246
FXUS63 KGRB 181736
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1236 PM CDT Fri Sep 18 2026

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- A period of moderate to heavy rainfall is expected this
  afternoon through Saturday, with the heaviest rain occurring
  tonight. A 20 to 40 percent chance of rainfall exceeding 2
  inches exists roughly south of a Marshfield to Appleton line.

- A quieter and drier weather pattern will settle into the region
  early to mid next week.

&&

.DISCUSSION...
Issued at 115 AM CDT Fri Sep 18 2026

The latest RAP analysis and radar and satellite imagery show
strong high pressure centered over northwest Ontario and extending
south across the western Great Lakes early this morning. Weak
cold advection is occurring, with patchy ground fog developing
where skies are clear. However, this fog will be short-lived early
this morning as mid and high clouds thicken from the west. These
clouds are already moving into the region ahead of shortwave
energy and broad warm advection occurring over the central and
northern Plains. Showers and thunderstorms are blossoming on the
nose of a low level jet across northern Nebraska and South Dakota
in a region of strong moisture convergence and moisture transport
coincident with mid-level frontogenesis. This activity is
occurring ahead of surface low pressure centered over northeast
Colorado, which will be the primary weather driver for the region
over the next 36 to 48 hours.

Heavy Rainfall Potential Today through Saturday:
As this shortwave energy and associated low pressure lift
northeast, the potential for heavy rainfall becomes the primary
focus of this forecast. The low level jet will continue to lift
northeast today, with strong moisture convergence developing on
the nose of a 30 to 40 kt low level jet beneath the right rear
quadrant of an upper jet streak. This will lead to rain spreading
east from South Dakota across Minnesota and into Wisconsin this
morning. The rain will start out light as it enters north-central
Wisconsin by late morning, with intensity increasing this
afternoon as the nose of the low level jet becomes centered across
central Wisconsin. In general, the band of rain will be weakening
as it progresses across the region alongside a weakening low
level jet. The rain will continue for much of tonight through
Saturday and into Saturday night as isentropic ascent and mid-
level frontogenesis continue to be centered across Wisconsin. A
few thunderstorms will be possible tonight and Saturday, but no
severe weather is expected.

The signal for the heaviest rainfall continues to shift south and
generally lies from southeast Minnesota to southern Wisconsin.
This trend makes sense since this is where the highest
precipitable water values near 2 inches, the most unstable CAPE in
excess of 1000 j/kg, and a persistent low level jet will be
located. As a result, the risk of flash flooding has decreased
considerably as the rainfall rates do not look supportive. The
latest guidance shows the risk of 2 inches of rain in 6 hours is
under 5 percent during the peak rainfall period late tonight.
Overall rainfall amounts from this afternoon through Saturday
afternoon have decreased to 1 to 2 inches over central and east-
central Wisconsin. Most of the rain will gradually diminish on
Saturday night.

Confidence in rainfall amounts is generally lower than normal for
this time range of the forecast due to uncertainty in the area of
heaviest rainfall and potential for locally heavy downpours in
thunderstorms.

Quiet Weather Next Week:
Looking ahead to next week, shortwave energy originating from the
western North American trough will slowly migrate east from the
central Plains to the southern Great Lakes and eventually the east
coast. Therefore, it appears that most of the rain will remain
south of the region. There are only small chances of rain up to 20
to 30 percent forecast for Monday over parts of central and east-
central Wisconsin. Otherwise, strong high pressure over northern
Ontario and northern Quebec looks to keep ample dry air across
northern Wisconsin. Other than Monday, no precipitation is
forecast next week.

With high pressure maintaining dry and quiet conditions for the
bulk of the extended period, temperatures will be undergoing a
slow warming trend. Daytime highs ranging from the low to mid 60s
on Monday will gradually warm each day, reaching the low to mid
70s by Thursday and Friday.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1236 PM CDT Fri Sep 18 2026

Low pressure will be slowly moving towards the area from the west
over the next 24 hours. The initial band of rain will affect the
central and north-central TAF sites early this afternoon before
moving east towards east-central Wisconsin. Steadier rain is then
expected to track through later this afternoon into early this
evening and continue through Saturday. CIGs early this afternoon
currently range between VFR and MVFR as the low level moisture is
starting to take root across the region. CIGs should eventually
continue to fall and settle in at IFR by tonight and continue into
Saturday. Reduced visibilities to high end IFR to low end MVFR
are likely in the heavier downpours. Thunderstorms will likely
remain south of the TAF sites, but a few storms could reach as far
north as KATW and KMTW. East winds will likely be rather steady
in the 5-10 kt range with gusts to 15 kts for much of the TAF
period.

&&

.MARINE...
Issued at 115 AM CDT Fri Sep 18 2026

Gusty easterly winds ahead of and during the passage of the
aforementioned low pressure system will build wave heights
significantly on Lake Michigan from late tonight through the
Saturday afternoon. This will create hazardous conditions for
small craft and increase the risk for dangerous rip currents along
beaches south of Algoma. Will be considering issuing a Small
Craft Advisory and Beach Hazards Statement later this morning.

Another round of hazardous marine conditions looks likely Sunday
through Monday due to gusty northeast winds 20 to 30 kts and high
waves.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......Kurimski
MARINE.........MPC

NWS GRB Office Area Forecast Discussion