Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 041752
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1152 AM CST Mon Dec 4 2017

.UPDATE...For 18Z Aviation discussion below

&&

.SHORT TERM...(Today and tonight)
Issued at 419 AM CST Mon Dec 4 2017

Take a minute to appreciate 50 degrees in December, because after
today you can expect temps to be well below freezing for an extended
period of time. Later today a powerful winter storm will bring
northwest wind gusts of 40 to 50 mph, along with rain changing
over to snow. A few inches of accumulations are possible across
western and central Minnesota.

From a pure meteorological standpoint, this storm is going to be a
beauty and has a lot of things going for it that will allow it to
rapidly intensify into a mature synoptic cyclone over the next 24
hours. A positively tilted shortwave trough will take on a negative
tilt as it emerges from the Colorado Rockies and heads towards the
Great Lakes. A west-to-east jet on the southern side of this trough
will strengthen as the polar and subtropical jet phase together. The
combination of these two will allow the positive vorticity to
congeal and increase in magnitude and lead to upward vertical motion
that will cause a surface low to lose about a millibar per hour as
moves from eastern Nebraska towards Lake Superior.

From a sensible weather standpoint, this storm is going to be huge
wind maker along with falling temperatures. Rain will change over to
snow today, and could see a quick 1 to 3 inches across western
Minnesota, with lesser amounts as you head east. Although these
totals are not very high, the combination of the strong wind
together with the falling temperatures are enough to support winter
storm warnings and winter weather advisories across wester/central
Minnesota. Areas to the east will have less snow, and a wind
advisory covers these locations. Forecast soundings across the board
show 50 kts at the top of the channel, so we shouldn`t have any
trouble getting gusts near 50 mph at the surface. Again, this
storm system is quite powerful, so from a big picture perspective
could it see reaching its full potential and producing winds that
are on the high side of guidance.

The big question was wind advisory vs winter weather advisory. There
is a tremendous amount of warm air with this system. For example,
temperatures are in the lower 50s at most locations across the
region. Forecast soundings and HiRes models show the precip
remaining rain as the deformation band develops and sweeps through
the region. If this happens, there will only be a small window of
snowfall, some of which will melt on contact, so totals may be
negligible for many locations in eastern MN and western WI. The
only concern would be reduced visibility with the falling snow,
and temperatures going below freezing which could lead to slick
spots on untreated roads that were previously wet. All things
considered, the wind is the highest confidence so went with a wind
advisory across the remainder of the forecast area.

One last thing of interest was the potential for thunder with this
system as it matures and a trowal develops. HiRes models show
enhanced reflectivities as that deformation band shifts through the
region, and a closer look at forecast soundings shows around 200
J/kg of MUCAPE over MSP around 01-02Z tonight, so could actually
have some upright instability and a few convective elements with
thunder.

Temperatures will continue to fall overnight and should see
northwest wind gusts of 30 to 40 mph prevail into the morning hours.

.LONG TERM...(Tuesday through Sunday)
Issued at 419 AM CST Mon Dec 4 2017

Unless you are interested in cold weather, there is not much of
note in the extended part of the forecast. An upper level ridge
will set up across the west coast with a deep longwave trough over
the central United States. Northerly flow will continue across
the Upper Midwest brining down repeated rounds of cold air from
Canada, some of which actually can be traced across the north pole
back to Siberia. Needless to say air of this origin this time of
year is going to be cold and dry.

A few shortwaves traverse the region, but they are quick moving
and moisture-starved so shouldn`t see anything more than a few
snow showers from time to time. At this point, most locations will
be below freezing through the weekend into early next week and
beyond.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1153 AM CST Mon Dec 4 2017

MVFR conditions will degrade to IFR/LIFR this afternoon as a
storm system arrives. Rain will change over to snow as the cold
front passes, with a narrow band of heavy snow with very low
visibility. Snowfall rates could approach 1.5-2 inches per hour,
but would only last for 1-2 hours. Temperatures will be near the
freezing mark which could hold back accumulations slightly, but
falling temperatures overnight into Tuesday will freeze any
untreated surfaces. Winds will shift to the northwest and increase
behind the front. Sustained speeds of 25 to 35 knots with gusts
to between 40 and 50 knots will prevail through most of the
period.

KMSP...
We expect the ceiling to lower below 1800 feet by 21z, then lower
below 1000 feet by 23z. Drizzle/rain will transition to snow
between 02z and 03z, then diminish shortly after 06z. Could have
a brief 1-2hr window of moderate to heavy snow, with vsbys
lowering to near 1/2SM. Melting should hold back accumulations
toward 2 inches. Winds will increase later this evening with
gusts over 40 kts likely between 03z and 07z.

/OUTLOOK FOR KMSP/

Tue night...VFR. MVFR possible. Wind NW 15-25kts.
Wed...VFR. Wind NW 10-15G25kts.
Thu...VFR. MVFR possible late with Chc -SN.Wind NW 5-10kts.
Fri...VFR. MVFR possible early with Slgt Chc -SN. Wind NW 5-10kts


&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Wind Advisory from 6 PM this evening to 6 AM CST Tuesday for
     WIZ014>016-023>028.

MN...Wind Advisory from 3 PM this afternoon to 4 AM CST Tuesday for
     MNZ051-052-059>063-066>070-074>078-082>085-091>093.

     Winter Storm Warning until midnight CST tonight for MNZ041-042-
     047-048-054>056-064.

     Winter Weather Advisory from 3 PM this afternoon to 3 AM CST
     Tuesday for MNZ043>045-049-050-057-058-065-073.

     Wind Advisory from 6 PM this evening to 6 AM CST Tuesday for
     MNZ053.

&&

$$

SHORT TERM...JRB
LONG TERM...JRB
AVIATION...LS



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