Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 171754
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1254 PM CDT Sun Sep 17 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 402 AM CDT Sun Sep 17 2017

Temperatures returning close to seasonal normals for the start of
the work week, then likely warming back to unseasonably warm
levels for the middle to latter part of the week.

The upper pattern of a western trough and eastern ridge will
persist across North America throughout the forecast period,
though the amplitude of the pattern will vary considerably. The
pattern is currently flattening as a shortwave ejecting out of the
trough drives into the ridge position. But strong reamplification
will begin during the upcoming work week, peaking late in the
week or next weekend.

A cool front trailing south from the cyclone generated by the
ejecting shortwave will cross the area early today, ushering
a Canadian air mass into the area. Temperatures will be cooler
than yesterday, but still above normal. A couple days with
seasonable temperatures will follow as the modifying Canadian air
mass shifts through the region, then readings will return to well
above normal as a strong southwest upper flow develops over the
area. Precipitation chances remain difficult to gauge due to model
differences in the proximity of the eastern ridge to the forecast
area.
&&

.SHORT TERM...Today...Tonight...and Monday
Issued at 402 AM CDT Sun Sep 17 2017

Scattered showers along with a few clusters of thunderstorms were
shifting across the area early this morning, in advance of the
cold front pushing in from the west. The main band of convection
was east of far north-central Wisconsin, but the radar mosaic
showed some precipitation lingering over east-central Minnesota,
and local mesoplots showed that the tail of the strong QG forcing
with the main shortwave may brush north-central Wisconsin this
morning. So can`t totally give up on additional showers in that
area yet. Otherwise, generally followed the trend of the models in
shifting the precipitation east of the are by late morning. Cooler
air will be advecting in today, but the area will be off to a very
warm start, so edged maxes a little above guidance.

A weak anticyclone will shift across the north tonight. Light
winds across that area could lead to the formation of some fog and
maybe some frost. Meanwhile, the passage of the high to the north
will bring winds around to the north and then northeast across
the southern and eastern part of the area, and likely result in a
secondary cold front/wind shift sliding down the lakeshore. Winds
staying up and having an onshore component should hold mins up
some in the southeast part of the area.

Models were in pretty good agreement in showing an area of
precipitation pushing into at least the western part of the area
Monday afternoon in the return flow on the back side of the
anticyclone. There`s middle and upper support too, as the forecast
area will be in the RRQ of a small upper speed max in the Lake
Superior vicinity, and a mid-level shortwave will be riding east-
northeast across the area. The main concern is that dry low-levels
may take a while to saturate and slow the arrival of the
precipitation some. The blended guidance product produced chance
PoPs over the west for the afternoon, and that seemed reasonable.

.LONG TERM...Monday Night Through Saturday
Issued at 402 AM CDT Sun Sep 17 2017

Models continue to advertise a highly-amplified mean flow
consisting of an upper ridge over the eastern Pacific, a deep
upper trough over western NOAM and a second upper ridge over the
eastern CONUS. The models are trying to get into better agreement
with regards to height falls/movement of a cold front across the
central CONUS mid-week. This is followed by another strong system
diving into the western CONUS late week acting to strengthen the
upper trough. For northeast WI, this places a broad southwest flow
aloft into WI with more above normal temperatures and periodic
rain chances.

The mid-level shortwave trough continues to track northeast across
WI Monday evening, eventually reaching Lower MI Monday night. Even
though instability is lacking with this system, there is good
frontogenetical forcing, with lift provided by the right entrance
region of the upper jet. May need to raise pops Monday evening
with the main focus shifting east after midnight. Min temperatures
to range from the upper 40s to around 50 degrees north, lower to
middle 50s south. This system departs on Tuesday, leaving the
region on the southwest fringe of Canadian high pressure located
near Hudson Bay. Anticipate a mix of sun and clouds through the
day with temperatures slightly above normal. Look for readings in
the upper 60s to around 70 degrees north/near Lake MI, lower to
middle 70s south.

As the new piece of energy (over the Pacific Northwest) dives
southeast into the main upper trough Tuesday night, this will kick
out a large piece of energy into the northern Plains. This in turn
is forecast to move a cold front into the Upper MS Valley with a
push of WAA into WI. Expect clouds to increase Tuesday evening
with a chance of showers/thunderstorms arriving after midnight.
Highest pops to be placed over central WI where the isentropic
lift is strongest. Min temperatures to be in the middle to upper
50s north, upper 50s to lower 60s south. There continues to be
some timing issues with regards to the timing of the cold front
into WI at mid-week. The GFS has the cold front reaching eastern
WI by 00z Thursday, while the ECMWF only has the boundary reaching
western WI. This timing difference would obviously impact
precipitation chances, especially over the east where the ECMWF is
dry through the daylight hours. Prefer to play the middle of the
road and bring the front into central WI late on Wednesday with
only slight chance pops east/chance pops central WI. Too early yet
to determine the strength of these storms, especially with the
uncertainty of frontal timing. Max temperatures for Wednesday
should warm ito the lower 70s north-central/near Lake MI, middle
to upper 70s elsewhere.

The ECMWF does catch up to the GFS with regards to the cold
front`s location Wednesday night (over northeast WI) as the front
runs into the upper ridge situated over the east-cetnral CONUS.
The concern now is that the boundary will stall over the area, or
even begin lifting north as a warm front as the prevailing
southwest flow aloft continues into WI. The dry forecast may now
be in doubt. Have decided to introduce a small chance pop for this
possibility for now, although confidence on this front`s location
is rather low. Max temperatures on Thursday appear to be similar
to Wednesday`s, or about 5 degrees above normal.

The forecast for the end of the week will come down to the
location of the upper ridge in proximity to WI. The models range
from the East Coast to the eastern Great Lakes. A farther east
location would allow for another cold front to focus precipitation
into WI as pieces of shortwave energy eject northeastward from the
primary upper trough. A farther west solution of the upper ridge
would keep both the cold front and individual shortwaves to our
west, thereby dry and warm conditions for the forecast area.
Another possible player(s) could be the two expected hurricanes
over the western Atlantic that could bog down systems trying to
move east across the CONUS. Until this muddled picture starts to
clear, will have to at least acknowledge a chance of showers/
thunderstorms for both Friday and Saturday. Temperatures will
remain above normal, no matter which solution ends up correct.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1254 PM CDT Sun Sep 17 2017

Mainly VFR conditions behind a departing cold front. MVFR ceilings
in the east should improve to VFR while MVFR ceilings in the far
north persist. IFR/LIFR fog is expected to develop in parts of
northern Wisconsin late tonight into early Monday morning before
fog burns off and VFR conditions prevail across the area by
midday.
&&

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

$$

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



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