776 FXUS63 KGRB 211730 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1230 PM CDT Fri Jul 21 2023 Updated aviation portion for 18Z TAF issuance .SHORT TERM...Today...Tonight...and Saturday Issued at 339 AM CDT Fri Jul 21 2023 Skies were mostly clear early this morning, along with some patchy fog across the north. Temperatures were in the middle 40s to lower 50s north and in the 50s to around 60 south and along the Bay of Green Bay and Lake Michigan shorelines. For today, skies will start out mostly sunny then turn partly to mostly cloudy depending on your location. There is a chance of showers and thunderstorms during the afternoon primarily driven by daytime heating. Bufkit soundings only indicated a few hundred J/KG of CAPE and shear values around 10 knots, thus not expecting any severe weather. A weak upper level disturbance will move across the area tonight bringing the chance of isolated showers and perhaps a thunderstorm as mid level lapse rates are around 6 C/KM. Lows should be mainly in the 50s. A few readings across the north may dip into the upper 40s. On Saturday, the northwest flow aloft will continue. The combination of daytime heating another disturbance dropping southeast will bring another chance of showers and thunderstorms. CAPE values off the bufkit soundings were around 500 J/KG using the mixed layer values and effective shear values only around 20 knots would support just ordinary storms. The risk of severe would be extremely low. High temperatures should reach the upper 70s to middle 80s. .LONG TERM...Saturday Night Through Thursday Issued at 339 AM CDT Fri Jul 21 2023 Not much change in the long term forecast from the previous couple forecast packages. The main impacts will come midweek as temperatures and humidity are likely to rise, with some of the warmest weather of the year so far. Saturday night...Scattered showers and thunderstorms remain possible Saturday evening as a surface trough crosses the area. Some models show instability up to around 1000 J/kg, but with weak shear and lack of any upper support severe weather potential appears low. However, cannot rule out some hail or gusty winds from any showers or storms Saturday evening. Instability wanes quickly with a few showers lingering into the night, otherwise clearing skies expected into Sunday morning. Sunday and Monday...Once again, a few showers and thunderstorms are possible during the afternoon and evening each day, primarily driven by weak shortwaves from northwest upper flow and daytime instability. Temperatures will be near normal across the Northwoods, and a few degrees above normal across central Wisconsin and the Fox Valley. Rest of the week...As an upper trough across the Great Lakes moves east, a ridge across the southwest extends eastward into the upper Mississippi River Valley. This ridge has been responsible for sweltering heat across the southwest U.S. the past week. While temperatures will not be that warm in Wisconsin, highs in the upper 80s to middle 90s are appearing more likely from Tuesday through Thursday, peaking on Thursday. A rise in humidity could also mean heat index values may approach 100 in a couple locations. If you have outdoor plans during this time, start preparing for very warm weather. One other point to mention is smoke. Smoke models only go out about 48 hours, but latest runs show smoke returning to the northern Plains Saturday night. Do not know where smoke may go from there or for how long, but there could be potential for smoke aloft or at the surface Sunday or next week, which could impact temperature forecasts. Meanwhile, on and off chances for showers and thunderstorms remain in the forecast through the week. This will be driven by weak surface boundaries and/or shortwaves, making it difficult to pin down any specifics. The majority of the time will be dry at any given location. && .AVIATION...for 18Z TAF Issuance Issued at 1230 PM CDT Fri Jul 21 2023 Generally good flying weather is expected through the taf period with mostly VFR conditions, high cloud bases, and light winds. The only exception will be some possible patchy MVFR/IFR fog across north- central Wisconsin late tonight into early Saturday morning. The fog should be mainly limited to the KRHI site for TAF purposes. Scattered to broken convective clouds will be present this afternoon into early evening. Spotty, pop up showers are possible this afternoon, but coverage is expected to be low enough to exclude from the forecast. Then additional mid-level clouds and isolated showers will be possible tonight when an upper level disturbance moves across the region. Light winds will be favorable for lake and bay breeze development. Expect a wind shift at KMTW early this afternoon and possibly KGRB late in the afternoon. .KOSH... Mostly all of the general aviation discussion applies to Wittman Regional Airport. Continued to include a light lake breeze off Lake Winnebago for 4 hours late this afternoon into early this evening. Coverage of pop up showers looks low enough to exclude from the forecast. Looking ahead to Saturday, shower potential is expected to be higher during the afternoon hours. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM.....Eckberg LONG TERM......KLJ AVIATION.......MPC