938 FXUS63 KGRB 271149 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 649 AM CDT Sun Jul 27 2025 Updated aviation portion for 12Z TAF issuance .KEY MESSAGES... - 20-40% chance for thunderstorms this evening over far northern Wisconsin. Another round of storms is possible late this evening and overnight mainly over central WI. Some storms may become severe, with damaging winds as the primary hazard. Additional thunderstorms are possible Monday through Tuesday night, some of which could be strong. - Hot and humid today and Monday. A few locations will see heat indices reach 100 today. More widespread heat indices around 100 are expected Monday, when a Heat Advisory may be needed. - Drier, cooler, and less humid conditions return later Wednesday through next weekend. && .DISCUSSION... Issued at 428 AM CDT Sun Jul 27 2025 Short Term...Today...Tonight...and Monday Thunderstorm chances and heat are the primary focuses in what is shaping up to be a complicated short term. In the near term it is a warm, muggy morning with patchy fog. Fog has not been widespread as our area missed out on showers Saturday. Smoke aloft moves in through the day which likely will hold down max temps from what they could be with full mixing. No surface smoke impacts are expected. Incorporated slightly cooler guidance to account for this. High temps this afternoon mid 80s to lower 90s with expected dewpoints in the lower 70s resulted in heat indices mainly below 100F. Held off on any Heat Advisory headline today as a result. In terms of storms today, we`ll have to watch far northwest WI late today as convection may redevelop on tail of shortwave that is causing robust storms from North Dakota into northwest MN this morning. Not agreement here though as some models, including CAMs, restrict this to late evening at the earliest. Next question is how convection evolves that forms on the edge of mid-level capping and extreme instability (MUCAPE of 4000-5000 j/kg from MO/IL into much of MN) this evening. Low-level jet aimed into MN will result in initiation of storms, but it is VERY unclear how far east these storms will track late evening into the overnight hours. Generally favored a NAM/HREF idea (supported by 06z HRRR) which rides the CAPE gradient over western WI, but other guidance keeps complex of storms farther southwest and/or well to the north tied more to the shortwave crossing MN/Lake Superior and less tied to the higher instability. Effective shear of 30-40 kts and ample instability could certainly lead to severe storms. 0-1km shear seems sufficient to maintain cold pool balance, allowing an even elevated complex of storms to produce damaging winds at the sfc, especially given the extent of instability present. Not all models show that though. Some just completely dampen out the complex of storms all-together while other guidance track it all the way into IL on Monday morning, so there is that. Ultimately, this still looks like mainly an issue for north-central to central WI and less of issue farther east. At least that idea is holding right now, but given the pattern we are in, even that could be subject to change. Whatever convection is around first thing on Monday should quickly shift south/southeast out of the area. Most models are then benign with pops/thunder chances most of Monday morning. Afternoon development may occur due to building instability with very warm and humid airmass in place as MLCAPEs rise to 2000-3000j/kg. Also could stay capped in subsidence behind the late night storms. Maybe a bit better signal that tail of shortwave along with lake breeze convergence may lead to higher pops over northern Bay of Green Bay/far northeast WI mid to late afternoon. Some of this activity if it develops could try to build more toward Green Bay area given the instability that will be available. If storms develop on Monday afternoon, they could be severe with effective shear over 40 kts and more than sufficient instability. Unless convection and associated cloud cover is much more widespread, Monday will see highs reach the upper 80s to lower 90s again, this after an adjustment downward from full mixing potential due to more elevated wildfire smoke. No surface smoke, impacts are expected. Dewpoints are slightly higher, though not looking to be as high as what was shown a few days ago. Regardless, looks like more of the area (especially Waushara to Waupaca counties into parts of the Fox Valley, far northeast WI) will see heat indices reaching upper 90s to around 100. Heat Advisory could be needed, but there is still some uncertainty regarding convective trends as outlined above. Will let later shifts issue if it still looks needed. Through the next two days to beat the heat, be sure to stay hydrated by drinking water, even if you don`t feel thirsty, and take frequent breaks in the shade or in air-conditioned locations if spending time outdoors. Long Term...Monday Night Through Saturday Precipitation/Thunderstorm Chances...A zonal upper-level flow will bring embedded shortwaves to the region through Wednesday before evolving into a northwest flow midweek. Precip chances will be focused with any embedded shortwave energy interacting with a lingering warm/moist air mass and surface features. The first embedded shortwave of interest slides east across southern MN Monday evening into WI late Monday night into Tuesday, with an eventual MCS developing. The better forcing and severe parameters will be focused further to the west of the forecast area; however, there is potential the MCS will still have some fuel (sufficient elevated instability, steep mid-level lapse rates, and deep-layer shear) as it enters central WI late Monday night/early Tuesday morning. While there are some models indicating the MCS will track eastward across the forecast area through Tuesday morning resulting in showers and storms across almost the entire area, other models indicate the MCS will drop southeastward instead, which would leave the majority of the forecast area dry, except along the far southern portions of the CWA. Either solution, the MCS would be following along the CAPE gradient. Determining when and where convective initiation develops and where the CAPE gradient sets up will be the main features to watch. Unfortunately, this is another case of predictability being low until observed trends are apparent. Additional showers and storms may develop Tuesday afternoon and evening, but coverage would be more isolated/scattered. The second embedded shortwave of interest appears to be slightly further south than the first, tracking east/southeast across IA late Tuesday night into the WI/IL area Wednesday morning/afternoon. Trends continue to indicate the MCS that develops with this shortwave should remain mainly south of the forecast area, including the severe potential. However, portions of central and east-central WI could see some rain from being on the northern periphery of the thunderstorm complex on Wednesday. Finally, the upper-level pattern becomes much less active after Wednesday, evolving into a northwest flow with surface high pressure spreading over the region from the north. The high pressure will dominate over the region through next weekend, resulting in dry weather and plenty of sunshine. Temperatures...The warm/humid air mass will gradually exit the region Tuesday with the departure of the morning MCS. However, temperatures will remain on the warmer side with highs in the low to mid 80s, but with less humid conditions. As the high pressure gradually slides into the region on Wednesday, temperatures will lower even more with highs in the mid 70s to low 80s. These temperatures will continue through the remainder of the week and into the weekend. Beginning Wednesday night, overnight low temperatures will also be much cooler with readings mainly in the 50s, but a few spots in far north-central WI could see values in the 40s. && .AVIATION...for 12Z TAF Issuance Issued at 648 AM CDT Sun Jul 27 2025 Shallow fog, locally dense, will diminish by 13-14z. VFR today with just some scattered cu. Smoke aloft will make the sky hazy. Winds will be light yet again. Main concern late today will be if storms developing over northwest WI make it to RHI. It is uncertain, but close enough to include PROB30 group. Another potential for thunderstorms will occur overnight from north- central to central WI. Have addressed this low-end potenital with PROB30 groups at RHI, AUW and CWA. At this point, it does not appear that showers and storms will make it farther east through end of the TAF period. Some more fog may form late tonight at GRB/ATW/MTW and eventually RHI. .OSH...Fog has not developed yet and think this risk is done. Balance of day will feature scattered cu this afternoon and increasing high clouds. Smoke aloft will result in a hazy sky. Light winds through tonight. Possible that light fog (MVFR vsby) may develop late tonight. Any showers and storms that occur are at this point expected to remain west of OSH through end of TAF period, but this will have to be monitored. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....JLA/Kruk AVIATION.......JLA