161
FXUS63 KGRB 262308
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
608 PM CDT Fri Jul 26 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- There is a chance of thunderstorms Sunday afternoon through
  Tuesday. The severe weather potential is low.

- Very warm and humid weather is expected this weekend and much of
  next week. Heat index values could approach or exceed 90 degrees
  at times starting on Sunday.

&&

.DISCUSSION...
Issued at 154 PM CDT Fri Jul 26 2024

Short Term...Tonight and Saturday

Mostly clear skies are expected tonight across the region. Some
modest surface warm air advection will keep temperatures a bit
warmer tonight, with lows generally ranging from 55 to 60 degrees.

A stratocumulus deck, seen on satellite imagery over eastern
Minnesota, is expected to track east over central and north-
central Wisconsin Saturday morning and linger across that area
through the afternoon. Further east, mostly sunny skies are
expected across northeast and east-central Wisconsin. This deck of
clouds should keep temperatures a few degrees cooler across the
west, with highs in the lower 80s. Further east, highs are
expected to rise into the upper 80s away from Lake Michigan with
highs in the 70s near the lake. In addition to the warmer
temperatures, dew points will rise into the 60s, allowing
humidities to rise across the western Great Lakes.

Long Term...Saturday Night Through Friday

Concern surrounding thunderstorm potential Sunday into Monday
with prolonged heat and humidity lasting into the week continue
to be the main focus for this forecast period.

Precipitation: By Saturday evening, the ridge aloft partially
responsible for our tranquil weather will erode with an amplified
but sluggish shortwave drifting into the region from the Mid-
Mississippi Valley. This feature will help draw moisture from the
Gulf of Mexico north, introducing the potential for showers and
thunderstorms to close out the weekend. Chances start increasing
more appreciably Sunday afternoon when more noteworthy height fall
aloft accompanying the shortwave begin to impact eastern
Wisconsin and diurnal instability will maximize. That said, the
forcing itself is relatively weak, which will likely keep
convection scattered at best Sunday afternoon and evening. Deep-
layer shear continues to be relatively weak (15-20kts) among
available guidance, which will hinder thunderstorm strength and
overall severe weather potential. With precipitable water values
approaching 1.50 inches in most deterministic and ensemble
solutions, which approaches the 90th climatological percentile,
heavy downpours will be a concern in addition to typical
thunderstorm hazards.

The stronger mid-level forcing will persist overnight Sunday, but
with a loss of diurnal instability, there is likely to be a lull
in convective activity overnight into Monday. Another threat for
thunderstorms will develop Monday afternoon, by which point the
forcing aloft becomes more nebulous. However, a weak cold front
approaching from the west will lend better low-level lift and
better thunderstorm coverage Monday afternoon and evening into
early Tuesday when the front exits. Again, the shear parameter
space is rather weak but abundant moisture will threaten heavy
downpours. Stronger ridging aloft and a weak surface high take
hold later Tuesday into Wednesday, leading to a break in
precipitation. Another wave approaches along the periphery of the
ridge from Canada late Wednesday into Thursday, introducing
another rain threat, though guidance differs wildly on its speed
and intensity.

Temperatures: The stretch of near-normal, comfortable weather
we`ve enjoyed will be a distant memory by Sunday, with 850mb
temperatures jumping into the low 20s Celsius accompanying an
expanding upper-level ridge. Along with the increased humidity
streaming north into the Upper Midwest will send heat index values
into the upper 80s from Sunday onward, approaching 90 degrees in
some spots through the week. This level and longevity of heat and
humidity will threaten minor to moderate impacts, particularly to
those sensitive to heat. There is some degree of uncertainty
surrounding how long the heat will last, owing largely to
increased rain chances Wednesday into Thursday. While there is
relative agreement on an amplified mid-level wave disrupting the
ridge overhead Wednesday into Thursday (and possibly beyond),
there is no confidence in the timing or location of this wave. The
track and evolution of this wave will factor greatly in how warm
we get. If the wave directly impacts us, our current forecast may
be too warm. If it is relegated further north and the ridge
remains intact, the heat and humidity will persist into the late-
week.

&&

.AVIATION...for 00Z T AF Issuance
Issued at 553 PM CDT Fri Jul 26 2024

Precipitation: None.

Clear skies will continue until around 15z Saturday. Increasing
low level moisture will result in stratocumulus clouds developing
between 15z and 18z Saturday and continue through Saturday
afternoon. A broken deck with CIGS developing between 2500 and
3000 feet during the morning should rise to around 3500 feet
during the afternoon is expected at KRHI/KAUW/KCWA/KMFI/KISW. A
scattered deck is expected across northeast Wisconsin which should
start out around 2800 feet rising to around 3500 feet during the
afternoon.

VSBY trends: Could see patchy ground fog in river valleys of far
northeast and east central WI late tonight into early Saturday
morning, but confidence remains too low to include at GRB/ATW/OSH/MTW.
Low level winds increase tonight across north-central Wisconsin
which should prevent any fog from forming.

Smoke Trends: Elevated smoke to remain across the forecast area
through the TAF period, but little or no surface impacts are
expected.

Winds: Light S-SE winds expected tonight, with S winds gusting to
15-20 kts by late Saturday morning and then continue through
Saturday afternoon.

.OSH...VFR conditions expected through the TAF period, with only a
very low (10%) probability of MVFR vsbys in fog late tonight.
Clear skies will prevail through 15z Saturday, then stratocumulus
should develop around 2800 feet between 15z and 18z, rising to
3500 feet Saturday afternoon. South winds around 5 knots should
prevail tonight, then increase to around 10 knots with gusts to
around 17 knots late Saturday morning and afternoon.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....Kurimski/MRB
AVIATION.......Eckberg