Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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982 FXUS63 KGRB 060338 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1038 PM CDT Fri Jul 5 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Flooding will continue along the Little Wolf River downstream of the Manawa dam. Do not drive across flooded roadways if traveling through this area. - Patchy dense fog is possible over north-central Wisconsin after midnight. - On and off chances for showers and thunderstorms will continue from Sunday through Wednesday. Although the severe weather threat is low, locally heavy rainfall will be possible. && .DISCUSSION... Issued at 227 PM CDT Fri Jul 5 2024 Short Term...Tonight and Saturday The latest RAP analysis and satellite/radar imagery show surface low pressure moving east between Sheboygan and Milwaukee early this afternoon. Clusters of showers and thunderstorms have developed north of this low track in the 925-850mb moisture convergence zone ahead of the 850mb low and base of the shortwave trough. These clusters of showers/storms have been augmented by up to 800 j/kg of mixed layer cape and precipitable water values (pwats) of 1.30 inches, which is about the 75th percentile. As a result, and in combination with relatively slow storm motion (around 12 kts), storms have been prolific rain producers of 1-3 inches per hour and have been locally much higher (Waupaca was estimated over 5" per hour). The heaviest precipitation will likely exit east-central Wisconsin by around 4 pm. A Flash Flood Watch is in effect until 7 pm this evening. Tonight...Low pressure will continue to track east over the Lower Peninsula of Michigan and Lake Huron. Cyclonic flow around the low will continue this evening with low levels remaining moist beneath the upper trough. A few showers will therefore remain possible into the evening, but heavy rainfall will not be a threat. Then at least partial clearing is expected late in the evening and overnight from northwest to southeast. The clearing skies and light winds will promote ground fog formation over north-central and central Wisconsin as forecast temps fall below the cross-over temps by early overnight. Lows will fall into the 50s except near the Lake Michigan shoreline. Saturday...Flattened troughing aloft will remain over the area. Should see strong solar insolation during the morning, which will lead to convective cloud development by late morning. Convective temps are quite low, and only in the middle 70s. With highs in the upper 70s to near 80 degrees, upwards of 1000-2000 j/kg of surface based cape is expected which should result in shower/thunderstorm develop by early to mid afternoon. CAMS indicate areas along the lake breeze and bay breeze convergence stand the highest chance showers/storms, but cannot rule out spotty showers elsewhere either. Long Term...Saturday Night Through Friday Focus in this part of the forecast centers around severe and heavy rainfall potential during the Sunday through Tuesday time frame. Sunday through Tuesday...Upper troughing will remain in place from the northern Plains to the western Great Lakes. Impulses moving through the trough will provide periods of showers and storms. The first of these troughs is located north of Montana and poised to reach central Minnesota by 7 am Sunday. This trough with help from daytime instability of 1000-2000 j/kg will help trigger showers and storms. An isolated severe thunderstorm threat may develop in the afternoon given the deep layer shear around 30 kts. The heavy rainfall risk looks low given the forward storm motion around 20 kts and pwats just slightly over the mean for the date. Secondary shortwave impulses are forecast to cross the area on Monday and Tuesday. Slightly less favorable convective parameters for severe weather will be in place on Monday due to less shear aloft. But low convective temps will likely spell an uptick in coverage and intensity of showers and storms with daytime heating. More diurnally enhanced showers and storms will also be possible on Tuesday. Deep layer shear is even weaker by this point, so the risk of organized severe weather is low. Rest of the forecast...Relatively quiet mid-Summer weather is expected for the rest of the week as the upper flow flattens. Temperatures should trend slightly above normal by the middle of next week. && .AVIATION...for 06Z TAF Issuance Issued at 1037 PM CDT Fri Jul 5 2024 Clearing skies and wet ground could result in areas of dense ground fog or low clouds late tonight. The areas most likely to have dense fog or low clouds late tonight should be north of a Merrill to Iron Mountain line, and possibly the area west of a Green Bay to Oshkosh line and east of a Clintonville to Waupaca line. If low clouds or fog materialize, it should dissipate by 13z. A few hours of MVFR ceilings are possible between 13z and 16z Saturday, otherwise VFR conditions and light surface winds are expected. There could be a stray shower or thunderstorm in the afternoon or early evening west of a Iron Mountain to Oshkosh line. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....MPC/MPC AVIATION.......RDM