Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 141638
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1138 AM CDT Thu Sep 14 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 406 AM CDT Thu Sep 14 2017

Continued very warm and mainly dry for a couple more days, then a
little cooler with some rain by the latter part of the weekend.

The main westerlies across North America are primarily across
Canada, but beginning to sag southward across the West where a
longwave trough is beginning to form. The trough will briefly
weaken during the weekend as a strong shortwave ejects toward the
downstream ridge, then reform a little farther west next week.
Models differ on the exact position of the trough and ridge next
week, and that has important implications for the forecast.

Backing upper flow downstream from the developing trough will
continue to steer a very warm air mass from the Plains into the
area, keeping temperatures significantly (10-18 F degrees) above
normal through the weekend. They`ll drop back next week, but
probably remain 5-10 F degrees above normal. Increasing upper
ridging and a dry air mass will initially be unfavorable for
significant precipitation, though some weak convection could brush
the far north/northwest at times. The most significant chance for
rain will occur during the latter half of the weekend as a strong
shortwave ejects out of the western trough and drives a cool
front through the region. Precipitation chances beyond that depend
on the exact positions where the trough and ridge reform. They
could range anywhere from only minor rain chances (ECMWF) to a
rather wet pattern (GFS/Canadian NH). As such, I don`t feel it`s
possible to provide a meaningful estimate of precipitation amounts
for the next 7 days.
&&

.SHORT TERM...Today...Tonight...and Friday
Issued at 406 AM CDT Thu Sep 14 2017

The fog is less widespread this morning, likely due to stronger
flow in the boundary layer. Will continue to have patchy ground
fog across the area through daybreak. Of more concern is an
expanding area of FG/ST spreading north from southeast WI. The
progress of the FG/ST shows up well on the GOES-16 RGB Advanced
Nighttime Microphysics product. The fog has surged into the
southern Lake Michigan NSH zone, and will likely reach the next
zone as well. So will post a marine fog advisory for those areas.
Inland, it`s progress toward Manitowoc county appears to have
slowed in the past 1/2 hour, so will hold off on an advisory for
that area for now. Figuring out when the fog goes away is more
difficult. Guidance indicates it could last all day and into
tonight. But the lake is near it`s annual temperature peak (mostly
around 65F), and with dew-point temperatures only around 60F the
fog may behave differently than it does when the water is very
cold in the spring. The fog over the lake yesterday lingered into
mid-day and then dissipated. Will run the advisory through 15Z
(matching MKX) for now, and let the day shift reassess.

A little stronger surface pressure gradient and stronger boundary
layer winds tonight suggest fog won`t be an issue, except for
possibly over the lake again.

The overall trend on the models is to keep precipitation confined
to near the frontal system draped to our north. That seems
reasonable give limited moisture and capping around 800 mb. But
some weak isentropic lift and steep mid-level lapse rates could
result in high-based convection atop the inversion. Just brushed
the far N/NW with PoPs tonight into Friday to account for this.

Little change in air mass suggests persistence will yield a good
high temperature forecast for today, with readings perhaps just a
bit warmer due to most areas getting off to a warmer start. But a
strengthening gradient south flow up Lake Michigan will keep the
lakeshore a little cooler. Readings will probably edge up another
degree or two Friday as even warmer air arrives in the area.
Stayed close to a blend of top-performing guidance for mins
tonight. The guidance has been too warm for mins lately, but a
little more wind, some high clouds, and a little added moisture
will result in conditions less favorable conditions for
radiational cooling than the past few nights.

.LONG TERM...Friday Night Through Wednesday
Issued at 406 AM CDT Thu Sep 14 2017

Models have come into better agreement with the timing/movement of
an upper trough/cold front combination that will impact northeast
WI Saturday night into Sunday. Once this system departs, focus
turns to the next impressive looking upper trough forecast to dig
into the western CONUS early next week. The main question is
whether downstream upper ridging over the east-central CONUS will
hold off any pieces of energy ejecting northeastward in the mean
southwest flow aloft. If the answer is no, northeast WI could see
precipitation return as early as Monday night. If the answer is
yes, northeast WI could be dry through next Wednesday. After a
brief cool down on Monday, temperatures will return to above
normal conditions for both Tuesday and Wednesday.

