000 FXUS63 KGRB 211121 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 621 AM CDT Thu Sep 21 2023 Updated aviation portion for 12Z TAF issuance .SHORT TERM...Today...Tonight...and Friday Issued at 410 AM CDT Thu Sep 21 2023 The main forecast focus will be on fog development at night, well above normal temperatures, and precipitation chances. Abundant low level moisture trapped under a steep low level temperature inversion has once again led to some dense fog across portions of northeast Wisconsin early this morning. Currently the worst conditions appear to be across isolated locations; however, the fog is expected to be a bit more ubiquitous as the morning wears on. The fog does not appear to be widespread or persistent enough for a Dense Fog Advisory at this time but will continue to monitor through the morning. Any fog will dissipate a few hours after sunrise as the inversion mixes out. Another warm day is on tap for northeast Wisconsin with 850 mb temperatures of 12 to 14 Celsius. Highs today are expected to rise into the upper 70s to lower 80s away from Lake Michigan with lower to middle 70s near the lake. Although some models continue to develop some light precipitation across the region today, the general overall trend has been dry. MUCAPEs rise to 500 to 1000 J/kg during the afternoon; however, CAPE profiles are very long and skinny with abundant dry air in the sounding with PWATs of only around 1 inch. Therefore, will keep the forecast dry given the lack of forcing to create any showers during the day. Fog looks to once again be an issue tonight with abundant low level moisture still in place along with light winds and mostly clear skies. The models indicate the worst conditions would likely be across north-central and northeast Wisconsin. Lows tonight are expected to range from the lower to middle 50s across the north with middle to upper 50s across central and east-central Wisconsin. Surface winds turn southeasterly on Friday, which should push temperatures down a few degrees but still well above normal for this time of year. Highs on Friday are expected to generally be in the middle to upper 70s away from Lake Michigan, with lower to middle 70s near the lake. The main story on Friday will be a chance for showers and thunderstorms across central and north- central Wisconsin as a mid level shortwave lifts north through western Wisconsin. Mid level moisture will be a bit better; however, MUCAPEs will only be a few hundred J/kg along with long and skinny CAPE profiles and bulk shear around 10 knots. Therefore, if any storms do develop they are not expected to be severe or long lived. .LONG TERM...Friday Night Through Wednesday Issued at 410 AM CDT Thu Sep 21 2023 The extended forecast will focus on the an approaching upper low that will move across the Upper Plains this weekend. Overall, the forecast continues to look drier than previously forecast. The main features for the beginning of the extended will be an area of broad high pressure over far northeastern CONUS and accompanying ridging aloft while an upper low approaches from the west. Consensus remains in line with previous forecasts on the track of the upper low, bringing it from the Rockies into the Dakotas Friday night to Saturday night, before stalling out to our west. The low then remains mainly west of the region before gradually merging with the main flow again by the end of the weekend. This means that the heavy precipitation associated with the occluding low will stay well west of the area, focused mainly across Minnesota. For our area, this leaves only weak moisture advection into the mid- levels Friday night into Saturday but will lack any focused sources of lift. Thus, while this could still produce a stray sprinkle or two, felt reasonably confident 70%+ in lowering pops for the weekend as most will see far more dry conditions than any rainfall; this is especially true for areas across eastern Wisconsin. Additionally, removed thunder for any showers that form as instability and sources of lift will be fairly lacking. Suffice to say, any sprinkles that remain in the forecast are expected to be fairly light. Finally, confidence is growing that if the drier trend holds through the next forecast cycle, could foresee lowering pops further and creating a much drier forecast for the weekend. The changes in the weekend forecast will also impact early next week. While the exact dissolution of the low is still unclear, none of the options bring a focused round of precipitation to the area. Thus, pops continue to fall in the Monday to Tuesday timeframe as well. Behind this, upper ridging and high pressure are expected for the middle of next week. Temperature-wise, expect highs to start around 5 degrees above normal Saturday before coming to near normal by early to the middle of next week. && .AVIATION...for 12Z TAF Issuance Issued at 621 AM CDT Thu Sep 21 2023 Low clouds and fog will continue to plague the TAF sites this morning; mainly north of a line from KGRB to KAUW. LIFR/VLIFR conditions will prevail for a few more hours this morning until the fog mixes out after sunrise. After the fog dissipated VFR conditions are then expected the rest of today into this evening. Models indicate additional fog is expected to form later this evening into tonight as surface moisture gets trapped beneath an inversion. KRHI looks like it would be impacted the most with LIFR/VLIFR conditions with some fog possible at KGRB and KMTW but to a lesser extent. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM.....Kurimski LONG TERM......Uhlmann AVIATION.......Kurimski