Area Forecast Discussion
Issued by NWS Green Bay, WI

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274
FXUS63 KGRB 231757
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1257 PM CDT Wed May 23 2018

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 248 AM CDT Wed May 23 2018

The main forecast concerns for the next 36 hours to be on
precipitation chances later tonight into Thursday and how warm can
we get, especially on Thursday.

The 07Z MSAS surface analysis showed high pressure over the Great
Lakes region with a weak frontal boundary located from Upper MI
eastward into southeast Canada. IR satellite imagery showed mainly
high clouds streaming across WI, however surface obs indicate some
low clouds due to areas of fog that has developed.

The patchy fog across central WI will burn off by mid-morning,
otherwise high pressure to be situated over the eastern Great
Lakes today with a return flow gradually developing across WI. An
upper ridge axis will be situated from the Gulf Coast states
northward into the Upper Midwest. The main focus for precipitation
will be near this upper ridge as the low-level jet weakens this
morning and where better instability to reside this afternoon. For
northeast WI, look for fair weather cumulus to develop, along with
the passing high clouds from convection to our west. Max
temperatures will be a touch warmer than yesterday as winds back
to the south. Look for readings in the lower to middle 70s near
Lake MI, generally upper 70s to lower 80s elsewhere (although a
few middle 80s are possible over far northeast WI where the air
mass is drier). See the fire weather section below for additional
information on low afternoon relative humidity values.

The upper ridge is forecast to move into the western Great Lakes
later tonight and this leaves central WI vulnerable for a chance
of showers/thunderstorms as the low-level increases and a
shortwave lifts northeast along the backside of the upper ridge.
Eastern WI should remain dry through the night with only a gradual
increase in mid-high clouds. Min temperatures tonight to range
from the lower 50s near Lake MI, to around 60 degrees over parts
of central WI.

The overnight showers/thunderstorms are expected to move northeast
and weaken Thursday morning as the low-level jet weakens and the
shortwave lifts farther northeast. By Thursday afternoon, a warm/
unstable air mass to be situated over northeast WI. Models
indicate CAPE values from 1-2K J/KG (highest values central WI)
with little CIN to speak of. However, bulk shear values are very
weak (under 20 knots) and there is a lack of a trigger. The
unknown for Thursday afternoon is whether any boundary could be
left behind by the earlier thunderstorms. Due to this unknown,
prefer to leave a small chance pop in the forecast mainly for the
northwest half of the forecast area. Thursday will definitely have
a summer feel to it with max temperatures in the lower to middle
80s (away from Lake MI) and dew points in the upper 50s to lower
60s.

.LONG TERM...Thursday Night Through Tuesday
Issued at 248 AM CDT Wed May 23 2018

Forecast concerns mainly revolve around thunderstorm chances on
Friday into the weekend as a Summer-like pattern moves into the
region.  Will continue a blend of the ecmwf/gfs.

Thursday night through Friday night...Due to lingering diurnal
instability, scattered showers and storms could linger into Thursday
evening, particularly north and west of the Fox Valley.  Shear is
pretty meager, so not thinking there will be a severe threat.  Skies
should then clear for a time as additional thunderstorm activity
occurs upstream over Minnesota and far western WI.  Eventually some
clouds from this system will push into north-central WI by late
Thursday night into Friday morning.  As weak shortwave energy
interacts with building surface based instability up to 2000 j/kg on
Friday afternoon, should see scattered showers and storms redevelop
over much of the area.  Though 0-6 km bulk shear will only range
from 20-25 kts, cannot rule out an isolated severe storm given the
magnitude of instability that is projected to develop. The storms
should die off relatively quickly on Friday evening as the shortwave
exits to the east and diurnal instability is lost.  Should see at
least partial clearing take place over a mild southwest wind.  Temps
on Friday will likely be the warmest of the season to date, with
highs in the mid 80s away from Lake Michigan.

Rest of the forecast...The Summer-like airmass will hang around into
the weekend despite winds aloft turning to the northwest.  Another
shortwave will move into the region on Saturday, and could trigger
additional storms on Saturday afternoon and evening as it interacts
with daytime instability.  Appears there will be more in the way of
shear aloft, so another isolated severe threat could develop,
particularly over northern and northeast WI where lake breezes could
provide a low level focus.  This shortwave will hang around over
northern WI on Sunday, though coverage of showers and storms on
Sunday afternoon looks considerably less than on Saturday.  Dry
weather remains on track to return early next week, as ridging
builds in aloft and high pressure will reside at the surface.  Temps
holding in the 80s away from Lake Michigan.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1257 PM CDT Wed May 23 2018

VFR conditions are expected through the period with high clouds
drifting across the area. Daytime cumulus clouds will continue to
develop across much of the area early this afternoon, then
dissipate through the late afternoon hours. A few showers or
possibly a storm is possible across central and north central WI
late tonight into Thursday, but confidence too low to include in
the TAF. Patchy fog possible tonight as well overnight into early
Friday, especially at CWA/AUW/RHI, but decided to keep out for
now. If fog develops or showers/storm does develop, CIGS/VSBYS in
the MVFR or briefly IFR would be expected.
&&

.FIRE WEATHER...
Issued at 248 AM CDT Wed May 23 2018

Dry conditions will continue across northern WI today, with
relative humidities dropping into the 20s again during the
afternoon. An increase in shower and thunderstorm chances are
forecast for the end of the week which could provide much needed
relief from the dry conditions and ease the fire weather threat.
&&

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

$$

SHORT TERM.....AK
LONG TERM......MPC
AVIATION.......Bersch
FIRE WEATHER...AK



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