638
FXUS63 KGRB 231139
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
639 AM CDT Tue May 23 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 401 AM CDT Tue May 23 2017

An upper low will drop south into Iowa today and then to Missouri
by early Wednesday. It should produce good upper diffluence today
and early tonight but then will lose it`s influence as it moves
further away late tonight and tomorrow. Closer to the ground, an
850 mb low moves from around Dubuque this morning to near Chicago
by late tonight. It will produce low level warm advection and
bring up moisture across the forecast area today and early
tonight.

There could be enough forcing for non severe elevated thunderstorms
today across the southern two thirds of the area. The showers
will diminish from west to east tonight, with just a chance of
showers across the east Wednesday.

Highs today will be 5 to 10 degrees below normal due to clouds and
occasional rain. Lows tonight will be a few degrees above normal
due to cloud cover. Highs Wednesday will be held down by a cool
northeast wind near the lake and bay.

.LONG TERM...Wednesday Night Through Monday
Issued at 401 AM CDT Tue May 23 2017

Initially, the mean flow will consist of an eastern Pacific upper
ridge, two distinct nearly closed upper lows over southern
Saskatchewan/TN Valley and another upper ridge over the western
Atlantic. By the end of the weekend, both upper ridges will have
shifted east (into the western CONUS/farther into the Atlantic),
while the Saskatchewan upper low eventually reaches southern
Ontario. It is this upper low that will be our main weather
feature that should not only bring warmer conditions to northeast
WI, but also bring precipitation chances headed into the upcoming
holiday weekend.

It is going to be a close call across eastern WI for precipitation
chances Wednesday night as the entire system slowly tracks east
over the eastern third of the CONUS. A surface low is forecast to
move northeast into southern/central sections of the Great Lakes
and it is possible for a little moisture to wrap-around/get thrown
back toward eastern WI. May need to add a small pop for this
potential. Otherwise, a very weak ridge of high pressure will
approach WI and begin to clear out the clouds over northern and
central WI. Min temperatures to be in the upper 30s to lower 40s
north, middle to upper 40s east-central WI. This weak ridge pushes
into WI on Thursday and should provide for at least partly sunny
skies and warmer temperatures. The GFS hints at some WAA
precipitation, but lift appears too weak to kick off any rain,
thus prefer keeping the forecast dry. Look for max temperatures to
range fro the lower 60s near Lake MI, to the lower 70s over parts
of central WI.

By Thursday night, the Saskatchewan upper low (and its associated
surface low) are forecast to move into Manitoba with a cold front
extended southeast into northern MN, then southwest all the way to
the southern Plains. Expect to see clouds gradually increase over
the forecast area with a small chance pop reaching into central WI
toward daybreak. Min temperatures should settle into the lower to
middle 40s north, middle to upper 40s south. This cold front does
push east into WI on Friday, but the tendency in the models is to
have this front weaken with time as it encounters a weak upper
ridge over the Great Lakes. Prefer to carry a chance of showers/
slight chance of thunderstorms during the day. Even with some
clouds and the precipitation threat, temperatures are expected to
warm close to normal levels for late May as readings reach the
middle 60s lakeside, around 70 degrees north-central and lower to
middle 70s elsewhere.

A weak bubble of high pressure will briefly interrupt the
precipitation chances Friday night, but the approach of another
cold front could set off showers/thunderstorms, especially
Saturday afternoon with the help of daytime heating. Models also
begin to diverge with respect to the movement of the upper low/
shortwave trough with the ECMWF faster/moving southeast versus the
slower/dropping south solution of the GFS. These differences
become more magnified through the rest of the holiday weekend. Max
temperatures on Saturday will be similar to Friday (close to
seasonal normals).

A separate issue arises Saturday night as the GFS develops a
surface wave on the cold front and sends a slug of moisture into
WI, including locally heavy rain. Meanwhile, the CMC keeps this
surface wave well to our south and the ECMWF doesn`t even have
this surface wave. Such a variety of possible solutions makes for
a low confidence forecast for the end of the forecast period. For
the time being, will continue to have a small chance pop over
central/east-central WI Saturday night and then continue with the
chance pop forecast into Sunday as the shortwave trough swings
through the region. Cool air aloft with the trough overhead may
provide enough instability to carry thunder in the forecast on
Sunday. Temperatures will cool on Sunday with readings in the
middle 60s north-central WI/lakeshore, upper 60s to lower 70s
elsewhere.

Depending on the location of the mid-level shortwave trough,
Memorial Day`s weather could range from partly cloudy with near
normal temperatures, to cloudy/rainy/below normal temperatures.
Hard to see this trough simply racing through the region with
upper ridging in the western Atlantic, thus will need to linger
precipitation chances over northeast WI through Memorial Day with
temperatures a few degrees below normal.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 639 AM CDT Tue May 23 2017

An upper level low moving south across Minnesota will combine
with a weak surface low moving across southern Wisconsin to
produce some showers today along with ceilings lowering to IFR by
afternoon. The showers should diminish from west to east tonight,
with IFR gradually becoming MVFR towards daybreak. There is still
a chance of showers Wednesday, with MVFR conditions in most
places.

&&

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

$$

SHORT TERM.....RDM
LONG TERM......Kallas
AVIATION.......RDM