FXUS63 KGRB 140551 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI Issued by National Weather Service Milwaukee/Sullivan WI 1251 AM CDT Wed May 14 2025 .KEY MESSAGES... - Areas of dense fog along the lakeshore this evening may affect eastern counties overnight. Marine Dense Fog Advisory for nearshore areas south of Sturgeon Bay. - There is a chance (30-40%) Wednesday afternoon for central WI. - Showers and storms are most likely (50-80% chance) Thursday afternoon and evening. Severe storms are possible with the passage of a cold front with all hazards possible. && .DISCUSSION... Issued at 856 PM CDT Tue May 13 2025 Dense fog of 1/4 mile visibility or less is slowly spreading inland from Lake Michigan in Kewaunee and Manitowoc Counties. A Dense Fog Advisory is in effect for these counties through 10 AM CDT Wednesday. This fog bank may continue inland into the Fox Valley tonight, but confidence is lower for widespread dense fog in this region due to advective winds weakening and warmer temperatures persisting in the valley itself. As mid-level clouds clear late tonight into early Wednesday morning, temperatures will fall to near dewpoint temperatures and areas of fog, locally dense (1/4 to 1/2 mile visibility), will develop across much of eastern Wisconsin, with patchy fog at 1 to 3 mile visibility across central Wisconsin. If temperatures cool quickly enough tonight, Dense Fog Advisories may also be needed in the Fox Valley and Door County late tonight into Wednesday morning. PREVIOUS DISCUSSION A closed upper low spinning over the Upper Great Lakes region will continue to bring moist, southeast flow into eastern WI through Wednesday night. Dewpoints have increased into the lower 60s east to the mid 50s central and clouds are widespread. There are a few light showers out there west of the Fox Valley. Marine fog is looming over the open waters of Lake Michigan, and the light easterly breeze tonight and abundant low level moisture is expected to help usher that fog inland overnight. The lakeshore cities and towns may see fog as early as 9pm, but it should take a little longer for it to overspread the rest of the Fox Valley, probably after midnight. For areas west of the Fox Valley, fog should be more patchy and less dense. A Dense Fog Advisory may be needed tonight for eastern areas. Another round of pop-up showers and storms is expected farther inland from the lake stabilizing flow on Wednesday afternoon as sfc based CAPE increases to 500 to 750 J/kg from central to north- central WI. Again with limited effective shear, no severe storms are expected. With more clouds to start the day and limited mixing, highs on Wednesday will come down across the board with readings in the 70s all areas, though staying in the lower 60s near the Lake Michigan shore. MRC Thursday through Tuesday: The main focus for Thursday remains on the potential for severe thunderstorms across most of Wisconsin. Questions remain about storm coverage and mode, especially across the southern portion of the state. A strongly capped elevated mixed layer will be building into Wisconsin starting Thursday morning, as a warm front surges north across the region. This EML will feature steep mid level lapse rates on the order of 7.5-8 deg C/KM, atop an inversion centered between 850 and 750 MB. A few showers or perhaps a thunderstorm or two may be ongoing along the leading edge of the EML early Thursday, mainly across the northern portions of the state, but for the most part the strong cap should limit shower/thunderstorm activity through at least early afternoon. At the surface, south to southeast flow will support warming temperatures away from Lake Michigan, with mid to upper 80s possible over south central Wisconsin, and upper 70s to low 80s over the central and northern portions of the state. The southeast flow will result in a gradient with temperatures stretching from Racine and Milwaukee northwestward toward Green Bay, with progressively cooler readings east of this line (i.e., toward the lake), with locations right along the lakeshore likely remaining in the 60s. Surface moisture will also increase through the day, with dewpoints expected to rise into the upper 50s to mid 60s, highest with southward extent. This is expected to result in a large region of 2000+ J/KG of MLCAPE across much of the state by early afternoon. A shortwave associated with the upper low centered over South Dakota will surge eastward toward the region during the afternoon hours, along with a cold front at the surface, with the front currently expected to reach the Mississippi River between 1 and 4 PM. At the same time, increasingly fast flow at 500 and 300 MB will be overspreading the area from the west, resulting in 30-50 kt of deep layer shear across the region. Mid level dry air remains the greatest wrinkle in the forecast for Thursday. While most model guidance suggests quite a bit of dry air, especially in the 700-500 MB layer, there are differences among the various models. Ultimately this will boil down to if and where the combination of large scale lift and a warm and moist low level airmass is sufficient to overcome the dry air. With all of that said, if/where convection is indeed able to overcome the dry mid levels, convective initiation is expected to occur during the early to mid afternoon hours along and just ahead of the front. Initial mode would likely be discrete to semi- discrete, with more than sufficient shear profiles for supercells. Tornadoes and large to very large hail will be possible with any sustained supercells. With time, convection should become more linear in nature, likely from north to south, since frontal and upper level forcing will be stronger across the northern half of the state. The risk for straight line winds will increase with any linear segments, along with a continued hail and a QLCS tornado risk. Convection is expected to move east-northeastward, and if current trends hold would be expected to cross into the cooler lake-induced airmass and eventually over Lake Michigan by late evening (10 PM- Midnight). Quiet weather is expected Thursday night and early Friday morning as a subtle surface ridge passes through the area. By later Friday, additional convection will be possible as the upper low begins to pivot southeastward into Minnesota, and associated surface energy pushes across the state. While dewpoints will only be in the low 50s, continued steep lapse rates and fast flow aloft will support 1000+ J/KG of MLCAPE and around 30 knots of deep layer shear Friday afternoon. Hail should be the greatest risk with this activity, though a potentially well mixed boundary layer (i.e., relatively high cloud bases) may also be supportive of downbursts. The upper low is then expected to finally move through the region on Saturday, with continued chances for showers, before rain chances end Saturday night. High pressure is expected to build south for Sunday and potentially into Monday. By the early portion of next workweek, additional shortwave energy will bring renewed chances for showers and thunderstorms to the western Great Lakes. Boxell && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1225 AM CDT Wed May 14 2025 Light east to southeast winds tonight with dense fog being the primary impact early this morning. VSBYS less than a mile and as low as quarter-mile have pushed as far west as Brown and Calumet counties but looks likely to push a bit further west through the early morning hours. There does seem to be a limit on how far north the limited VSBYs have been able to reach but Door County and even parts of far eastern Oconto and Marinette counties closer to the Bay may see VSBYs under a mile. This is also a low cloud deck with VLIFR CIGS as well. KATW/GRB will likely start to see impacts from this over the next hour or so. This will likely remain in place through daybreak with the sun gradually allowing it to dissipate in the morning. Otherwise into the day expect some potential for a few storms toward central and north central WI with northeastern and eastern WI likely remain more so on the dry side. All eyes will then turn to Thursday will potential for strong to severe storms in the afternoon for central WI with continuing potential into the evening north northeast WI. Breezy southeast winds also expected Thursday, particularly toward central WI. && .MARINE... Issued at 1019 AM CDT Tue May 13 2025 Expanded the Marine Dense Fog Advisory northward one zone to include areas south of Sturgeon Bay. The marine fog is expected to stick around through at least Wednesday morning due to light, moist easterly flow over the cold open waters. An additional northward expansion to include the Door Peninsula will be possible, but more uncertainty remains. && .GRB WATCHES/WARNINGS/ADVISORIES... WI...Dense Fog Advisory until 10 AM CDT this morning for WIZ038>040- 048>050. MARINE...Dense Fog Advisory until 1 PM CDT this afternoon for LMZ542- 543. && $$ DISCUSSION... AVIATION... MARINE...