000
FXUS63 KGRB 252339
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
639 PM CDT Thu Apr 25 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Elevated fire weather conditions are possible on Friday along
  the Upper Michigan border as relative humidity values fall to 25
  to 30 percent combined with increasing southeasterly winds of
  15 to 25 mph.

- Two systems will result in several chances for showers and
  thunderstorms Friday afternoon through Monday. Confidence is
  increasing in severe weather potential Saturday afternoon with
  large hail and damaging winds the primary threats.

- Storms through early next week could produce periods of moderate
  to heavy rain, resulting in chances for isolated flash
  flooding.

&&

.DISCUSSION...
Issued at 214 PM CDT Thu Apr 25 2024

Short Term...Tonight and Friday

High pressure will slide east of the area tonight, which will turn
surface winds to a southeasterly direction. This should allow for
warmer temperatures tonight as lows only fall into the upper 20s
to lower 30s across the north, with middle 30s across central and
east-central Wisconsin.

Surface winds will increase on Friday as the high moves further
east and low pressure approaches from the west, resulting in a
tight pressure gradient across the western Great Lakes. Clouds
will also increase on Friday as the low approaches, with a chance
for showers steadily increasing throughout the afternoon. Although
the high will be off to the east, it will still influence the
weather across northeast Wisconsin as it continues to pump dry air
into the region. This will delay the onset of precipitation from
the previous forecast, with all of Friday morning expected to be
dry and even the early part of the afternoon, especially across
northeast Wisconsin. The further delay of precipitation will mean
fire weather concerns will continue into Friday across north-
central and northeast Wisconsin given the warm temperatures, gusty
winds, and low relative humidity values. Highs on Friday are
expected to mainly be in the middle to upper 50s, with a few 60
degree readings possible.

Long Term...Friday Night Through Thursday

Focus for the extended remains on two systems that are expected to
trek from the central Plains into the upper Mississippi Valley
through early next week. Several chances for rain and storms will
accompany these systems, with confidence increasing in potential
for severe weather on Saturday. Current setup would support large
hail and gusty winds as the primary threats for any severe storms,
although there are still several hindrances to the forecast this
far out. A marginal risk for excessive rainfall remains in place
through Monday morning, with the WPC placing around 1.5 to 2
inches of cumulative 5-day QPF across the forecast area.

Saturday storm chances... Best timing for any severe weather
looks to be early Saturday afternoon as a cold front sweeps east
across the warm sector, providing a source of lift for surface-
based convection. The current suite of model runs would suggest a
mixed mode scenario, with best chances for discrete cell formation
occurring with an earlier onset of convective initiation. Any
storms that pop up during this time may be able to ingest decent
low-level helicity (surface to 1 km SRH values ~200 m2/s2) during
peak heating and instability (CAPE values between 1000 and 1500
J/kg). Low-level wind shear exceeding 30 knots would also favor
some right-moving storms across central Wisconsin. Model soundings
also indicate that hail would be a threat with some of the
stronger storms given steep lapse rates, favorable CAPE in the
hail growth zone, and wet bulb zero levels approaching 9500 to
10000 feet. This being said, there are still several sources of
forecast uncertainty. Remnant mid-level dry air from departing
Canadian high pressure combined with daytime mixing has hinted
that it might be difficult to get surface dew points up where they
need to be. This will be directly countered by how much deep Gulf
moisture this system is able to tap into. Additionally, whether
or not the near storm environment Saturday afternoon has enough
time to recover following the first round of precip Friday night
into Saturday morning is up for debate. Storm mode will likely
transition to linear/quasi-linear by late Saturday afternoon as
the frontal boundary encounters stable air over the lake and
convective elements become less organized.

Sunday/Monday precip... A brief lull in precip is expected to set
in Saturday night before a second system trails from the Four
Corners region into the upper Midwest by Monday morning. There is
much more uncertainty regarding severe weather risk on Sunday,
although confidence is increasing in periods of moderate to heavy
rainfall. Any storms that pop up ahead of the warm front will have
to contend with Canadian high pressure sitting and spinning over
Ontario, so there could be a sharp cutoff in the northern extent
of any convective development. Short to medium-range models should
paint a better picture in the next day or two.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 631 PM CDT Thu Apr 25 2024

Dry conditions will prevail until late in the TAF period, as
Canadian high pressure gradually shifts east and low pressure
approaches the western Great Lakes region. Clear skies will give
way to increasing high/mid clouds on Friday, with showers expected
to overspread the region from mid-afternoon through early
evening. Widespread showers and some embedded thunderstorms will
impact the region Friday night.

Southeast winds will increase and become quite gusty by late
Friday morning. LLWS is possible at the western TAF sites late
tonight into early Friday, then regionwide Friday night.

&&

.FIRE WEATHER...
Issued at 214 PM CDT Thu Apr 25 2024

Elevated fire weather conditions are possible on Friday as the
rain showers are further delayed across the region. North-central
and northeast Wisconsin will continue to see elevated fire weather
conditions as relative humidity values fall to 25 to 30 percent
during the afternoon, particularly near the Michigan border, as
rain is not expected until the late afternoon or early evening.
In addition, southeast winds will gust to 15 to 25 mph along with
temperatures in the upper 50s.

&&

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

&&

$$

DISCUSSION.....Kurimski/Goodin
AVIATION.......Kieckbusch
FIRE WEATHER...Kurimski

NWS GRB Office Area Forecast Discussion