National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS63 KGRB 140348
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1048 PM CDT Sat Apr 13 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- There is a slight chance of thunderstorms tonight across
  northern Wisconsin. Small hail is possible with some storms.

- Dry and very mild weather is expected on Sunday, with relative
  humidities of 20 to 30 percent inland from Lake Michigan in the
  afternoon. This may create conditions conducive for grass fires.
  Therefore, there will be an elevated fire weather concern on
  Sunday.

- Showers and a few embedded thunderstorms are expected from
  Tuesday into Wednesday. Severe thunderstorms are not expected at
  this time, but heavy rainfall will be possible.

- Strong east to southeast winds expected Tuesday into Tuesday
  night, with gusts to 30 to 45 mph. Gusts to near gale force
  possible on Lake Michigan Tuesday night.

&&

.DISCUSSION...
Issued at 250 PM CDT Sat Apr 13 2024

Short Term...Tonight and Sunday

Main forecast concerns to be on thunderstorm chances tonight and
fire weather concerns on Sunday.

The 19Z MSAS surface analysis showed a ridge of high pressure that
extended from the TN Valley through the Great Lakes. An area of
low pressure was located over north-central SD with a warm front
that stretched southeast into southern MN and a cold front that
stretched from the low pressure westward into the Rockies. Visible
satellite imagery indicate a swath of mid-clouds from ND into
Upper MI.

The weak area of low pressure is forecast to move east this
evening and pull the warm front northeast into WI. This warm front
will combine with a weak mid-level shortwave moving across
northern sections of the Great Lakes and bring a chance of showers
mainly to northern WI. Despite minimal CAPE, there is a slight
chance of thunderstorms as mid-level lapse rates are quite steep
(7-8 C/KM) and 0-6km shear is 50-60 knots). Since any storms will
likely be elevated, small hail will be possible. The trailing cold
front is then expected to sweep across the area overnight and veer
the winds to the northwest by daybreak. Min temperatures to be in
the upper 30s to lower 40s far north/along Lake MI, middle 40s to
around 50 degrees south.

This system to already be in the process of pulling away from
northeast WI at the start of Sunday with high pressure building
into the region. Plenty of sunshine in the morning with fair
weather cu for the afternoon as northwest winds increase to 10 to
15 mph with higher gusts. Another push of dry air behind the cold
front will send afternoon relative humidity values down into the
20 to 30 percent range away from the lake and when combined with
forecast max temperatures in the lower to middle 60s north-
central/near Lake MI, upper 60s to lower 70s elsewhere, elevated
fire weather conditions are expected.

Long Term...Sunday Night Through Saturday

The main forecast concerns during the extended forecast will
occur in association with a strong low pressure system Tuesday
into Wednesday. Widespread shower activity, heavy rainfall,
isolated to scattered thunderstorms, and strong east to southeast
winds are anticipated during this period.

High pressure will dominate our weather early in the period, with
dry conditions prevailing Sunday night and Monday, and quite
possibly through Monday night. Would not be surprised if we need
to slow down the arrival time of precipitation late Monday night
into Tuesday. Have lowered dew points and relative humidities
into Monday, leading to continued concern for elevated fire
weather conditions.

Vertically stacked/deep low pressure will move from the Central
Plains to western IA on Tuesday, then into southwest WI Tuesday
night. A developing 50 kt low-level jet ahead of this system will
bring a surge of moisture (PWATs 1-1.3 inches) into the forecast
area, and generate strong isentropic ascent over a tight
baroclinic zone. Widespread shower activity will overspread the
region Tuesday afternoon and night, along with potential for heavy
rainfall. Somewhat underwhelmed by the paltry instability (surface
and aloft) during this period, so suspect that only isolated to
scattered embedded storms will develop, mainly across the southern
half of the forecast area Tuesday night. On Wednesday, the
occluding surface low will push through the southeast part of the
forecast area during the day, accompanied by moderate shower
activity, and a few storms along the occluded front in the
southeast CWA. Strong east to southeast winds will develop ahead
of the low Tuesday into Tuesday night, with gusts to 30 to 45 mph
(strongest in central WI), and near gale force gusts on Lake
Michigan Tuesday night.

The remnants of this low lift north into Ontario and eventually
phase with additional energy in central Canada to produce a weak,
slow moving low that tracks to our north later in the work week.
This will bring a chance of rain or mixed rain/snow showers to
mainly northern WI.

Temperatures will start off well above normal for the early to mid
part of the week, then trend closer to normal late in the week.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1045 PM CDT Sat Apr 13 2024

VFR conditions prevailed across the region during the late
evening with some mid clouds making it down into northern
Wisconsin along a warm front. Scattered showers and elevated
thunderstorms moved east across northern Wisconsin through the
evening, mainly affecting RHI. Three quarter inch hail and gusty
winds were reported with some of the stronger cells. Unsure how
far south these storms will make it, but given frequent
lightning and re-development to the west, opted to add a TEMPO
group mentioning thunder for AUW. Kept mention of thunder in for
RHI until 07Z. LLWS remained a concern south of RHI during this
time as surface winds rapidly decoupled and 850 mb winds began to
ramp up.

Cloud cover will quickly clear out toward sunrise Sunday morning
with daytime mixing, leaving mostly clear skies and VFR
conditions through the TAF period. Surface winds will then veer
to northwesterly and increase Sunday afternoon behind a trailing
cold front. Gusts between 20 and 25 knots will be possible at
most TAF sites after 18Z.

&&

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

&&

$$

DISCUSSION.....Kallas/Kieckbusch
AVIATION.......Goodin