Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 182116

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
316 PM CST Mon Dec 18 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 315 PM CST Mon Dec 18 2017

A quiet short term period ahead before the weather turns more
interesting in the long term period.  Many locations overachieved on
temperatures this afternoon in western MN, in large part thanks to
the momentum of the southwest winds coming off the buffalo ridge.
Granite Falls was 51 degrees as of this writing.  Eastern MN and
western WI on the other hand held in the 30s all day.

We`ll continue to see mid and high clouds through the evening but
clearing skies are expected overnight.  With this pacific airmass in
place, dew points will remain in the mid to upper 20s tonight and
limit overnight lows to those same values.  A cold front will drop
through overnight with the clearing skies, but its more of a wind
shift and influx of drier air behind the front with a lack of actual
cold air behind the front.  Highs on Tuesday should be in the 30s.

.LONG TERM...(Tuesday night through Monday)
Issued at 315 PM CST Mon Dec 18 2017

Main concerns in the long term are for snow later this week and the
impending cold arrival this weekend.

The first round is expected to move in Wednesday afternoon and last
into the overnight hours.  This is a warm air advection/isentropic
lift area of snow that could produce broad 2-4" across much of
northern MN.  For our local area, the metro looks to get grazed with
I-94 being a good line of delineation between snow vs. no snow,
with higher amounts as you go north from there.

The main system looks to emerge from the Colorado Rockies Wednesday
night and evolve northeast with time.  The models tend to agree that
the surface low bottoms out early in the process and begins
weakening by the time it reaches the upper midwest, which is not a
typical evolution of a Colorado Low.  The complicating factor is
another system that will drop southeast out of Canada behind it, and
how and when the two phase together.  This type of setup typically
yields a lot of model spread and it has once again today.

The biggest change in the model guidance today was by the
operational GFS, which pushed the system south, mostly leaving the
metro out of the snow on Thursday-Friday, but putting the metro more
in the path of a few inches Wednesday afternoon and night due to the
aforementioned warm air advection wing.  The big point to make here
is that the operational GFS was near the top of the GEFS members
guidance last night, and now its near the bottom of the chart.  So,
the GEFS members did not change much overall and the mean solution
still paints a 4-6" snowfall across much of southern MN and even the
southern and eastern metro area.  The ECMWF takes the low north of
the operational GFS as does the FV3.  The NAM is even further north
but doesn`t really seem plausible.

All in all, this is a complicated forecast and given that it is only
Monday, we did not have the confidence to issue any type of winter
weather headline given the uncertainty and spread in possible
outcomes. Continue to check back for updates.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1214 PM CST Mon Dec 18 2017

Other thank KEAU, VFR conditions have returned and will remain
that way for this forecast period. Just mid and high clouds will
pass through this afternoon and evening, before sunshine returns
tomorrow. Winds will be southwesterly this afternoon, turning
westerly overnight.  Just scattered high clouds tomorrow. additional concerns.

Wed...VFR. Chc IFR/-SN. Wind ESE 5-15kts.
Thu...IFR/SN developing. LIFR/+SN possible. Several inches
      possible. Wind NE 10kts.
Fri...IFR/-SN ending.  Wind NW 10kts.




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