Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 201657
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1057 AM CST Mon Feb 20 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 409 AM CST Mon Feb 20 2017

Unseasonably warm through mid-week, with rain this afternoon and
tonight. Then turning colder with some rain and snow as a potent
storm system crosses the area late in the week.

The split flow that has been dominating North America will
gradually transform into a single dominant band of westerlies.
That will begin to occur as the current southern stream
strengthens and expands northward across the CONUS during the
early part of the forecast period, and come to completion as much
of the remaining northern stream energy merges south late in the
period.

Temperatures will remain well above normal (possibly breaking
additional records) the next several days, then return closer to
seasonal normals as the primary frontal boundary settles south of
the region allowing polar air to dominate the area. The
developing weather pattern will be active. A fairly substantial
rain (for February) is anticipated late today and tonight. A
powerful cyclone is likely to affect the area late in the week.
Large-scale southwest upper flow is forecast to persist for some
time after the passage of the initial cyclone. That will probably
allow for at least a couple opportunities for additional
precipitation as waves ride northeast along the baroclinic zone
stretched out near the area. The pattern could also support one
or two additional strong cyclones during the following week.
Precipitation amounts for the next week are likely to end up well
above normal.
&&

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 409 AM CST Mon Feb 20 2017

The main short-term forecast issues are again temperatures and
the timing of the precipitation. Clouds will probably hold
temperatures back some today, but readings will still be well
above normal. Southerly flow will keep readings from falling too
much tonight, then clearing on Tuesday should allow for another
day with readings well into the 50s and possibly reaching the low
60s in a few spots. Trended above guidance.

The onset of the precipitation now looks to be a little slower
than expected yesterday at this time. Adjusted the forecast
accordingly. Opted to add a mention of thunder across about the
southwest 2/3 of the area as the heaviest rains move though. Model
elevated CAPE values and mid-level lapse rates are marginal at
best. But dynamics are fairly strong and the LDS has already
indicated lightning strikes over eastern Minnesota early this
morning.

.LONG TERM...Tuesday Night Through Sunday
Issued at 409 AM CST Mon Feb 20 2017

Forecast concerns revolve around temperatures midweek, and potential
for a large cyclone on Thursday night into Saturday.  Model
differences in regards to the track and timing of this cyclone
remain relatively large.  Will try to mitigate these differences by
taking a blend of the gfs and ecmwf.

Tuesday night through Wednesday night...While a front stalls out
over Lake Superior, ample dry air will ensure a mostly clear to
partly cloudy conditions, and a quiet Tuesday night.  This changes
on Wednesday when low pressure travels east along the front and
across northern Wisconsin.  This system has been trending stronger
over the past few days, and will provide a chance of showers along
and to the north of the surface low track, mainly for northern
Wisconsin. To the south, warm air will surge northward ahead of the
low, and will likely bring some record highs to central and east-
central Wisconsin.  Following the better performing ecmwf guidance,
temps could reach into the low to mid 60s.  A cold front associated
with the low pressure will swing southeast across the area on
Wednesday night.  Think precip chances will remain across far
northern WI along the front, but cold air will push south into
central and east-central WI late.

Rest of the forecast...That front will stall over the southern Great
Lakes on Thursday, with colder air to the north dropping high temps
back into the 40s.  As a large cyclone intensifies across the
plains, precipitation is expected to return northward from late
Thursday afternoon into Thursday night.  Thermal profiles remain
difficult to nail down, due to track differences of the surface low
in the models. Precipitation could start out as rain over central
and east-central WI and a mix over northern WI, but then change to
more snow/wintry mix as temperatures cool Thursday night.  Moderate
to heavy precipitation will then continue for much of Friday and
Friday night.  At this time, the most likely scenario is for heavy
snow to occur north and west of the Fox Valley, and a wintry mix
from the Fox Valley to the lakeshore, but this is subject to change
pending the track of the low.  Regardless, a significant winter
storm remains possible for at least parts of the area.  Expect
differences in the track to continue for several more days due to
the more complex nature of the split flow pattern across North
America.  Behind the system, will see colder and windy conditions
move in as the snow ends on Saturday.  Temperatures close to normal
will then remain present into early of the next week.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1052 AM CST Mon Feb 20 2017

Main concern for this period will be showers and perhaps a few
thunderstorms later this afternoon into tonight, increasing low
level wind shear this afternoon and evening, then low clouds
and fog late tonight into Tuesday morning.

For this afternoon, bufkit soundings indicating wind shear
around 2000 feet increasing to around 55 kts this afternoon into
early Tuesday morning. Meanwhile, a band of showers and perhaps
a few thunderstorms will move into central/north-central Wisconsin
by mid to late afternoon, and then to the lakeshore by early to
mid evening. The probability of thunder at each taf site is too
low to include them in the 18z taf issuance. Once the showers
and few storms exit the region later tonight into Tuesday morning,
low clouds and fog are expected with conditions at IFR or lower.
Any low clouds and fog should burn off by mid to late morning on
Tuesday.

&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......MPC
AVIATION.......Eckberg



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