Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 162115

Area Forecast Discussion
National Weather Service Green Bay WI
315 PM CST Thu Feb 16 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Friday
Issued at 315 PM CST Thu Feb 16 2017

Quiet weather continues across the area tonight into Friday as
high pressure builds into the western Great Lakes. Only forecast
concerns will be how clouds/fog behave under a strengthening
inversion and how temps respond as warmer air arrives aloft.

Radar returns across the area tonight are mainly virga, but a few
flurries are possible in the strongest returns. This activity
looks to be diminish late this afternoon. Trying to get a handle
the development/movement/exit of stratus clouds during this type
of pattern can be a challenge and tonight looks to be one of those
times. MAV/MET/SREF guidance showing best chance for a low
stratus deck being over northern and central WI. This makes sense
since that is where the snow pack resides, a SE upslope wind is in
place, and the inversion will be the strongest, so although it
isn`t a slam dunk, will lean on stratus forming later tonight for
this forecast. Would like to have seen some low stratus last night
to our south/west, but the lack of snowpack and presence of a
higher cloud deck likely kept it from forming. Fog is another
issue tonight, but think dewpoints in the 20s (not 30s) will keep
fog in check, although some 3-5 mile fog could develop if we clear
out for a period overnight into Friday morning, but did not add
it to the forecast at this point. The cloud concerns continue into
Friday morning, with the main question being how long (or if) the
stratus/fog will linger. Think ridging moving into the Great
Lakes and dry air will win out allowing the stratus/fog to mix out
during the morning hours.

Low temps tonight will not fall much as warm air advection really
kicks into gear (850mb temps climbing 6C to 9C from 00z to 12z
Fri), mid clouds over the area through at least the evening hours
and the stratus deck developing over portions of the area. There
is some concern the strong inversion and lingering low clouds
and/or fog could keep many locations from getting as warm as
previously advertised on Friday. Interesting to note the NAM does
not get highs out of the 30s to low 40s across the entire area,
with the GFS on the colder side as well. On the other hand, the
ECMWF has some middle to upper 50s. One example, NAM high for
Green Bay is 36 while the ECMWF is 55! If there was no inversion
in place, 850mb temps would support highs in the 70s! Will lean
toward the lower end of guidance, but confidence is low due to the
questions regarding how clouds behave and how far we can mix into
the warmer air aloft.

.LONG TERM...Friday Night Through Thursday
Issued at 315 PM CST Thu Feb 16 2017

Models continued to show a short wave, in the northern branch of
mid level flow, and a surface low passing north of the state on
Friday night. Any associated precipitation is forecast to remain
north of the forecast area. After that, a mid level ridge is
forecast to amplify as it moves from the Plains to the Great Lakes
over the weekend. This should result in a warming trend, with low
temperatures generally above the normal highs into the middle of
next week. Highs look to be mainly in the upper 40s to middle
50s, which could end up breaking a some record highs.

There was a little better agreement among the 12Z models regarding
a system for early next week. The GFS, ECMWF and Canadian all
bring rain, associated with some mid level short wave energy and a
cold passing across Wisconsin. Best chance for rain looks like
Monday afternoon and night. Rainfall totals could be around an
inch at some locations. After that, there were PoP issues since
the Canadian was a little slower than the ECMWF and GFS, and the
ECMWF developed another area of QPF over Wisconsin Tuesday into
Tuesday night.

There is another chance for precipitation Wednesday/Wednesday
night but temperatures will be a bit colder, so precipitation type
Wednesday night is a bit uncertain.

.AVIATION...for 18Z TAF Issuance
Issued at 1155 AM CST Thu Feb 16 2017

VFR conditions will continue into the early evening hours as mid
clouds continue to push across the area. Then, some uncertainty
enters the forecast tonight as a strengthening inversion limits
mixing, snow on the ground (mainly over central and northern WI),
and a weak SE upslope wind coming off of Lake Michigan should
lead to the development of a low stratus cloud deck (low IFR or
LIFR). Negatives will be the dry weather over the past several
days and dewpoints only in the lower 20s. The MET shows the
stratus forming for AUW/CWA/RHI and the MAV now shows some in RHI.
So confidence is increasing there will be stratus tonight. In
these cases, it is usually all or nothing with the stratus. So it
would either be overcast or clear (with possibly some fog). That
will likely be the case tonight, although there will be some high
clouds still over the region, especially over eastern WI.
Confidence is high enough at this point to add a broken deck of
low clouds to AUW/CWA and RHI late tonight into Friday morning,
but will keep them out of GRB/ATW/MTW. Moisture and pollutants
trapped under the inversion will also result in some MVFR
(possibly IFR) fog development overnight into early Friday, with
the lowest vsbys over central and north central WI. The low clouds
and fog look to mix out during the morning hours; however, model
soundings show this might be tough to do over north central WI.

The threat for LLWS looks minimal as 2000 ft winds remain at or
below 20 knots through early Friday afternoon.



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