Scientific Forecaster Discussion
fxus63 kgrb 200417
Area forecast discussion
National Weather Service Green Bay WI
1017 PM CST Tue Nov 19 2019
Updated aviation portion for 06z taf issuance
issued at 704 PM CST Tue Nov 19 2019
Low-level moisture across the area, along with a temperature
inversion aloft, has led to increasing fog formation across
central into north-central Wisconsin. Surface dewpoint depressions
are steadily decreasing across those locations along with very
light winds, which will lead to increasingly dense fog through the
late evening and overnight hours. Hi-res model guidance continues
to indicate visibilities dropping below a half to a quarter mile
at times overnight into the early morning hours. Farther east,
confidence in visibilities dropping below a mile is much lower;
therefore, east-central and far northeast Wisconsin has not been
included in the advisory. Will monitor observation trends in those
locations for potential expansion later tonight. In addition to
the fog, surface temperatures are expected to drop to near or just
below freezing. This may lead to some freezing fog development
and may cause slippery travel on untreated and elevated surfaces.
Short term...tonight and Wednesday
issued at 228 PM CST Tue Nov 19 2019
The latest rap analysis and satellite/radar imagery show weak
shortwave energy moving across the northern Mississippi Valley
early this afternoon. Spotty light returns on area radars are
reaching the ground, primarily over north-central WI, in the form
of light snow. Little to no signs of freezing drizzle/drizzle and
visibilities have slowly improved through the day. Some of the
lowest visibilities reside within a weak surface trough over the
western part of the state. Otherwise, widespread low overcast
continues to prevail across the region and extends across
Minnesota and eastern Dakotas. As temperatures cool tonight,
forecast concerns revolve around fog/freezing fog potential,
followed by cloud trends on Wednesday.
Tonight...the weak surface trough over western Wisconsin is
expected to dissipate later today or this evening, but ample low
level moisture will remain beneath the subsidence inversion.
Forecast soundings show inversion heights falling and with
nocturnal cooling, so should see cloud heights fall as well.
Statistical and probabilistic guidance also supports falling
visibilities over central and north-central WI. Unsure if a dense
fog advisory will be needed, but with temps falling below
freezing over central and north- central WI, freezing fog onto
Road surfaces may lead to slippery or even icy stretches. Will add
a mention to the hazardous weather outlook. Just like the past
several nights, temps will not fall much, and leaned on the
previous nights lows heavily once again.
Wednesday...low stratus and fog will likely persist through much
of the morning as a surface ridge axis moves into central WI.
With daytime heating, increasing anticyclonic flow, and warming
low level temps, it's possible that partial clearing will occur
during the afternoon. On the other hand, ample mid and high clouds
will be streaming into the region ahead of the next system, so
will likely see filtered sunshine at best where low stratus
erodes. Stayed close to the previous forecast for high temps, but
may have to be adjusted pending cloud cover. Most locations will
see highs ranging from the upper 30s to low 40s.
Long term...Wednesday night through Tuesday
issued at 228 PM CST Tue Nov 19 2019
A strong low pressure system is expected to arrive Wednesday
night into Thursday, and the GFS/gefs/ECMWF are in very good
agreement on timing and a track through east central Wisconsin.
Low-level thermal profiles and thicknesses suggest all rain
across the forecast area Wednesday night. The rain/snow line is
expected to gradually shift southeast across north central WI
on Thursday, but the heavier precipitation should be tapering off
as this occurs. Could see 1 to 3 inches of accumulation over
Vilas County by evening, aided by lake-enhancement late in the
day. Farther southeast, a surge of Gulf moisture, with pwats
exceeding 1 inch, will lead to moderate rainfall late Wednesday
night into Thursday morning, with rainfall totals likely reaching
0.75 to 1.0 inch. With frost depths of 4+ inches over the forecast
area, runoff from the rain could cause minor flooding issues.
There is also some concern for minor Lakeshore flooding and
erosion on the Lake Michigan shoreline, as 6-9 foot waves batter
the coast late Wednesday night into Thursday morning. The
locations most susceptible to flooding and erosion would be the
Eastern Shore of Door County, and the Two Rivers area, where wave
action will be most perpendicular to the coast. Winds will become
offshore Thursday afternoon, so impacts will lessen with time.
This event is not expected to be nearly as impactful as the event
on October 21st.
As the low pressure system exits, there will be potential for a
brief period of light lake effect snow showers over north central
WI Thursday night.
The weekend looks quiet, with systems tracking north and south of
the forecast area. Looks like a generally dry weekend, with
temperatures near seasonal normals.
Low pressure developing near the Texas/OK Panhandle early in the work
week will lift toward the region on Tuesday, bringing a chance of
rain or mixed rain/snow. Models show this system strengthening
as it lifts into the western Great Lakes region Tuesday night
into Wednesday. The current track would appear to be too far west
for a significant snowfall in grb cwa, but we are 7-8 days out,
so significant changes are likely to occur. At a minimum, this
system should impact early travel for Thanksgiving day.
Aviation...for 06z taf issuance
issued at 1016 PM CST Tue Nov 19 2019
Widespread low stratus and reduced visibility continues to linger
across much of the area. MVFR to IFR conditions are expected to
linger through much of the taf period with some LIFR visibility
possible across central and north-central WI overnight into
Wednesday. This would mainly impact the County Warning Area/auw/rhi taf sites. In
fact, those locations could see visibilities drop as low as a
quarter mile late tonight. Some freezing fog is also possible;
however, not overly confident in this occurring as temperatures
are near to slightly above freezing. Otherwise, some improvement
will likely occur, especially in visibilities, during the day
Wednesday into Wednesday evening. Low level wind shear may also develop at each of
the taf sites as a temperature inversion strengthens across the
dense fog advisory until 9 am CST Wednesday for wiz005-010-018-