Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 211732
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1232 PM CDT Sat Oct 21 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 418 AM CDT Sat Oct 21 2017

Warm today with scattered sprinkles or light showers early,
then more widespread showers tonight and Sunday. Much cooler
next week.

The upper pattern across North America will be in transition the
next couple days as a trough from the intermountain West shifts
into the Great Lakes region and is replaced by a building ridge
along the West Coast. The amplitude of the new pattern will peak
Tuesday, then additional changes will follow as the primary
features reform back farther west and reamplify by next weekend.

Unseasonably warm weather will continue today, then temperatures
will trend downward, but should still be modestly above normal
through Monday. Readings much closer to normal are anticipated for
the remainder of the work week. Unlike this past work week which
was completely dry. the new pattern will result in some
precipitation. But amounts will depend on the specifics of the
evolution of the pattern and models are struggling with that as
they often do when retrogression occurs. The best guess is that
amounts will end up near normal for the period, though that`s a
low confidence forecast.
&&

.SHORT TERM...Today...Tonight...and Sunday
Issued at 418 AM CDT Sat Oct 21 2017

What was most striking at the start of the shift was the lack of
precipitation across the Plains. The 00Z RAOBs detected rather
limited moisture, with the moisture axis still way back to the
west. That was despite deep strong southerly flow ahead of the
upper trough advancing in from the west. The surface front was
also still pretty far back to the west. Radar returns have since
increased a little, but are still limited to just a few clusters
of convection and some sprinkles. Strong southerly flow ahead of
the front will continue to advect moisture northward and into the
area. But considering where we are starting at it seems likely the
main rain band along the front will arrive more slowly than
previously anticipated. The guidance seems to have latched onto
that, and the primary change with the new forecast was to slow the
arrival of the main rain band tonight.

Prior to the arrival of the main rain band, some sprinkles or
light showers will be possible this morning as the fairly steep
mid-level lapse rates and the LLJ remains across the area. Not
sure how much of this will be measurable since cloud bases will be
very high. A disturbance lifting northeast from the Plains was
generating a cluster of convection over IA, and that may clip
north-central Wisconsin later in the morning, so carried chance
PoPs there.

The primary rain band with the front probably won`t arrive in the
area until around midnight. Forcing will initially be weakening,
so expect the precipitation to do the same. The day one convective
outlook from SPC kept the far western part of the forecast area in
the marginal risk area for severe storms. Instability will be
limited and decreasing, but strong winds above the surface could
be mixed down if the precipitation is heavy enough.

After the lull in forcing late tonight, frontogenetic forcing and
upper divergence will increase as the front reaches the east
Sunday morning as an upper speed max shifts from the Plains into
western Ontario. That should cause the band of showers along and
behind the front to increase again. Carried categorical PoPs in
east-central Wisconsin to account for that.

The slower arrival of the precipitation suggested max temps today
will again be above guidance. With atypical diurnal trends
expected tonight and Sunday, built hourly temperature grids from a
blend of the guidance products, the generated the Max/Min grids
from those.

.LONG TERM...Sunday Night Through Friday
Issued at 418 AM CDT Sat Oct 21 2017

500mb ridge that has brought the mild weather to the area during
the past several days will flatten, and then be replaced with a
deepening trough across the western Great Lakes region on Monday.
The northern stream system is expected to interact with a southern
stream system across the Ohio River Valley, resulting in a closed
upper low across the region by Tuesday morning. The system is
expected to open up and move east of the area on Wednesday. How
to the two systems interact leads to a low confidence in the
forecast with respect of the strength of the system and its
impacts across the forecast area. The ECMWF was the most aggressive
model the last several nights with respect to the strength of the
system deepening over Lake Huron. The ECMWF has backed off on the
magnitude of the system. The GFS ensemble members with the
surface low position were in a little better agreement from last
night, but still offer many varying solutions on the eastern
envelop of the surface low tracks between the Canadian, ECMWF and
the GFS operational model.

The chances of showers will arrive Monday afternoon and continue
at times into Wednesday. The highest chances of rain appear to
be on Tuesday into early Tuesday evening. For the remainder of
the period, weak disturbances moving in the mean flow will bring
chances of showers at times Thursday and Friday. Temperatures
will be above normal on Monday, then temperatures should be or
even slightly below normal for the remainder of the work week.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1231 PM CDT Sat Oct 21 2017

Pockets of MVFR cigs will be possible across central WI this
afternoon as a mid-level shortwave lifts northeast into western
Ontario. Eastern sections of WI will remain VFR with not much in
the way of any cloud cover. Flying conditions are expected to
gradually deteriorate later tonight as a cold front moves east and
reaches central WI around 12z Sunday. Expect a broken line of
showers and isolated thunderstorms to accompany the front and send
cigs down to the low-end MVFR category. Cigs could also briefly
dip to IFR conditions for a few hours immediately behind the cold
front. Have also kept the LLWS for all TAF sites tonight as winds
just above the surface could reach 40-45 knots. The showers and
possible thunderstorms would then continue to track slowly to the
east on Sunday, bringing the lower cigs into eastern WI. Expect
the precipitation to end over central WI by mid-afternoon Sunday
with cigs becoming VFR.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Eckberg
AVIATION.......Kallas



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