Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 251702
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1202 PM CDT WED MAY 25 2016

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 405 AM CDT Wed May 25 2016

Precipitation trends, severe tstm potential and temperatures are
the main fcst concerns.

A weak frontal boundary was sagging south across central and east
central WI early this morning, but was too shallow to generate
any convection. Low clouds had moved into north central and
northeast WI, and were surging southward toward the northern Fox
Valley. Areas of fog had developed, especially over north central
WI, where the more significant rainfall occurred yesterday. Fog
was also developing over the bay and northern Lake.

Expect a cooler day today, due to low clouds in the morning and
east flow off Lake Michigan. The southern part of the forecast
area should still get close to 80, but much of northern WI will
only get into the lower to middle 70s, and that may be a bit
optimistic. Most of the morning should be dry, but the arrival of
a H8 warm front and a short-wave trof will bring showers and tstms
back into the southwest two-thirds of the forecast area during the
afternoon.

Showers and tstms will continue this evening as the H8 warm front
and short-wave trof move through, then diminish from sw to ne
overnight. Elevated instability increases tonight, with H8 LI`s
of -3 to -6, and steep mid-level lapse rates of 7 to 8 C/km. This
suggests that a few strong to marginally severe tstms with hail
are possible. Locally heavy rainfall is likely, as precipitable
water values increase t0 1.25 to 1.5 inches, and H8 dew points
climb to +10 to +13 C.

On Thursday, the surface warm front is expected to shift north of
the region by afternoon, and there is little indication of any
other forcing mechanism to provide a trigger for organized
convection. Kept low chance pops anyway, as sfc-based instability
will become fairly strong during the afternoon, with CAPE
increasing to 1500-2000 j/kg. Though widespread severe tstms are
not anticipated, the degree of instability supports the potential
for isold strong to severe tstms. Clouds are expected to decrease
in the afternoon, allowing temps to soar into the 80s over most of
the region. Dew points will also surge into the lower to middle
60s, offering a taste of summertime humidity.

.LONG TERM...Thursday Night Through Tuesday
Issued at 405 AM CDT Wed May 25 2016

Mean flow consisting of an Eastern Pacific upper ridge, Rockies
upper trough and Western Atlantic upper ridge is expected to hold
nearly steady through the extended forecast. This pattern will
bring a persistent SW flow aloft into the Great Lakes which would
allow for a warm and unsettled regime to set up over NE wI. The
main forecast problem will be trying to determine which periods to
hit showers/thunderstorms harder. This could prove difficult due
to the unknown location of a quasi-stationary front in the
vicinity and timing of individual shortwaves lifting NE out of the
upper trough.

The quasi-stationary front appears to stretch from northern
sections of the Great Lakes SW through the Upper MS Valley
Thursday night...thus any pcpn chances over NE WI would generally
have to come from individual shortwaves ejecting NEWD from the
Rockies upper trough. Latest model guidance would favor one such
shortwave to move into the Upper MS Valley after midnight,
therefore higher chc pops placed over Central/North-Central WI.
Instability would be waning by the time any thunderstorms would
reach the forecast area, so severe threat appears to be low
Thursday night. Min temperatures will again be summer-like with
readings in the mid to upper 50s north/lakeshore...lower to middle
60s south.

The upper trough is forecast to edge eastward into the Plains on
Friday with a persistent southerly flow from the surface through
8H mb bringing copious amounts of gulf moisture into WI (PW values
approach 1.75"). Even though plenty of clouds are expected to
limit the amount instability (Capes end up between 500-1500 j/kg),
mid-level lapse rates are fairly steep with additional shortwaves
lifting NE through the region. Have maintained the likely pops
across the entire forecast area, although the afternoon hours
would seem to be more prolific for rain chances as instability
increases. Anticipate max temperatures to be quite variable
depending on when/where convection fires. For now, have readings
in the lower to middle 70s near Lake MI, upper 70s to lower 80s
elsewhere.

Shower/thunderstorm chances should diminish to a degree Friday
night as instability weakens. However, will need to at least carry
chc pops through the night as low-level moisture tranport persists
and weak lift lingers over the region. Min temperatures to again
be in the mid to upper 50s north/lakeshore, lower to middle 60s
south.

Models continue to show a large piece of this upper trough will
lift NE toward the Midwest on Saturday, the Upper MS Valley/
Western Great Lakes Saturday night and into the rest of the Great
Lakes on Sunday. While the models do show general agreement in the
movement of this shortwave trough, exact timing differs just
enough to place doubt as to which period to place the higher pops.
Expect to see mid-level forcing gradually increase as the
shortwave trough approaches, thus this would favor later Saturday
into Sat nightfor the higher pops with precipitation trends going
down on Sunday as the trough exits the area. Severe potential
would be dependent on whether sufficient instability can be
reached with expected mostly cloudy skies. Too early yet to make
this determination. Just something to watch for the upcoming
holiday weekend. Little change in the air mass should provide a
warm/humid weekend as well as max temperatures both days in the
lower 70s near Lake MI, mid to upper 70s north and upper 70s to
around 80 degrees south.

There should be a break in the precipitation chances Sunday night
as a weak shortwave ridge moves into the Western Great Lakes. That
being said, NE WI to still be under a SW flow aloft with a
potential frontal boundary nearby at the surface. In addition,
low-level moisture does not get stripped away as PW values to
still be around 1" on Monday. We could see some instability
showers and thunderstorms develop on Monday, especially during the
afternoon hours. Max temperatures for Memorial Day to remain
consistent with previous days which would be around 10 degrees
above normal.

Unfortunately, cannot completely rule out precipitation chances
even on Tuesday as the atmosphere reloads with gulf moisture ahead
of the next prominent shortwave trough located over the Northern
Rockies. Higher pops will be west of the forecast area, thus
Tuesday will not be a wash-out.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1202 PM CDT Wed May 25 2016

A warm front will lift northeast across the region later
this afternoon and this evening.  Showers and thunderstorms are
expected to increase in coverage through the day...as they move into
central Wisconsin this afternoon and northern Wisconsin this
evening.  Some strong storms cannot be ruled out...which could
generate wind gusts above 40 mph and dime size hail.  Some
uncertainty with the timing of the arrival of the storms...but IFR
conditions will be possible as the storms pass.  In the wake of the
precipitation...LIFR/IFR cigs and IFR/MVFR vsbys are anticipated to
impact the taf sites from late tonight thru mid-morning before the
clouds scatter.
&&

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

$$

SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......MPC



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