Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 191708

Area Forecast Discussion
National Weather Service Green Bay WI
1208 PM CDT TUE JUL 19 2016

Updated aviation portion for 18Z TAF issuance

Issued at 353 AM CDT Tue Jul 19 2016

Turning hot and humid. Periods of showers and thunderstorms for
the middle to latter part of the work week.

Subtropical ridge dominates much of the CONUS, anchored by a
closed upper high centered over the Southern Plains. Trough
positions within the westerlies flowing along the northern
periphery of the ridge are located off the West Coast and over
eastern NOAM--bracketing mid-continent ridging. The main changes
during the forecast period will be for the westerlies to gradually
sag southward and then become progressive. This will allow the
upper high center to shift back toward the Desert Southwest. This
is a somewhat different evolution than appeared likely last week
when it seemed the upper high would settle in over the middle
Mississippi valley. The new scenario is much less favorable for
prolonged heat over the area.

Temperatures will warm to above normal and then remain there into
early next week before settling back closer to seasonal normals.
The hottest weather is likely to occur during the latter part of
this week. Humidities will also reach uncomfortable levels at
times through this weekend, then drop back a notch next week.
Precipitation amounts will depend on the behavior of convection,
and thus are tough to judge. With abundant moisture, any
convective event could easily generate an inch or two of rain.
Given that, it looks like there will be enough opportunities for
precipitation for most areas to receive at least normal amounts
during the period.

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 353 AM CDT Tue Jul 19 2016

Lots of forecast details to work out. Narrow band of low-middle
clouds over northern Wisconsin has been quite persistent, and
edged a bit south the past several hours. That may be at least
partly related to dissipating boundary lingering over the area.
Some of the models generated a little precipitation with the
feature. But have seen no signs of that on radar yet, so will
stick with a dry forecast into the evening.

Timing and location of precipitation chances starts to become
much more of a challenge tonight. Models all offered up something
a little different in terms of placement and timing of convection.
In general, preferred the ECMWF since its at least possible to
follow along with why it ends up with the precipitation pattern it
generates. Convection should be focused to our south and west
today and this evening, then it will probably begin to advance
east into at least the southwest part of the area during the night
as the low-level jet starts to veer and becomes directed into the
forecast area. Showers and storms are likely Wednesday. The LLJ,
though weakening, will continue the influx of moisture. Forcing
will also be generated by mid-level shortwave crossing the area.
This features can be traced back to convective cluster now over
northeast Montana. The severe threat will probably be held back by
limited destabilization ahead of the incoming complex and modest
shear. The assessment of a marginal risk of severe storms across
the area in the SPC day 2 convective outlook seems right on track.

.LONG TERM...Wednesday Night Through Monday
Issued at 353 AM CDT Tue Jul 19 2016

Model guidance continues to show the large upper-high pressure
area to build into the Western Great Lakes through at least
Thursday night before getting shoved south, then west this
weekend. To the north of this upper-high, the models remain at
odds with mid-level energy running into northern sections of the
Great Lakes and the timing of both warm and cold fronts through
WI. Potential clouds/precipitation from thunderstorm complexes
could put a crimp on the impending heat wave for Friday into
Saturday and make potential headlines a tough call. This remains
the main forecast challenge: heat wave vs. potential showers/

Precipitation chances look to continue into Wednesday night,
especially north and east as the warm front slowly lifts northeast
across WI. Dew points in the middle 60s to around 70s degrees and
PW values at 1.5 inches could bring locally heavy rains to parts
of the forecast area. An increasing mid-level cap building into
central WI behind the warm front should see the precipitation
diminish overnight. Min temperatures should be fairly uniform with
most locations in the upper 60s to lower 70s.

Thursday is where the forecast gets both interesting and
frustrating as the models debate on how far north the warm front
can get, as well as whether any MCS from Wednesday night to our
northwest could roll our way. The latest 00z model output seem to
now point to the warm front only reaching central/east-central WI
with a possible MCS moving across northern WI. This scenario could
seriously impact temperatures, especially if any convective clouds
drop south. If there is a concern with the heat/humidity, it would
be central WI, south of the warm front that could still warm into
the lower 90s with dew points in the mid 70s. This would produce
heat indices right around 100 degrees or the lower threshold for a
heat advisory. Due to the uncertainty of both the warm front and
any potential thunderstorm complex, prefer to hold off on any heat
heat headlines. That being said, Thursday will be at least a very
warm and humid day with lower to middle 80s near Lake MI, middle
to upper 80s north and upper 80s to lower 90s south.

Models have come to a somewhat similar solution for Thursday night
by dropping a weak cold front through northeast WI. This front may
be too weak to even break the cap over most of WI, thus
precipitation chances appear minimal as of right now. 8H
temperatures hardly even drop behind this frontal boundary, thus
another warm/muggy night can be expected with min temperatures in
the upper 60s to around 70 degrees north, lower to middle 70s
south. This cold front is forecast to stall in the vicinity of the
WI/IL border on Friday as it encounters the large upper high over
the central CONUS. Under the assumption that the front actually
can drop that far south, that would keep the better rain chances
to our south and potentially allow temperatures to warm into the
lower to middle 80s lakeside, middle to upper 80s north and upper
80s to lower 90s south. Heat indices could again approach heat
advisory criteria over parts of central WI.

The big question for the Friday night/Saturday time frame will be
the location of this frontal boundary as there are signals that
the boundary will try to return north. Problem is the models have
different timing which would affect not only precipitation
chances, but also temperatures on Saturday. A faster northward
shift would bring showers/thunderstorms back into the forecast
area Friday night, while a slower movement could hold the
precipitation off until Saturday afternoon. Since model
consistency has been poor from run-to-run, prefer to just go with
low chance pops Friday night and higher chance pops for Saturday.
Max temperatures on Saturday are expected to be in the lower to
middle 80s north/lakeshore, middle to upper 80s south.

Shower and thunderstorm chances would continue Saturday night as a
stronger cold front pushes into the Western Great Lakes region.
Even as this cold front pushes to our southeast on Sunday, a
trailing shortwave trough could kick off some showers, especially
over northern WI. Slightly cooler/less humid conditions should
prevail on Sunday with max temperatures in the upper 70s to around
80 degrees north/lakeside, lower to middle 80s south.

High pressure is progged to build into the region for the start of
the work week, bringing more seasonal temperatures back to
northeast WI. This would place max temperatures on Monday in the
middle to upper 70s north/near Lake MI, upper 70s to lower 80s

.AVIATION...for 18Z TAF Issuance
Issued at 1207 PM CDT Tue Jul 19 2016

Vfr conditions are expected to prevail through late tonight.  Then a
shortwave riding over the top of a strong mid-level ridge combined
with an approaching warm front will lead to a chance of showers and
storms arriving, mainly after 12z Wed.  Confidence is rather low in
the exact timing and coverage of showers and storm potential, so
left thunder out of the forecast. Ifr conditions will be possible
within any thunderstorms.



SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......MPC is the U.S. government's official web portal to all federal, state and local government web resources and services.