795
FXUS63 KGRB 151150
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
650 AM CDT Sat Jun 15 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Well above average temperatures are expected from Sunday through
  Tuesday. Heat indices may surpass 90 degrees at times. The
  warmest temperatures and highest heat index values are expected
  across central into northeast Wisconsin.

- A round of showers and thunderstorms is expected to move across
  the area tonight through Sunday morning. Heavy rainfall is
  likely, especially west and north of the Fox Valley, where amounts
  over an inch are possible. High rainfall rates could result in
  localized street flooding and ponding of water on roads. Severe
  thunderstorms are not expected.

- Thunderstorms are possible Sunday afternoon and Sunday night.
  Some of these storms could become severe with damaging winds and
  large hail, especially over north central Wisconsin Sunday
  night.

- Thunderstorms are possible at times next week. Due to the heat and
  humidity, some stronger storms and locally heavy rainfall may
  be possible at times from Monday through Wednesday. Confidence
  is low in the timing of any rounds of storms.

&&

.DISCUSSION...
Issued at 430 AM CDT Sat Jun 15 2024

Short Term...Today...Tonight...and Sunday

The forecast area remained under the influence of a Canadian high
pressure system, which was centered over eastern Ontario early
this morning. Developing return flow and WAA over southern MN and
northern IA was generating some showers, though much of this
appeared to be virga or light sprinkles falling from a mid-level
cloud deck.

Dry weather should prevail over most of the forecast area this
morning, with a few sprinkles arriving in central WI toward
midday. Increasing WAA and a weak short-wave trough should
generate scattered showers across mainly C/NC WI this afternoon.
Despite increasing clouds, highs should still reach into the 70s,
except upper 60s near Lake Michigan.

Precipitation chances ramp up tonight, especially overnight as a
45 to 55 kt low-level jet develops, a potent short-wave trough
moves through, elevated instability arrives, and a warm front
approaches the region. The strong dynamics, combined with PWATs
increasing to 1.5-1.8 inches, supports a heavy rainfall threat,
with amounts of an inch or more possible west and north of the Fox
Valley. Several models show two main QPF maxes during the
overnight period; one affecting NC WI, and another in central WI.
There is support for this, as NC WI will be under the influence of
the RRQ of an upper level jet, while central WI will get the main
surge of elevated instability. WPC has outlooked most of the
forecast area in a Marginal risk of excessive rainfall, with parts
of C/NC WI in a Slight risk. Suspect the main issue will be
localized flooding of urban areas. The late arrival of elevated
instability and widespread shower activity (leading to a lack of
discrete cells) should preclude a significant threat of severe
storms during the overnight period. Have raised min temps several
degrees, as southeast winds will be ramping up through the night.
Lows should be in the upper 50s to middle 60s.

On Sunday, the main surge of showers and storms should depart
from SW to NE during the 14z-18z period. Stabilization from the
morning rainfall, plus lingering cloud cover, slight ridging aloft
and an inversion/capping evident in the 850-800 mb layer, all
suggest a relative lull in convective activity during the
afternoon. However, with mid-level lapse rates increasing to 7.5-8
C/km and CAPE eventually building to 2000-3000 j/kg, I`m not
brave enough to go with a dry forecast. Will keep slight chances
over the southern part of the area, and chance pops across
northern WI, where some models suggest the warm front will reside
later in the day. Cannot rule out an isolated severe storm, but
the best chance appears to be over north central WI Sunday night.
Highs should range from the lower 80s north to the middle 80s to
around 90 south. Locations along the immediate Lake Michigan
shoreline should remain in the 70s. South to southwest winds will
gust to 25 to 30 mph, especially in C/EC WI.

Long Term...Sunday Night Through Friday

500mb ridge across the eastern United States with a trough across
the Rockies will result in an active weather pattern for much of
next week with periodic chances of thunderstorms. Heat and
humidity will be the main story across much of the area early in
the week, with Monday and Tuesday being the warmest days. A weak
cold front will work across the area Wednesday, bringing
temperatures closer but still slightly above normal for the end
of the work week.

For Sunday night, the models are showing a weak disturbance at
500mb across the northwest half of the state that some of the
models are depicting convection firing during the evening. Bufkit
soundings indicated CAPE values of 3,000 to 4,000 J/KG and 0-6 km
shear values around 25 kts to support strong or severe storms
with large hail and damaging winds. The greatest risk of severe
storms would be across north-central Wisconsin closer to the upper
level feature. There is a second feature at 500mb that moves into
western Wisconsin shortly after 12z on Monday that could bring
stronger storms to our northern counties Monday morning. On
Monday, it will very warm or hot depending on your location.
Temperatures across portions of central and northeast could climb
into the lower 90s with heat index values as high as the middle
90s across east-central Wisconsin. With the active southwest flow
pattern aloft and building ridge, much of the convection Monday
night and Tuesday may remain west of the forecast area. With
thunderstorm complexes to the west of the area during this time
frame, it would not surprise me if a complex snuck into our
northwest counties at some point. Also, several models are
depicting diurnally drive convection Tuesday afternoon and
evening. If storms do develop, there is plenty of CAPE to support
strong or severe storms. Damaging winds would be the primary
threat, but could not rule out some large hail despite the very
warm temperatures at the surface.

On Wednesday, the models are depicting a weak cold front moving
across the area which will bring continued chances of storms. Some
of the storms could be strong or severe as well depending on how
much sunshine we see before the storms arrive. The 00z ECMWF is
indicating continued chances of showers and storms as a frontal
boundary lingers across the area. Being this far out, there is
always concern that one of the thunderstorm complexes could push
the front south of the area to bring a break in the chances of
rain. Will resolve these concerns over the next several days.
Temperatures will be well above normal early next week, then turn
closer to normal but still above normal late in the week.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 638 AM CDT Sat Jun 15 2024

VFR conditions are expected through early evening, with light
showers arriving at mainly the western TAF sites this afternoon.
Widespread moderate showers should develop over the northwest
half of the forecast area during the evening, then gradually
spread southeast across the region during the overnight period.
Elevated instability will move into the region overnight, leading
to a chance of thunderstorms and associated heavy rainfall. Flight
conditions will deteriorate to MVFR/IFR west and north of the Fox
Valley during the late evening and overnight hours. MVFR vsbys may
also accompany any storms, and these may reach the eastern TAF
sites toward the end of the TAF period.

Southeast winds will gradually increase today into tonight. LLWS
will develop this evening and continue through the overnight
period.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/Eckberg
AVIATION.......Kieckbusch