947
FXUS63 KGRB 051136
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
636 AM CDT Sun May 5 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Patchy dense fog this morning, mainly across central and
  northern WI. Otherwise, quiet conditions expected today into
  Monday.

- Relative humidity dropping into the upper 20s and 30s this
  afternoon and Monday afternoon. Recent rain, lack of gusty
  winds and continued green-up will limit fire weather concerns.

- Next round of showers and storms arrives Tuesday morning. Best
  chance for non-severe thunderstorms will be Tuesday afternoon
  and evening.

&&

.DISCUSSION...
Issued at 345 AM CDT Sun May 5 2024

Short Term...Today...Tonight...and Monday

Quiet early May weather on tap for the region today into Monday.
Only weather concerns will be fog/frost potential and low RH`s
each afternoon.

Precip / Clouds / Fog:

High pressure will build across the Great Lakes today, slowly
sliding to our east on Monday, bringing dry conditions. Fog,
locally dense, will continue across parts of central and northern
WI early this morning. May need a SPS if the fog can become a
little more dense/widespread. So far satellite and webcams are
showing more patchy ground fog. The low clouds and fog will
lift/burn off after sunrise, leaving sunny skies. Then look for
fair weather cumulus to develop later this morning into the
afternoon as we hit our daytime convective temps. These will fade
in the mid-late afternoon, with clear skies expected tonight. Fog
potential tonight looking very low as the lower levels dry out
even more and winds just off the surface remain between 10-20 kts.
Plenty of sunshine is expected on Monday with only a few fair
weather cumulus expected in the late morning and afternoon. As
flow turns back to the south/southeast on Lake Michigan late
tonight into Monday, a few lake clouds or even a little fog will
be possible.

Winds / Relative Humidity:

Northwest winds will shift to the west and gust up to around 15
mph today. As return flow sets up on Monday, southeast winds
could gust up to 20 mph. Afternoon relative humidities are
forecast to drop into the upper 20s and 30s this afternoon and
Monday afternoon, lowest in the sandy soil regions. Recent rain
and lack of gusty winds will limit any fire weather concerns today.
Slightly elevated fire weather conditions are possible on Monday
as soils dry out and slightly warmer temps are expected, especially
over northern WI where the green-up has been the slowest.

Temps / Frost Potential:

925/850mb temps support highs in the 60s to around 70 today. A
lake breeze will keep afternoon temps cooler near Lake Michigan.
Lows tonight are forecast to drop into the 30s across much of the
Northwoods. Some of the colder spots in central WI could also drop
into the upper 30s. Frost is looking likely over northern WI, but
frost/freeze headlines have not begun yet. Further south, some
patchy frost will be possible in the cold spots of central WI.
925/850mb temps increase slightly on Monday, so most spots should
be a a couple/few degrees warmer than today.

Long Term...Monday Night Through Saturday

Monday night through Tuesday...Focus of the extended forecast
remains on a well organized system shifting over the northern
Plains Tuesday. A warm air advection regime and deep layer
moisture (PWAT values 1-1.3") out in front of this system will
likely bring an initial round of rain to the region Tuesday
morning spreading from southwest to northeast. There will be some
dry air to overcome in front of this system, so areas in east-
central and northeast WI may not see rain arrive until Tuesday
afternoon. Best chance for thunderstorms comes Tuesday afternoon
and evening as models show a thin band of instability (MUCAPE 200-
500 J/kg) building over center and east- central WI. Additionally
mid-level lapse rates approach 7-7.5 C/km during this period.
Forcing comes in the form of a left exit region and 500mb PV
anomaly. The strength of these features will be key in determining
if severe storms develop. Currently, confidence in any storms
becoming severe is low with the better chance for severe storms
remaining south toward the WI/IL boarder. In terms of rainfall
amounts the long term grand ensemble (GEFS, ENS, and GEPS) shows a
50-70% chance for over 0.5" of QPF Monday to Tuesday night.
Probabilities drop to about 10-30% when looking at the chance for
greater than 1".

Wednesday through Thursday...Ensemble models have come into
decent agreement with a period of dry weather Wednesday morning
as dry air behind an occluding front filters over the region. Dry
conditions may not last long as the weakening cyclone moving east
out of the norther Plains merges with a developing southern
stream system returning rainfall to the region late Wednesday
into Thursday. The GFS in particular shows the potential for a
band of heavier rainfall Thursday over central to southern WI as
these systems merge. The potential for any convective activity is
low during this period as the region resides of the non-favorable
northwest side of the low.

Rest of the extended...The details beyond Thursday become difficult
to pin down as as the GFS and Euro show a trough digging down from
Ontario keeping scattered shower and storms possible, while the
Canadian has a ridge developing over the region. One note to make
is depending on if skies clear out early Friday morning temperatures
across northern WI may drop to or near freezing. With Frost/Freeze
headlines starting for northern WI May 7th this will be a period
to monitor, particularly for those who may have gotten an early
start to the growing season.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 636 AM CDT Sun May 5 2024

Patchy ground fog will lift/burn off across central and northern
WI early this morning. Otherwise, VFR conditions are expected
through the TAF period. Look for fair weather cumulus to develop
later this morning, then dissipate late this afternoon and
evening. Very low chance of a little ground fog early Monday
morning, especially over north-central WI.

Northwest to west winds will generally stay under 10 kts, but
a few gusts up to 15 kts are possible later this morning and
afternoon. A lake breeze will turn winds south/southeast across
eastern WI this afternoon. Light and variable winds are expected
tonight.

&&

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

&&

$$

DISCUSSION.....Bersch/GK
AVIATION.......Bersch