Area Forecast Discussion
Issued by NWS Green Bay, WI

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949
FXUS63 KGRB 271020 AAA
AFDGRB

Area Forecast Discussion...UPDATED
National Weather Service Green Bay WI
520 AM CDT Thu Apr 27 2017

Updated to add short term portion for morning forecast issuance

.SYNOPSIS...
Issued at 515 AM CDT Thu Apr 27 2017

Very chilly with rain and some wintry precipitation today, then
a brief break before rain returns this weekend.

The powerful southern branch of a split flow regime will dominate
the CONUS during the period. Initially, a rather flat ridge will
be near the West Coast with a sharper ridge along the East Coast.
In between, a deep trough will be over the southern Plains. The
features will remain in place for about the first half of the
period, though the western ridge will sharpen. The features are
expected to begin a slow progression next week.

This is a wet weather pattern, as a series of powerful shortwaves
will eject out of the mean trough position and interact with
abundant Gulf moisture to produce widespread heavy precipitation.
Amounts across the forecast area will be well above normal for
the period. Daytime highs will run about 8 to 15 F degrees below
normal for much of the period, with some moderation late. Much of
the cooling will be due to clouds and precipitation, so the
departure for nighttime lows won`t be nearly as great.
&&

.SHORT TERM...Today...Tonight...and Friday
Issued at 515 AM CDT Thu Apr 27 2017

A large precipitation shield has overspread just about the whole
forecast area in advance of a strong negatively-titled shortwave
ejecting out of the southern Plains upper trough. The surface front
has stalled over central Lake Michigan in response to a rapidly
deepening cyclone lifting through southeast Wisconsin. The
cyclone will race northeast, reaching eastern Upper Michigan by
mid-morning. The precipitation will continue this morning, then
end from southwest to northeast this afternoon as the cyclone
departs.

The main forecast issue is precipitation type over the northwest
part of the forecast area. Surface observations/web cams indicate
mainly snow thus far in north-central Wisconsin/adjacent areas of
Upper Michigan, implying cold air is deeper/more entrenched in
than depicted on any of the forecast models. While the warm air
could fight its way back west a bit the next few hours as the
cyclone approaches, temperature profiles will collapse with the
arrival of the strong upper system later this morning. So,
trended the forecast more toward snow, with freezing rain and
sleet mainly confined to the early to mid-morning period. The
advisory issued by the evening shift still looked good. Although
snows will eventually get south and east of the advisory area,
insolation due to the relatively high late-April sun angle should
prevent much accumulation on roads during the late morning and
afternoon hours. The snow that falls during that time is expected
to primarily accumulate on grassy areas. Will need to monitor the
situation closely--especially as the heavier precipitation
arrives in the north--but thus far forecast is working out pretty
well.

Added snow or at least a rain/snow mix much farther southeast than
previous forecast. Also pulled back on temperatures as highs today
in the east will occur early in the day and be followed by
falling temperatures.

Quieter weather is expected tonight into Friday. The next system
lifting out of the Plains looks like it will track close enough to
the area to warrant low PoPs late Friday.

.LONG TERM...Friday Night Through Wednesday
Issued at 354 AM CDT Thu Apr 27 2017

Long-range models continue to show good agreement with the overall
mean flow consisting of a broad upper trough located over the
central CONUS, surrounded by upper ridges over the eastern Pacific
and western Atlantic. The main system of interest is forecast to
impact northeast WI starting Saturday night and could bring
precipitation of various types to the area through Monday night.
This system still looks strong with plenty of lift, forcing and
moisture to make for a messy forecast, especially at night as
temperatures cool and precipitation type issues arise.
Temperatures are expected to run below normal through the entire
extended forecast.

