Gale Watch 5, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
257 PM CDT Thu Oct 5 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Friday
Issued at 257 PM CDT Thu Oct 5 2017

Isolated to scattered showers and sprinkles are possible through
this evening as a jet streak tracks through the northern Great
Lakes region. Rain chances will diminish through the evening as
this feature pulls away with some drier air advecting in from the
north. This drier air will cause clouds to clear out across the
far north late tonight, causing temperatures to plummet into the
mid 30s, with some low 30s possible in low lying areas. These
temperatures, combined with light winds, will bring the threat for
frost across far north-central Wisconsin late tonight. Therefore
will issue a frost advisory for the far north late tonight into
early Friday morning. This could very easily not be reached if
clouds do not clear out later tonight, however satellite imagery
does show the edge of the clouds is already fairly close to these
areas. The rest of the cwa will experience lows mainly in the 40s,
with some areas around 50 degrees near the Great Lakes. The clear
skies and light winds will also bring some patchy fog to the area,
mainly across north-central Wisconsin where skies will have a
better chance to clear out.

Rain chances will increase on Friday as better dynamics and
moisture advect north across the western Great Lakes. Recent model
runs have pushed back on the timing of this precipitation,
therefore will adjust POPs to reflect slower timing. The best
chances for rain will be across central Wisconsin during the
afternoon hours. Some modest instability will bring the chance for
some thunder at times. Highs on Friday will generally be in the
lower to middle 60s.

.LONG TERM...Friday Night Through Thursday
Issued at 257 PM CDT Thu Oct 5 2017

Model handling of two upper troughs across the central CONUS
continues to be a forecast problem in the extended period with a
variety of solutions possible. The model spread with trough #1 is
not significant for northeast WI, but enough to question when to
hit the pops the hardest and how long to carry pops this weekend.
The bigger issue is with trough #2 and whether this trough will be
progressive or dig more into the Rockies before lifting northeast
during the Tuesday/Wednesday time frame. Model run-to-run
discrepancies makes for a poor confidence toward the middle of
next week.

Showers/slight chance of thunderstorms will continue to overspread
northeast WI Friday evening as the warm front lifts northward into
central/east-central WI. During the overnight hours, a surface
wave is forecast to lift northeast along an eastward-moving cold
front that will push across western WI. Increasing levels of lift
from both frontal boundaries, combined with mid-level forcing from
the trailing upper trough (over the Upper MS Valley/Midwest by 12z
Saturday) and an abundance of low-level moisture, all point to
categorical pops across the forecast area. Due to a lack of
instability, any thunderstorms would remain sub-severe and very
isolated. The bigger concern would be precipitation amounts as PW
values are forecast to surpass 1.5 inches. Anticipate locally
heavy rainfall to occur with minor street flooding possible,
especially if storm sewers get blocked with leaves. Min
temperatures to be in the lower to middle 50s north, middle 50s to
around 60 degrees south.

The showers/possible thunderstorms will continue through at least
Saturday morning as the cold front reaches central WI by midday,
then exits the forecast area into Lake MI late Saturday afternoon.
Much of northeast WI could see another third to half-inch of
rainfall with totals reaching between 1.00 and 1.50 inches by the
time the rains taper off. Since much of the region has been dry
over the past couple of weeks, flooding should not be a concern
although river/stream levels could make a quick rise in some
locations. Max temperatures to range from the lower 60s north-
central WI, to the upper 60s across east-central WI.

This system exits to the east Saturday evening according to the
NAM and GFS, however the CMC and especially the ECMWF insist that
the trailing deformation zone would linger some showers into
Saturday evening. May leave a small chance pop mainly north and
east as the system departs. Advection of drier air into WI behind
the cold front should eventually clear out the clouds for the rest
of Saturday night. Min temperatures to be in the 45-50 degree
range central WI, 50-55 degree range eastern WI. Northeast WI to
reside on the western fringe of high pressure centered over the
eastern Great Lakes on Sunday. The development of an inverted
surface trough to our west could bring a few clouds into central
WI Sunday afternoon, but still expect a mostly sunny day with
above normal temperatures. Look for readings in the middle 60s
north-central, lower 70s eastern WI (away from Lake MI).

This inverted surface trough is progged to move into
western/central WI Sunday night and then almost hold stationary on
Monday as a new upper trough digs across the Rockies. This is
where the models begin to diverge as the ECMWF has this upper
trough move east into the Plains on Monday, while the other models
do not. Therefore, the models disagree on whether to introduce
pops into WI Monday or hold a dry forecast through the day. Based
on the way the ECMWF keeps changing from run-to-run, prefer the
majority here and would like to keep the forecast dry for now. Max
temperatures on Monday to cool a bit as winds switch to the
northeast. This would keep readings in the lower 60s north-
central, middle to upper 60s elsewhere.

Forecast confidence is minimal from Monday night through Wednesday
depending on which model trend ends up being correct. If there is
a positive here, the new 12z ECMWF has slowed its progressive
nature from earlier runs, but we will have to wait and see if it
keeps this thinking. For now, have followed the consensus forecast
which brings a chance of showers into central WI Monday night and
across the rest of the forecast area on Tuesday. This would be
followed by the passage of the trailing upper trough on Wednesday
with another chance of showers. Temperatures would also be
impacted by this uncertainty, but one would expect a downward
trend due to clouds/precipitation.

Models do attempt to get back into sync by next Thursday as a new
upper trough digs toward the West Coast and a modest downstream
upper ridge sets up across the central CONUS. High pressure at the
surface will bring dry weather with temperatures moderating to
above normal levels.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1228 PM CDT Thu Oct 5 2017

Some light showers or sprinkles may fall at times from a mid deck
of clouds from 6-9k ft this afternoon and evening. Despite the
showers VFR conditions are expected during the TAF period given
the low level dry air. Some fog is possible tonight across north-
central Wisconsin, mainly affecting the RHI airport.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Frost Advisory from midnight tonight to 9 AM CDT Friday for
WIZ005-010>012.
&&

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

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