Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
1011 PM CST Thursday Dec 25 2014
Updated aviation portion for 06z taf issuance
Short term...tonight and Friday
issued at 308 PM CST Thursday Dec 25 2014
A weak cold front over Minnesota along with weak impulse moving
into confluent upper flow will move into WI overnight and early
Friday. Band of light snow associated with weak frontogenetic
forcing has developed in northwest WI.
Front will likely become stationary over southern Wisconsin
Friday as it becomes parallel to the upper flow. Brief break from
low clouds across far southern part of forecast area per satellite
and latest runs of hrrr...but mainly cloudy as this next system
moves in. Some light snow expected across the northwest half of
the area tonight associated with aforementioned system. Freezing
drizzle also possible a bit later this evening as middle level
moisture is stripped away. Clouds should hold temperatures up so
continued forecast of several degrees above latest MOS
guidance...close to model blend of bias corrected numbers.
Slight chance for a few flurries or freezing drizzle Friday
across all but Fox Valley and Lake Shore. Better chance for
precipitation just beyond this forecast period...associated with
stronger shortwave/jet maximum that ejects from the SW U.S. Longwave
trough and surface low that moves along front.
Long term...Friday night through Thursday
issued at 308 PM CST Thursday Dec 25 2014
Focus in the long term will be on the system moving through Great
Lakes region Friday night into Saturday...with precipitation type issues
in east central Wisconsin and snow accumulations northwest of the
Low pressure is expected to develop in the Southern Plains and
ride northeast along a cold front. A progressive shortwave will be
swinging through the upper Midwest Friday night into
Saturday...which will strengthen the surface low as it slips through
southern Wisconsin Saturday.
Tended to use the GFS when working with this system...as it has
had the most run to run consistency...though models were in
okay agreement with most aspects of the system through the event
time-frame. Precipitation should overspread the area after midnight
Saturday as warm air advection/southwest flow increases ahead of the main system.
The main system precipitation should then push into the area Saturday
morning...as the center of the low moves over southern Wisconsin.
As it does so...it will sync up with a decent 500mb shortwave. This
will cause the surface low to quickly intensify before it surges
northeast into lower Michigan. Precipitation will end west to east
as the system moves away from the area Saturday afternoon...with
precipitation ending along the Lakeshore 00z Sunday.
Precipitation type may be an issue in eastern Wisconsin. Previously
mentioned warm air advection overnight Friday will cause a warm layer near the
surface. With no snow cover what so ever over the area...it wont take
long to rise above freezing. Models in some disagreement on how
much warm air will be present at the surface...which a few degrees in
either direction making the difference between mix or all snow in
the east. Decided to trend with the warmer guidance...as models
tend to underestimate the low level warm layer when it is that
shallow and to account for the lack of snow cover. This would
result in a rain snow mix along the Lakeshore Friday night...with
the mix pushing inland through the Fox Valley Saturday morning as
winds turn east. Door County should remain mostly snow as they will
be far enough north and winds in Northern Lake Michigan dont veer
Heaviest precipitation will be through Saturday morning as a shortwave
swings into the state and forcing increases. A general 1 to 3
inches possible north and west of the Fox Valley...with lesser
amounts south and east where rain will mix with snow.
After snow winds down Saturday evening...quiet weather can be
expected into next week. An amplified western ridge will allow for
Arctic high pressure to push down out of Canada...bringing much
colder air with it. Temperatures will begin to drop starting
Sunday...with well below normal temperatures expected by midweek.
Cold pattern holds tight and looks to be in place for the
long haul...with bitterly cold temperatures welcoming the New Year.
Aviation...for 06z taf issuance
issued at 1008 PM CST Thursday Dec 25 2014
Expected low cloud trends have worked out pretty well so
far...though ceilings across the north have lifted some. Expect that to be
temporary...especially as better forcing and snow shifts north. The arrival of
low clouds in central will be delayed on the upcoming 06z tafs.