102
FXUS63 KGRB 090252
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
952 PM CDT Thu May 8 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- A Frost Advisory has been issued for tonight with lows forecast
  to fall in the upper 20s to middle 30s over portions of central,
  east-central, and northeast Wisconsin. A few locations could
  see a few hours with temperatures at or below freezing.

- Low relative humidities 20-35% are forecast from Friday through
  Saturday away from Lake Michigan and the Bay of Green Bay.
  Watching Sunday into early next week for next potential of near
  critical fire weather due to low RH values, gusty south to
  southwest winds, and temperatures warming into the lower 80s
  inland from Lake Michigan.

- Below normal precipitation is expected over the next week.
  Isolated showers and storms will be possible on Friday night.
  Otherwise, the next chances for rain will not arrive until next
  Wednesday or Thursday.

&&

.DISCUSSION...
Issued at 124 PM CDT Thu May 8 2025

Short Term...Tonight...Friday...and Friday Night

The latest RAP analysis and satellite/radar imagery show high
pressure centered from James Bay to the western Great Lakes early
this afternoon. While winds have been breezy over northeast WI
thanks to northeast onshore flow, generally quiet conditions under
clear skies prevail elsewhere. Humidities have fallen into the
10-20% range across north-central Wisconsin thanks to a very dry
airmass in place (precipitable water values 25% of normal).
Looking upstream, clear conditions prevail across the northern
Plains until Montana where there is a front heading east. With a
cool/dry airmass in place, focus of the forecast centers first
around frost and fire weather potential. Then attention turns to
rain chances along a cold front for Friday night.

Frost Potential Tonight: The surface high pressure will remain
centered across the region. With low pwat air (25% of normal),
statistical guidance points towards winds becoming calm after
midnight. As a result, lowered min temperatures closer to MOS
statistical guidance which put lows from the upper 20s to middle
30s where the growing season has started. Have therefore issued a
Frost Advisory from 1 am to 8 am Friday.

Fire Weather: The dry airmass will linger across the area through
Friday afternoon leading to increased fire danger. After min
humidities from 10 to 30% over central and north-central WI
today, some modification of the airmass is forecast for Friday.
But at the same time, warmer temperatures will be invading the
region with highs ranging from the upper 60s to middle 70s. As a
result, humidities are still expected to fall to the 20-30% range
in the afternoon. Fortunately, sustained winds under 10 mph are
forecast across the region which will mitigate some of the fire
weather concerns.

Rain and Storm Potential Friday Night: A cold front will drop
southeast across the area from late Friday evening through early
Saturday morning. An instability axis aligned along the front in
the afternoon will diminish as it approaches north-central WI in
the evening. But 200-500 j/kg may be enough to support isolated
storms into north-central WI in the evening. The front will not
have much in the way of moisture to work with. Therefore, precip
chances will remain low (less than 20%).

Long Term...Saturday Through Thursday

The latest ensemble means indicate that split flow will be the
prevailing pattern across the lower 48 from this weekend through
the middle of next week. By next Wednesday, southern stream energy
will be lifting into the eastern Great Lakes in response to a deep
trough digging over the western CONUS. This trough will provide a
chance of precip during the Thursday-Friday timeframe next week.
Otherwise, focus of this forecast remains on fire weather.

Fire Weather: A dry airmass (PWATs 70% of normal) will settle
across the region behind a cold front on Saturday, which will
stall over the southern Great Lakes. While the driest air is
forecast to reside over the Upper Peninsula, decent mixing to
about 800mb should result in dewpoints falling into the middle or
upper 20s and humidity levels around 25% across the north-central
WI on Saturday afternoon.

The front will return north on Sunday and moisture in the boundary
layer is forecast to increase, which should offer some reprieve
from the low humidities (30-40%).

However, by Monday, the dry air gets recycled north and across the
western Great Lakes. While not excessively dry, and mixing is not
projected to be overly deep, a tight pressure gradient and warm
south winds will lead to low humidities around 25% across far
northern WI. Given the gusty winds aloft, think mixing could be
more robust than models indicate. There is a 40-60% chance of
sustained winds greater than 15 mph over north-central WI on
Monday afternoon. With highs forecast near 80 degrees, conditions
will be near critical levels for fire weather.

By Tuesday and Wednesday, deeper Gulf moisture is forecast to move
into the region which will alleviate the elevated fire weather
conditions.

Precipitation: The cold front, which settles over the southern
Great Lakes on Saturday, will return north across the region on
Sunday. Ensemble means generate 200-500 j/kg of instability across
the region as the front advances northeast on Sunday, but
moisture profiles vary amongst model guidance leading to a low
confidence forecast regarding shower potential. Given the dry
airmass, rising mid- level heights, the risk of rain is low (less
than 15%).

Spotty shower activity, perhaps diurnally based, appears possible
by next Wednesday as gulf moisture will be present across the
area. A higher chance of rain will arrive Thursday into Friday
with a cold frontal passage.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 951 PM CDT Thu May 8 2025

VFR conditions expected through the TAF period. Some upper level
clouds will drift across the region at times overnight. Light
winds are expected overnight, with winds switching to the
southwest/west on Friday and remain under ~10 kts.

A band of mid-clouds will spread across the area from northwest to
southeast Friday afternoon/evening as a cold front drops across
the state. These clouds could produce a few showers/sprinkles,
but moisture is limited and no impacts to aviation are expected.
Very limited instability across central and north central WI will
keep thunder chances under 15% so will not include.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Frost Advisory from 1 AM to 8 AM CDT Friday for WIZ020-022-030-
031-035>040-045-048>050-073-074.
&&

$$

DISCUSSION.....MPC/MPC
AVIATION.......Bersch

NWS GRB Office Area Forecast Discussion