NWS-Green Bay Forecast Discussion

 

						FXUS63 KGRB 200017
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
717 PM CDT Wed Sep 19 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 226 PM CDT Wed Sep 19 2018

A surface warm front will lift north tonight and Thursday,
producing several rounds of showers and thunderstorms with
abundant isentropic lift north of the front combined with an
increasing low level jet and mid level PV anomalies. Heavy rain
will be possible at times given the deep moisture layer and the
slow moving front.

Models indicate that an inch or two of rain could fall during this
time, which could cause localized flooding. However given most
areas, except for Brown County, have been fairly dry recently the
threat for Flash Flooding does not appear to be that high. However
some urban and small stream flooding is possible from the heavier
rains.

As the front lifts north Thursday afternoon the southern cwa will
actually break into the warm sector with continued rain across
north-central Wisconsin north of the warm front as isentropic lift
continues to cause elevated convection. This will bring warm
temperatures and uncomfortable dew points back to central and
east-central Wisconsin. This warm, moist air will also cause a
surge in the instability as MUCAPEs rise to 2000 to 3000 J/kg with
bulk shear values of 50 to 70 knots. If any storms can get going
in the warm sector Thursday afternoon there is a good chance for
severe weather given the wind and instability profiles. That being
said the best potential appears to hold off until Thursday night
when a cold front approaches from the west given a capping
inversion in the warm sector Thursday afternoon. If any storms can
get going Thursday afternoon the main threat would be damaging
winds given the strong winds just above the surface, with hail
and being a secondary threat. The high degree of turning in the
low level hodographs also indicates that tornadoes could be a
threat as well.

Lows tonight will generally be in the 50s. Highs on Thursday will
range from the middle to upper 70s across central and east-
central Wisconsin south of the warm front and generally south of
highway 29, with highs only in the lower to middle 60s further
north, north of the warm front.

.LONG TERM...Thursday Night Through Wednesday
Issued at 226 PM CDT Wed Sep 19 2018

The main concerns for the extended period will be the severe
weather potential Thursday night and the potential for frost
Friday night into Saturday morning across northern Wisconsin.

The potential for severe storms will concentrated towards Thursday
evening to the early overnight hours, as the cold front moves
through the region. MUCAPE still varies from 1000-1500 on the
ECMWF to 2500-3000 on the NAM. Bulk layer 0-6km shear values
approach the 45-55 knots while low layer shear remains strong
around 25-35 knots. Couple these with the strong low level jet and
even the low end MUCAPE will be sufficient for thunderstorm
development and mixing of winds to the surface. Therefore, expect
the primary threat for the area to be damaging winds, with a few
brief tornadoes and large hail as secondary concerns. High
rainfall totals up to around 1-2 inches by the end of the event
will also pose localized flooding concerns on the roads. The front
is expected to be fast moving in the area, pushing through the
region by early on Friday.

Friday, additional shower to isolated thunderstorm activity will
be possible, as wrap around moisture continues to enter northern
Wisconsin behind the departing low pressure system. No severe
potential is expected during this time. By Friday evening, active
weather will be out of the region as high pressure moves into the
region.

Depending on the speed at which cloud cover exits behind the
system Friday night, frost may become more of a concern across the
area, especially in the typical cold spots in northern Wisconsin.
Stuck to a blend of model guidance for now, leaving lows in the
middle 30s, as the active weather ahead of Friday will help
dictate the temperatures trend at that time.

Beyond that, quieter conditions will persist across the area for
the weekend before we see the next shot for active weather arrive
Monday night into Tuesday.

Temperatures for the time period will be fairly close to normal,
with a brief period of above normal temperatures on Monday ahead
of the next round of active weather.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 714 PM CDT Wed Sep 19 2018

IFR/LIFR conditions will prevail across the area into tomorrow as
a frontal boundary to the south slowly begins to work its way
north. LLWS will also develop during the day tomorrow as winds
above the surface begin to increase ahead of a cold front heading
in from the west.
&&

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

$$

SHORT TERM.....Kurimski
LONG TERM......Uhlmann
AVIATION.......Skowronski