Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 221029
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
429 AM CST Thu Feb 22 2018

.SHORT TERM...(Today and tonight)
Issued at 428 AM CST Thu Feb 22 2018

Widespread snow will give everyone in the forecast area at least a
couple of inches starting later today, with most locations seeing
around 2-4 inches by the time the snow ends Friday morning. The
heaviest amounts will be north of I-94. Winds will be light with
this snow, so little if any blowing or drifting is expected.

The main driver behind this precip is isentropic ascent ahead of an
approaching shortwave that is currently rounding the base of a
longwave trough over CO/NM. Broad lift will develop ahead of this
wave, which is evident by the surface pressure dropping from around
1040mb this morning, to roughly 1025mb Friday morning.

There is not a strong Fgen signal, meaning there is little potential
for any focused area of ascent and banded precipitation. There will
certainly be some mesoscale features that appear on radar later this
evening, but the potential for any location to pick up 6 or 8 inches
of snow is very low. Instead should see widespread light to moderate
snow develop and lift northeast today, with the heaviest snow
falling later this evening.

Forecast soundings show light winds at the surface but there is a
small dendritic growth zone and most of the lift is focused below
it. For that reason shouldn`t see outrageous snow ratios, with
generally 13:1 expected. Forecast soundings also a loss of ice later
tonight, so have some freezing drizzle mixed in with the snow. Any
ice accumulations should range from a light glazing, to none at
all.

.LONG TERM...(Friday through Wednesday)
Issued at 428 AM CST Thu Feb 22 2018

The main concern in the long term period is the winter storm
potential for Saturday.  Currently, an amplified trough is digging
across the Pacific Northwest.  It will continue digging today and
slowly transition eastward through Friday.  By Friday night, surface
cyclogenesis will be ongoing along the lee of the rockies, with a
surface low emerging and advancing eastward across the state of
Kansas early Saturday.

Strong low-level warm air advection will lift into the area,
along with the upper level PV feature and trough becoming negatively-
tilted.  There is decent agreement among the models that the
Colorado low will curve northeast through eastern Iowa and into
Wisconsin.  There are small differences in the exact path of the
low, but that doesn`t change the fact that we will see accumulating
snow Saturday through Saturday night.

When the surface low turns northeast, it`s being driven by the upper
trough becoming negatively-tilted, and an increasing jet streak on
the southern flank.  The response is a deepening surface low as the
system moves across the midwest, so there will certainly be strong
winds with this system.  Heavy snow combined with strong winds is
going to make for very difficult travel Saturday night for much of
the area.  In terms of amounts, the QPF is looking more impressive
and given the reasons mentioned above, forcing should be strong with
this system.  Generally a range of 0.4"-0.8" of QPF will yield snow
amounts in the 5 to 9 inch range in the heaviest band (eastern MN
and western WI). Near whiteout conditions will be likely during
the falling snow with wind potential up to around 30 MPH, but
possibly stronger based on exact path and pressure of the surface
low.

Beyond that, near normal temperatures and dry conditions are in the
forecast for early next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1107 PM CST Wed Feb 21 2018

Only change to thinking from the 00z TAF was to slow the arrival
of snow per trends from the HRRR. When main band gets here vis of
3/4sm to 1 1/2sm will be predominate vis, though some 1/2sm vis is
certainly possible, though that looks more like the exception as
opposed to the norm. Again, any sub-vfr cig restriction look to be
tied to having snow actively falling.

KMSP...HRRR continues to point to a later arrival of the snow,
with it now looking to arrive between 20z and 22z per the GFS and
NAM, with the heaviest snow still expected between 1z and 7z.
Still looks likely for 3-4 inches, with max rates of 3/4" per
hour.

/OUTLOOK FOR KMSP/
Fri...MVFR cigs in mrng. VFR in aftn. Wind W 5-10 kts.
Sat...VFR in mrng. MVFR or lower with SN in the afternoon.
      Significant accums possible Sat night. Wind E 5-10 kts.
Sun...MVFR early, VFR late. Wind W 10-15 kts.


&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 6 PM this evening to 9 AM CST
     Friday for WIZ014>016-023>028.

MN...Winter Weather Advisory from 4 PM this afternoon to 6 AM CST
     Friday for MNZ041-042-047>049-051-057>060-062-066>070-
     076>078-085.

     Winter Weather Advisory from 6 PM this evening to 9 AM CST
     Friday for MNZ043>045-050-052-053-061-063.

     Winter Weather Advisory from 1 PM this afternoon to 3 AM CST
     Friday for MNZ054>056-064-065-073>075-082>084-091>093.

&&

$$

SHORT TERM...JRB
LONG TERM...SPD
AVIATION...MPG



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