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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1119 am CST Thursday Dec 18 2014

Short term...(today and tonight)
issued at 421 am CST Thursday Dec 18 2014

Quiet short term period expected with the only chance for
precipitation being flurries or light snow showers passing through
eastern Minnesota and western WI.

At the surface...high pressure remains in control across the north.
This time of year however...cloud cover is a concern yet again
today. A solid shield of stratus has been slowly advancing west
through western WI into eastern Minnesota overnight. This cloud shield will
only get so far under the presence of anti-cyclonic flow.
However...as this cloud line halts across east central Minnesota
today...additional cloud cover will spread in from the west. Early
morning water vapor imagery depicted two items of interest. One
upper level circulation pushing into the western Dakotas from
Montana...and another circulation lifting northeast from norther
Kansas into eastern Nebraska. Although...nothing too exciting will
come of there waves. Cloud cover will increase from western Minnesota
eastward today thanks to the upper level saturation...but no
precipitation is expected this far north. Winds will be light today
as the surface high moves slowly east...but we'll see a southerly
component develop by tonight. Seasonal temperatures expected...with
a warmer overnight expected tonight thanks to the southerly flow and
increased cloudiness.

Long term...(friday through wednesday)
issued at 421 am CST Thursday Dec 18 2014

There have been a few changes to the long term forecast...
particularly regarding freezing drizzle potential Saturday night
and probability of precipitation next week. There remains quite a bit of uncertainty
surrounding the Christmas evening/day storm but confidence is
increasing there should be a very large storm system affecting
much of the eastern half of the Continental U.S. During this time.

Closer to the present time...surface high pressure will slowly
build eastward to the East Coast late this week. Warm air advection and
increasing low level moisture will contribute to considerable
cloudiness Friday and Saturday. Models are picking up on quantitative precipitation forecast
during this time...but are probably overdoing it given the shallow
depth of moisture. Eventually the moisture depth will be
sufficient to allow for the development of freezing drizzle across
southern/eastern Minnesota and western WI Saturday night into Sunday morning. It
doesn/T look like much...but it doesn/T take much to create some
problems. Moisture depth continues to grow Sunday...and freezing
drizzle may turn to light snow with the dgz moistening. This
moisture will lift northeast and a brief dry period is expected
Sunday afternoon and evening.

The next system...which will be a player for the remainder of the
forecast period will slide southeast from Canada as an innocent
clipper system Monday. There are timing differences of not only
the low...but of when precipitation develops with this system. Most
guidance is showing snow developing Monday morning across the eastern
County Warning Area with not much but wrap around snow showers or flurries for western
Minnesota Monday afternoon and beyond. Models are more consistent in
showing the eastern County Warning Area precipitation chances...so felt it was prudent to
increase probability of precipitation into the likely range in these areas. Wet bulb temperatures
do inch above freezing Monday afternoon...so should see some rain
mixing in...especially across southern Minnesota.

The rest of the forecast period revolves heavily on the evolution
of this clipper system. That evolution in large part will depend
on the phasing of it with a sharply deepening trough over the
plains and MS River Valley. Another low pressure system will
develop Monday night in the Southern Plains near the left exit
region of a 150-175kt upper jet...and gain strength as a 100+ knots
500mb jet streak rounds the base of the trough Tuesday night. As
the jet streak begins to lift north...the resulting system will
undergo very rapid intensification. The highly negatively tilted
system and strong jet streak surrounding the southern half of the
trough will keep a very strong system from moving much Christmas
evening and day. Ensemble members are all highly suggestive of such a
system somewhere near or just east of the Great Lakes...and some
members have deepened the system to less than 950 mb. Obviously if
this were to occur...high winds would be an issue. Think most of
the serious precipitation will remain east...but persistent snow
showers under cyclonic flow are possible. By the time all is said
and done...models suggest several inches of snow across most of
the County Warning Area next week but this will be spread out across a few days.

&&

Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1100 am CST Thursday Dec 18 2014

Earlier this morning a cloud layer around 1.5 to 2.5k above ground level moved
from wc WI...into ec Minnesota as boundary layer winds slowly became
east-southeast/southeast this morning. As temperatures in the cloud layer held near -10c to
-12c...ice Crystal formation continued...plus the added affect of
weak convergence in this layer...led to -sn. Temperatures will slowly
warm this afternoon and winds will veer to the S/SW...so the MVFR cloud
deck should move off to the north/NE and only affect krnh/Keau through
the next 6 hours. Elsewhere...kstc will see VFR conds by 21-23z. After
00z...another area of MVFR ceilings will move from NE/Iowa back into SW
Minnesota and cause krwf/kaxn to lower to MVFR ceilings once again later
tonight. Confidence remains high on the MVFR ceilings for ec Minnesota/wc
WI...with lower confidence on the return of MVFR ceilings across SW/wc
Minnesota after 00z. This is mainly a timing issue...not so much of will
it occur or not. Light winds from the east-southeast/southeast will slowly increase
by the end of the taf period.

Kmsp...

Last batch of -sn should move through the Airport terminal area by
18-19z. MVFR ceilings will slowly erode from the west/SW and advect into
the metropolitan area by 20-22z. Timing remains the may problem...but
confidence is high enough to introduce VFR conds by 21z. MVFR ceilings
will likely come back into ec Minnesota Friday...but timing remains the
main dilemma. Will continue with VFR conds until we have a better
handle on timing. Winds will hold from the east-southeast/southeast through the
period...and slowly increase from 3-5 kts this afternoon...to 6-8 kts
by the end of the taf period.

/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.
&&

$$

Short term...speed
long term...borghoff
aviation...jlt

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