A wave of low pressure is forecast to ride along the recently
passed cold front across the southern Great Lakes region Friday
night. Models differ a bit on the exact track this wave takes with
the NAM farther north than the GFS/CMC/ECMWF. This is critical for
precipitation chances with the NAM bringing higher chance pops
into central/east-central WI, while the ECMWF keeps precipitation
well to our south. Will keep a low chance pop in the forecast for
east-central WI for Friday evening, but anticipate the bulk of any
precipitation to stay to our south. Otherwise, partly to mostly
cloudy skies to prevail with min temperatures in the middle to
upper 20s north, around 30 degrees central/far northeast WI and
middle to upper 30s east-central WI. Saturday is still looking
like a pleasant day across northeast WI as an area of high
pressure passes by to our north. This would keep a cool north-
northeast wind over the region, thus despite a mix of sun and
clouds, temperatures will remain below normal. Look for max
temperatures to range from the middle to upper 40s near Lake MI,
around 50 degrees north and lower to middle 50s south with the
warmest readings west of the Fox Valley.

Attention turns toward the southern Plains where a deep upper
trough to reside, accompanied by a strengthening area of low
pressure over southern MO/northern AR Sunday night. Increasing
southerly winds ahead of these features will not only draw copious
amounts of gulf moisture northward, but also begin to lift a
stalled boundary northward as a warm front (reaching central
sections of IL and IN by 12z Sunday). A surge of WAA into WI will
bring increasing precipitation chances, especially to central/
east-central WI after midnight. Depending on thermal profiles when
the precipitation arrives, either a rain/snow mix or all light
snow will be possible across central/northern WI. There might be a
minor snow accumulation, however the higher QPF amounts are
expected to be over east-central WI where the precipitation type
would remain all rain. Min temperatures to be in the lower 30s
north, middle to upper 30s south. The system is progged to lift
northeast on Sunday with the upper trough closing off into an
upper low over the central Plains and the surface low lifts north
into the Midwest. The warm front will remain to our south,
reaching northern sections of IL and IN by 00z Monday. Gulf
moisture to also continue flowing north with PW values surpassing
1.00" by Sunday afternoon. Persistent low-level WAA, coupled with
mid-level frontogenetical forcing, should bring a wet day to
northeast WI with any mixed precipitation or snow changing to all
rain by mid-morning. Max temperatures to only reach the middle 40s
over most locations due to thick clouds, precipitation and
blustery northeast winds.

This system still is expected to become vertically-stacked across
eastern IA/northwest IL/southwest WI Sunday night, before moving
northeast across WI on Monday. PW values around 1.25" across
eastern WI may bring locally heavy rainfall Sunday night and will
have to watch for both potential for flash flooding and river
flooding as ground conditions are already saturated from earlier
precipitation. The bigger question Sunday night will be
precipitation type over northern/central WI with a range from all
rain to all snow depending on how much warm air can get pulled
west by the approaching system. Models have shifted a bit farther
west, especially the CMC. This overall trend toward warmer would
lessen the risk of accumulating snow (other than the deformation
zone over MN/NW WI). Therefore, have adjusted precipitation type
more to a rain/snow mix over northern and parts of central WI
Sunday night. The rain showers will continue through Monday as the
system crosses the Badger state.

Wrap-around moisture and cyclonic flow to linger over the western
Great Lakes into Monday night, thus at least chance pops are
necessary. A shot of CAA pulled into WI should turn any rain to a
mix or all snow beginning late Monday evening through the
overnight hours. Deeper moisture to have pulled away by this time,
so any snow accumulations over the north appear minor at this
time. It may take until later Tuesday to finally get rid of the
effects of this system as weak surface ridging pushes into WI.
This surface ridging is expected to hold over the Great Lakes
through Wednesday, keeping any precipitation to our west or south.
Northwest flow aloft will keep temperatures below normal through
Wednesday.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 515 AM CDT Thu Apr 27 2017

Expect mainly IFR conditions this morning, with a slow improvement
from southwest to northeast this afternoon as the cyclone pulls
away from the area. MVFR ceilings could linger tonight, especially
over the north.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon CDT today for WIZ005-010-011-
018.
&&

$$

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



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