Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 230927
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
427 AM CDT SAT JUL 23 2016

Forecast discussion for routine morning forecast issuance

.SYNOPSIS...
Issued at 423 AM CDT Sat Jul 23 2016

A round of thunderstorms tonight, possibly producing strong winds
and torrential rains. Hot and humid the next couple days, then
temperatures and humidities gradually returning to more normal
levels for late July.

A zonal band of westerlies will remain over the northern CONUS
and southern Canada throughout the period, while the subtropical
ridge holds across the southern CONUS. The ridge will consolidate
some over the western CONUS late in the period, causing the
westerlies to tilt a little more west northwesterly.

Hot and humid weather will persist through the weekend.
Temperatures and humidities will edge down a little early in the
upcoming work week, then more substantially during the latter half
of the week, returning close to seasonal normals by next weekend.
High moisture values in place as an MCS crosses the area tonight
should yield another substantial rainfall at most locations.
Precipitation chances thereafter are less clear, but tonight`s
rainfall and the overall pattern seem sufficient to result in at
or above normal totals for the period.
&&

.SHORT TERM...Today...Tonight...and Sunday
Issued at 423 AM CDT Sat Jul 23 2016

The focus of the short-term forecast is squarely on thunderstorm
potential. Weak frontal boundary that dropped south of the area
and was reinforced by thunderstorm outflows stretched across
Illinois and Iowa early this morning. The boundary will start to
drift north during the day. Very moist air has pooled near the
front with a large area of surface dewpoints above 70F, and even a
pocket of dew points AOA 77F from southeast South Dakota into
northwest Iowa. Meanwhile, a cold front was beginning to push east
across the northern Plains. At upper levels, a major shortwave
trough was advancing east from the Rockies.

The primary forecast issue is whether the main convective
development today and tonight occurs along the warm front or back
to the west with the upper system and cold front. As is typical,
the models offered varying ideas. Most favored the latter, having
the primary complex come across from the Dakotas and Minnesota with
scattered clusters developing near the warm front. Of some
concern however is that the NCEP meso-WRFs both favored the warm
front. Those models have been doing much better than most of the
rest of the guidance with recent MCS development. Given the split
opinion, was not comfortable banking on a specific scenario and
built forecast to handle either possibility.

Structured forecast with PoPs increasing from the west, and
categorical PoPs at all locations at some point. Exact timing may
need adjustment once convective evolution becomes clearer. Also
added heavy rain attribute to the grids as PWATs climb to AOA 2
inches. Will play up the potential for torrential rains and
possible flooding in the HWO. We may need to consider a flash
flood watch at some point, but again, need to have higher
confidence in exact convective evolution before issuing.

Shear and instability tonight are forecast to be a little more
modest than with the Thursday morning MCS, but still appear
sufficient for a severe weather. The primary hazard will be
damaging winds. Primary development coming in from the
west/northwest will be more favorable for severe than if primary
development occurs on the warm front lifting up from the south.

Secondary question is the potential for redevelopment Sunday
before the cold front clears the area. 850 mb flow is progged to
become westerly, which will limit moisture convergence and thus is
unfavorable for getting more than scattered storms. Models also
differed some, with the NAM the slowest with the front but showing
minimal forcing. The GFS on the other hand cleared the front
through fairly quickly, but showed some QG forcing with the main
shortwave and RRQ of speed max overspreading the area during the
late afternoon. The ECMWF came in as a compromise. It seems
guidance often tries to dry things out too quickly in this type of
situations, so maintained low PoPs in the east during the
afternoon. Although the coverage of storms would be limited, any
storms that form could certainly still pose a severe risk.

The latest guidance was a little lower with temperatures for
today, but with satellite showing convective debris from the west
thinning as it moves east, stuck with highs closer to yesterday`s
forecast. Also edged dew points up a bit based on latest obs. Also
went a little above consensus guidance products for highs Sunday.

.LONG TERM...Sunday Night Through Friday
Issued at 423 AM CDT Sat Jul 23 2016

Upper level flow is forecast to be rather flat across the region,
before starting to amplify a bit heading into next weekend.

Any storms should be out of the area by Sunday night, and surface
high pressure and a weak mid level ridge will keep things dry
through Monday night. Things get messier after that as another mid
level short wave and a frontal boundary move into the region. The
best chance for showers and storms appears to be Tuesday night
through Wednesday night. There are plenty of slight chance POPs
and some chance POPs through the end of the upcoming work week,
with the surface front hanging around the area and model
differences in timing and location of mid level short waves.

Temperatures should be cooler, but still several degrees above
normal for the start of the new work week. Highs are expected cool
off a bit more and be within a couple of degrees of normal for
mid to late next week.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 423 AM CDT Sat Jul 23 2016

Good flight conditions are expected after patches of early
morning fog burn off an hour or two after sunrise this morning.
Thunderstorms late today--or more likely overnight--will pose a
significant risk to aviation. Anticipate ifr conditions and gusty
surface winds with the storms.
&&

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

$$

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



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