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Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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444 FXUS63 KGRB 160402 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1102 PM CDT Thu Aug 15 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Periods of showers and thunderstorms through Friday night will produce an additional 0.5 to 2 inches of rain. Rain will be heavy at times, and may result in localized flooding in urban, low-lying and poor drainage areas. - A few strong storms are possible later this evening over central Wisconsin. Torrential rainfall and brief gusty winds are the main hazards. A few strong to severe storms may also develop over central and east central WI Friday afternoon. - Conditions hazardous to small craft will continue on the Bay and Lake through Friday morning. Dangerous swimming conditions due to high waves and strong currents are expected on Lake Michigan beaches. - Quiet weather conditions and near-normal temperatures in the mid 70s to low 80s are expected Sunday into much of next week. && .DISCUSSION... Issued at 250 PM CDT Thu Aug 15 2024 Short Term...Tonight and Friday Precipitation trends, and heavy rainfall and severe thunderstorm potential, will be the main forecast concerns. A vertically stacked low pressure system was situated over far NE ND/NW MN, and was drifting ESE toward the forecast area this afternoon. A short-wave trough wrapping around the low was generating a large band of moderate rain across the NE half of WI. This band was still filling in over far NE WI, while the back edge was moving through central WI. The rain band will shift out of far NE WI early this evening. Another short-wave over eastern ND and far NE Nebraska will move into central WI this evening, and lift northeast through the rest of the forecast area overnight. By the time this wave arrives, an unstable air mass (MUCAPE of 600-1200 j/kg) will be in place, so thunderstorms will become more likely. Not expecting any severe storms due to the nighttime arrival, high wet-bulb zero heights (poor hail potential) and weak shear (15-25 kts), but torrential rainfall will be likely due to continued high PWATs (1.3-1.7 inches). Convection will wane with the departure of the wave late tonight into Friday morning. Yet another short-wave will sweep across C/EC WI Friday afternoon, and trigger another round of strong storms with heavy rainfall. Could see an isolated severe storm or two with damaging winds, as temperatures warming to near 80 and dew points in the upper 60s yield SBCAPE of 1-2K j/kg. Although shear values have trended downward slightly, models still show 20-25 kts over C/EC WI, and CAMs show some evidence of better convective organization over the southern part of the CWA. Farther north, scattered to numerous showers and a few thunderstorms should also develop beneath the upper low. These will be slow moving, with potential for locally very heavy rainfall. Southeast winds will gradually taper off late this afternoon and evening, but current marine and beach hazard headlines still look to be on target. Lows will drop into the middle to upper 60s tonight. Highs on Friday will be in the 70s to around 80 range. Long Term...Friday Night Through Thursday Slow moving stacked low pressure system only slowly exits the western Great Lakes by late this weekend. Rain chances will not end until Sunday. As upper level ridge persists over the southwest and southern Plains, northwest flow will become established south central Canada to the lower Mississippi River valley. This will yield surface high pressure and mainly dry weather over the western Great Lakes Sunday through midweek with small chances for rain returning late in the week. This pattern will keep temps near normal with highs most days in the mid to upper 70s and low temperatures in the 50s. Showers/storms: Mid-level low over northern WI and western Upper Michigan on Friday evening settles slowly to lower Michigan Saturday afternoon. Shortwave rotating along southern edge of the low will result in a period of showers and storms Friday afternoon and these will carry into Friday evening. Based on being on north edge of stronger winds H85-H5/5kft-20kft and effective shear, the MLCAPEs over 1000J/kg forecast could be sufficient enough for a stronger storm into early Friday evening over east-central to northeast WI. Gusty winds the primary hazard. locally heavy rain remains a hazard as well though with PWATs near 1.75 inches and warm cloud depths over 11kft. CAMs point to convection that will have some motion west to east due to the shortwave, but any training convection would pose an issue and could cause issues for areas that see the most rain today and Friday as well as poor drainage, low-lying and urban areas. Kept chancy pops into the overnight hours over northeast to east-central WI as the low will be too close to assume showers will taper even in wake of the late afternoon/early evening shortwave. Finally, given the deep moisture and weakly cyclonic/neutral low- level flow still in place on Saturday along with daytime heating, instability, no need to alter chance pops over much of the area through the day. Pops finally taper off Saturday evening, allowing dry conditions to return on Sunday. Winds: As the low tracks to the east, southerly winds shift more north-northwest on Saturday. Pressure gradient between exiting low and approaching high pressure system will remain tight across Lake Michigan. Strongest winds/gusts will be later Saturday night through Sunday. Possible, but not a sure thing, that a Small Craft Advisory will be needed over southern zones on Lake Michigan (south of Sturgeon Bay). With the offshore wind direction, highest waves will be directed toward the western shoreline of lower Michigan and northern Indiana, so there will not be the need for any Beach Hazard Statements for our forecast area. Peak wind gusts on land may reach 25 mph on Sunday, highest over east-central WI closer to Lake Michigan. && .AVIATION...for 06Z TAF Issuance Issued at 1102 PM CDT Thu Aug 15 2024 Low cigs and fog have produced mainly LIFR flying conditions across the area late this evening. Expect this to continue through 12-13Z/Fri or so, at which time fog will begin to break up and cigs will rise. Meanwhile, still expecting scattered showers and a few thunderstorms overnight (as is occurring at the moment on radar); however, coverage will likely be less than originally forecast. Removed precip from the KAUW, KCWA, and KRHI TAF sites as chances here are low. However, fog will be more prevalent in central and north-central WI, so did include a TEMPO group for the period of greatest vsby reduction to around 1/2SM Friday morning. East-central WI has some ongoing showers around, with CAMs showing potential for some short lived showers or storms in the 7-10Z/Fri range. Kept a TEMPO group for TSRA at KGRB, KATW, and KMTW. A lull in the precipitation is expected throughout Friday morning. Some improvement in flight conditions to MVFR/VFR are likely during this lull. Then, more showers and thunderstorms develop Friday afternoon. Winds: Wind continue to settle this evening and will generally be around 5 to 8 kts overnight. SE winds will veer SW overnight, and eventually NW on Friday. && .GRB WATCHES/WARNINGS/ADVISORIES... Beach Hazards Statement until 10 AM CDT Friday for WIZ022-040- 050. && $$ DISCUSSION.....Kieckbusch/JLA AVIATION.......KLJ