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000
FXUS63 KGRB 161756
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1256 PM CDT Sat Sep 16 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 439 AM CDT Sat Sep 16 2017

Still very warm today, then turning cooler for a few days before
much above normal temperatures return by the middle of next week.

The upper flow across North America will be dominated by a western
trough and eastern ridge throughout the period, with the main
variability being amplitude. The flow will flatten some the next
couple days as a strong shortwave ejects out of the trough and
drives into the downstream ridge. But strong reamplification will
occur next week as a powerhouse pacific jet dives southeast into
the trough from the Gulf of Alaska.

Very warm temperatures are expected today, then readings will
settle back closer to normal by early next week. But the cool down
will be short-lived as reamplification of the upper flow will
drive warmer air from the Plains back into the area for the middle
to end of next week. Most of the area will get some precipitation
tonight or early Sunday, but amounts will probably be limited by
an unfavorable (late night/early morning) timing of the FROPA and
the fact that the bulk of the upper forcing will pass well
northwest of the area. Beyond that, it would seem the pattern
with a trough to the west and a ridge to the east should
eventually lead to some wetter weather. But it`s just not clear
if that will happen during the forecast period or beyond since
models continue to have differences in handling the details of the
pattern.
&&

.SHORT TERM...Today...Tonight...and Sunday
Issued at 439 AM CDT Sat Sep 16 2017

Much like yesterday, isentropic lift atop the inversion was
resulting in weak high-based convection at the edge of the
elevated mixed layer. It seems showers/sprinkles in regimes such
as this typically dissipate a couple hours after daybreak. That`s
what happened yesterday, and developed forecast based on a similar
expectation for today. Once the showers/sprinkles end, the models
were generally dry for today. But will need to closely watch a
cluster of storms lifting northeast from SE MN. Those could clip
the northwest part of the forecast area during the mid to late
morning.

A frontal system currently separates seasonably cool air
over Canada from the very warm air mass over much of the eastern
CONUS. That boundary will remain northwest of the area today, then
surge southeast across the area tonight and Sunday morning as
upper energy ejecting out of the western trough generates a strong
cyclone over western Ontario. The front crossing the area late
tonight and early Sunday seems to favor having limited
precipitation with the front. The models suggest that as well as
their QPFs generally decrease as the precipitation moves east
during the night. Kept likely PoPs over the west, but limited them
to the chance category in the east. The front should clear the
area by mid-day Sunday. The overall situation does not look
favorable for severe weather, though given the dew points in the
60s a few strong storms can`t be ruled out.

.LONG TERM...Sunday Night Through Friday
Issued at 439 AM CDT Sat Sep 16 2017

After a brief break down this weekend, the mean flow becomes
highly-amplified next week with a deep upper trough over the
western CONUS and strong upper ridging over the eastern CONUS.
The main forecast uncertainty remains with tropical system `Norma`
near Baja CA and whether its remnants get pulled northeast through
the central CONUS for unsettled weather over WI mid-week OR will
its remnants stay over the southwest CONUS, leaving WI on the
western edge of the upper ridge. There are also model differences
with regards to the depth of the western CONUS and how fast a cold
front will push east mid to late week. Until all of this gets
sorted out, there are plenty of question marks as far as
precipitation trends are concerned. After a brief cool down early
next week (closer to normal), temperatures will go above normal
for much of the mid to late week.

High pressure quickly moves across WI Sunday night and while clear
skies will start during the evening hours, high clouds to already
begin pushing back into WI from convection over the central
Plains/Midwest. Depending on how much rain falls Saturday night
into Sunday, there could be some patchy late night fog as winds
become light, especially over north-central WI. Min temperatures
will range from the lower 40s far north, to the lower 50s across
the east. If these high clouds stay away long enough, there could
be a few upper 30s over the colder spots of the northwoods.

Models now appear more settled on the northeast movement of a
mid-level trough into the Midwest on Monday with sufficient
forcing to bring a chance of showers and thunderstorms to parts of
northeast WI during the afternoon hours. More cloud cover would
also negatively impact temperatures and may need to lower values
from the previous forecast. For now, will have readings in the
middle to upper 60s north, upper 60s to lower 70s south.

Model differences begin to show up on Tuesday with respect to the
amount of digging of the new upper trough over the western CONUS.
The GFS is flatter with this trough, thus this model is already
sending the next cold front across the northern Plains, with a
warm front lifting north into WI providing for a small chance of
showers/thunderstorms. The majority of other models are digging
this upper trough much deeper into the central Rockies, thus a
slower eastward movement of the cold front and the warm front to
be much farther south with no precipitation in WI. Based on the
strength of the upper jet diving southeast into the Pacific
Northwest (140 knots), prefer the majority solution here and go
mainly dry Tuesday afternoon (after lingering morning
precipitation chances depart). Max temperatures to range from
around 70 degrees near Lake MI, to the lower 70s north and lower
to middle 70s south.

Northeast WI to reside in a broad southwest flow at mid-week with
modest WAA building into the region. Cannot rule out a few showers
Tuesday night or Wednesday, but any precipitation appears to be
spotty at best. On a side note, it now seems the models agree that
the remains of `Norma` will not head our way and be any factor in
our weather. Temperatures will continue to slowly warm with
readings reaching the lower 70s near Lake MI, generally 75-80
degrees elsewhere.

The aforementioned cold front is progged to slowly push east,
reaching the Upper Midwest Wednesday night and edging toward
western WI on Thursday. The forward speed of the cold front will
be hampered by both Hurricane Jose in the western Atlantic and the
upper ridge over the east-central CONUS. In fact, it is
conceivable that this front may wash-out before even reaching WI.
At this point, prefer to downplay pops a bit for both Wednesday
night and Thursday. Max temperatures for Thursday should be
similar to Wednesday, or about 10 degrees above normal.

Assuming all of this timing is correct, the next chance of a more
widespread rain event may hold off until Friday night when the
next cold front tries to push east toward the Great Lakes.
However, due to the great uncertainty for the latter half of next
week, prefer to go with continuity here and keep a chance of
showers and thunderstorms in the forecast on Friday.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1255 PM CDT Sat Sep 16 2017

VFR conditions should persist across the area the rest of the
afternoon into early evening. MVFR and some IFR ceilings, along
with showers and thunderstorms, will accompany a passing cold
front. Ceilings should become VFR again by late morning, once the
front has passed to the east of Wisconsin.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 11 PM CDT this evening for WIZ022-
040-050.
&&

$$

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