Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 121750
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1150 AM CST Tue Dec 12 2017

.SHORT TERM...(Today and tonight)
Issued at 422 AM CST Tue Dec 12 2017

A weak surface high pressure ridge to start the day will be quickly
replaced by the arrival of a low pressure trough tonight. Clouds
will increase notably by mid-day as isentropic lift develops east of
the warm front aligned along the MN/Dakotas border. This is first
noted across central MN by 18z, and then gradually drifts
east/southeast during the evening. Have therefore included 20-30
POPS. Temperatures will moderate into the low/mid 30s along the
Minnesota River Valley, with areas to the east seeing cooler highs
in the 15 to 25 degree range. Have also increased lows for tonight
given west/southwest flow ahead of the approaching trough and
clouds should limit radiational cooling.

.LONG TERM...(Wednesday through Monday)
Issued at 422 AM CST Tue Dec 12 2017

The potent upper wave dropping south into the Upper Mississippi
Valley Wednesday morning will become more neutrally tilted by
midday. This, with continued isentropic lift, will strengthen the
snow band from west central WI southeastward at the beginning of
the period. Models haven`t been displaying the best consistency
with the placement and intensity of the snow with this system so
confidence isn`t the best. It is possible 2 to 4 inches could fall
under that band before the system shifts to the southeast by
afternoon.

Another issue to contend with is freezing drizzle. The surface low
will pass over eastern MN late tonight and Wednesday morning with
the mid level dry slot accompanying it. Somewhat deep low level
moisture extending up to 7kft with some lift through that layer
could generate areas of freezing drizzle. This risk will be
enhanced during the late morning as strong CAA steepens low level
lapse rates and the moisture layer becomes unstable. The low
levels will be warm enough that this moisture won`t extend into
the DGZ until afternoon when the column cools. Therefore, the
forecast for most of the Minnesota counties calls for a chance of
snow initially given the uncertainty in placement of the morning
snow band, followed by a chance of freezing drizzle as the mid
level dry slot moves in, and then back to a chance of snow showers
during the mid to late afternoon with reintroduction of ice
aloft. There shouldn`t be too much ice accumulation, if any,
especially with temperatures rising to near freezing by late
morning.

Strong winds are expected again behind this clipper, mainly across
western and southern Minnesota. Forecast soundings are quite a
bit more impressive with wind gust potential with this one than
with the one for Monday. GFS Bufkit soundings show mixed layer
gusts ranging from about 40 kt in the lower levels to 50 kt at
the top of the mixed layer. NAM is even stronger at the top,
reaching 55 to 60 kt. A Wind Advisory will eventually be needed,
but a High Wind Warning may also be possible if later trends favor
the NAM.

The clipper train continues later in the week, but predictability
with each of these disturbances is low at this range. Maintained
low chance PoPs for now.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1139 AM CST Tue Dec 12 2017

Main impact this period is the fast moving clipper system that
will move quickly through the area late tonight through tomorrow
morning. This clipper will bring a mix of snow and freezing
drizzle through the region, and even some rain in southwest
Minnesota once temperatures rise above freezing. This clipper will
move in after 06Z and does have a warm side and dry mid level area
across much of southern MN, which will lead to light freezing
drizzle moving through. Ceilings will dip for a period in the east
this afternoon, but rise to VFR again this evening. Overnight,
ceilings will quickly drop ahead of the quick moving clipper,
prior to the onset of light precip.

The other concern is the wind for tomorrow. It will continue to be
light today, but will increase substantially tomorrow after the
surface low moves through. Northwest winds of 15-20kts with gusts
around 30kts will be likely by mid day tomorrow, with even
stronger gusts across western MN.

KMSP...highest risk of freezing precip is during a bad time, the
morning rush. Looks like 11Z-14Z is the best chance.

/OUTLOOK FOR KMSP/
Thu...Chc MVFR. Wind NW 10-15 kts.
Fri...Chc MVFR. Wind WSW 5-10 kts.
Sat...Chc MVFR. Wind SW 5-10 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

$$

SHORT TERM...LS
LONG TERM...BORGHOFF
AVIATION...SPD



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