Area Forecast Discussion
Issued by NWS Green Bay, WI

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

FXUS63 KGRB 171803

Area Forecast Discussion
National Weather Service Green Bay WI
103 PM CDT MON OCT 17 2016

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 354 AM CDT Mon Oct 17 2016

Precipitation trends, the potential for severe weather this
afternoon and evening, dense fog on the lakeshore and unseasonably
warm temperatures, are the main forecast concerns.

Showers and embedded thunderstorms were moving through northern
WI, due to forcing associated with ab 850 mb warm front and
short-wave trof. Despite MUCAPE increasing to 1000-1400 j/kg
and impressive deep layer shear, the storms have remained below
severe limits, except for one very intense storm over the western
Upper Peninsula. This batch of rain is expected to shift out of
far northeast WI shortly after 12z, followed by a general lull in
the precipitation until mid-afternoon.

The surface warm front is expected to lift into central and
northeast WI this afternoon, reaching a MFI-EZS-SUE line by
evening. Models show convection redeveloping near the warm
front in the mid to late afternoon, then lifting into northern
WI in the early evening. With deep layer shear of 60 to 75 kts
and Helicities of 200-400 mb, supercells are a possibility.
SPC has expanded the marginal risk of severe thunderstorms to
most of GRB CWA, and upgraded parts of central and north central
WI to a slight risk. This event is more typical of a mid to late
September event, as evidenced by CIPS analogs, which showed
several similar events (mostly mid-late September )with multiple
severe reports in GRB CWA, and even a couple tornadoes.
Significant rainfall will be possible across northern WI as a
surface low moves through this evening, when PWATs are around 1.5
inches. A trailing cold front will swing through the forecast area
overnight, followed by a decrease in shower activity.

As dew points have climbed into the upper 50s and lower 60s
overnight, marine advection fog has developed on Lake Michigan
and the lakeshore counties. Based on observations and soupy
conditions on lakeside web cams, have decided to issue a dense
fog advisory for Door, Kewaunee and Manitowoc counties, and for
the nearshore waters on Lake Michigan. These headlines will expire
at 15z, but think that areas of fog will persist on Lake Michigan
through this evening.

Unseasonably warm temperatures will push into the region with
the warm front today, with highs getting into the lower to
middle 70s over central, east central and parts of northeast WI.
These readings are about 15 degrees above normal. Despite the cold
frontal passage tonight, highs on Tuesday will still be above
normal, in the lower to middle 60s.

.LONG TERM...Tuesday Night Through Sunday
Issued at 354 AM CDT Mon Oct 17 2016

Main weather feature through mid-week will be the movement of a
longwave trough across the central CONUS and any potential impact
over northeast Wisconsin. This trough is progressive and will
reach the eastern CONUS late week, leaving a fast west-northwest
flow over the rest of the CONUS. For northeast Wisconsin,
precipitation amounts will be below normal with only spotty
precipitation chances as the longwave trough moves by and perhaps
next weekend associated with weak Pacific systems. Temperatures
are expected to settle closer to normal through the week into next

Weak surface high pressure is expected to nose into southern
Wisconsin Tuesday night, while a weak surface trough moves across
the Upper MS Valley. Skies should hold mostly clear to partly
cloudy with relatively light winds. Depending on how much
precipitation fell on Monday/Monday night, we could see patchy fog
develop later Tuesday night. Min temperatures to range from around
40 to the lower 40s north, middle 40s to around 50 degrees south.
The weak high pressure will stretch across southern sections of
the Great Lakes on Wednesday, while the models essentially wipe
out the surface trough. This should leave a mix of sun and clouds
over the region with max temperatures in the middle to upper 50s
north-central, mainly 60-65 degree range elsewhere.

The aforementioned longwave trough is progged to extend from
western Ontario south-southwest to the southern Plains Wednesday
night. Most models keep Wisconsin dry through the night, but will
have to watch for the potential of a sheared shortwave that could
move into the Great Lakes after midnight. Have kept the dry
forecast intact for now with min temperatures to range from the
middle to upper 30s north-central, to the middle 40s along Lake
MI. The longwave trough reaches the western Great Lakes on
Thursday, bringing cool air aloft into the region with 8H
temperatures in the zero to -4C range. Models continue to focus
the bulk of the precipitation to our east along an old cold front
stretched from the Gulf Coast to the eastern Great Lakes. Cannot
rule out a few instability showers or even a few lake effect
showers over the north in the afternoon, but anticipate a large
majority of northeast Wisconsin to be dry on Thursday. Max
temperatures to range from the upper 40s north-central, to the
middle 50s east-central Wisconsin.

Models are consistent in taking the longwave trough east to the
eastern Great Lakes on Friday, bringing a weak cyclonic flow to
Wisconsin. Air mass aloft is still quite cool and with the warm
waters over the Great Lakes and a prevailing northwest wind
Thursday night into Friday morning, we could continue to see the
potential of some lake effect light rain or even mixed rain/snow
over far northern Wisconsin. Once again, most of the forecast area
will remain dry under a partly cloudy sky. Max temperatures on
Friday to be in the 45-50 degree range north, 50-55 degree range

Forecast becomes a little more uncertain for next weekend as the
mean flow turns west-northwest over Wisconsin due to a very weak
upper ridge over the Rockies. Weak little systems tend to race
through a fast near-zonal flow like this and models tend to
struggle timing these little features. Precipitation chances are
possible late Friday night/Saturday morning as warmer air aloft
pushes into Wisconsin and possibly again Saturday night into
Sunday morning with the passage of a weak Pacific system. Have
followed the consensus solution for now due to the uncertainty
involved with only slight chances pops mentioned over parts of the
forecast area. Max temperatures over the weekend should be close
to normal with lower 50s north, middle to upper 50s south.

.AVIATION...for 18Z TAF Issuance
Issued at 102 PM CDT Mon Oct 17 2016

MVFR conditions prevailed across most of the forecast area at
midday, with mainly IFR ceilings and fog across north central
Wisconsin. The entire forecast area was in the warm sector of a
surface system. There was a surface inversion evident on the 12Z
GRB sounding and the atmosphere was saturated below a stronger
inversion just below 700mb. Mesoanalysis showed SB CAPEs in excess
of 1000 j/kg across much of the area at 17Z but there was still
plenty of CIN.

Little or no improvement is expected the rest of the day and the
chance for thunderstorms increases later this afternoon with an
approaching jet max and mid level short wave, and a surface low
over the Plains moving northeast along the cold front. Showers and
storms should exit the area late tonight but some low clouds and
fog may hang around for a while. VFR conditions should prevail on



SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......MG is the U.S. government's official web portal to all federal, state and local government web resources and services.