Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 240220

Area Forecast Discussion
National Weather Service Green Bay WI
920 PM CDT Thu Mar 23 2017

New Information added to update section

Issued at 913 PM CDT Thu Mar 23 2017

Moved up the start time of the advisory by an hour, as numerous
observations were showing temperatures in the 32-34 range across
the advisory area, so some minor icing is probably occurring in
localized areas.

The main band of showers and isolated tstms associated with strong
moisture convergence was shifting rapidly northeast this evening,
with decreasing pcpn in it`s wake. Made some adjustments to pops,
but will need to take another swipe within the next hour. Expect
pops to increase again late tonight as the cold front arrives,
and upper level forcing increases.

Issued at 638 PM CDT Thu Mar 23 2017

Made some adjustments to pops about an hour ago to account for the
line of thunderstorms that will impact parts of central and east
central WI through about 04z. Current indication is that the
stronger storms will remain to our south, though some small hail
is not out of the question.

Surface wet bulb temps remain at or a little below freezing across
northern WI, so the potential for freezing rain remains over most
of the advisory area. It should take a few more hours for
temperatures to drop to the freezing mark there, but a few
locations are already 33-34 degrees, so it`s possible the start
time may need to be moved up an hour or two.

.SHORT TERM...Tonight and Friday
Issued at 245 PM CDT Thu Mar 23 2017

Plenty of forecast concerns over the next 24 hours with main
emphasis to be on freezing rain potential across the north,
chances for thunder and amount of precipitation that could fall.

The 19z MSAS surface analysis showed an area of high pressure
along the East Coast, an area of low pressure still organizing
over the central High Plains, a warm front stretched from the
central Plains southeast into the Mid-MS Valley and a cold front
located over the northern Plains. Northeast WI saw some light
snow and flurries accompany the WAA this morning. Now, the radar
mosaic indicated the next surge of moisture (in the form of
rain/few thunderstorms) pushing across the Upper MS Valley.

The main trigger for precipitation tonight will be associated with
both frontal boundaries as the warm front lifts north into the
Midwest and the cold front marches across the Upper MS Valley.
These boundaries will interact with increasing moisture transport
to start a rather unsettled period of weather for northeast WI.
Precipitation type remains a concern, primarily for northern WI
where surface temperatures are expected to be a couple of degrees
either side of freezing. Ground temperatures are still below
freezing, thus even if all rain did fall, it could freeze on
contact. Did not see anything on the 12z model output to change
this possibility, thus a good chance for freezing rain across the
north where the winter weather advisory is in effect. Have also
kept thunder wording in the forecast due to very steep mid-level
lapse rates and a little elevated instability. Central/east-
central WI will only see rain tonight since forecast soundings
indicate the lower levels of the atmosphere to be warm enough to
keep precipitation all liquid. Min temperatures are expected to be
in the lower 30s north, middle to upper 30s south.

The cold front is progged to settle into southern WI on Friday and
essentially pull up nearly stationary by late in the day. Main
precipitation shield should be located along/just north of this
boundary, thus plan on highest pops to be situated over central
and east-central WI. The threat for freezing rain over the north
should end by mid-morning as temperatures begin to diurnally warm.
Winds are also forecast to start increasing late on Friday as the
pressure gradient tightens between low pressure over the central
Plains and high pressure over southeast Manitoba/southwest
Ontario. Max temperatures will range from the upper 30s to lower
40s north, 45-50 degrees west of the Fox Valley.

.LONG TERM...Friday Night Through Thursday
Issued at 245 PM CDT Thu Mar 23 2017

Models in good agreement on general track and timing of the slow
moving low pressure system that will be affecting the forecast
area for the start of the long term period. Still a few
differences in exact QPF fields, but a slight southerly trend
seems to be evident in the last few runs. Forecast challenge
remains focused around precip chances this weekend, timing of
overnight cooling, and precip type transitions.

Scattered rain will be ongoing in parts of the area Friday evening,
especially to the south. At this time Canadian high pressure will
also aid in bringing cooler and drier air into the north. Tough
to pinpoint a definitive pcpn/no pcpn line, but the last several
model runs have hinted at more drying in the north Friday night
into Saturday. Temperatures are expected to drop below freezing in
the north Friday night, but with the lack of deep moisture any ice
accumulation looks minimal, with perhaps a couple hundreths
overnight. Rain is more likely further south where temperatures
should remain above freezing.

Low pressure will be passing through Missouri and approaching
western Illinois on Saturday. This continues the on and off rain
chances for the southern forecast area, but with the continued
influence from the Canadian high would expect a more prolonged
dry period in the north on Saturday. Therefore, cut back on PoPs
in the north during this time period.

Moisture starts to return Saturday night and Sunday as the low
moves towards southern Lake Michigan. The latest ECMWF is the
slowest at bringing pcpn back to the north and is still dry at
12Z Sunday. The GFS, Canadian, and NAM all suggest some pcpn by
12Z Sunday. Exactly when pcpn returns to the north will affect
the potential for overnight icing. Boundary layer temperatures and
moisture profiles would certainly support freezing rain/drizzle
if the pcpn moves in early enough, but if it holds off (as the
ECMWF suggests) the window for freezing pcpn would be minimal once
temperatures warm above freezing mid-morning Sunday. For the
southern forecast area precipitation type during this time still
looks like rain.

Scattered precipitation chances continue Sunday as the low tracks
east. A second system pushes out of the southwest right behind the
first low, which keeps the chance for pcpn into Monday and Monday
night. The pattern of daytime rain with overnight mixed precip
also continues. Models currently suggest a quiet period Tuesday
afternoon and Wednesday, then diverge on when/if another system
will affect us late next week.

Temperatures this weekend and through much of next week don`t
fluctuate much. Overnight lows will be in the upper 20s to middle
30s, with daytime highs in the upper 30s and 40s.

.AVIATION...for 00Z TAF Issuance
Issued at 638 PM CDT Thu Mar 23 2017

There are numerous aviation concerns with this set of TAFs,
including a threat of thunderstorms, LLWS, freezing rain
potential across northern WI and flight conditions deteriorating
to IFR/LIFR later tonight into Friday.

A line of thunderstorms moving through southwest WI may impact
the GRB/ATW/MTW TAF sites between 02z-04z, so have added a TEMPO
group for those locations.

LLWS will be ongoing at all of the TAF sites at the beginning
of the period, and should gradually end from northwest to
southeast between 07z-10z/FRI.

Periods of rain will develop across the region this evening.
Surface temperatures are expected to drop below freezing across
northern WI, including the RHI TAF site, so will carry several
hours of FZRA on the RHI TAF. The best potential for FZRA appears
to be from 04z-13z/FRI. Temperatures should warm above freezing
Friday morning, bringing an end to the threat.

Look for conditions to drop to MVFR this evening, then IFR/LIFR
late tonight into Friday. Some improvement in flight conditions
is anticipated across northern WI on Friday afternoon as the rain
tapers off and ends.

Winter Weather Advisory until 9 AM CDT Friday for WIZ005-010>013-


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