Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 160006

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

Issued at 542 PM CST Fri Dec 15 2017

Updated to include 00z aviation discussion below.


.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 00Z TAFS through 00Z Saturday evening)
Issued at 542 PM CST Fri Dec 15 2017

A fairly wide band of light-moderate snow will slide southeast
across MN and WI through this evening into the early morning
hours. Ceilings will generally run in the MVFR range for all
terminals, with KAXN-KSTC most susceptible to IFR ceiling both
this evening and again during the first half of tomorrow.
Visibilities will generally run in the VFR range but isolated
heavier bursts of snow may well produce brief bouts of IFR
visibility. Once the main swath of snow diminishes during the
early morning hours, scattered flurries and snow showers will
persist mainly over the WI TAFs through daybreak.

KMSP...Light snow will commence around initialization time and
persist through 04z-06z, heaviest in the 00z-03z timeframe. Will
look for mainly MVFR visibility but IFR visibility certainly
cannot be ruled out. Ceilings will drop into MVFR range shortly
after snow commences and will remain in MVFR range throughout this
TAF set.

Sun...Mainly VFR. Chc morning MVFR. Wind variable 5 kts or less.
Mon...VFR. Wind SW 10 kts.
Tue...VFR. Wind NW 10-15 kts.




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