Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 202321
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
621 PM CDT Thu Apr 20 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Friday
Issued at 231 PM CDT Thu Apr 20 2017

Main forecast challenge to be on possible precipitation type
issues across northern WI later tonight as temperatures cool to
around the freezing mark and any rain may become mixed with snow
or freezing drizzle.

The 19z MSAS surface analysis indicated an area of low pressure
near Sheboygan with a cold front extended south, then southwest to
the Mid-MO Valley. There was also a trough that stretched
northwest from the low pressure into the Upper MS Valley. High
pressure was located over the northern High Plains. Plenty of low
clouds covered the Midwest/Great Lakes this afternoon with the
radar mosaic showing thunderstorms to our southeast and light rain
or drizzle over the western Great Lakes.

Even though the surface low is pulling away from the region
tonight, a trailing mid-level shortwave trough is expected to move
across WI. This trough, coupled with wrap-around moisture and
cyclonic flow, should keep plenty of clouds around northeast WI.
Precipitation trends are forecast to gradually diminish tonight as
moisture becomes more shallow and overall lift weakens with time.
That being said, anticipate temperatures to be cool enough to have
snow mix with any rain over northern WI this evening or even
briefly change to all snow or a little freezing drizzle before
diminishing late tonight. Nothing more than a dusting of snow
should fall as the ground is both warmed up and will be wet from
earlier rainfall. Min temperatures to range from near 30 degrees
north, middle to upper 30s east-central WI.

Look for improving conditions as high pressure builds east from
the Upper MS Valley on Friday. Clouds should still be around at
the start of the day, but as drier air begins to advect into the
region, a clearing trend will be noted from west to east with
everyone seeing mostly sunny skies Friday afternoon. Mean flow
aloft still from a northerly direction, thus temperatures will be
able to get back to normal levels for late-April. This would bring
readings into the lower to middle 50s north/lakeshore, middle to
upper 50s south.

.LONG TERM...Friday Night Through Thursday
Issued at 231 PM CDT Thu Apr 20 2017

A quiet start to the long term portion of the forecast, with active
weather returning next week when a couple systems pass through
the Midwest. Temperatures will generally be around normal for this
time of year, with highs a little cooler during cloudy/rainy
days, and a little warmer when the sun is present.

Models in good agreement that a surface high pressure will build
in Friday evening with dry air present in the mid and upper
atmosphere, too. Expect clear skies Friday night which will lead
to temperatures cooling a little below normal. Saturday morning
lows may need refining in later updates as MAV and MET guidance
would suggest lows a bit cooler, but continued with a blend for
now.

Saturday is setting up nicely for a pleasant day, with sunny
skies and highs in the middle 50s along Lake Michigan, and lower
60s elsewhere. A cold front associated with a system passing
through Canada may sink far enough south to trigger a few showers
in northern Wisconsin late Saturday night or early Sunday
morning. Temperatures should stay warm enough to see only rain,
but a few snowflakes are not out of the question. Surface high
pressure will keep conditions quiet for the rest of Sunday, with
more clouds and slightly cooler temps than Saturday.

Warm air advection ahead of a low pressure system in the southwest
may trigger a few showers Sunday evening and Sunday night,
primarily in north and northeast Wisconsin. Temperature profiles
support rain showers Sunday evening, but as temperatures cool snow
or a rain/snow mix is possible Sunday night. Any snow would melt
quickly as temperatures warm up Monday morning. The bulk of the
moisture and energy with this system arrives Monday night and
into Tuesday as a 500mb shortwave and surface low pressure
approach and pass through Wisconsin. Models differ on exact
timing, placement, and strength of these features, therefore did
not stray much from previous forecast with a chance of rain
showers through this time.

There is a brief dry period later Tuesday into early Wednesday,
then another system moves up from the southwest during the
Wednesday/Thursday time frame. Models differ on timing this far
out, but do indicate temperature profiles are cold enough to
support overnight snow in the north. Otherwise stuck with a chance
of rain showers for most of the area Wednesday and Thursday until
differences get sorted out.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 621 PM CDT Thu Apr 20 2017

Mainly MVFR cigs expected tonight into early Friday morning.
Patchy IFR cigs and MVFR vsbys due to light rain or drizzle will
linger this evening while light snow may mix with the light rain
across the far north. Conditions expected to improve to VFR late
Friday morning into the afternoon as high pressure builds into
the area from the northwest.
&&

.HYDROLOGY...
Issued at 231 PM CDT Thu Apr 20 2017

Heavy rainfall took place last night with most locations in the
0.50 to 1.50" range, with isolated amounts of 2" over parts of
central WI. While little additional rainfall is anticipated over
the next 48 hours, rivers across northeast WI are running high
with some already over bankfull stage. The additional rains will
only add to the high flows of local rivers and streams, as well as
bring some rivers above flood stage. People living along rivers or
streams should keep alert to rising levels and be prepared to move
to higher ground if any flooding does occur.
&&

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

$$

SHORT TERM.....Kallas
LONG TERM......KLB
AVIATION.......TDH
HYDROLOGY......Kallas



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