Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 241132

Area Forecast Discussion
National Weather Service Green Bay WI
632 AM CDT Thu May 24 2018

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Friday
Issued at 219 AM CDT Thu May 24 2018

Main forecast challenge to center on convective potential,
especially on Friday as some of the storms may become strong
during the afternoon hours. Otherwise, warm and somewhat humid
conditions to persist into the upcoming holiday weekend.

The 07Z MSAS surface analysis showed an area of high pressure
centered over northwest Lower MI. A weak area of low pressure was
located over southeast MT with another low pressure over southeast
CO. The radar mosaic picked up several clusters of showers/storms
over the Plains and the northern half of MN.

An upper ridge axis to be situated over the Great Lakes region
today with increasing moisture transport riding up the backside of
the ridge. All short-term models indicate at least a chance of
showers/thunderstorms across northern WI between the moisture
transport, weak isentropic lift and passage of a weak shortwave.
Shear values remain very weak (< 20 kts), thus no strong/severe
storms for today. A mild start to the day, coupled with a
persistent south wind, will bring another warm day to northeast WI
with max temperatures in the lower to middle 70s near Lake MI,
lower to middle 80s for inland locations.

This initial chance of precipitation will diminish this evening,
however additional shower/thunderstorm chances will be on the
increase later tonight as a mid-level trough moves into the Upper
Midwest. The approach of this trough is expected to enhance the
moisture transport/isentropic lift into WI, as well as add mid-
level forcing from the trough itself. Based on the latest timing
among the models, central WI to have the highest pops, while
eastern WI may not see any precipitation chances until daybreak.
Thickening cloud cover and south winds will bring a mild night to
the area with min temperatures only in the upper 50s near Lake MI,
lower to middle 60s elsewhere.

The mid-level trough moves into the western Great Lakes on Friday,
accompanied by a weak trailing cold front. These features will run
into a moist, unstable air mass over WI with MUCAPES of 1500-3000
J/KG, LI`s as low as -8 and steep mid-level lapse rates. The one
big negative is the weak shear values of 20-25 kts. Therefore,
while an isolated severe storm or two will be possible Friday
afternoon with hail/damaging winds being the main threats, a
widespread severe event is not anticipated. There could also be
some locally heavy rainfall with dew points in the 60s and PW
values around 1.5". Despite the clouds/precipitation chances,
Friday will be another warm day with max temperatures in the upper
70s near Lake MI, lower to middle 80s again for inland locations.

.LONG TERM...Friday Night Through Wednesday
Issued at 219 AM CDT Thu May 24 2018

Forecast concerns continue to revolve around thunderstorm chances
followed by above normal temperatures.  A blend of the ecmwf/gfs
will suffice.

Friday night through Saturday night...As weak shortwave energy and
main area of elevated moisture transport exits to the east on early
Friday evening, a small chance of thunderstorms will continue,
mainly for eastern and north-central WI.  But should see any
lingering convection diminish in the evening with loss of diurnal
instability. Partial clearing and light winds could lead to some
patchy ground fog overnight, especially in areas where there are
decent rainfall totals.

A secondary shortwave will drop in from the northwest on Saturday.
Though progged soundings indicate inverted V-soundings, with
dewpoints mixing out into the low or mid 50s, these soundings also
show SB capes up to 1500 j/kg developing over northern WI in the
afternoon, with mid-level lapse rates of 6.5-7 c/kg.  Therefore
could see a few strong storms developing in the afternoon into early
evening, especially with deep layer shear reaching upwards of 30
kts.  Like on Friday night, partial clearing overnight could lead to
another round of fog where decent rainfall occurs.

Rest of the forecast...Shortwave energy will be off to the east by
Sunday.  Will likely see clouds build as diurnal instability
develops, but lack of a trigger should keep any convection rather
isolated and tied to peak heating as temps approach their convective
temps.  Then high pressure will build into the region for Monday and
Tuesday.  With a light east wind, temps will fall back some, but
still remain near 80 degrees for most locations.  The next chance of
showers and storms will then occur in the Wed-Thu time period as
shortwave energy approaches from the west.  Some possibility that
the storm track could push most of this energy to the northwest of
the region.

.AVIATION...for 12Z TAF Issuance
Issued at 632 AM CDT Thu May 24 2018

Scattered showers/thunderstorms may impact the RHI TAF site for a
few hours this morning before moving out/weakening. There is a
slight chance for additional showers or a few storms across the
north this afternoon, however primarily VFR conditions are
expected across northeast Wisconsin today. The weather becomes
more unsettled for later tonight into Friday as a mid-level trough
and weak cold front push into a warm, moist air mass over
Wisconsin. The best chance for thunderstorms will be Friday
afternoon into Friday evening with a few stronger storms possible.
An increasing low-level jet into Wisconsin will also lead to
adding LLWS into the forecast for later tonight. Vsbys may drop
into the MVFR category under any of the thunderstorms.

Issued at 219 AM CDT Thu May 24 2018

Moist air flowing over the cool waters of Lake Michigan will
continue to produce areas of dense fog. Not much change in
dewpoints or wind direction is expected today, so marine fog is
likely to continue.



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