Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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227 FXUS63 KGRB 040903 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 403 AM CDT Thu Jul 4 2024 Forecast discussion for routine morning forecast issuance .KEY MESSAGES... - Next round of widespread showers/storms arrives from the west this afternoon and evening, becoming more widespread overnight. Some storms across central Wisconsin may become strong to severe, with the main threats being flooding, gusty winds, and small hail. - Elevated potential for flooding this evening into Friday. River and stream levels will likely be on the rise, with some urban flooding also possible depending on where swaths of heaviest rain occur. - On and off chances for rain/storms continue through early next week as upper-level flow remains unsettled. && .DISCUSSION... Issued at 400 AM CDT Thu Jul 4 2024 Short Term...Today...Tonight...and Friday Severe and heavy rain potential are main concerns through tonight. Shortwave trough over the northern plains moves to MN by tonight while weak sfc low and warm front reach western WI this evening. As has started to become more evident, unsettled weather is going to be present in portions of our area late today into this evening that will impact 4th of July activities, including fireworks displays. Ahead of this though, through at least early afternoon conditions will stay mainly dry with temperatures pushing into the low to mid 80s. Unlike yesterday though, winds will be considerably lighter. The dry weather will come to an end though. Storms are already developing as close as southwest MN this morning in warm/moist advection ahead of the low. Isolated convection could make it into western WI late this morning, though greater chances hold off until mid to late afternoon over IA/MN and southwest WI as mid- level jet arrives and interacts with building instability in the vicinity of the warm front and on nose of modest low-level jet. This convection will try to roll east, at least partially, as upper trough and H85 low tied to it remain progressive. As these features arrive, expect showers and some storms to expand first into central WI late afternoon into early evening and then eventually to east-central WI by late evening. The activity over central WI could be strong or even severe, at least in isolated fashion. Greatest risk for severe would be between 3p-8p. Locally heavy downpours will also be a factor with 0.5-1.0/hr rainfall rates. Leftovers of this activity will shift east/northeast the rest of the evening. Mixed signals still on extent of the showers and storms by time they reach east-central WI. If convection is more robust, heavy rain and localized flooding could occur with local amounts over an inch occurring pretty easily given higher PWATS over 1.25 inches shifting across along with warm cloud depths rising over 11kft. Most guidance dampens this convection though. 00z HRRR and Canadian are the most aggressive with east-central to northeast WI showers and storms through late evening. Better chances for widespread moderate to heavy rain will already be setting up farther northwest, from central to north-central WI within mid-upper diffluence and large scale Q-vector convergence and stronger low-level convergence near the H85 low. Though weaker whatever instability is left by this point (MUCAPES up to 400j/kg) will be feeding into this stronger lift to produce waves of moderate to heavy rain. Decent agreement from deterministic models and ensembles that greatest risk of 2 inches of rain tonight will occur from central to north-central WI. Debated whether to issue a flood watch in these areas, but did not as these parts of the cwa have largely missed out on the heavy rain that occurred recently on Tuesday night and thus the flash flood guidance is higher compared to the still saturated areas of east- central WI (where rainfall amounts tonight should remain under an inch). Even so, if rainfall amounts end up toward the reasonable high-end amount (90th percentile) of 2.5-3.0 inches or HREF max values of 3-4 inches, there would be certainly be some issues with flooding in low-lying and urban areas. Into Friday, it looks like a washout with showers for much of the time over much of the area as the upper trough slides right across and sfc low tracks from central WI to northern Lake Michigan, putting the majority of our area within moist, cool cyclonic NE-N flow. Lowered temps from NBM as this pattern would suggest it will be a struggle to reach low 70s. Being beneath the upper and sfc lows poses some concern for rotating storms as there will be ambient low-level vorticity present though current HREF projections for 0-1km/0-3kmSRH are very muted and there is no signal in terms of the NST. MLCAPES increase to 500-750j/kg during the afternoon centered on northeast and east-central WI so though there could be some rumbles of thunder, seems greatest risk will be from locally heavy rain. Long Term...Friday Night Through Wednesday Main focus will revolve around heavy rain/flooding potential through Friday evening as robust shortwave energy drags a surface low and accompanying rain chances across the upper Midwest. Confidence is increasing in periods of moderate to heavy rainfall Friday afternoon and evening, which may kindle flash flood concerns across portions of central to east-central Wisconsin. Otherwise, the remainder of the extended sees unsettled upper- level flow bringing several additional rounds of rain/storms to the forecast area through mid-week. Friday evening rain... As decaying low pressure treks through the western Great Lakes Friday afternoon, precip will be ongoing across much of eastern Wisconsin by Friday evening. Current ensemble guidance shows a more aggressive rainfall scenario than previously suggested, highlighting a 60 to 80 percent signal for exceeding one inch of rainfall north and west of the Fox Valley. However, there are still some questions with how progressive this system will be, and hence how much rain we end up getting. Flash flooding may be a concern across portions of east-central Wisconsin into Friday evening where recent periods of heavy rain have dropped flash flood guidance (6-hour guidance ~1 to 1.5 inches in the Fox Valley). Rest of the extended... Saturday looks to be slightly drier, although some wrap-around shower activity will be possible during the afternoon and evening as the surface low departs to the east. Coverage will likely be spotty due to the influence of high pressure to the south and lack of any sustaining surface features. Nonetheless, a swath of surface dewpoints in the low to mid 60s and resultant CAPEs exceeding 1,000 J/kg may be enough to generate some pulse storms during peak heating. On and off rain/storm chances will then continue through mid-week as another shortwave digs across the northern Plains. A first glance shows yet more heavy rain concerns as a corridor of Gulf moisture and high PWATs (~1.75 inches) make their way up into Wisconsin. Expect temperatures this weekend through early next week to run right around average for this time of year, plateauing in the upper 70s to low 80s. && .AVIATION...for 06Z TAF Issuance Issued at 1002 PM CDT Wed Jul 3 2024 Mainly VFR conditions expected through Thursday afternoon, with deteriorating flight conditions occurring over north central and central WI Thursday evening. Clear skies, light winds and wet ground from recent rainfall could lead to patchy ground fog overnight, but confidence is too low to add to the TAFs. An approaching low pressure system and warm front will bring a chance of showers and storms to mainly central WI late Thursday afternoon, though timing is still a bit uncertain. More widespread showers and storms should overspread the region Thursday night, possibly reaching the eastern TAFs by late evening. Light west to southwest winds will become southeast to east Thursday afternoon and evening. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....JLA/Goodin AVIATION.......Kieckbusch