Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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265 FXUS63 KGRB 050803 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 303 AM CDT Fri Jul 5 2024 Forecast discussion for routine morning forecast issuance .KEY MESSAGES... - Areas of showers and some thunderstorms are expected today. The primary threat from storms will be locally heavy downpours. - River and stream levels will remain elevated into the weekend. - On and off chances for rain/storms continue through early next week as upper-level flow remains unsettled. && .DISCUSSION... Issued at 302 AM CDT Fri Jul 5 2024 Short Term...Today...Tonight...and Saturday Shower and thunderstorm coverage into this evening is primary focus in the short term. Mid-level low is spinning over southern MN while weak sfc low is over southwest WI. Arc of showers and isolated thunder is crossing northeast WI with area of steadier and heavier showers over northern WI into central MN. Thus far this system has drastically underperformed in our CWA in terms of rainfall amounts and heavy rainfall potential, so that limits the flooding risk going forward. Think lighter QPF thus far is due in part to limited instability to boost rainfall rates by time the showers reached our area last evening and also PWATS barely up to 1.50 inches which is only up to 125 percent of normal. Do expect showers to increase this morning into the afternoon as the mid- level low tracks across the area. Thunder has been quite limited so far but do expect an uptick in thunder toward daybreak this morning as theta-e advection and elevated instability combine ahead of the mid-level low and sfc low. Best chance for thunder impacting more of the area though will occur late morning into the afternoon as the lift from the mid-level low and steeper lapse rates/MLCAPEs building to 800J/kg interact with some daytime "heating" (relative term as highs today will be below normal in the mid 60s north to the lower 70s south). Following MLCAPEs, expect thunder potential all areas through midday then shifting to mainly south of highway 29 the rest of the afternoon. Wind fields are not strong with the system as mid-upper jets are displaced to the east and southeast over the central Great Lakes. Thus effective shear is weak so am not expecting any severe weather. Heavy downpours would be the main threat from storms as PWATs don`t change much from what they are early this morning. Lingering showers end early, then it turns mainly dry tonight. Cool advection in wake of the trough and departing sfc low will keep skies partly to mostly cloudy. Hint of some sprinkles at times northeast WI, ending by daybreak Sat. Majority of Sat looks dry, but with approaching shortwave trough and weak warm air advection resulting in MLCAPEs reaching 500-1000J/kg, did include small chances for showers and storms most areas during peak heating. Also is signal for convergence along Michigan lake breeze which could also support pop-up shower or storm later Saturday, very isolated coverage. That said even with these small chances for rain, plenty of dry time is expected on Saturday with highs bouncing back into the mid 70s lakeside to low 80s inland. Long Term...Saturday Night Through Thursday Not many changes to the extended as an active pattern remains through the middle of next week. Next chances for moderate rain will arrive Sunday afternoon, becoming more robust into Monday morning. There is still uncertainty regarding rainfall amounts, although current ensemble guidance seems to hone in on east- central Wisconsin to receive the bulk of the precip. Definitely worth keeping an eye on given that east-central Wisconsin is currently vulnerable to flooding given recent bouts of heavy rain. Weekend precip chances... As upper-level ridging over the Pacific struggles to build onshore, much of the central CONUS will be left under a persistent troughing regime through mid-week. This pattern will maximize opportunities for perturbations/weak impulses to bring chances for rain and storms to the forecast area. Best chances look to be Sunday afternoon through Monday morning as a surface low and attendant cold front track across the upper Mississippi Valley. Moisture content appears to be favorable for periods of moderate to heavy rain during this time as a surge of Gulf moisture places a narrow corridor of 1.75 inch PWATs (~125% of normal) along the lakeshore. Some diurnally-driven storms will also be possible Sunday afternoon and evening as resultant instability exceeds 1,000 J/kg. However, the big question will be whether or not the bullseye of highest rainfall misses off to the east over the lake. Rest of the extended... Rain/storm chances will continue through Wednesday as several embedded impulses circulate through a longwave trough. An end to the current wet pattern may then be in sight toward the end of the extended as long-range models show upper-level ridging settling in across the CONUS. This may signal the beginning of a warmer/drier summer pattern, although only time will tell. && .AVIATION...for 06Z TAF Issuance Issued at 1037 PM CDT Thu Jul 4 2024 An upper level low will move slowly from southern Minnesota to northern Wisconsin and produce occasional showers and isolated thunderstorms tonight and Friday. The activity will diminish Friday evening. Flight conditions will lower overnight, with IFR ceilings expected from around 09z through 15z Friday west of a ESC to DLL line, with MVFR weather to the east. Ceilings and visibilities will improve Friday afternoon and evening. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....JLA/Goodin AVIATION.......RDM