Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
500 PM CST Thursday Jan 29 2015
Updated aviation portion for 00z taf issuance
Short term...tonight and Friday
issued at 223 PM CST Thursday Jan 29 2015
Main forecast issues center around how long these low clouds will hang
around tonight...impact on min temperatures and Minor Lake effect event
across north-central WI.
The 20z msas surface analysis showed a broad area of low pressure over the
eastern Great Lakes and the Quebec/Ontario border and a broad area of
hi pressure that stretched from Lake Winnipeg southward to the Southern Plains.
In between these systems...a tight pressure gradient existed across
the upper Midwest with gusts occasionally surpassing 30 miles per hour. Some
snow showers or flurries were found across Upper Michigan and NE Minnesota...but
the precipitation had pretty much ended for NE WI. Vsbl satellite imagery did
indicate some breaks or thin spots in the overcast to our north
The snow shower threat for north-central WI will continue through the evening hours
as favorable trajs...hi Delta-T values and weakening cyclonic flow
to persist. That being said...snow accumulations look to be minor
due to lowering inversion heights to 3k feet and lowering dendritic
growth zone heights. The northwest half of Vilas County could get around an
inch of snow with lesser amounts farther south and east. An area
of hi pressure is forecast to move into the upper MS valley overnight and send
enough dry air into WI to essentially shut-off the lake effect.
Trying to time when to break up the low clouds remains the biggest
forecast problem...especially since it is possible that the clouds may
hold tough per model time sections. Since vsbl satellite is picking up
some breaks in the clouds already...have remained optimistic that
we will be able to go at least partly cloudy later tonight. Of course...
the amount of clearing will play a large role in how far temperatures drop
tonight. The gusty northwest winds will diminish overnight as the hi pressure
approaches...thus at least partial clearing would allow temperatures to
plunge. For now...have min temperatures around zero north...5 to 10 above
zero south with the warmer values near Lake Michigan.
This area of hi pressure pushes across WI Friday morning and be just to
our east Friday afternoon. Winds are expected to slowly back to the
SW during the day with weak warm air advection developing across the region. Look
for middle and hi clouds to steadily overspread NE WI on Friday...thus
mostly cloudy skies expected by late in the day. Despite the warm air advection...
temperatures will be much cooler than today with readings only in the 15 to
20 degree range.
Long term...Friday night through Thursday
issued at 223 PM CST Thursday Jan 29 2015
No major storms in the long term as upper level flow remains west-northwest
across the western Great Lakes...keeping the Gulf of Mexico closed
for business. Main forecast issues will be with timing the best
chances for light snow (both synoptic and lake effect) and cold
Clipper system still on track to push across the northern Great
Lakes Friday night into Saturday. The majority of the snow with
this system looks to stay north of the state...closer to the
parent upper trough and shortwave. However...models do advertise a
weaker shortwave on the southern edge of the trough crossing Lake
Superior Friday night. This should help produce some light snow or
flurries...mainly across northern WI. Lake effect snow will be
possible behind this system as 925-850mb winds waver from northwest-NE
Saturday and Sunday. With no persistent favorable fetch and
inversion heights under 3000-4000ft...do not expect a significant
event for north-central WI. Current forecast has these scenarios
handled pretty well...so no major changes planned.
Models have come into better agreement with the weekend storm
system...and keep the majority of the accumulating snow south of
the County Warning Area...as high pressure pushing south of Canada looks to win
the battle and pushes the baroclinc zone into the Ohio Valley.
This will keep the best frontogenetic forcing...synoptic
lift...and moisture south of the County Warning Area. Even the Gem now holds the
precipitation from southern WI southward. Have removed probability of precipitation from the
grids and went dry for Saturday night and Sunday. The exception
will be over far north-central WI and east-central WI...where lake
effect flurries and/or snow showers will be possible. Wind
trajectories on Lake Michigan point to higher chances for lake
effect activity from Kewaunee south...especially on Sunday when
winds become a little more out of the NE.
High pressure settles over the western Great Lakes on Monday with
dry but cold conditions expected. The high quickly shifts east of
the area Monday night as a weak shortwave pushing into WI. Some
light snow is possible with this feature. Another...possibly more
potent...shortwave and upper trough is forecast to push across the
area on Tuesday. This system will need to be watched as it could
bring accumulating snow to the entire area...but model not in the
best agreement to increase probability of precipitation at this point.
Temperatures will approach normal on Saturday (20s)...then much colder
temperatures are expected for most of next week as surges of cold/Arctic
air spreads across the Great Lakes behind each weather system.
Coldest periods look to be Sunday into Monday...and Tuesday
night into Thursday.
Aviation...for 00z taf issuance
issued at 457 PM CST Thursday Jan 29 2015
Main aviation forecast issue is timing of the dissipation of the
low clouds. Flow across Lake Superior will probably cause clouds to
linger a little longer than forecast by the models. But the arrival of
drier air and low-level subsidence should eventually allow clouds to
break. Expect a hole to open up over northestern WI and then expand...and
clouds to our west to begin to decr. North-c will probably be the last area