DISCUSSION
Issued at 329 AM CDT Fri Sep 20 2024

Short Term
Today
Tonight...and Saturday

Main forecast concerns will the exiting showers this morning, chances for additional showers/storms on Saturday and fog potential early this morning and again late tonight into Saturday morning. Above normal temps continue today into Saturday.

Shower/Storm Chances & Clouds:

Shortwave trough and cool front will continue to push west to east across the area this morning. Most of the shower activity will come to an end by 4AM, but there could be some additional development along/ahead the front through mid-morning. Still enough elevated instability (up to ~1000 J/kg of MUCAPE) early this morning to keep a thunder mention. Dry conditions are expected this afternoon into tonight. Low clouds will continue to form ahead of the front early this morning due to the moist south/southeast flow, but as winds turn to the west behind the front, look for the clouds to clear out as drier air works in later this morning, with only a few daytime cu expected this afternoon.

A few CAMs are trying to bring an area of light rain or sprinkles into parts of central and/or north central WI in association with a push of WAA and LLJ poking up into western WI. Won't bite on this just yet, but some low-end PoPs may need to be added if this feature does materialize. Another frontal boundary, shortwave and jet streak will then approach the area Saturday afternoon, with chances for showers and storms arriving across north central WI after ~4pm. A ribbon of decent instability (500-1200 J/kg MUCAPE)
and shear (25-35 kts) is forecast to push into north central WI in the afternoon (but uncertainty remains on just how unstable it will get ahead of the front), bringing the chance of a strong storm with gusty winds, brief heavy rain (PWATs climbing to ~1.3")
and small hail. But the severe weather threat will remain low through sunset.

Fog Potential/Trends:

Areas of dense fog has developed across parts of southeast MN and southwest WI early this morning. Expect the fog to continue to expand a little eastward through sunrise as skies clear and with recent rain and winds remaining light. Some spots will likely see a low stratus deck rather than fog, as winds just off the surface remain strong enough to limit fog formation. While it will be drier tonight, recent rains, light winds and a low-level inversion could lead to patchy ground fog development.

Temperatures:

The long stretch of above normal temps will continue today and Saturday. Despite some weak CAA behind the front, 925mb temps remain between 19-21C, supporting highs mainly between 75-85 degrees, warmest across east central WI. A touch of humidity will remain in the air today, mainly across eastern WI, where dewpoints in the low to mid 60s are expected much of the day.
Temps tonight will be cooler but still remain above normal, with lows ranging from the upper 40s across north central WI to the mid and upper 50s across much of east central WI. Temps at 925mb get a 1-2C bump on Saturday, allowing highs to be a couple/few degrees warmer than today in most spots.

Long Term...Saturday Night Through Thursday

500mb pattern indicated a zonal flow at the beginning of the period. Early next week, a 500mb ridge is expected to amplify across the western United States with downstream trough expected over the middle portion of the country. The models diverge on where the trough, or more likely a closed 500mb low develops by the middle to end of next week that leads to uncertainty in the precipitation trends beyond Tuesday. At this point, will lean toward a dry or mainly dry solution Wednesday and Thursday based on the expected location of the upper low along with surface high pressure expected across the area.

After a warm afternoon on Saturday, a cold front will bring a chance of showers and thunderstorms to the area Saturday night, then linger into Sunday morning across eastern Wisconsin. The models have come into agreement with the north dry out Sunday night, with lingering chances of rain across portions of central Wisconsin east to the lakeshore including Door County. The Canadian model actually had a wave moving across Lake Michigan Sunday night, which would result in higher precipitation totals and stronger northerly winds. This model appears to be the outlier at this time with the other models not showing a closed surface low across Lake Michigan. Confidence is low on the rain chances Monday night into Tuesday as the models have come in with a drier solution.

For temperatures, the GFS mos guidance has come in cooler than the European guidance for lows Monday morning. If the GFS guidance is correct, will need to add frost to the forecast in the next day or two. The old European numerical guidance was a few degrees warmer which leads to doubt in how much frost will be around Monday morning. Temperatures for the most part should be pretty close to normal next week, if not a tad bit above normal at times.

AVIATION
for 12Z TAF Issuance Issued at 645 AM CDT Fri Sep 20 2024

An area of low clouds and fog will bring MVFR/IFR/LIFR conditions this morning. As dry air works in from the west, conditions will improve to VFR as the clouds and fog dissipate. Look for mostly sunny skies this afternoon with only a few daytime cumulus clouds possible. Can't rule out a brief shower across far eastern WI this morning as a cool front completes its passage through northeast WI. Otherwise, look for dry conditions. Some patchy ground fog will be possible tonight. Winds will shift to the west behind the front, gusting to 20 kts later this morning and afternoon. Winds will become light and variable tonight.

CLIMATE
Issued at 329 AM CDT Fri Sep 20 2024

Record stretch of 80 degree weather for September is expected to continue today at Green Bay and maybe at Wausau. If the high reaches 80 degrees in Green Bay today, it would mark the 10th consecutive day reaching 80 degrees in September, breaking the old record of 9 days in a row from Sep 1-9, 1933. At Wausau, the high on Thursday reached 84 degrees, marking the 9th consecutive day reaching 80 degrees. The old September record was 8 days in a row set from September 4-11, 1978 and from September 17-24, 1908. The forecast high for Wausau today is 80 degrees, so uncertainty exist if the record would be extended.

Except for portions of central Wisconsin, the rainfall overnight did little to make up for the rainfall deficits over the last month to a month and a half, especially across northern and far northeast Wisconsin.

GRB WATCHES/WARNINGS/ADVISORIES
None.