Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 200353
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1053 PM CDT Fri May 19 2017

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Saturday
Issued at 250 PM CDT Fri May 19 2017

Clouds continue to thicken as a low pressure system develops over
the central plains. Despite warm air advection showers on Doppler
Radar across southern Wisconsin and the Minnesota/Iowa border,
very dry air being pumped in by the high pressure system across
James Bay is eroding these showers rather rapidly. Therefore will
scale back on the shower activity for the afternoon and overnight
hours, opting for a dry forecast for the area.

This dry air will also play a role in precipitation chances on
Saturday as the low lifts northeast towards the upper Mississippi
Valley region. Despite a fairly solid forecast for rain in
previous forecasts, recent model runs indicate the aforementioned
dry air will delay the onset of rain showers by several hours.
Central and east-central Wisconsin look to get rain sometime in
the morning hours, while north-central Wisconsin holds off until
the afternoon hours as the rain moves in from south to north. The
new forecast reflects this trend in the models and current dry
airmass in place. Some elevated instability could bring a rumble
or two of thunder across central and east-central Wisconsin during
the afternoon hours with the rain showers. When the rain finally
does arrive it could be moderate or heavy at times, exacerbating
the already swollen area rivers. Persons near these already
flooded areas will want to monitor forecasts and warnings closely
as this additional rain falls this weekend.

Abundant cloud cover will keep temperatures a few degrees warmer
tonight. Despite temperatures falling into the middle 30s across
far north-central Wisconsin, frost should not be much of an issue
given the gradient in place as winds are not expected to die down
completely. The abundant clouds and rain showers on the cold side
of the approaching low will keep highs muted to the lower to
middle 50s on Saturday, a good 10 to 15 degrees below normal for
this time of year.

.LONG TERM...Saturday Night Through Friday
Issued at 250 PM CDT Fri May 19 2017

Northeast WI to still be dealing with a strong nearly vertically-
stacked system through the end of the weekend with precipitation
chances hanging around the north through Sunday night. The mean
flow early next week to consist of upper ridges over the West
Coast and western Atlantic, with upper troffing to reside over
east-central North America. Additional mid-level energy will allow
this trough to sharpen toward mid-week with more precipitation
chances arriving for northeast WI from Monday night through at
least Tuesday night. Model uncertainty remains for late next week
with respect to the West Coast upper ridge moving east and
weakening with time. Temperatures will continue to run below
normal (on average 5-10 degrees) through Thursday before perhaps
finally reaching or possible surpassing normal readings next
Friday.

Showers/isolated thunderstorms will continue through at least
Saturday evening as the surface low tracks north-northeast into
the western sections of the Great Lakes and a cold front swings
across WI. Precipitation trends will tend to diminish later
Saturday night over central/east-central WI as a dry slot rotates
northeast into the region. PW values to be in the 0.75 to 1.00"
range central WI/1.00-1.25" range eastern WI through the evening
hours, thus an additional 0.10-0.33" of rain may fall over the
forecast area. Temperatures in the east will hold steady or even
climb a couple of degrees before the frontal passage occurs. Look
for min temperatures to eventually fall into the lower to middle
40s north, middle to upper 40s central and around 50 degrees
east-central WI.

The surface low (and its associated/trailing mid-level shortwave
trough) will lift slowly north-northeast into Lake Superior/
Upper MS Valley respectively on Sunday. Wrap-around moisture
coupled with a cold pool aloft, may kick off some additional rain
shower activity over northeast WI. Any rain would be light and not
be a further issue for rivers/streams that are running high across
the state. Otherwise, plenty of clouds and a blustery west wind
can be expected for Sunday with max temperatures only in the lower
to middle 50s north-central WI, around 60 degrees over east-
central WI. This would be about 10 degrees below normal.

Shower chances to persist into Sunday night as the system is
forecast to only reach southern Ontario, a surface trough to
rotate southeast over WI overnight and an individual shortwave to
move southeast through the northwest flow aloft across WI. The air
mass overhead will be quite chilly by late May standards with 8H
temperatures approaching 0C across northern WI. Would not be
surprised to see a few snowflakes mix with any rain late Sunday
night in the northwoods. Min temperatures to range from around 40
degrees north-central, to the middle 40s east-central WI.
Northeast WI may get a brief break in the precipitation chances on
Monday as we finally get rid of the initial system and before we
start feeling the effect of the next system dropping south into
the Upper MS Valley (essentially dumbbelling around the initial
system). Still anticipate a decent amount of cloud cover across
the area, but any sunshine should help raise max temperatures to
around 60 degrees north-central, lower 60s near Lake MI and middle
to upper 60s south.

This latest system, in the form of another mid-level shortwave
trough, is progged to rotate southeastward and strengthen with
time into a longwave trough that will eventually stretch from
James Bay to near the Gulf of Mexico during the Tuesday-
Wednesday time frame. Precipitation chances will exists Monday
night and continue into Tuesday (especially over central/east-
central WI) as the shortwave trough moves through WI and a surface
wave develops on the old cold front that moves east-northeast
across southern sections of the Great Lakes. Cyclonic flow on the
backside of the developing longwave trough could keep rain chances
in the forecast across eastern WI through Tuesday night and may
not completely end until Wednesday afternoon. Central parts of WI
could see a weak ridge of high pressure arrive on Wednesday and
provide for some sunshine. Temperatures to remain below normal
through mid-week, although Wednesday will be a little closer to
normal compared to Tuesday`s readings.

Toward the end of the work week, the mean flow will have undergone
some changes as the east-central North American trough is forecast
to shift toward the East Coast and the West Coast upper ridge to
shift east into the central CONUS, but weaken in the process. The
end result would be an almost zonal flow from the West Coast to
the east-central CONUS which would allow milder Pacific air to
flow east into WI. Exactly how warm we can get will depend on
clouds/precipitation chances as there should be a chance of
showers/thunderstorms Thursday as the warmer air arrives (warm
front) and possible on Friday as a piece of mid-level energy rides
the mean flow into the Great Lakes and interacts with increasing
instability. For now, will have Friday as the warmest day of the
extended forecast with max temperatures in the lower 60s near Lake
MI, middle 60s north-central and middle to upper 60s elsewhere.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1052 PM CDT Fri May 19 2017

VFR conditions through about 15 UTC on Saturday
with mainly mid-level cloud deck overnight. Then flying conditions
will deteriorate steadily from SW to NE as a low pressure system
brings a fairly contiguous rain shield and IFR conditions,
starting Saturday afternoon and continuing WELL into Saturday
night. East winds gusting up to 25 kts is also expected especially
over eastern Wisconsin as the low pressure system approaches.
Showers should end Sunday with conditions improving to MVFR by
afternoon.
&&

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

$$

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



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