Forecast Discussion for GRB NWS Office
000
FXUS63 KGRB 220005
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
705 PM CDT Sat Oct 21 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Sunday
Issued at 209 PM CDT Sat Oct 21 2017

The main forecast concerns center around timing of precipitation
later tonight, convective potential and temperatures for Sunday as
colder air arrives.

The 19z MSAS surface analysis a cold front which extended from
near Lake Winnipeg south through extreme western MN, far eastern
NE and points south. A tight pressure gradient existed across WI
in advance of the front with gusts of 20 to 30 mph at times.
Visible satellite imagery showed clouds over the western half of
WI, while eastern WI enjoyed mostly sunny and warm conditions.
Finally, the last of the morning showers from a mid-level
shortwave are exiting northern WI.

Models are still in good agreement with sending the cold front
into WI later tonight (but remaining west of the forecast area).
the models also bring MUCAPES to around 400 J/KG into central WI
this evening, thus cannot rule out a few storms approaching
central WI. Despite in instability weakening through the night,
this would be countered by increasing shear and lift as the front
gets closer. Due to the shear, the chance of seeing thunderstorms
will continue through the night with gusty winds possible, but
remaining sub-severe. Highest pops placed over central WI, while
lakeshore counties may not see any precipitation until the pre-
dawn hours. Min temperatures to remain mild for one more night
with readings in the lower to middle 50s central WI, upper 50s to
lower 60s eastern WI.

The cold front is expected to slowly push east across central and
eastern WI on Sunday, partially impeded by an upper ridge situated
over the East Coast. It may take until mid-afternoon before the
front reaches Lake MI, thus showers to persist over the entire
forecast area Sunday morning, then gradually diminish over central
WI during the afternoon. Have kept the slight chance of thunder
over eastern WI into Sunday morning, but not overly confident in
this. Have also slowed the rate of clearing over central WI Sunday
afternoon due to the slow movement of the front. Max temperatures
may easily occur in the morning as colder air starts to sweep into
WI. This would place readings in the middle to upper 50s central,
lower 60s eastern WI before slowly holding steady or slowly
falling in the afternoon.

.LONG TERM...Sunday Night Through Saturday
Issued at 209 PM CDT Sat Oct 21 2017

The stretch of dry and mild weather provided by an upper level
ridge will be replaced by an active, cooler weather pattern as a
long wave trough establishes itself across the Great Lakes region
during the upcoming week. While the models seem to be in better
agreement regarding a large low pressure system moving through the
central Great Lakes Monday and Tuesday, there are discrepancies
on how quickly the low pulls out of the Great Lakes region. The
speed of the low makes the difference between drier and warmer
weather or rainy and cooler conditions. Given the model
discrepancies did not differ much from the SuperBlend.

Another low pressure system will track through the western Great
Lakes area during the late part of the week. Again the models are
in the general vicinity but the exact placement of the low is off
amongst the models, making for a low confidence forecast that far
out. Models such as the Canadian and ECMWF keep the main low to
the north with little to no Qpf over our area while the further
south GFS has a swath of QPF across the northern cwa. This slow
moving low will linger into next Saturday, continuing the
precipitation chances across the region.

The aforementioned cooler air will mean snow could mix in at times
during the mid to late week storm systems. However given the
fairly warm ground and mixing with rain not much if any snowfall
accumulation is expected.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 704 PM CDT Sat Oct 21 2017

Ceilings and visibilities will slowly fall from west to east
late tonight as a cold front approaches from the west. It will be
accompanied by showers and possibly some thunderstorms. Weather
conditions will improve Sunday afternoon and evening after the
front exits.
&&

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

$$

SHORT TERM.....Kallas
LONG TERM......Kurimski
AVIATION.......RDM

NWS GRB Office Area Forecast Discussion