Forecast Discussion for GRB NWS Office
661
FXUS63 KGRB 231716
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1216 PM CDT Thu May 23 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Showers are possible at times this afternoon, mainly across
  northern and central Wisconsin.

- A few rivers over central and eastern Wisconsin will remain
  above bankfull today into Friday.

- Widespread showers and thunderstorms are expected Friday into
  Friday night. Some of the storms could be strong to severe
  Friday afternoon and Friday evening.

- There are more chances of rain later in the Memorial Day
  weekend.

&&

.DISCUSSION...
Issued at 259 AM CDT Thu May 23 2024

Short Term...Today...Tonight...and Friday

Mostly sunny skies and warmer temperatures are expected across the
region today as a semi-stationary front develops across northern
Wisconsin and Upper Michigan. Some modest instability is expected
to build throughout the day, as SBCAPEs rise to around 500 J/kg.
While the semi-stationary front should be the main focal point for
showers across the north with the best chances (30 percent) the
instability across central and east-central Wisconsin will provide
a small chance (20 percent) for showers given the lack of a
capping inversion during peak heating. Highs today are expected to
range from the lower 70s across the north, to the upper 70s across
the rest of the area away from Lake Michigan.

Any showers lingering into the early evening should dissipate with
the loss of daytime heating, with mostly clear skies expected
tonight. Lows tonight are expected to range from the 40s across
the north, to the lower 50s across central and east-central
Wisconsin.

Two rounds of showers and thunderstorms are forecast to track
through the forecast area on Friday. The first round will track
through Friday morning and despite bulk shear values of 40 to 50
knots, instability will be de minimis as MUCAPEs will at best only
be a few hundred J/kg. Therefore, this first round of showers and
thunderstorms is not expected to be severe.

The second round of convection is slated to develop during the
afternoon ahead of an approaching occluded front as the warm front
lifts over southern Wisconsin. Although central and east-central
Wisconsin are expected to stay north of the warm front, MUCAPEs
are expected to rise to around 1000 J/kg during the afternoon as
bulk shear values remain around 40 knots. Any storms that track
through central and east-central Wisconsin could contain damaging
winds or large hail (5-14 percent probability) with the more
vigorous updrafts given the instability and shear; however, the
tornado threat is expected to stay further south in the warm
sector. Highs on Friday are expected to range from the middle 60s
across the north, to the lower 70s across east-central and
portions of central Wisconsin.

Long Term...Friday Night Through Wednesday

Models indicate the next northern stream shortwave trough will
begin to impact northeast WI Sunday afternoon and likely continue
through the morning hours of Memorial Day. Still enough nuisance
differences among the models as to where/how much precipitation
will fall with this system, but locally heavy rains are possible.
Toward mid-week, an upper ridge builds over the Rockies with a
northwest flow into WI. A secondary shortwave trough could bring
more showers to the area around Tuesday. Temperatures for the most
part will be at or below normal through the period.

Friday night and Saturday..
Showers/thunderstorms to be ongoing at the start of the period as
a surface low/cold front and trailing mid-level shortwave trough
move through the region. Even though instability will be
diminishing upon sunset, bulk shear of 30-40 knots could maintain
a few stronger storms into the evening. Precipitation will
gradually end from west to east late evening/overnight behind the
cold front. Skies could actually become mostly clear over central
WI toward daybreak as high pressure begins to build east from the
Midwest. Min temperatures to range from the lower 40s over north-
central WI, to the upper 40s to lower 50s. This high pressure
drifts across WI Saturday morning and reside over Lower MI
Saturday afternoon. Mostly sunny skies are expected for most
locations to start the day, but some increase in clouds may occur
over central WI later in the afternoon as a new system organizes
over the central High Plains and a mid-level shortwave trough
moves into the northern Plains. Saturday looks like the nicest day
of the holiday weekend, with max temperatures in the middle 60s
north-central, upper 60s to lower 70s elsewhere.

Saturday night and Sunday...
Quiet conditions should hold through Saturday night as a surface
low moves across the central Plains and the shortwave trough
shifts into the Upper Midwest. Look for clouds to increase/thicken
through the night with min temperatures in the middle 40s north,
upper 40s to lower 50s south. Forecast concerns come into play on
Sunday as to how much interaction will occur between the surface
low and shortwave trough. More interaction could deepen both
weather features and potentially bring another round of showers/
locally heavy rains to parts of central and east-central WI. Less
interaction would keep each feature on the weaker side with more
precipitation staying to our south. For now, have kept higher pops
south of HWY 29 and see how subsequent model runs handle this. Max
temperatures Sunday to be in the middle to upper 60s.

Sunday night and Memorial Day...
Chance of showers/isolated thunderstorms would continue into
Sunday night, but the extent of the precipitation is still in
doubt depending on the intensity of the surface low and shortwave
trough. If the phase of these two features comes to fruition,
heavy rains may occur which could then lead to localized flooding.
A cyclonic flow is forecast by the models on Memorial Day which
would keep a good amount of clouds around the area and a continued
chance of showers. Max temperatures on Memorial Day look to be in
the middle to upper 60s with a few sites perhaps reaching 70
degrees.

Monday night through Wednesday...
As an upper ridge begins to build over the Rockies toward the
middle of next week, one more shortwave trough is progged to sweep
into the Great Lakes in the developing eastern CONUS upper trough.
Models are handling this shortwave trough differently when it
comes to timing, but not enough to impact Tuesday`s weather with
additional chances for showers. Drier conditions should arrive on
Wednesday as this shortwave trough departs and is replaced by high
pressure. Temperatures are forecast to return to near normal
readings on Wednesday with upper 60s to middle 70s.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1207 PM CDT Thu May 23 2024

VFR CIGS will continue through the afternoon. Isolated
to scattered thunderstorms will be possible, especially across
far northeast Wisconsin and then across central into east-central
Wisconsin. These storms will be high based, allowing for the
potential for gusty winds at the surface to around 40 knots
and may produce some small hail. Any showers or storms should
end by 02z. The next round of showers will arrive late tomorrow
morning across central and portions of north-central Wisconsin
and into eastern Wisconsin by early afternoon. A round of strong
or severe storms are possible later Friday afternoon into Friday
afternoon.

&&

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

&&

$$

DISCUSSION.....Kurimski/Kallas
AVIATION.......

NWS GRB Office Area Forecast Discussion