Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
Issued by NWS Milwaukee/Sullivan, WI
546 FXUS63 KMKX 190220 AAA AFDMKX Area Forecast Discussion...UPDATED National Weather Service Milwaukee/Sullivan WI 920 PM CDT Mon Aug 18 2025 .KEY MESSAGES... - Scattered showers and storms with moderate to heavy rainfall south of I-94 will gradually end from west to east overnight. Could see some localized flooding with any heavier showers persisting over the same area. Rises on area rivers will also be possible with the evening activity. - Drier and less humid by midweek. && .UPDATE... Issued 920 PM CDT Mon Aug 18 2025 The ongoing MCV that is crossing southern WI and northern IL will continue to drift eastward through the night. There is a back edge to the precip and it is should exit southeast WI by 8 AM if the current timing holds, but will likely exit or diminish earlier. We issued two flash flood warnings over the southern third of Rock and Beloit counties during the evening, where the rain rates were on the order of 2 inches per hour. We are no longer seeing those rain rates out of this system (isolated storms with half to 1 inch rates now), so the flash flood concerns are diminishing. Low clouds and some light fog will dominate central and most of southern WI overnight into Tuesday morning. Patchy dense fog is possible, but not expecting widespread. Cronce && .SHORT TERM... Issued 334 PM CDT Mon Aug 18 2025 Tonight through Tuesday night: Initial line of storms along the remnant outflow boundary from this morning continue to push out over the Lake. This has worked over the environment across southern WI for this afternoon, especially for areas north of I-94. However, instability gradient is showing signs of steadily recovering with 1000-2000 J/kg of SBCAPE strewn across areas south of I-94, likely due to the increase low-level moisture and a bit of sun peaking through. Thus starting to see some additional redevelopment of showers along this gradient as the mid- level shortwave trough and remnant surface MCV track across southeast MN and northeast IA and into southwestern WI. Overall the potential for strong storms and heavier rain looks further south into IL, but there seems to be enough forcing with this MCV feature and moisture to bring additional rounds of showers and storms into the evening along and just north of the WI/IL border. Overall the stronger/severe threat is limited, but cannot rule out an isolated stronger storm or two though into the evening with the main concern being localized damaging wind gusts. The main concern will be the potential for heavy to moderate rainfall along the west to east orientated aforementioned instability gradient where this activity is suggested to develop into the evening. Thus this will lend way more to a flooding concern with any repeated rounds of storms over the saturated areas. So will need to monitor where this activity sets up as 1-2" per hour rate are not out of the questions. Localized urban flooding and rise on area rivers will be possible through the early evening. Then expecting the shortwave trough and MCV to push eastward later this evening bringing a drier airmass to southern WI overnight. However, low-level moisture and light winds may lead to some patchy fog development overnight. For Tuesday will see northerly winds as high pressure gradually builds into the Upper Midwest. While cannot rule out a few stray showers on the backside of the departing system, thinking it will be dry enough airmass to keep any activity pretty isolated. Wagner && .LONG TERM... Issued 334 PM CDT Mon Aug 18 2025 Wednesday through Monday: Long story short, the entire long term forecast is looking mostly quiet and pleasant. The ridge axis holds to our west (over the Dakotas / Minnesota) mid week, leaving northwest flow in the jet overhead, favoring subsidence and high pressure over the upper Great Lakes region Wednesday and Thursday. A north to northeast breeze (at the surface) throughout the region forms Wednesday, and becomes a tad gusty along the Lake Michigan shoreline. As such, elevated waves and High Swim Risk conditions may develop Wednesday, especially further south towards the Illinois border. The breeze also holds daytime high temps to the mid 70s to around 80 degrees. Winds decelerate and veer further east into Thursday, allowing temps to moderate back up to the upper 70s / low 80s. Shallow moisture may form some scattered to broken stratocumulus clouds at low altitudes Wednesday / Thursday, and a sprinkle (trace) of rain from these cannot be completely ruled out, but the atmosphere is completely stable above this layer. Hence, no chance for thunderstorms or measurable rainfall based on the latest guidance. Moving on to Friday, both the polar and subtropical jet streaks at 300-250mb begin to trough southward into the midwest, bringing the jet stream overhead for much of the extended, perhaps even well south of the region into early next week. With the trough axis setting up along or east of us, the flow aloft remaining out of the northwest, and the jet stream remaining mostly laminar, precip chances are relatively low each day. A cold front crosses the region on Friday, and could potentially support a brief round of convection along it, but with the progressive nature of the front coupled with stiff northwesterlies aloft moving in behind it, any storms would move out quickly (no potential for training convection or heavy rain). We only afford it very low (generally less than 15%) chances of producing any measurable precip at all, given the complete lack of moisture return (no gulf moisture) in advance of the front. Our best guess is that it passes through as a dry cold front. Quiet weather continues into the weekend. Temperatures hold around 80 / low 80s into Friday before the cold front, then gradually fall under the influence of gentle northwesterly cold air advection. By Sunday, daytime high temps could fall to 70 degrees flat. Sheppard && .AVIATION... Issued 920 PM CDT Mon Aug 18 2025 IFR ceilings have made it into central WI on light northeast winds, and are expected to spread into all of southern WI during the early overnight hours as weak low pressure finally exits WI. The ongoing showers and thunderstorms across far southern WI will slowly exit from west to east overnight, possibly not ending until closer to 12Z in far southeast WI. Patchy fog is possible tonight, although there is higher probability of widespread LIFR ceilings below 500 ft. The IFR ceilings should improve to MVFR by midday, then VFR by mid afternoon, and overall cloud cover may not start to scatter out until later in the afternoon. Areas toward western WI have the best chance of clearing earlier. Cronce && .MARINE... Issued 920 PM CDT Mon Aug 18 2025 Weak low pressure crossing southern Wisconsin this evening will cross southern Lower Michigan on Tuesday. High pressure sitting over Ontario will remain in place through much of the week. A period of elevated northerly winds is expected Tuesday, then light and variable winds will develop as the high glides over Lake Michigan later in the week. Cronce && .MKX WATCHES/WARNINGS/ADVISORIES... WI...None. LM...None. && $$ Visit us at weather.gov/milwaukee Follow us on Facebook, X, and YouTube at: www.facebook.com/NWSMilwaukee www.x.com/NWSMilwaukee www.youtube.com/NWSMilwaukee