There remains some concern for a few showers/possible
thunderstorms across northern WI Friday night as a quasi-
stationary front to be locate just to our north and a shortwave
rides northeast across the Upper MS Valley. Prefer to keep any
pops on the low side for now. The rest of the forecast area should
be dry under partly to most cloudy skies. Min temperatures to
generally be in the upper 50s to lower 60s. As the initial upper
trough begins to lift northeast across the northern Rockies toward
the northern Plains on Saturday, the associated cold front is
forecast to be slowly moving across the Upper MS Valley. A lack of
any trigger and a capped atmosphere appears to keep any
precipitation away during the daylight hours. Persistent southerly
winds, with 8H temperatures in the upper teens (C), will bring an
unseasonably warm day to northeast WI. Look for max temperatures
to be in the middle to upper 70s near Lake MI, around 80 degrees
north and lower to middle 80s south.

The upper trough will continue to lift northeast across the
northern Plains Saturday night, while low pressure also moves
northeast from southern MN to western sections of Lake Superior.
The cold front is forecast to approach central WI by 12z Sunday
and be the primary focus for showers/thunderstorms. The threat for
any severe storms still appears to be quite low due to minimal
instability and stronger shear hanging back behind the frontal
boundary. Min temperatures to range from the upper 50s north-
central WI, to the lower to middle 60s elsewhere. Showers and
thunderstorms chances would then carry over into Sunday morning
central WI, Saturday afternoon eastern WI as the cold front
continues to march east through the rest of the state. The onset
of CAA behind the frontal passage will usher a cooler air mass
into the region. Look for max temperatures on Sunday to range from
the upper 60s to around 70 degrees north-central, to the middle
70s over eastern WI (slightly cooler than Lake MI).

An area of high pressure is forecast to build into the western
Great Lakes region Sunday night and bring clearing skies/cooler
conditions to northeast WI. This high pressure begins to shift
east on Monday and some models try to develop a little
precipitation associated with a weak mid-level shortwave. Hard to
determine whether there would be enough forcing or moisture
availability to allow for any precipitation development. May end
up placing a slight chance pop in the forecast for Monday, by
confidence is rather low. Max temperatures for Monday will be more
typical of mid-September with readings mainly in the upper 60s to
lower 70s.

The forecast becomes more uncertain as we head toward the middle
of next week, partially due to the unknown movement of tropical
system `Jose` over the western Atlantic and partially due to the
location/strength of upper ridging over the east-central CONUS.
Both the GFS and CMC prefer a weaker ridge axis, therefore
allowing the old cold front to light back north as a warm front
Monday night with showers/thunderstorms overspreading the
forecast area. The chances for additional showers/storms would
continue into Tuesday (assuming this scenario plays out). Problem
is that the ECMWF keeps the upper ridging in place and prevents
any precipitation from reaching northeast WI Monday night through
Wednesday. Due to all the question marks, have deferred to the
consensus solution which focuses the highest pops during the
Monday night and Tuesday night time frames, but anticipate the
models to waffle on these times in the coming days. Temperatures
are expected to warm once again as a southwest flow aloft takes
over the central CONUS. Max temperatures by next Wednesday could
again be 10 degrees above normal.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1138 AM CDT Thu Sep 14 2017

VFR conditions to prevail this afternoon into tonight
across more locations. The exception includes areas of LIFR/IFR
cigs and vsbys which continue to drift northward over Lake
Michigan and the adjacent shoreline. Increasing south winds this
afternoon into tonight may serve to lift the fog to mainly IFR
cigs this afternoon and tonight. The increasing south winds
should aid with reducing the coverage and duration of the fog
tonight.
&&

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

$$

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



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