Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
348 PM CST Wednesday Dec 18 2013
Short term...(this evening through thursday)
issued at 348 PM CST Wednesday Dec 18 2013
Early afternoon water vapor imagery together with 500mb heights and
winds depicted broad zonal flow spanning the entire Continental U.S....with two
nearly symmetric waves each located off either coast. The wave across
the northwest will be the driving force behind a powerful weekend
storm set to affect the eastern half of the country. Closer to
home...a weak surface low located across northern Minnesota...with a
trailing cold front extending west southwest. Ahead of this cold
front...afternoon temperatures were in the upper 30s and lower 40s
across western Minnesota..and lower 30s elsewhere.
The aforementioned cold front will move southward through the region
overnight...with northerly winds gusting 20-25mph behind it. Despite
the cold air advection...the stronger winds should keep the boundary
layer mixed and prevent temperatures from completely bottoming out
overnight. On Thursday temperatures will struggle to rebound as
northerly winds persist. By late afternoon a weak shortwave will
move up from the Desert Southwest and cause a band of light snow to
develop over the upper Midwest.
Long term...(thursday night through wednesday)
issued at 348 PM CST Wednesday Dec 18 2013
The snow will become steadier and heavier Thursday evening as the
disturbance races to the east. Snow will overspread the area
quickly early Thursday evening...with most all of it tapering off
around or shortly after midnight. The best isentropic lift will be
found along the I-90 corridor...however the best saturation and
the depth of the will dgz will be displaced just to the north from
roughly Redwood Falls to msp and Eau Claire. Snowfall rates could
reach an inch an hour for a time by middle evening before drier
air pushes in and snow tapers off during the early overnight.
Accumulations of 1 to 2 inches still looks good.
High pressure builds overhead Friday and begins to move east
Saturday. Warm air advection late Friday night may set off a light band of snow
across eastern Minnesota and western WI along a warm front. Amounts look minimal.
Seasonably cold temperatures will prevail through Friday night.
Milder air returns Saturday in southeast flow as a powerful winter
storm begins developing over the Southern Plains. The big weather
story in the long term will be how this system evolves and where
it tracks. A deep upper low over socal/northern Baja California will begin
ejecting eastward Friday and Friday night. An anomalously large
and strong ridge over the Caribbean will feed copious amounts of
moisture northward from the eastern Pacific and equatorial
Atlantic. This ridge will steer the system northeastward late
Saturday and Saturday night along a stationary boundary.
Widespread heavy precipitation will stretch from the Southern
Plains into the Ohio Valley and northeast. In addition...broad
troughiness over Canada and the northern tier of the U.S. Will
begin to amplify as another piece of energy dives southeast into
the northern plains Saturday night. This subtle feature will play
a role in how much the system deepens and precisely how far west
it can track Sunday. A more aggressively digging trough would
favor a track closer to our area that would result in a negatively
tilted system...a more organized trowal...and thus a broader band
of accumulating snow. A weaker and more progressive trough would
keep a more positively tilted system and the precipitation would be
confined generally along the stationary boundary draped across the
middle MS valley and Northern Ohio valley. Systems such as these
generally tend to shift westward with time...so I would not be
surprised to see some precipitation at least across the WI counties -
hence the chance probability of precipitation. Models have indeed shown this trend with the
12z runs. However...all models are generally in agreement with the
track and sensible weather impacts. Therefore...not anticipating
any drastic changes from here on out...especially that some
lingering differences in upper air features have not resulted in
many differences at the surface. Interestingly...the operational
GFS was the furthest west member in the gefs but it matches well
with the 00z ece means. CMC and 18z NAM are further west...but
these are typical biases. Bottom line...this system will have a
band of heavy deformation snow accompany it somewhere nearby with
significant accumulations and it will need to be watched in the
Following this system...deepening trough set to bring colder temperatures
for Sunday night and Monday before ridging over the western Continental U.S.
Spreads eastward a bit. A northwest flow aloft remains...so
periodic light snow chances should continue making an appearance next
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1131 am CST Wednesday Dec 18 2013
VFR conditions should today ahead of an approaching cold front
from the north. Once the front passes through winds will increase
out of the north...with gusts near 20kts at times. There is
indication that MVFR clouds will develop behind the front as
well for Wednesday morning. Looking ahead...snow will develop
during the afternoon.
frontal passage should be around 06z...with northerly winds
thereafter. MVFR clouds cloud develop Post frontal...so have
broken deck after midnight.
/Outlook for kmsp/
Wednesday evening...MVFR/IFR and -sn. Winds north at 10kts.
Thursday...lingering MVFR/IFR and -sn. Winds northwest at 10g20kts.
Friday...VFR. Winds SW at 05kts.
Sat...VFR. Winds northwest at 10kts.