Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 220344

Area Forecast Discussion
National Weather Service Green Bay WI
1044 PM CDT Tue Mar 21 2017

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Wednesday
Issued at 243 PM CDT Tue Mar 21 2017

The main forecast issues over the next 24 hours will be on lake
effect clouds/flurries as winds veer from north-northwest to
north-northeast tonight, and temperatures as a good radiative
cooling night is on tap IF clouds stay away.

The 19z MSAS surface analysis showed a strong area of high
pressure centered just north of Lake Winnipeg, with a couple weak
area of low pressure over the central Rockies. Visible satellite
imagery indicated an area of low clouds associated with lake
effect and daytime heating over the northern half of Great Lakes.
Another band of mid/high clouds was situated across the southern
Great Lakes associated with a mid-level shortwave. The radar
mosaic and surface observations were also picking up scattered
snow showers/flurries downwind of Lake Superior into northern WI.

The Canadian high pressure is expected to settle southeast into
the western Great Lakes by late tonight, allowing for skies to
become mostly clear, winds to become light and leading to a cold
night across the region. There will be some lake clouds initially
over northern WI this evening, followed by lake effect clouds for
eastern WI as winds veer north-northeast overnight. There could
also be some flurries in the east with delta-T values in the mid-
teens. The main story though for tonight will be the cold
temperatures with readings ranging from around zero over the
typical cold locations of north-central WI, to 15 to 20 degrees
over east-central WI.

This surface high slowly moves toward the eastern Great Lakes on
Wednesday and should bring a decent amount of sunshine to the
forecast area. Winds will gradually veer from the east-northeast
in the morning, to the south-southeast in the afternoon, so there
could still be some lake clouds near Lake MI through the day.
Otherwise, the onset of weak WAA will bring some mid/high clouds
to the rest of northeast WI. Temperatures are forecast to be on
the cool side with readings only able to reach the lower to middle
30s near Lake MI, middle to upper 30s elsewhere. A few locations
west of the Fox Valley could hit 40 degrees with enough sun.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 243 PM CDT Tue Mar 21 2017

Fairly active long term portion of the forecast as the main flow
brings a couple surface lows along with shortwave energy into
Wisconsin. Main focus is the system Thursday afternoon through
the weekend, timing trends, and precip types. Models still
struggle on agreement with timing, but have decent confidence in
getting above average precipitation during the period, with
questions remaining on temps and resulting pcpn types.

Latest 12Z model runs show some signs of light QPF, likely in the
form of snow, in the western forecast area early Thursday
morning. This appears to be associated with the surface low moving
east across Canada, but there is also a stronger surge of WAA at
this time. Added a slight chance PoPs to the far northwestern
forecast area for this time, but overall thinking is that there
may still be too much dry air to overcome for much of anything to
reach the ground.

Models generally agree that precipitation will begin Thursday
afternoon/evening as south winds ahead of the main low bring in
more moisture and strong WAA takes over. 12Z NAM appears to be the
outlier with a slower progression and much less QPF. Surface temps
are warm enough to support mainly rain on the onset Thursday. The
types become messy overnight as temps cool, however, models also
vary on exactly how cold it will get Thursday night. GFS is the
warmest and would suggest very little mixed precipitation. Other
solutions are cooler, especially near the surface, which would
mean a transition to freezing rain or mixed pcpn, especially in
the north. Kept a blended solution which supports mixed pcpn
overnight, and rain during the daytime/warmer hours.

Periods of rain likely to continue throughout Friday afternoon.
Pcpn becomes tricky again Friday night and Saturday. As the
surface low slowly passes south of Wisconsin models suggest pcpn
tapering off in the north, with the best chance for continued rain
to the south. If pcpn does continue in the north, this area is
more likely to get into wintry mix territory again Friday night
and early Saturday. Looking at rain as the precip type Saturday
afternoon, then another wave of switching to a wintry mix Saturday
night and Sunday. Moisture seems to pull back around into the
area during this time as the low moves from about northern
Illinois to southern Michigan. Rain is possible again Sunday as
the surface low moves off to the east, primarily in eastern
Wisconsin, before coming to an end Sunday evening. Liquid amounts
of 1 to 2 inches from Thursday through Sunday are possible in
areas where heavier rain bands setup, but again this is highly
dependable on the slower timing and track of the low.

Similar to previous runs, there is a brief break through the first
part of Monday. Then another system brings a rain/mixed pcpn
event Monday night into Tuesday.

Temperatures through the period look to waffle around average and
will likely depend on how continuous the rain is. Daytime highs
may need lowering if persistently wet forecast pans out.

.AVIATION...for 06Z TAF Issuance
Issued at 1038 PM CDT Tue Mar 21 2017

Patchy stratocumulus may brush far northeast WI and the Lake
Michigan shoreline areas early in the TAF period, but the clouds
will break up as high pressure and much drier air builds into the
region overnight. An increase in high/mid clouds is anticipated
during the afternoon and evening. VFR conditions will prevail.




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