Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 310200

Area Forecast Discussion
National Weather Service Green Bay WI
900 PM CDT MON MAY 30 2016

Updated aviation portion for 06Z TAF issuance
and new information added to update section

Issued at 844 PM CDT Mon May 30 2016

Three-hour pressure rises of 3+ mb over eastern lake Superior
suggest boundary over eastern Upper Michigan will begin to surge
southward overnight. Will add a mention of the wind shift to the
HWO, and bump winds up some over marine areas for late tonight.
Luckily, the wind shift will occur overnight when the number of
boats on the lake and bay will be low, but the wind shift will
still pose a hazard to any small craft still on the water.

00Z GRB sounding was still quite dry. That favors slowing the
arrival of the precipitation a little more. 01Z meso plots showed
about 100 j/kg of MUCAPE over north-central WI, with stable air
elsewhere. Some weak radar returns have developed at times in that
area, and the latest run of the HRRR had light precipitation over
that area. Will handle in the forecast by adding a mention of
sprinkles to the forecast for north-central Wisconsin.

A few storms with a large cluster of convection over northeast
Iowa have become severe. That precipitation area will move toward
our area overnight, but mesoplots show little northeast shift in
the instability yet, so the storms should weaken significantly
before they arrive.

Updated product suite incorporating these ideas will be out
within about 1/2 hour.

.SHORT TERM...Tonight and Tuesday
Issued at 224 PM CDT Mon May 30 2016

The latest rap analysis and satellite/radar imagery show a cold
front sagging south over far northern Lake Superior.  Weak high
pressure is centered across the state between this cold front, and a
warm front extending from a surface low over western South Dakota to
west-central Illinois.  Earlier showers and storms have fizzled
along this warm front, but more potent thunderstorm activity is
occurring over the western Dakotas.  As the low pressure system
occludes and sends a cold front into the western Great Lakes on
Tuesday, precip trends and severe chances are the main forecast

Tonight...The cold front moving into northern Lake Superior will
back door its way into north-central and northeast Wisconsin after
midnight.  No precip is anticipated along the front, but it will
shift winds to the east or northeast and drop low temps into the
upper 40s to lower 50s.  Meanwhile, a shortwave over Montana and
associated surface low will close off over the eastern Dakotas by
late tonight. Mid-level warm/moist advection will ramp up ahead of
the low while its surface warm front will shift into far southwest
WI by 12z Tue.  The moisture transport is sufficient for a chance of
showers over central and north-central WI after midnight.
Instability is rather meager, but enough for a thunderstorm threat.
Warmer lows in the mid 50s over central WI and the southern Fox

Tuesday...Occluded low pressure will meander east across the
Dakotas.  The cold front will stall over central WI during the
morning, then begin to lift north late in the afternoon at the same
time as a cold front from the west moves across western WI.  Showers
and isolated storms remain likely during the morning over north-
central WI, which will be situated in the moisture transport zone.
Eastern WI may escape the precip due to drier and more stable air in
place.  Some guidance suggests a lull in precip chances could occur
centered around midday before precip chances increase again during
the afternoon when the cold front draws closer.  After modifying
progged soundings for central WI, ml capes 700-1000 J/kg appear
possible with sufficient heating.  But 0-6km bulk shears are only
expected to range from 20-25 kts, which suggests only an isolated
severe storm is possible.  Due to the slower timing of the cold
front, the marginal severe storm threat has been pushed west to
cover mainly Wood and Marathon counties. Highs ranging from the low
70s north to around 80 south.

.LONG TERM...Tuesday Night Through Monday
Issued at 224 PM CDT Mon May 30 2016

Forecast issue Tuesday night into Wednesday will be precipitation
trends and severe potential. Surface to 850 low moves little
Tuesday night and deepens over eastern North Dakota before
drifting just north of the MN/Canada border into Wednesday. A cold
front extending from this low will work slowly into western
Wisconsin Tuesday night then into the eastern half of the state
through Wednesday. Increasing moisture with pwats from 1.30 to
1.50 work ahead of the cold front through Wednesday morning.
nearly persistent north to south RRQ upper jet region parallel to
the surface front will provide large scale ascent near the front
for a period of heavy rainfall and stronger storms over parts of
central and north central Wisconsin. With this slow trend, it is
possible eastern Wisconsin will not see much precipitation until
starting Wednesday morning. Progs continue to push a dry slot
across the area Wednesday to diminish the precipitation from west
to east Wednesday. But depending on timing and available
instability and amount of clouds, a few storms may be on the
strong side again over eastern Wisconsin Wednesday afternoon as
the cold front slides through.

Will keep a Thursday afternoon diurnal chance of showers across
the north due to weak cyclonic flow and wrap around moisture with
the low pressure system sliding to the north. Later shifts can
remove if this system departs quicker to the north.

After a period of quiet with a near zonal flow for the start of
friday, next short wave from the northern plains begins to work
toward the region. Recent GFS run has shifted to a stronger
frontal surface and closed 850 low dropping into the western
Great Lakes region later Friday night into Saturday. The ECMWF is
not as robust but does produce a steeper trough into the area
during the same period.

.AVIATION...for 06Z TAF Issuance
Issued at 844 PM CDT Mon May 30 2016

Good flight conditions will persist for much of the night. The
chance for showers and thunderstorms will slowly advance in from
the west overnight. But relatively dry air should allow cigs and
vsbys to hold up fairly well even once the initial precipitation
arrives. Expect conditions over the northwest part of the area to
gradually deteriorate tomorrow as showers become more numerous



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