647 FXUS63 KGRB 210852 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 352 AM CDT Sat Aug 21 2021 Forecast discussion for routine morning forecast issuance .SYNOPSIS... Issued at 347 AM CDT Sat Aug 21 2021 Warm and humid with scattered showers and thunderstorms today, then cooler and less humid tomorrow. Turning rather warm and humid again for at least the first half of the upcoming work week. Changes to the large scale pattern during the next week will be somewhat subtle. A longwave trough will remain over the Intermountain West at least until mid-week while an upstream ridge holds back over the eastern Pacific. Downstream ridging over the Great Lakes region will tend to flatten, allowing a seasonably strong west or west-southwest upper flow to become established from the Rockies to New England. Consistency among the medium range models breaks down thereafter, especially related to the possible eastward progression of the trough. Temperatures will remain above normal today, drop back closer to seasonal normals tomorrow, then warm back above normal. How long the warmth lasts is unclear as it depends on the resolution of aspects of the large scale pattern with which the models are struggling. It will be quite humid today, more tolerable tomorrow, then turn somewhat more humid again next week. Precipitation will probably end up from near to somewhat below normal, though with the typical spatial variation common with convective precipitation in the summer. && .SHORT TERM...Today...Tonight...and Sunday Issued at 347 AM CDT Sat Aug 21 2021 A vigorous cyclone will track from south of Lake Winnipeg to near Hudson Bay today. A cold front trailing south of the cyclone will wrap east across the western Great Lakes region, crossing the forecast area during the afternoon. A narrow but impressive axis of high PWATs has developed along the front. But instability was waning as the front surges east. The overall trend of the intensity of the convection in the frontal zone has been downward, though a narrow band of more vigorous showers and storms redeveloped in the past couple hours and was nearing the western part of the forecast area. Anticipate that band will begin to weaken during the next hour or two as it continues to shift east more rapidly than the more buoyant air along the front. So scattered showers and storms will push across the western part of the area this morning. The main question is to what extent convection redevelops this afternoon. Although clouds will overspread the east this morning, some destabilization is likely before the front arrives this afternoon. Expect some uptick of lingering convection or a redevelopment of storms in the 16-22Z time frame. Shear will be marginal at best (from around 30 kts over the far NE to 20 kts in E-C WI), but would not be surprised to get a few strong storms with gusty winds. An anticyclone will build into the area tonight in the wake of the front and then linger on Sunday. A brief period of gusty NE winds is possible Sunday morning in E-C WI as the cooler and drier air surges down the Bay. Otherwise, generally quiet weather is expected for the remainder of the short-term portion of the forecast. .LONG TERM...Sunday Night Through Friday Issued at 347 AM CDT Sat Aug 21 2021 Focus revolves around thunderstorm chances through much of the coming work week. Meanwhile, both daily high and low temperatures will be above normal through the period, with a brief return to near normal readings Thursday and Friday. Sunday evening through Monday...Initially quiet weather as high pressure departs to the east. Then, as shortwave and coinciding surface low slide across southern Manitoba into central Ontario, which brings a surface boundary across MN and into western WI. Scattered showers and storms are progged to develop ahead of this boundary in MN Sunday evening, and could reach central and north- central WI after midnight. Instability is minimal and forcing is weak-- with some LL WAA the main driver. Therefore, expect limited coverage of any showers/storms Sunday night. Farther east dry weather is expected overnight. A few showers could linger into mid-morning on Monday, but with very little to sustain the activity most places will be dry. The more noticeable weather on Monday will be the increasing temps and humidity, with dew point temps climbing 10 degrees into the mid to upper 60s by the afternoon. There is some elevated instability which develops Monday afternoon, but with the lack of a trigger and increasing temps aloft putting a cap in place around 5kft think a dry afternoon is likely. CAMs would also suggest a dry scenario. Monday night through Wednesday...Already some differences in the models, especially the GFS, so leaned closer to an ECMWF/CMC solution. This slowly brings a few showers and storms back into the forecast area from south to north late Monday night into Tuesday morning as a warm front moves northward. By Tuesday afternoon, more showers and storms develop ahead of a cold front moving across MN. As this activity approaches the GRB forecast area instability will have been able to build to 1000 to 2000 J/kg with bulk shear around 30 kts. Late Tuesday afternoon into Tuesday night looks like the best opportunity to see some strong or severe storms during the week, but will need to see how the situation evolves. Showers may linger into Wednesday, but the thunderstorm potential lowers as most of the instability is wiped out. Thursday through Saturday...Somewhat messy forecast, leaning more towards the ECMWF at this point. High pressure likely builds in on Thursday leading to dry weather, and a brief return to seasonable temps and comfortable humidity. Beyond Thursday there are significant differences in the features driving any precipitation, with a blended model solution leading to chance PoPs Friday and Saturday. This seems reasonable for now given there will likely be some precip in this window, but the timing details will need to be refined as the days get closer. && .AVIATION...for 12Z TAF Issuance Issued at 347 AM CDT Sat Aug 21 2021 The main aviation weather issue will be timing the precipitation with the front crossing the area. Initial thoughts are to carry thunder in the eastern TAF sites this afternoon but omit it from the west. But will make the final decision at issuance time. LLWS will also persist across the west early this morning. && .GRB WATCHES/WARNINGS/ADVISORIES... Beach Hazards Statement from 10 AM CDT this morning through this evening for WIZ022. && $$ SYNOPSIS.......Skowronski SHORT TERM.....Skowronski LONG TERM......KLJ AVIATION.......Skowronski