Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 181046
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
546 AM CDT MON JUL 18 2016

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 313 AM CDT Mon Jul 18 2016

High pressure will build in across the western Great Lakes today
behind a departing cold front. Although temperatures will only be
slightly cooler, the main difference will be drier air as dew
points fall a good 5 to 10 degrees from yesterday. Overall a
rather pleasant day should ensue as highs range from the middle to
upper 70s across the north...with low 80s across central and east
central Wisconsin away from the lake. Mostly sunny skies should
rule the day with only scattered diurnal cumulus clouds in the sky
associated with surface heating.

The high will slowly drift east starting tonight, with mostly
clear skies and light winds bringing the coolest temperatures of
the week to northeast Wisconsin as lows dip to around 50 across
the north, with lows in the upper 50s to around 60 across the
south.

Return flow on the backside of the highs will begin a warmup on
Tuesday that will carry through to the rest of the week. Highs on
Tuesday will increase to around 80 across the north, with lower to
middle 80s across central and east central Wisconsin away from the
lake.

Despite some light qpf produced by the models at times late
tonight and into Tuesday, inspection of the model soundings
indicate the atmosphere is quite dry through much of the column.
In addition there isn`t much in the way of forcing to produce
shower activity during this time period save for some shortwaves
that are outside of the cwa. Therefore will maintain a dry
forecast for the short term portion of the forecast.

.LONG TERM...Tuesday Night Through Sunday
Issued at 313 AM CDT Mon Jul 18 2016

Models continue to show an expansive area of high pressure aloft
over the central CONUS through Thursday, before the ridge gets
flattened by a series of mid-level shortwave troughs. The main
forecast problems remain from Friday through next weekend with
timing not only with these troughs, but also frontal boundaries
trying to push south. Day-to-day model inconsistencies also do not
help the confidence factor. Northeast WI will get hot and humid
from Wednesday through Friday before a slight cool down/slightly
less humid conditions return for the weekend. Will need to watch
heat indices Thursday/Friday as some locations could see readings
push 100 degrees which is the threshold for heat advisories.

Increasing WAA, coupled with a 30-40 knot low-level jet
transporting gulf moisture northward, a surface warm front lifting
north toward WI and a mid-level shortwave trough moving into WI,
all add up to bringing the threat for showers and thunderstorms
back into northeast WI Tuesday night. Most of this activity should
hold off until after midnight when the stronger lift/forcing
arrive. Early indications are that any storms would be sub-severe,
however locally heavy rains are possible with PW values greater
than 1.75 inches. Increase in clouds will help to hold
temperatures up Tuesday night with readings in the lower to middle
60s north, middle to upper 60s south.

The warm front is expected to lift north into the Western Great
Lakes region on Wednesday with the aid of the expanding upper-
level high pressure area over the central CONUS. Expect at least a
chance of showers/thunderstorms to continue into Wednesday
morning until the low-level jet weakens and the shortwave trough
exits the area. Increasing heat in the mid-levels of the
atmosphere will build a cap over WI Wednesday afternoon and then
to suppress further convection from developing. Max temperatures
on Wednesday could be tricky depending on how fast (if) the
convective clouds can dissipate. For now, have gone with lower 80s
near Lake MI, middle 80s north-central WI and middle to upper 80s
elsewhere. If we can get enough sunshine, parts of central WI
could hit the 90 degree mark.

Very warm and moist air will continue to spread across WI
Wednesday night with 8H temperatures climbing into the lower 20s
(C) and PW values pushing 2 inches across central and east-central
WI. The persistence of the WAA regime into WI could allow for
additional showers/thunderstorms to move across parts of WI
through the night, but trying to determine exactly where remains a
challenge. Any ripple topping the upper ridge or subsidence behind
a convective complex could determine precipitation or no
precipitation Wednesday night. Too uncertain at this point to drop
the pops, thus have kept chance pops in the forecast. Min
temperatures to be in the upper 60s to lower 70s.

The heat wave should hit northeast WI on Thursday as the upper
ridge is forecast to build into the Western Great Lakes. Models
continue to try and generate some precipitation, however with 7H
temperatures around 14C, it appears that any convection would have
difficulty breaking this stout cap. May keep some kind of slight
chc pop in the forecast, especially in the east where perhaps the
lake breeze could initiate something. Otherwise, the main focus to
be on the hot temperatures and potentially dangerous heat indices.
Max temperatures away from the lake and bay should be able to
reach the lower 90s, with a few middle 90s possible over central
WI (assuming we get a good amount of sunshine). Heat indices to
flirt with 100 degrees, especially over central and east-central
WI. Will continue to mention this in the HWO.

Forecast uncertainty grows toward the weekend as the models are
really struggling with the strength of the cap, movement of a cold
front and location of precipitation. Friday appears to be another
hot/humid day similar to Thursday with the cold front still to our
northwest. The issues become apparent by Friday night as the ECMWF
drops the cold front into WI, then stalls it with a MCS spreading
into the forecast area late Friday night into Saturday morning.
Meanwhile, the GFS and GEM do not bring the cold front into
northeast WI until roughly Saturday afternoon and keep the front
progressive. Since the models are at odds here and either solution
is plausible, prefer to go with persistence here and keep a chance
pop in the forecast for Friday night and Saturday. Since the upper
ridge is progged to weaken and get shoved south on the weekend,
the cold front should be able to push far enough south to go with
a mainly dry Saturday night/Sunday with cooler/less humid
conditions.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 546 AM CDT Mon Jul 18 2016

A weak surface high pressure will bring light westerly winds and
clear skies during the taf period with VFR conditions expected.
&&

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

$$

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



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