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