Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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128 FXUS63 KGRB 201805 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 105 PM CDT Sat Jul 20 2024 Updated aviation portion for 18Z TAF issuance .KEY MESSAGES... - Showers and thunderstorms are likely across northern WI this afternoon. Severe storms are not expected, but locally heavy rainfall may occur. - A chance of showers and thunderstorms will be possible at times from Sunday afternoon through the middle of next week. The severe thunderstorm potential is low for Sunday and Monday, but is too early to determine for Tuesday and Wednesday. Locally heavy rain is possible. && .DISCUSSION... Issued at 356 AM CDT Sat Jul 20 2024 Short Term...Today...Tonight...and Sunday High pressure brought quiet conditions to the forecast area early this morning, with partly cloudy skies and patchy fog in a few locations. A cold front was dropping south through southern Ontario and northern Lake Superior. This front will become the focus for shower and thunderstorm development as it moves through GRB CWA this afternoon and tonight. The back door cold front will sag south through northern WI this afternoon. A weak upper trough over the region won`t provide any forcing, as the main short-wave will remain well to our west. However, increasing moisture (dew points in the lower to middle 60s) and instability (CAPE around 750-1000 j/kg) along the front should lead to numerous showers and scattered thunderstorms across north central and far northeast WI. With PWATs increasing to 1.25 to 1.75 inches and weak flow (5 to 10 knots) aloft leading to slow movement of any storms, there is potential for locally heavy rainfall. Severe weather is not anticipated, as deep layer shear is expected to be weak (5 to 15 knots). Highs will be mostly in the upper 70s to middle 80s. The cold front will move through the rest of the forecast area tonight, but with the diurnal decrease in instability and a lack of any other forcing mechanism, suspect that precipitation coverage will be isolated to scattered at best. Low temperatures will range from the lower to middle 50s far north to the lower to middle 60s in C/EC WI. On Sunday, the weak front should be situated south and west of the forecast area, but models show scattered showers and storms redeveloping in the southwest part of the forecast area during the afternoon. The best chance should be southwest of a line from Merrill to Oshkosh, where CAPE values of 750-1000 j/kg will reside. Weak shear and light steering winds aloft will again support a localized heavy rainfall threat with any storms. Farther northeast, plenty of sunshine and a more comfortable air mass will make for a nice day. Highs will be in the 75 to 80 range. Long Term...Sunday Night Through Friday Northeast Wisconsin will remain on the west side of a broad upper trough through midweek, and then transition to weak ridging by the end of the week. A surface boundary residing over the area on Monday, combined with daytime heating, will bring a chance for scattered showers and isolated thunderstorms. MLCAPES are as high as 1000 to 1500 J/kg, but weak shear and lack of upper forcing results in a low severe thunderstorm potential. Pwats of 1.0 to 1.5 inches could bring locally heavy rain. There are higher chances (40-70%) for showers and thunderstorms on Tuesday and Wednesday as a mid-level shortwave approaches from the northwest, along with the sfc front still over the area. CAPE values are similar, but 0-6km shear increasing to 15-30 kts. Pwats are in the 1.0 to 1.5 inch range. May need to keep a closer watch for a few isolated stronger storms, but this will also depend on the position of the front and timing of the shortwave. Models are not in great agreement on this at the moment. Models continue to diverge late in the week, with the GFS quick to move the front out and allow upper ridging to build. The ECMWF and Canadian are slower with this. Sometime late Wednesday-Thursday any remaining precipitation should clear out with dry days to follow to end the week. Temperatures will be near to slightly below normal, with daily highs in the mid 70s to low 80s, and daily lows in the 50s to low 60s. && .AVIATION...for 18Z TAF Issuance Issued at 105 PM CDT Sat Jul 20 2024 Generally VFR conditions will prevail through this afternoon and evening. Exception being across northern WI were a slow moving line of thunderstorms may drop vsbys to MVFR/IFR. Framed out a window of 21Z-00Z during which thunderstorms would be most likely in the RHI TAF. Behind the cold front, driving the line of thunderstorms, winds will become north to northeasterly. For areas south of a RRL to NMN line this afternoon a developing cumulus field will result in SCT to BKN cigs around 4.5-6kft. As the aforementioned cold front pushes further south this evening and overnight scatter showers are expected to move across central WI, however, lightning active is expected to wane after sunset. As mentioned above as the cold front moves through winds will veer and become north to northeasterly through Sunday morning. Additionally, areas of fog may develop across portions of central and northern WI after midnight. Any fog that does develop should burn off by 12-13Z Sunday. Lastly, there is an area of wildfire smoke moving in behind the cold front as well, however, comparing current surface vsbys with satellite trends it appears most of the smoke is remaining elevated. Will need to monitor these trends as smoke arrives to see if adjustments to vsbys will be needed. An area of non-severe thunderstorms may return to central WI around mid-day Sunday, but confidence in how far north and east this storms will develop is too low to mention in the CWA and AUW TAF at this time. .OSH...A borderline SCT to BKN cumulus field has developed over OSH early this afternoon with cigs around 5kft. The cumulus field will persist over the area through this afternoon with cloud bases rising to around 6kft. Depending on when the cloud deck developing ahead of a slow southward advancing cold front arrives skies may clear out for a period this evening, otherwise this cloud deck will bring BKN cigs around 5kft tonight. Current trends suggest that any precipitation associated with the cold front will stay north and west of OSH. As the cold front moves through overnight winds will veer from the west and become northeasterly for Sunday morning. There is an area of wildfire smoke moving in behind the cold front as well, however, compering current surface vsbys with satellite trends it appears most of the smoke is remaining elevated. Will need to monitor these trends as smoke arrives to see if adjustments to vsbys will be needed. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Kieckbusch/KLJ AVIATION.......GK