Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 200354
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1054 PM CDT Sat Aug 19 2017

Updated aviation portion for 06Z TAF issuance

.SYNOPSIS...
Issued at 240 PM CDT Sat Aug 19 2017

Seasonably warm and humid the next couple days. A couple rounds
of thunderstorms late Sunday into Tuesday, then cooler, drier, and
less humid for the remainder of the week.

A seasonably strong band of westerlies will persist across the
northern CONUS and southern Canada throughout the period. The flow
will initially be relatively flat, then undergo amplification with
the upper trough position over eastern North America.

The period will begin with above normal temperatures, and it will
become rather humid the next day or two as well. Temperatures
will drop back to a little below normal mid-week, then return
close to normal by next weekend. A couple rounds of thunderstorms
are likely as a frontal system slowly crosses the area the next
few days. Given ample moisture and upper support, that should
result in above normal amounts for the period despite the dry
conditions expected for the latter half of the forecast period.
&&

.SHORT TERM...Tonight and Sunday
Issued at 240 PM CDT Sat Aug 19 2017

Any showers or thunderstorms in far northeast Wisconsin this
afternoon will quickly die off with loss of daytime heating. After
that, fog potential is the concern tonight while timing the onset
of showers and storms is the main issue on Sunday.

Cumulus that developed today will dissipate by sunset, leaving
mainly clear skies across the forecast area overnight. Winds are
expected to be light with surface high pressure over the region.
The light winds and lack of clouds will combine with dew points
in the middle 50s to lower 60s to produce at least patchy fog
tonight. Am a bit concerned about dense fog, but think it will be
confined to low spots so have decided that an advisory
unnecessary.

Models had a mid level short wave moving across Wisconsin in
relatively zonal flow in a broad upper trough. Exact timing and
location differ slightly. A surface cold front is expected to
move into Wisconsin on Sunday and just about reach the forecast
area by the end of the day. The 12Z ECMWF kept the area dry on
Sunday but the Canadian, GFS and NAM all had QPF in at least the
northwest half part of the area. The front, upper system and
increasing instability on Sunday should bring at least a chance
for thunderstorms to the area so have a mention of storms
generally north and west of the Fox Valley. Seems like some strong
storms are possible considering the presence of a surface
boundary, support aloft, MUCAPE values of at least 1,200 J/kg and
bulk shear values around 25 knots.

Kept temperatures pretty close to what was in the previous
forecast.

.LONG TERM...Sunday Night Through Saturday
Issued at 240 PM CDT Sat Aug 19 2017

The main long term forecast issue is determining the risk of
severe storms and heavy rains with the frontal system that will
sag across the area early in the forecast period. Models agree
pretty well on the overall scenario, though (not surprisingly) not
on the details.

The tail of a strong shortwave surging eastward across Canada
will drive the cold front toward the forecast area, with the
front probably reaching north-central Wisconsin Sunday evening.
Models differed on the amount of upper support coming across the
area with the front, and the potential for storms near the
boundary. Most of the models held the shortwave back enough to
generate weak large-scale ascent across the north as the front
crosses that area, and generated some convection across the north
Sunday evening. The notable exception was the ECMWF, which peeled
the upper support off to the northeast more quickly and was
pretty much dry. Favor the majority idea at this point since it
seems unlikely a front reaching the area during the evening will
be dry, especially since mid-level lapse rates will be modest, so
capping does not look too strong. Given the above mentioned model
differences, the forecast area was not highlighted in the 5
percent (MARGINAL) or greater risk of severe in the SPC
convective outlook (SWODY2). But the severe risk is not zero.
Will attempt to detail this in the HWO.

The frontal boundary is likely to continue to slowly sag south
across the area later Sunday night into Monday, as weak mid-level
ridging advances across the area from the west. The result should
be a decrease in the coverage/intensity of storms near the front.
The lack of upper forcing may result in at least be some breaks
in the clouds Monday despite having the front in the area. That
offers at least some hope the solar eclipse won`t be totally obscured
by clouds. At this point the northwest part of the forecast area
looks to have the best chance for skycon allowing the eclipse to
be viewable.

Large scale amplification of the eastern NOAM upper trough will be
getting underway by the time the next shortwave crosses the region
Monday night into Tuesday. Upper support will also increasing as
the RRQ of an upper speed max pulls through the region. The
forcing may result in a wave riding along the front as well. So
there will likely be another round of storms. The possibility
exists that the front will have sagged far enough south by then to
keep the greatest severe threat south of the area. Unfortunately,
we will need to wait to see the details of how the pattern evolves
before we get a good feel for the severe risk across the area. But
regardless, anticipate some of the storms will produce heavy
rainfall.

Quiet weather is expected for the rest of the period as northwest
upper flow develops aloft, and a Canadian anticyclone slowly
crosses the region.

The standard forecast initialization grids based on a broad blend
of model output seemed reasonable, so no significant changes were
necessary.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1019 PM CDT Sat Aug 19 2017

Relatively light winds, few clouds and high dew points should
lead to the development of fog overnight. Expect areas of dense
fog but am not certain it will be widespread. Have MVFR fog at TAF
sites tonight with IFR/LIFR in a TEMPO group. Fog should mix out
rapidly on Sunday morning through mixing and increased winds.
Some showers and thunderstorms are possible Sunday afternoon and
evening as a cold front approaches from the west, particularly
across the western TAF sites. Therefore will introduce a PROB30
group for these sites during the late afternoon and evening hours
on Sunday. There is some doubt on whether this activity can hold
together and hit the eastern TAF sites given the timing will be
after peak heating therefore will keep these sites dry with this
issuance.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....MG
LONG TERM......Skowronski
AVIATION.......Kurimski



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