Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 261707
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1207 PM CDT THU MAY 26 2016

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Friday
Issued at 341 AM CDT Thu May 26 2016

Fcst concerns include fog trends, pcpn trends, severe tstm
potential and temperatures.

The main area of showers and tstms associated with the H8 warm
front and a short-wave trof moved ne of the area shortly after
midnight. After the rain ended and partial clearing occurred,
widespread stratus/fog developed. The fog was most extensive over
central and east central WI, where several sites reported vsbys
as low as one-quarter mile. Have been watching the next round of
convection over southern MN and IA early this morning. Current
indications are that the line of tstms will reach central WI
between 11z and 12z.

Plan to handle the short-term dense fog threat with a Special
Weather Statement and a mention in the Hazardous Weather Outlook.
Suspect that vsbys will quickly improve after sunrise, or when
the storms arrive.

The line of storms to our west, which is being driven by a weak
short-wave trof and some llvl moisture convergence, will likely
remain strong to marginally severe as it moves through the se
two-thirds of GRB CWA this morning and early afternoon. This
expectation is based on the presence of MUCAPE of 500-1000 j/kg,
steep mid level lapse rates of 6.5 to 7.5 C/KM, and deep layer
shear of 30 to 35 kts. Several of the meso-models show some
intensification as the convection moves into northeast WI later in
the morning, perhaps due to increasing daytime instability. After
the short-wave trof shifts east around 18z, the best chance of
additional convection should be over northern WI, where models
show the warm/stationary front lingering through the afternoon.
Partial clearing this afternoon should allow temperatures to
quickly warm into the lower to middle 80s, which will combine with
dew points in the low to mid 60s to boost SBCAPE to 1500-2000
J/KG in most areas. Thus, any additional tstms that develop may
also become strong to marginally severe.

Tonight should start off relatively quiet after any lingering
daytime convection wanes. However, another short-wave trof is
expected to arrive late, which should result in an increase
in showers and sct tstms late. Showers and storms will become
more numerous on Friday, as a very moist air mass (with PWATs
greater than 1.5 inches) returns to the region. Abundant cloud
cover should limit instability, with CAPES only expected to be
800-1200 J/KG, so the threat of severe tstms should be lower than
the previous couple days. Locally heavy rainfall will be a good
bet though. Temperatures will be a bit cooler, mainly in the 75 to
80 range. An easterly component to the winds will keep lakeshore
readings in the upper 60s to lower 70s.

.LONG TERM...Friday Night Through Wednesday
Issued at 341 AM CDT Thu May 26 2016

Mean flow is not expected to change much into early next week with
modest upper ridging over the Eastern Pacific/Western Atlantic and
broad upper trough over the Rockies. A persistent Sw flow aloft
into WI will provide for unsettled weather through much of the
upcoming holiday weekend. A change to the mean flow is forecast
toward the middle of next week as a new upper trough moves into
the Eastern Pacific and shoves the upper ridge eastward into the
Western CONUS. This would then push the Rockies upper trough east
into the Plains/Midwest and bring additional chances for
precipitation to NE WI. Overall, look for above normal
temperatures and above normal precipitation over the next 7 days.

A deep/persistent S-SW flow will continue to send copious amounts
of gulf moisture into WI ahead of a shortwave trough that is
forecast to eject NEWD from the Central/Southern Plains Friday
night into Saturday. A combination of moisture transport...theta-e
convergence and increasing thermodynamic forcing should lead to
several rounds of showers and thunderstorms that will move through
the forecast area. Severe potential looks minimal due to clouds
holding down instability and shear values are rather weak (around
20 knots). However, locally heavy rainfall is possible under any
convective activity with dew points in the upper 50s to lower 60s
and PW values generally in the 1.5-1.75 inch range. Min
temperatures Friday night to mainly range from the upper 50s to
lower 60s, with max temperatures on Saturday in the lower to
middle 70s north/lakeshore and upper 70 to around 80 degrees
south.

This shortwave trough is progged to lift NE and reach the Western
Great Lakes Saturday night with a continued potential for showers
and thunderstorms. There will also be a weak cold front moving
east into Western WI late Saturday night that would add lift for
precipitation chances. May need to raise pops up to likely
category with locally heavy rains again possible. The shortwave
trough exits the region on Sunday, but the cold front to still be
working east across the state. Therefore, we need to carry at
least a chance of showers/thunderstorms across all of NE WI on
Sunday, although parts of Central/N-Central WI could see
precipitation chances begin to drop off in the afternoon behind
the front. Prefer to not get too specific just yet regards to
timing. Tha air mass aloft does not chance much this weekend, so
have kept temperatures pretty close to what is expected Friday
night and Saturday.

There is an increasing potential for a dry period of weather
Sunday night into Memorial Day as a weak area of high pressure at
the surface and mid-level shortwave ridging move into the region.
The only concern on Memorial Day is for instability showers or
thunderstorms to pop up as low-level moisture never leaves the
area and CAPE values rise into the 1000-1500 J/KG range. Prefer to
keep a minimal pop in the forecast for Monday, but far from any
wash-out. Max temperatures for Memorial Day will remain above
normal with upper 70s to around 80 degrees north, lower 80s south
(cooler lakeside).

As the Eastern Pacific upper ridge pushes inland over the West
Coast on Tuesday, the downstream upper trough will shift east from
the Rockies into the Plains. This scenario is vaguely comparable
to the set-up from Friday night into Saturday with gulf moisture
surging northward and individual shortwaves lifting NE toward WI.
Expect clouds to thicken over the forecast area late Monday night
into Tuesday with rain chances increasing from west to east on
Tuesday. More clouds/higher pops will knock a couple of degrees
off of the temperatures with maxes on Tuesday ranging from around
70 degrees near Lake MI...to roughly 75-80 deg range elsewhere.

There is some timing differences with respect to how fast this
shortwave trough will work across the Central CONUS toward the
middle of next week. Ths GFS favors a stronger/slower moving
trough while the ECMWF prefers a weaker/faster moving trough. Have
leaned teh forecast toward the better verifying ECMWF for now...
although confidence for day 7 is rather low.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1207 PM CDT Thu May 26 2016

An area of showers and storms will exit northern
Wisconsin early this afternoon.  Will still have a warm front
positioned over far northern Wisconsin or the southern Upper
Peninsula though...which could fire off a storm or two late this
afternoon into early this evening.  The probability that a storm
will impact RHI is too low to mention.  A muggy airmass will be in
place tonight...which should lead to fog formation late in the
evening and overnight.  Fog could become dense in spots...like along
the Lake Michigan shoreline and also north-central Wisconsin.  In
addition...expectations are for another round of showers and storms
to move northeast across the region late tonight and Friday
morning...which could lead to ifr/mvfr conditions continuing at some
locations through midday.  Timing is uncertain...so some adjustments
are likely.
&&

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

$$

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



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