Forecast Discussion for GRB NWS Office
FXUS63 KGRB 201224 AAA

Area Forecast Discussion...UPDATED
National Weather Service Green Bay WI
624 AM CST Wed Feb 20 2019

New Information added to update section

Issued at 618 AM CST Wed Feb 20 2019

Higher radar returns have surged into east-central Wisconsin. There
is still an issue with dry air to overcome, as suggested by the
fact the visibility on surface observations is undergoing
substantial rapid fluctuations. But this effect should lessen with
time as saturation occurs. Will update the advisory to officially
begin immediately in east-central Wisconsin, and keep the
original 15Z start in the north. In a practical sense, this change
will be of limited meaning because our long-fused winter
headlines will always have "in effect" wording once we are within
3 hours of the official start time of the hazard.

Updated product suite will be out ASAP.

Issued at 347 AM CST Wed Feb 20 2019

An active weather pattern with a round of snow today; the
probability of a major winter storm this weekend is increasing.

The large scale pattern will continue to be dominated by high
upper heights over the northeast Pacific. The downstream flow will
be split, with the bulk of the energy composing a strong,
amplified southern stream that will encompass most of the CONUS.
The southern stream trough position will remain over the Desert
Southwest, with ridging over the southeast states.

This is a very active weather pattern for the forecast area as
energy ejecting out of the southwest U.S. trough will interact
with a strong baroclinic zone across the area and northward surges
of Gulf moisture to produce significant precipitation, resulting
in well above normal amounts for the period. The main
precipitation episodes will occur as an occluded cyclone crosses
the area today, and with a rapidly deepening cyclone expected to
cross the region during the weekend. The southwest upper flow will
result in AOA normal temperatures.

.SHORT TERM...Today...Tonight...and Thursday
Issued at 347 AM CST Wed Feb 20 2019

No changes to forecast headlines this morning. The previous
forecast still looked pretty much on track, with just some
adjustments made to keep it in line with the latest observational
trends (in terms of the onset of the snow) and expected snowfall
totals. The new totals are about an inch or so higher across about
the northwest 3/4 of the forecast area. That takes them right up
to the warning criteria threshold (6 inches/12 hours). Fully
expect some areas to reach that, but don`t anticipate them
exceeding it by a lot. In addition, this is not going to be a very
windy storm, and with just a modest increase in snow from the
previous forecast, the best course of actions seemed to be to just
stick with the advisory. The evolution of the comma head on
satellite imagery continues to suggest the heaviest snows will
fall NW and W of the area.

Low-level dry air lingering across the area from the departing
anticyclone was slowing the northward progress of the
precipitation early this morning. Local meso plots suggest the 850
mb RH over east-central Wisconsin was still around 10% at
forecast issuance time. The dry air is likely to slow the
northward progress for a few more hours, especially over the
northeast part of the area. The arrival of stronger lift and
veering low-level flow should allow for a rapid northward
expansion of the precipitation in the 12Z-16Z time frame. Once
saturation occurs, snow should continue for the rest of the day.
The possible exception is east-central Wisconsin where hi-res
models suggest a break may occur late morning before
precipitation fills back in for the afternoon. Thermal profiles
support snow across the entire area--except where saturation may
not be deep/cold enough to result in ice crystal formation.
That`s most likely to occur in east-central Wisconsin, though a
consensus of model forecast soundings was that conditions favoring
a loss of ice would only be marginal and periodic. Thus snow
should be the primary precipitation type.

The precipitation should taper off from south to north this
evening as the main shortwave lifts through. Light precipitation
could linger in the far north into early Thursday as a surface
trough hanging back from the departing system keeps low-level
cyclonic flow in that area, but do not anticipate much additional
accumulation with that feature.

.LONG TERM...Thursday Night Through Tuesday
Issued at 347 AM CST Wed Feb 20 2019

Active pattern across the northern United States as the 500mb
pattern will feature southwest flow aloft at the beginning of
the period, then transitions to a zonal flow early next week.
Temperatures at the beginning of the period will be at or
slightly above normal into the weekend, then should be at or
below normal next week.

The main storm system this weekend should come in two pieces.
The first piece will be in the form of light snow or light
freezing rain based on bufkit soundings as models show fairly
strong 850mb warm advection across the state. The precipitation
should start out as light snow across the entire region. Bufkit
soundings indicated that the snow could change over to freezing
rain across central into east-central Wisconsin where there is a
more pronounced warm layer.

There will be a lull or break in the precipitation later
Saturday morning into Saturday afternoon as the 850mb warm
advection shifts east of the state and system approaches from
the southwest. Models impressive with precipitation totals over
an inch across much of the region. The low pressure system is
expected to move from eastern Kansas (18z Sat) to near Chicago
(06z Sun) and then into Canada during the day Sunday. This track
is very favorable for heavy snow across northern Wisconsin. As the
low tracks northeast, it will be intensifying. It appears there
will be a band of 8 to 12 inches across central into far northeast
Wisconsin. Snowfall amounts will be problematic across the Fox
Valley and lakeshore region where precipitation starts as rain or
a wintry mix and then changes over to all snow later Saturday
evening into early Sunday morning. The change over should occur
last at Manitowoc. How quick the change over occurs will determine
snowfall amounts in this region. There are still some timing
differences in the change over, although the heavy precipitation
may change it over quicker than expected. The snow will be heavy
at times, with snowfall rates of 1 to 2 inches per hour are
possible across the entire region. Snow will linger into Sunday
morning, although may need to increase snow chances Sunday
afternoon over the next day or two. Gusty northwest winds are
also expected late Saturday night into Sunday, with gusts to near
40 mph possible across northeast and east-central Wisconsin.

Tranquil conditions are expected Monday and Tuesday as high
pressure controls the weather pattern. Next system approaches
from the west Tuesday night, bringing a chance of light snow.
Snow is expected on Wednesday as this system could another 3 to
6 inches to much of the area.

.AVIATION...for 12Z TAF Issuance
Issued at 347 AM CST Wed Feb 20 2019

Flying conditions will rapidly deteriorate to IFR and periodic
LIFR as snow overspreads the area this morning and continues into
the evening. Some improvement in conditions is expected overnight,
mainly as some breaks develop in the low clouds.

Winter Weather Advisory until 9 PM CST this evening for WIZ020-

Winter Weather Advisory until 9 PM CST this evening for WIZ005-


SHORT TERM.....Skowronski
LONG TERM......Eckberg

NWS GRB Office Area Forecast Discussion