Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
1145 PM CST Wednesday Dec 17 2014
Updated aviation portion for 06z taf issuance
Short term...tonight and Thursday
issued at 330 PM CST Wednesday Dec 17 2014
As snow ends this afternoon...the main forecast concerns will
become cloud cover and temperatures.
Lake effect snow showers and flurries will come to an end over
northern WI this late afternoon and evening as winds off of Lake
Superior become north-northeast (less favorable)...Delta T/S decline...and
deep moisture gets replaced by drier air. Another area of snow
showers associated with a departing upper level trough (and vorticity
max) will end over east central WI late this afternoon. Will
linger some flurries across the area through early evening to
account for any lingering effects of these features...along with
the fact the area remains in cyclonic flow with lingering moisture
in the low/middle levels...and the flurries sometimes take a few
extra hours to come to an end...especially over the north.
Focus then turns to cloud cover tonight and how it they will
impact temperatures. There was an area of partly/mostly sunny skies
extending from central WI into the southern Fox Valley...due to
some subsidence behind a 500mb vorticity maximum and minor downsloping
winds. This area should fill back in...as there are plenty of
clouds upstream...both at low and middle levels...over Upper
Michigan...Lake Superior...and Ontario headed into the area.
Models do advertise some drying working into the area as high
pressure builds into the western Great Lakes later tonight into
Thursday morning. This dry air should allow for some clearing in
the overcast. Partly sunny skies are expected on Thursday...
although if models are over-doing the low level moisture...it
could turn out to be a mostly sunny day as high pressure settles
over the region. If clouds clear out tonight...some temperatures will
drop into the single digits...mainly for locations where new snow
fell last night and/or today. Even with some sunshine on
Thursday...temperatures will struggle to get into the middle 20s over north-
central WI and to 30 degrees over the Fox Valley and Lakeshore.
Long term...Thursday night through Wednesday
issued at 330 PM CST Wednesday Dec 17 2014
Fairly quiet pattern expected early in the period...then it
appears stormy conditions can be expected across the eastern
third of the United States for the first half of next week
for Christmas Holiday travelers.
High pressure will dominate the weather pattern Thursday night
through Sunday night...bringing tranquil conditions to the
region. Did lower min temperatures Thursday night due to clear
to partly cloudy skies and light winds.
Attention then turns to two separate systems next week. First
low pressure system will bring precipitation to the area Monday
into Tuesday. Surface temperatures may be warm enough for rain
across the southeast third of the forecast area.
For Tuesday night into Christmas evening day...negatively tilted 500mb
trough will deepen and surface low rapidly to 961mb over Lake
Huron Wednesday evening. If this scenario plays out...at least
several of snow can be expected across northeast Wisconsin along
with strong west to northwest winds that could cause significant
travel problems for Christmas Holiday travelers. Usually do not
put a day 7 event in the hazardous weather outlook...but
consistency in the models and other offices are mentioning
it...thus warranting adding it. Left it vague due to the large
amount of uncertainties in the details of the system which will
not be resolved for several days.
Aviation...for 06z taf issuance
issued at 1145 PM CST Wednesday Dec 17 2014
Models were showing drier air moving into the area from the north
tonight and Thursday...but ceilings were MVFR and IFR across the
forecast area at 0530z. Visibilities were VFR. Have opted for a more
pessimistic approach to ceilings in the tafs through at least the
first part of Thursday morning. Have mainly kept MVFR
ceilings...except prevailing IFR at rhi early. The north should be the
first part of the area to scatter out but would not expect that
to be until the daylight hours...with the rest of the area
starting to scatter out around midday. Think skies should finally
clear by 00z Friday. Confidence level in ceiling forecast is not very
high with model time/height sections showing high relative humidity values near
the surface throughout the taf valid period.