778
FXUS63 KGRB 032347
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
647 PM CDT Wed Jul 3 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Next round of widespread showers/storms arrives from the west
  Thursday afternoon/evening, becoming more widespread overnight.
  Confidence is increasing these showers/storms will impact
  fireworks displays the night of the 4th, especially across
  central and north-central Wisconsin.

- Threat for flooding is increasing Thursday evening into Friday
  from this next round of heavy rain. Rivers and streams will
  likely rise, with some urban/rural flooding also possible.

- The unsettled weather pattern continues Sunday through (at
  least) Tuesday with on and off chances for rain/storms.

&&

.DISCUSSION...
Issued at 347 PM CDT Wed Jul 3 2024

Heavy Rain Threat / Precip Chances:

Showers have developed across northwest and north central WI
early this afternoon, so will continue with isolated showers this
afternoon and evening, especially over northern WI, as we reach
peak heating and the robust cumulus clouds track eastward. Still
can`t rule out a rumble of thunder as instability of nearly 1000
J/kg and steep low level lapse rates will continue this afternoon.
Dry conditions are expected later tonight into Thursday morning.

Attention turns to the arrival of the next round of showers and
storms on the 4th. Deterministic and CAMs still showing slightly
different scenarios, especially the HRRR which seems to be a
fast/more aggressive outlier. Would like to ignore this solution,
but it has captured some events in recent weeks, so won`t
completely throw it out. Leaning toward a model blend, have only
made minor changes to the going forecast for the 4th, bringing in
chance PoPs from west to east through the day, with one round
possible in the later morning and afternoon associated with a push
of some elevated instability and weak warm/moist advection, then
the main round in the evening into the overnight as the main
shortwave, low pressure and upper trough drop across the region.
Still some uncertainty on just how progressive this system will
be, which will likely determine how much rain we will get. While
PWATs won`t be as tropical as yesterday (when we had spots over 2
inches), still respectable values approaching 1.5 inches late
tonight into Friday will be supportive of additional heavy rain.
Additional rainfall of an inch or two is looking likely (40-70
percent of an inch or more, highest in central WI) from Thursday
afternoon into Friday evening, which could aggravate flooding
issues.

Considered a flood watch for parts of the area that have received
the most rainfall the past 2-3 days, mainly central and east
central WI, as the continued bouts of heavier rain have led to
flash flood guidance (FFG) to decrease and the overall flood
threat to increase with each event. Parts of the Fox Valley have
1-hour FFG around/just under an inch, with 6-hour FFG between 1-2
inches. Parts of central WI are closer to the 1.5-2.5" range.
The pockets of heaviest rain have been in different locations over
the past few days and there has been at least a day or two of dry
conditions to help rivers/low drainage areas to lower. With the
heaviest rain still 24+ hours out, and some uncertainty on where
the heaviest bullseye will be, will opt to hold off on any flood
watch at this time but continue to highlight the flood threat in
the HWO. The threat for severe weather will be low, as the better
surface instability and effective shear will remain to our
southwest. But some of the stronger elevated storms could produce
some hail. SPC did maintain a marginal risk for severe weather in
the far southwest forecast area.

Saturday is looking dry as high pressure slides across the region.
Then the pattern turns unsettled again Sunday into at least
Tuesday as another large trough slowly rotates across the western
Great Lakes. Some bouts of heavy rain will once again be possible
with PWATs climbing back over 1 inch, possibly creating additional
flooding concerns.

Winds / Fog / Temps:

Winds have struggled a little today as mixing was a little
hampered in spots, but west winds could gust to around 25 mph
the rest of the afternoon, then as mixing decreases, winds will
decrease toward sunset. Lighter winds are expected tonight into
Thursday.

Patchy ground fog will be possible late tonight as winds die off,
but SREF/HREF showing widespread fog potential under 5%. There
will be additional fog chances during the overnight and early
morning hours, especially after the heavier rain events and
if/when skies clear.

Above normal temps will continue into Thursday, then a brief cool
down arrives on Friday. Temps rebound close to normal for the
weekend and into next week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 634 PM CDT Wed Jul 3 2024

VFR conditions expected through the TAF period.

Clear skies, light winds and recent rainfall could lead to patchy
ground fog overnight, but confidence is too low to add to the
TAFs at this time. Will reevaluate for the 06z TAFs.

An approaching low pressure system and warm front will bring a
chance of showers and storms to mainly central WI late Thursday
afternoon, though timing is a bit suspect at this point. More
widespread showers and storms are expected to overspread the
region Thursday night, along with deteriorating flight conditions.

Gusty west winds will diminish by sunset, with light west winds
switching to southeast Thursday afternoon.

&&

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

&&

$$

DISCUSSION.....Bersch
AVIATION.......Kieckbusch