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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1111 PM CST Thursday Dec 18 2014

Update...
issued at 1107 PM CST Thursday Dec 18 2014

Updated to include 06z aviation discussion below.

&&

Short term...(this evening through friday)
issued at 307 PM CST Thursday Dec 18 2014

A narrow band of drier air around 92h has led to some clearing or
partly sunny skies across portions of central and southern Minnesota
early this afternoon. In far ec Minnesota/wc WI...earlier cloud layer and weak
convergence in this layer led to a period of -sn...which briefly
caused some minor accumulations across the Twin Cities. This area
of -sn has moved off to the north/NE and dissipated...or became
very light for radar not to detect it.

Although there will be a brief period of mostly clear skies moving
across the area this evening...it will quickly become cloudy
again as a weak shortwave moves across the region tonight from the
Dakotas. In addition to the increasing cloud cover...a weak area
of lift across central Minnesota may produce some light freezing
drizzle/flurries. Deeper moisture will be north of mpx County Warning Area...but
enough to include some light precipitation across the far northern
County Warning Area by Friday morning. No major problems with the freezing
precipitation is expected due to limited coverage and duration.

The strong inversion that developed last week and led to a long
period of cloudy skies...low ceilings and fog...is likely to develop
once again by Friday afternoon as temperatures above the boundary layer rise
from the warmer SW flow underneath the cloud deck.

Long term...(friday night through thursday)
issued at 307 PM CST Thursday Dec 18 2014

Main concerns during the long term remain fzdz potential
Saturday/Saturday night...followed by precipitation chances Sunday
through Wednesday night. Included in the precipitation chance next week
is the issue of p-type Sunday night into Monday.

For this forecast...model preference was a blend of the GFS/European model (ecmwf)
as the Gem quickly becomes an outlier on Monday next week.
Differences between the GFS/European model (ecmwf) are fairly minimal until
Thursday...but after that...we are beyond this forecast period.

The first problem with the forecast shows up Saturday as we get
into increasing srly flow. One thing that looks like a certainty
is more time spent beneath a deck of stratus. Still questionable
Sat/Sat night is if moisture is deep enough to generate dz. For
now...kept Saturday dry...but kept the fzdz mention going for Sat
night. The reasoning here is that during the day Saturday...we
will have shortwave ridging and subsidence working across the
area...while Saturday night...the next weak shortwave moving
across the area will provide a little burst of lift and deeper
moisture. Confidence is still low on if we will see any dz or
precipitation at all...but on Saturday night...most models show some
light quantitative precipitation forecast spreading across the area...which is indicative of dz.

For Sunday...srly winds will be increasing in strength...but we
will see the added benefit of seeing moisture return to the
dgz...along chances for precipitation other than dz. Moisture transport
increases as well on Sunday...but it is oriented toward eastern
areas...so continued to have chance probability of precipitation for western WI.

Sunday night...the upper wave that will be with US more or less to
Christmas makes its appearance. This will send a surface low across
southern Minnesota/northern Iowa Monday. The upper wave will still be taking on a
negative tilt and closing off...so the surface low will make it only
as far as Michigan by Wednesday. Also on Wednesday...a southern wave
will round the trough...with a coastal low taking over by
Thursday. The European model (ecmwf) still shows this low deepening to under 970
mb...but it is a bit farther east than earlier runs. Other notable
change with the system is in the temperature department. Because
the upper low gets so deep...the cold air gets stopped up in
Canada. With the 17.12 runs...the European model (ecmwf)/GFS were showing 925-850
temperatures cooler than -12c by Thursday...but with the 18.12 runs temperatures
in that same layer are forecast to be warmer than -10c...so we
only look to cool back down to about normal by the end of the
week...with the most noticeable change in the forecast for next
week being temperatures about 5 degrees warmer than what we had.

As for potential sensible weather next week...we still look to
spend a prolonged period of time within cyclonic flow...which
looks to result in a prolonged period of light to moderate snow
Monday night through Wednesday night. In addition...as the low is
moving across the area on Monday...it looks to bring a fairly warm
tongue of air up in the area...with the European model (ecmwf)/GFS showing surface wet
bulb temperatures increasing into the middle 30s Sunday night into Monday.
Forecast soundings from the GFS continue to show p-type being rain
Sunday night through Monday morning for much of the area outside
of central Minnesota and northwest WI. Once this system moves off to the
east...profiles still look to cool enough to switch things over to
snow...though the European model (ecmwf) is slow enough in bringing the cold air in
to push the threat for a rain/snow mix out into Tuesday in western WI.
Both the GFS and European model (ecmwf) continue to show several inches of
accumulation next week...but we get those several inches because
we look to pick up an inch or two of snow a day Monday through
Wednesday. Winds also look to pick up on Tuesday...though with the
sub 970mb low staying farther east in the latest models
runs...winds do not look like they will be as strong as they were
looking yesterday...but still would be strong enough to lead to
blowing snow issues depending on how much snow we get next week.
Still lots of uncertainty surrounding this system...but given its
timing during a busy travel week...still has the potential to
adversely impact much of the area in one way or another next week.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1107 PM CST Thursday Dec 18 2014

Fairly persistent low-end VFR to solid MVFR ceilings to remain in
place for just about this entire taf period as southerly winds bring in
low level moisture in advance of a weak upper level disturbance drifting
through overnight through tmrw morning. Precipitation should be very light
if not none...possibly the occasional flurry. Also not looking for
ceilings to drop into IFR range despite some MOS guidance hinting
as such. Winds will slowly veer from southeast to S tonight into
tmrw...with the WI taf sites going from light/variable to S as high pressure
ovhd slides east.

Kmsp...tricky start to the kmsp taf as ceilings are right at or
around the 1700 threshold. Am xpctg ceilings to remain at or slightly
below 1700 through early Friday morning then experience some slight
improvement for at least the first half of the day tmrw. With
ceilings so near categorical thresholds for much of this taf...
some frequent amendments may need to be made so will continue to
monitor.

/Outlook for kmsp/
Friday...VFR early...possibly MVFR by aftn/evening. Winds south-southeast 6-8 kts.
Sat...MVFR ceilings. Chance -fzdz late. Winds south-southeast 5 kts.
Sun...MVFR/IFR. Chance -fzdz/-ra/-sn. Winds south-southeast 7-10kts.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$

Update...jpc
short term...jlt
long term...mpg
aviation...jpc

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