Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Twin Cities/Chanhassen Minnesota
643 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
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 12z tafs through 12z Friday morning)
issued at 542 am CST Thursday Dec 18 2014
A stratus deck continues to linger in western WI and far eastern
Minnesota early this morning. Main concern this package is forecasting
the behavior of this deck. The edge of the clouds reached the
eastern vicinity ring of msp at 08z...but have made essentially
zero progress west since that time. It looks like the Airport will
see MVFR for a period...but leaned to the optimistic side for the
morning. The main message is...the MVFR deck will linger within
one County of msp...and may bounce between broken and scattered coverage
for much of the morning and even the afternoon. The same holds
true for St cloud. Rnh and eau are solidly under the stratus deck
and I expect that to remain in place all day. VFR ceilings for axn and
Kmsp...a dewpoint depression of just 3 degrees was observed on
the 12z radiosonde observation at mpx on the 925mb level. Skies were clear at the
time. So...a slight moistening of this level...to shrink that
depression by one degree...is likely all that is needed to get a
broken deck of stratus. So there is a fine line between forecasting
VFR or MVFR given the challenge of determining where that edge
sets up and how or if it moves. Part of the reason for leaning
optimistic was the fact that the cloud edge held steady for most
of the overnight when I expected it to make more progress west.
Flirting with the edge most of today means amendments will be
issued as necessary.
/Outlook for kmsp/
Friday...VFR. Winds south-southeast 5 kts.
Sat...VFR. Winds south-southeast 5 kts.
Sun...VFR. Slight chance -rasn. Winds south-southeast 7-10kts.