Forecast Discussion for GRB NWS Office
FXUS63 KGRB 060442

Area Forecast Discussion
National Weather Service Green Bay WI
1142 PM CDT Sun Jul 5 2020

Updated aviation portion for 06Z TAF issuance

Issued at 235 PM CDT Sun Jul 5 2020

Rather hot and humid with scattered showers and thunderstorms at
times through Friday, then a little cooler and much less humid
next weekend.

The upper flow across North America has settled into a rather
typical summertime pattern. The subtropical ridge was stretched
out across the southern CONUS, though the remnants of an old upper
trough were resulting in a weakness in the ridge over the Gulf
States. A modestly strong, fairly low-amplitude belt of westerlies
was located to the north, from the Pacific Northwest across the
far northern CONUS and southern Canada. Little change of
consequence is expected for the next few days, then during the
latter part of the work week an upper anticyclone will consolidate
within the ridge over the Four Corners region. As ridging builds
northward from that feature, troughing will gradually increase
over the eastern CONUS as height falls over the eastern Great
Lakes region merge with the lingering weakness over the southeast

The upper pattern favors very warm and humid conditions throughout
the upcoming work week, but temperatures will drop back closer to
seasonal normal and the humidity will return to more comfortable
levels as the upper flow turns northwest next weekend. Precipitation
will be convective and much of it will be driven by small scale
forcing, so amounts are likely to vary considerably in time and
space. When considering the precipitation averaged across the
whole area and totaled for the entire area, the overall pattern
probably supports near to slightly below normal rainfall.

.SHORT TERM...Tonight and Monday
Issued at 235 PM CDT Sun Jul 5 2020

The main forecast concern continues to be extent of thunderstorms
during peak heating.

The 19Z MSAS surface analysis indicated weak areas of high
pressure over the eastern and southern Great Lakes region. A cold
front was situated from eastern Manitoba south into eastern ND,
then southwest through central SD. Radar mosaic showed scattered
diurnally-driven thunderstorms dotted over the region. A few of
these storms had become locally strong at times with very heavy
rain and brief gusty winds.

As has been the case over the past several days now, expect the
diurnal convection to wane by mid-evening with the loss of daytime
heating. We could also see another round of patchy ground fog late
tonight, especially where any precipitation had fallen. One
synoptic difference will be the eastward movement of a cold front
across the Upper MS Valley tonight. Anticipate any showers/storms
to remain to our north and west closer to the frontal position,
thus dry/muggy conditions to persist overnight. Min temperatures
to be mainly in the middle 60s north, upper 60s to around 70
degrees south.

The majority of the models only take this cold front (and its
associated weak mid-level shortwave trough) into northwest WI on
Monday. Anticipate mainly dry conditions through the morning
hours, but as daytime heating peaks in the afternoon, look for
more pop-up showers/thunderstorms to develop over most of the
forecast area. The exception would be north-central/central WI
where thunderstorms from the approaching cold front could reach
that part of the area by later in the afternoon. MLCAPES from
1500-2500 J/KG and moderately steep mid-level lapse rates could
support some stronger storms. Shear remains weak, however cannot
rule out an isolated severe thunderstorm with damaging gusts and
hail possible. SPC has placed most of northeast WI under a
marginal risk of severe thunderstorms for Monday afternoon. High
temperatures for Monday will be completely dependent upon when/
where the precipitation falls. For now, have kept readings in the
middle 80s to lower 90s.

.LONG TERM...Monday Night Through Sunday
Issued at 235 PM CDT Sun Jul 5 2020

A weakening frontal boundary will linger across or near the area
through Tuesday. There will still be a diurnal component to the
precipitation chances, but the presence of the front and a subtle
mid-level shortwave warrant keeping some mention of precipitation
through the night. Limited PoPs to chance category for now, but
likely PoPs will probably be warranted once the location of the
boundary becomes more apparent. Given the high humidity, some of
the storms will produce heavy rainfall and brief gusty winds.

A return to more diurnally driven storms is likely Wednesday.
Another frontal system and stronger shortwave will approach the
area late Wednesday and cross the region Thursday into Thursday
evening. This will be the most favorable time frame for
precipitation during the period, so stuck with the likely PoPs
from the broad-based blend of guidance.

Although precipitation can`t be totally ruled out next weekend,
conditions will become less favorable for convection as the upper
flow tilts northwest and slightly cooler and less humid air
spreads across the area.

.AVIATION...for 06Z TAF Issuance
Issued at 1142 PM CDT Sun Jul 5 2020

VFR conditions are expected to prevail during the TAF period,
however a few locations may see some patchy ground fog toward

Another round of scattered showers and thunderstorms are expected
on Monday, especially during the afternoon and evening as a cold
front moves into northwest WI. Once again, vsbys will drop under
any of these storms. At this point will handle the activity with a
VCTS period in the afternoon and evening as coverage is expected
to be scattered.



SHORT TERM.....Kallas
LONG TERM......Skowronski

NWS GRB Office Area Forecast Discussion