Forecast Discussion for GRB NWS Office
FXUS63 KGRB 191202

Area Forecast Discussion
National Weather Service Green Bay WI
602 AM CST Wed Dec 19 2018

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 240 AM CST Wed Dec 19 2018

Main concerns in the short-term are the chances for
drizzle/sprinkles today through Thursday along with freezing
drizzle/light snow chances late tonight into Thursday morning over
the north-central portion of WI. Temperatures are expected to remain
well above normal through this time period.

Satellite and surface observations continue to show mid to high
clouds building across much of the Western Great Lakes region with
temperatures in the upper 20s to mid 30s. Radar is showing some weak
returns over western WI and northern IA, but no precipitation has be
observed by the automated stations in those locations.

Today: The aforementioned weak returns are expected to continue to
lift to the northeast throughout the day as a weak shortwave slides
through the area. Forcing is not overly impressive and the dry layer
aloft is progged to hold steady throughout much of the day. This
would keep the precipitation very light, likely in the form of
drizzle or very light sprinkles as the best moisture is expected to
remain shallow, with the exception of a few hours by late
morning/early afternoon. With that in mind, decided to go with light
rain showers today, but the overall impact should be limited with
the precipitation totals expected to be a hundredth of an inch or
less. It is not out of the question that some of the precipitation
may start as a very light wintry mix over central WI, but temps
should rise quick enough to limit any major impacts from this.
Again, confidence is not particularly high with any precipitation
falling before temperatures rise above freezing. Highs today are
expected to be in the mid to upper 30s to around 40.

Tonight into Thursday: A shortwave trough and associated cold front
are expected to approach the area from the west tonight and then
pass through the Western Great Lakes region during the day Thursday.
Along and ahead of the front there will be enough moisture in place
to allow for some very light drizzle to occur. Again, overall
moisture looks to be fairly shallow, so precipitation will again
remain light. With most of the moisture confined to the lower levels
of the atmosphere, the most likely precipitation type would remain
drizzle/sprinkles tonight through Thursday. The exception will be
right along the front, where moisture may be deep enough to allow
for some light rain showers to form. Again, not much impact expected
from the rain showers either way. The bigger impact may be from
colder temperatures working in from the northwest late tonight into
Thursday morning. The combination of colder surface temperatures and
aloft may allow freezing drizzle or light snow to form over north-
central Wisconsin where some lake effect/enhancement may occur.
This could make the early Thursday morning commute a bit slippery
at times, before temperatures warm above freezing by late morning.
Only expecting around 0.01 to 0.02 inches of QPF, so overall
impacts are not expected to be to high, although it only takes a
small amount of freezing drizzle to make travel slippery.
Overnight lows are expected to be in the low to mid 30s with highs
expected to be in the 30s.

.LONG TERM...Thursday Night Through Tuesday
Issued at 240 AM CST Wed Dec 19 2018

An amplified, progressive mean flow at the start of the period
quickly turns near zonal this weekend as a western CONUS upper
ridge moves eastward and flattens, while an eastern CONUS upper
trough moves into the Atlantic. A northern stream modest shortwave
trough sweeps across the Great Lakes this weekend and may kick out
some light snow showers. A series of Pacific systems are forecast
to push across the CONUS next week, but the models disagree on the
strength/timing of each system. This places serious uncertainty as
to what day to hit precipitation chances. It is conceivable that
we may not see any substantial precipitation until the middle of
next week when a more potent system takes aim at WI. Temperatures
are expected to be above normal into next week, although readings
will be closer to normal Sunday and Monday.

While synoptic precipitation will be over with by Thursday
evening, modest CAA and northwest winds may be enough to allow
lake effect snow showers to develop over north-central WI Thursday
night. The problem for any snow potential is that there is a dry
layer aloft, thus any precipitation could fall more as freezing
drizzle than snow. This has the look of only a minor event as 8H
temperatures are not that cold (-8 to -10C over western Lake
Superior). Otherwise, expect a good amount of clouds to hang
around the region with min temperatures in the lower 20s north-
central WI, to the upper 20s near Lake MI. Any light snow or
freezing drizzle over north-central WI will come to an end Friday
morning as a ridge of high pressure approaches and winds veer
north-northeast (unfavorable trajectories). The rest of northeast
WI should see sunshine return as drier air advects eastward.
Despite the sunshine, temperatures will be cooler than Thursday
with readings ranging from the upper 20s north-central, to the
lower 30s eastern WI.

The surface ridge and the remnants of the western CONUS upper
ridge reach the western Great Lakes region Friday night. While dry
conditions will persist, there will be a gradual increase in
clouds as a northern stream shortwave trough/associated cold front
move into the Upper MS Valley. Min temperatures to dip into the
mid teens north-central, lower 20s eastern WI. The cold front
outruns the shortwave trough into WI on Saturday with only a small
chance of light snow reaching central WI, mainly during the
afternoon. Due to a lack of moisture and stronger mid-level
forcing hanging back with the shortwave trough (still over the
Upper MS Valley), do not anticipate much of a problem for
travelers. The rest of northeast WI to be mostly cloudy with max
temperatures in the middle to upper 20s north, upper 20s to lower
30s south.

The cold front exits the region Saturday evening, while the
shortwave trough shifts east and moves into northern sections of
the Great Lakes Saturday night. Precipitation chances appear
pretty minimal now and focused more on northern WI closer to the
trough where forcing is marginally stronger. Sunday may end up
simply mostly cloudy with perhaps a minimal chance of seeing light
snow somewhere over the forecast area as the shortwave trough
tracks east across Lake Superior. Max temperatures on Sunday to
range from the middle 20s north-central, to around 30 degrees
east-central WI. Quiet conditions return Sunday night/Monday as
another surface ridge axis stretches across the western Great
Lakes. Look for a mix of sun and clouds on Monday with max
temperatures in the lower to middle 20s north-central, upper 20s
to around 30 degrees eastern WI.

The forecast becomes more iffy headed into Tuesday as the models
try to bring chance pops back into northeast WI, but for
completely different reasons. The GFS lifts a warm front into the
region, while the ECMWF brings a weak shortwave trough into the
Great Lakes. The ECMWF has also backed off on its QPF, essentially
keeping northeast WI dry on Tuesday which is a change from its
previous 12Z model run. Have followed the consensus solution which
keeps a low-end pop in the forecast for later Monday night into
Tuesday. Temperatures appear cold enough (middle 20s to lower 30s)
to keep precipitation type as all snow.

We will also have to watch for a potential of a bigger system to
impact the region toward the middle of next week. Plenty of
timing, strength and storm track issues will need to be settled
before getting too excited.

.AVIATION...for 12Z TAF Issuance
Issued at 602 AM CST Wed Dec 19 2018

VFR conditions early this morning are expected to give way to
MVFR/IFR ceilings by late morning or early afternoon. Western TAF
sites will see lower ceilings first then slowly shifting to the
eastern TAF sites during the afternoon hours. Intermittent light
rain or drizzle will be possibly through much of this TAF period,
with the best chances occurring late tonight into Thursday
morning. Some snow may also mix in with the drizzle at each of the
western TAF sites late tonight. Only limited visibility
restrictions are expected.



SHORT TERM.....Cooley
LONG TERM......Kallas

NWS GRB Office Area Forecast Discussion