Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 251743
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1243 PM CDT Fri May 25 2018

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Saturday
Issued at 240 AM CDT Fri May 25 2018

The main forecast focus to be on extent of convection over the
next 36 hours and the possibility of a few storms becoming severe
this afternoon into this evening.

The 07Z MSAS surface analysis showed a warm front lifting
northeast through WI, with a cold front over the eastern Dakotas.
The radar mosaic indicated several clusters of showers/storms to
our west and north associated with the low-level jet/stronger
moisture transport. Temperatures were quite mild for late May with
some sites still above typical normal max temperatures.

Models send a mid-level shortwave trough/attendant cold front
across the Upper MS Valley today with these features reaching the
MN/WI border by 00Z Saturday. The air mass over WI will be moist
and unstable with dew points in the 60s/PW values around 1.5" and
MUCAPES of 1-2K J/KG with LI`s as low as -6. The problem for
sustained thunderstorm activity continues to be very weak bulk
shear with only around 20 kts anticipated. Another concern would
the uncertainty of current convective activity and whether it will
diminish this morning or hold together with additional
development. Current thinking is for the ongoing activity to
weaken and then re-fire this afternoon across northern/western WI.
This band of showers/storms would then drive southeast toward
east-central WI late in the day. Due to the very weak shear,
expect thunderstorms to be pulse-type with a few strong to severe
storms capable of producing large hail or damaging winds, along
with brief heavy downpours. Another warm/humid day expected ahead
of the precipitation with max temperatures in the lower 80s
north-central WI/near Lake MI, middle to upper 80s elsewhere. Some
record highs will be in jeopardy, especially over eastern WI.

These showers/thunderstorms are forecast to continue into this
evening as the shortwave trough/cold front move across WI. A few
of these storms could still be strong into mid-evening before
instability starts to weaken. The activity is then progged to
diminish from northwest to southeast from late evening through the
overnight hours as the trough/cold front begin to pull away. This
would allow skies to become partly cloudy and leave the
possibility of some late night fog/low clouds. Min temperatures to
range from the upper 50s north-central/lakeshore, lower 60s for
everyone else.

After a quiet start to the day on Saturday, a secondary/weaker
shortwave trough is forecast to drop southeast across northeast WI
during the afternoon hours. The air mass over the region to still
be unstable with MUCAPES of 1000-1500 J/KG and there is a bit more
shear (25-30 kts). This should be sufficient to allow for another
round of showers/thunderstorms to develop in the afternoon with
the highest pops over far northeast WI where mid-level forcing to
be the strongest. Cannot rule out a stray stronger storm with
small hail/gusty winds. Max temperatures to range from the upper
70s near Lake MI, middle 80s to around 90 degrees elsewhere. These
values will be near record highs over most of the forecast area.

.LONG TERM...Saturday Night Through Thursday
Issued at 240 AM CDT Fri May 25 2018

Forecast concerns continue to revolve around thunderstorm chances
followed by the above normal temperatures.  In general, the pattern
will consist of weak upper flow over the western Great Lakes, with
the jet stream pushed well to the north into Canada.  Temperatures
are likely to be above normal while precipitation is expected to be
near to below normal through the period.

Saturday night through Sunday night...Scattered thunderstorms may
persist into the evening hours on Saturday night over far northeast
WI, where outflow boundaries may interact with lake/bay breezes and
lingering diurnal instability.  Should see coverage of thunderstorms
diminish relatively quickly, however, as shortwave energy exits to
the east and instability wanes.  Partial clearing is then expected
overnight.  With light winds, potential for ground fog to develop
where heavy rains occur within thunderstorms.

Mid-level heights then build aloft on Sunday.  Though will have
instability develop Sunday afternoon, rising heights and lack of a
trigger should prevent much of a chance of thunderstorm development.
Perhaps an isolated shower could develop along the lake breeze over
eastern WI during peak heating.  Focus for thunderstorm development
should then take place over northern Minnesota and Dakotas along a
cold front.  Some clouds will likely move into the area from blowoff
of these thunderstorms.  But otherwise, should see a mostly clear
and warm night.  Temps on Sunday will remain very warm, with highs
in the mid 80s to low 90s.  A few record highs will be possible.

Rest of the forecast...The ecmwf now projects that the cold front
over northern Minnesota will sag into northern WI on Monday. This
front may result in a few storms popping on Monday afternoon and
evening. Otherwise, dry and and very warm conditions should continue
through Tuesday.  Models still show the next upper trough to
approach the region on Wednesday, which looks like the next best
chance of thunderstorms. High temps should remain firmly in the 80s
away from Lake Michigan.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1234 PM CDT Fri May 25 2018

Isolated/scattered thunderstorms were developing over western WI
early this afternoon, and should become more numerous as
instability increases and an upper disturbance approaches later
in the day. Though a few storms may impact the RHI/AUW/CWA TAF
sites early in the TAF period, the main bulk of the convective
activity should occur from 21z/Fri-02z/Sat. After the storms move
through, attention turns to fog potential overnight. Partial
clearing, decreasing surface and boundary layer winds, and
abundant low-level moisture from earlier rains should allow areas
of fog to develop. The fog should be most extensive over north
central and central WI, where a weak ridge of high pressure will
reside late tonight.

The fog should dissipate by 13z-14z/Sat, followed by partly cloudy
skies. Additional thunderstorm activity is anticipated Saturday
afternoon, especially over northeast WI.

&&

.MARINE...
Issued at 240 AM CDT Fri May 25 2018

Warm and moist air flowing over the cool waters of Lake Michigan
may produce areas of marine fog today into tonight.
&&

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

$$

SHORT TERM.....AK
LONG TERM......MPC
AVIATION.......Kieckbusch
MARINE.........AK


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