Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 170418

Area Forecast Discussion
National Weather Service Green Bay WI
1018 PM CST Thu Feb 16 2017

Updated aviation portion for 06Z TAF 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 06Z TAF Issuance
Issued at 1005 PM CST Thu Feb 16 2017

The main aviation concern for this TAF period is the potential
for stratus development, especially late tonight into Friday
morning. The greatest potential appears to be over north central
and far northeast WI, where models all show the best low-level
saturation occurring. Positive factors include southeast
upsloping winds, lingering deep snow cover, and dew points in
the lower to middle 20s advecting toward the region from the
southeast. The main negative factor is a lack of melting snow on
Thursday due to sub-freezing temperatures. Have decided to stay
the course and mention a prevailing IFR deck at the RHI TAF site
overnight into Friday morning. One new area of concern is the
Lake Michigan shoreline, as patches of MVFR stratus have been
moving onshore in the lakeshore counties, especially over northern
Door county. May need to consider a TEMPO group for MVFR ceilings
at the MTW TAF site. The rest of the region should have mainly
VFR conditions, though some patchy light fog is possible overnight
into early Friday.

The threat of stratus should diminish as low-level winds become
southwest to west Friday afternoon and evening.



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