Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 122313
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
513 PM CST Mon Feb 12 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Tuesday
Issued at 218 PM CST Mon Feb 12 2018

Surface high pressure passed over the region today, keeping
things quiet and sunny. Return flow begins this evening, bringing
moisture to the upper levels and additional upper to middle level
cloud cover.

The challenge in the short term remains the same as the past
couple days, with the degree and depth of cloud cover largely
dictating overnight low temperatures. Cloud cover is expected to
thicken overnight, which should help mitigate a large drop in
temperatures in the early morning tomorrow. Thus started with the
ECMWF and MOS guidance, but then trended a few degrees warmer,
given the colder bias the past couple of nights.

Winds will pick up Tuesday morning, as the pressure gradient
tightens and high pressure exits the region. These will become
stronger over the early parts of Tuesday afternoon as diurnal
heating allows some better mixing to the stronger winds aloft.
Gusts up to 20 to 25 miles per hour will be likely over the land.
The stronger return flow will also bring a rise to our daytime
high temperatures, and with fairly consistent signals in the
models, stayed with the daytime highs ranging from the upper 20s
to lower 30s.

.LONG TERM...Tuesday Night Through Monday
Issued at 218 PM CST Mon Feb 12 2018

The mean flow through the end of the work week to consist of an
upper ridge over the eastern Pacific, an upper-level high pressure
over the Gulf of Mexico through FL and a positive-tilted longwave
trough from Hudson Bay to the southwest CONUS. Precipitation
chances for northeast WI appear limited during this time with
only small chances Wednesday and Thursday. By late week, this
upper trough swings eastward through the Great Lakes with the mean
flow becoming more westerly. A new upper trough then digs into the
western CONUS early next week and turns the flow into WI from the
southwest. This will typically lead to a more active pattern for
the Great Lakes. Roller coast ride still looks on track for
temperatures with mid-week warming, late week cooling, then up
again this weekend.

Quiet and not as cold conditions are expected across northeast WI
Tuesday night due to a southwest surface wind and westerly flow
aloft. The only concern would be the possibility of low clouds to
develop over parts of central/north-central WI as warmer air aloft
overrides the cooler snow-covered ground. With this in mind,
prefer to keep the partly cloudy skies after midnight for this
part of the forecast area with mostly clear skies holding in the
east. Min temperatures to range from the middle to upper teens
north-central WI, upper teens to lower 20s elsewhere. A weak
little shortwave trough pushes across WI Wednesday afternoon,
however forcing is too weak to kick off any precipitation.
Assuming we can keep clouds away, Wednesday will be a mild day as
max temperatures reach the upper 30s to lower 40s. If there is
enough sunshine, parts of central WI could hit the middle 40s.

A cold front moves southeast through the area Wednesday night,
however gulf moisture is locked over the OH/TN Valleys, thus only
a wind shift to the west is anticipated with no precipitation.
There will be more clouds across the north due to a secondary
surface trough that is forecast to drop south into northern WI by
daybreak. Min temperatures to range from the lower to middle 20s
north-central, to the upper 20s across east-central WI. This
surface trough sags through the rest of the forecast area Thursday
morning, but it is the afternoon hours that remain in question as
the models differ on which system could impact WI. The GFS focuses
precipitation across roughly the northern half of the state due to
a shortwave trough approaching from the west. The ECMWF focuses
precipitation more over the southern third of the state due to a
surface low riding east-northeast along the old cold front. Since
either solution is plausible, will mention small chance pops
across all of northeast WI. If the ECMWF is correct, we may see a
mix of precipitation types across central/east-central WI. Max
temperatures to range from the upper 20s far northern WI, to the
middle 30s over central/eastern WI.

The chance for light snow will continue into Thursday evening for
most of the region until the shortwave trough exits to the east.
Strong CAA takes over for the rest of Thursday night and with
northwest winds expected, will need to carry a chance of snow
showers into north-central WI. By Friday, an area of high pressure
is forecast to extend from the central Plains toward southern
sections of the Great Lakes. Winds will back to the west, thereby
ending the threat for any more lake effect. Even with a good
amount of sunshine, Friday will be a cold day with readings from
10 to 15 degrees north-central, near 20 degrees eastern WI.

Another modest shortwave trough is progged to swing through
northern sections of the Great Lakes late Friday night into
Saturday morning. There will also be the onset of WAA as winds
become southwest. Cannot completely rule out a few snow showers
and/or flurries during this time, but any precipitation would be
very light and not a big concern. By Saturday afternoon, the
shortwave trough is gone and we are left with southwest winds, a
mix of sun and clouds and warmer temperatures. Look for readings
to reach the upper 20s to lower 30s.

Models show a clipper system moving into the northern Plains
Saturday night with clouds starting to increase across the Upper
MS Valley into the western Great Lakes. The problem for Sunday is
that the GFS maintains a pretty strong surface low into southwest
Ontario, while the ECMWF weakens the surface low. There is also
the timing of the associated cold front into WI Sunday afternoon.
May need to bring a chance of a rain/snow mix into north-central/
central WI for starters and see how the models adjust as we get
closer to next weekend. A milder start to Sunday should allow max
temperatures to get into the middle 30s north-central, upper 30s
to around 40 degrees east-central WI.

This cold front is forecast to push southeast through northeast WI
Sunday night with a small chance of light snow. The questions for
Monday are how far south will the cold front get and where will a
surface wave develop along this front. The GFS takes this front
all the way to the Ohio Valley by 00z Tuesday, while the ECMWF
never gets this front through WI AND brings a surface wave along
the front. Therefore, we can have a potential of no snow at all or
have an accumulating snow somewhere within WI. Have simply carried
chance pops for now and wait for the models to settle on a
solution.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 454 PM CST Mon Feb 12 2018

VFR conditions will prevail through the TAF period. High clouds
over the region this evening will transition to mid clouds
overnight. Some wind shear will be possible late Tuesday morning,
affecting primarily RHI, AUW, and CWA.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....Uhlmann
LONG TERM......Kallas
AVIATION.......Kurimski



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