up decreasing snowfall amounts a little bit, but
still have higher confidence that light snow will indeed fall over
the next 12 hours, just not as much.

.LONG TERM...(Saturday night through Friday)
Issued at 301 PM CST Fri Dec 15 2017

Sunday will be dry and mild with most locations seeing highs in the
lower 30s. Another clipper system will bring snow across northern
MN/WI on Monday but most of this will miss the forecast area. The
relatively warm weather will continue Tuesday into Wednesday as well
as zonal flow continues across the region. Snow should develop to
the north Wednesday night and there is a chance it could spread
southward across the region on Thursday and Thursday night. The
Thursday system has lower confidence, but this storm has been
trending drier and weaker so at this point is appears that a winter
storm is unlikely for the latter part of next week.

A positively tilted trough will crawl down the west coast on
Wednesday, while a northern stream jet strengthens as it crosses an
upper level ridge over British Columbia. The GFS/ECMWF have been
stronger with this jet and this end up forming a cutoff low across
the Baja. Meanwhile the northern stream continues in a progressive
positively tilted upper level wave over the central Conus which is
not favorable for cyclone development. The 15.00 ECMWF brought an
area of vorticity across the Midwest which did produce a snowstorm,
but the GFS 15.12 resembles a more realistic scenario with a cold
front and some light snow moving though the region followed by cold
air for the weekend. The ECMWF 15.12 has backed off slightly as
well, but still brings a few inches of snow across the southern half
of Minnesota and most of Wisconsin, so continued with 40-50 pops.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1200 PM CST Fri Dec 15 2017

A band of light snow will move southeast across MN & WI this
afternoon & evening. Ceilings & visibilities are expected to stay
MVFR at area terminals, with the potential to drop to IFR at RNH
& EAU after midnight when the heaviest snow is expected.
Accumulations will still be light however with only 1-1.5" of
accumulation. IFR ceilings expected to develop at AXN & STC
overnight, with all other terminals remaining borderline VFR/MVFR
into tomorrow.

Light snow expected to begin around 22Z, ending by 06Z. Most
persistent snowfall will be between 00-03Z. Visibility shouldn`t
fall below MVFR, with only around 0.5" of accumulation expected.
Sun...Chc mrng MVFR. Wind NW 5 kts.
Mon...VFR. Wind SW 10-15 kts.




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