Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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560 FXUS63 KGRB 240406 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1106 PM CDT Thu May 23 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Scattered showers and some thunder is possible into early this evening. Isolated strong storms could produce gusty winds and small hail. - Showers and thunderstorms are expected Friday into Friday evening. Some of the storms could be strong to severe Friday afternoon and Friday evening. Severity of storms will depend on how much instability builds up mid to late afternoon on Friday. Heavy rain is also possible with the storms. - Memorial Day weekend starts off dry for Saturday, but rain chances increase later Sunday into Memorial Day. A soaking rain could occur during this time. - A few rivers over central and eastern Wisconsin will remain above bankfull through Friday. && .DISCUSSION... Issued at 223 PM CDT Thu May 23 2024 Short Term...Tonight and Friday Convection trends through Friday are the main focus. High pressure brought a dry morning but seeing signs that some will see some rain this afternoon. A weak wave shifting through in westerly flow aloft combining with MLCAPES up to 500j/kg is helping to trigger some showers as we have already breached our convective temps of 68-70F observed on the 12z GRB sounding. Have kept highest pops and better thunder chances over southern tier of CWA to the south of highway 29. Though shear is weak, 20kts or less, steep low-level lapse rates lower wet-bulb zero heights could support gusty winds and small hail in isolated stronger storms into very early evening. Otherwise, any lingering showers should dissipate with the loss of daytime heating mid to late evening, with mostly clear skies the rest of the night. Enough gradient wind ahead of low developing in the central Plains to restrict any fog. Decent range to lows, with low 40s north-central and low 50s central WI. On to Friday where some potential for strong or severe storms is still in the mix, but it is not clear cut. Showers and thunderstorms are expected to develop late tonight ahead of the low close to nearby warm front, higher instability on nose of low-level jet. This line of storms then shifts into WI on Friday morning, but though they may hold together some, they will be outpacing greater instability that doesn`t budge much from the upper mississippi river valley. Maybe a brief downpour and some gusty winds, but not expecting this activity to be severe so long as it arrives late morning/early afternoon. In wake of this the question is how much instability will build up mid to late afternoon ahead of the primary cold front sweeping in from the west and warm front lifting into southern WI. SE winds to north of the warm front will limit the instability with the flow off Lake Michigan and could limit northward progression of the warm front. Overall, think greatest risk of strong to severe storms ends up over southwest/south central WI perhaps into central WI. Effective shear sufficient for organized storms at least over 30 kts more likely 40 kts farther west into the cwa you go. So, if we realize sufficient insolation and realization of instability forecast (MLCAPES over 1000J/kg), severe storms could occur. SPC has the southern half of the forecast area within a marginal risk for severe with large hail and damaging winds the primary hazards (5-14 probability). The primary tornado threat is expected to stay further south in the warm sector near the warm front, but the probability for our southern areas is non-zero as the warm front will be close by. PWATS ramp up to 1.5 inches or 175 percent of normal so heavy rain is possible (WPC has area in marginal risk for excessive rainfall) but strong winds will result in quick storm motions so main issue in terms of heavy rain/flooding will be whether there is training of storms across repeated areas. Highs on Friday are expected to range from the middle 60s across the north, to the lower 70s across east-central and portions of central Wisconsin. Long Term...Friday Night Through Thursday A cold front will move across eastern Wisconsin Friday evening. Bufkit soundings would suggest the greatest risk of severe weather would be over central Wisconsin where CAPE values are over 1,000 J/KG and shear values around 40 knots. The severe weather parameters drop off as you head northeast to Green Bay where these values were only 20 to 30 knots with CAPE values of 1,000 J/KG or less. The Storm Prediction Center Day 2 Outlook has a Marginal Risk south of a Wausau to Antigo to Sturgeon Bay line. The amount of coverage and intensity in strong or severe storms will depend on how much instability there is by late afternoon. The strongest storms should exit northeast Wisconsin around 04z with some lingering showers across north-central and far northeast Wisconsin into early Friday morning. Saturday will be the day to spend outdoors with highs in the upper 60s to middle 70s under partly cloudy. Attention then turns to Sunday afternoon into the early morning of Memorial Day where a soaking rain is expected as low pressure moves across the region. There are still some uncertainty in the track of the storm that could impact the amount of thunder is seen across northeast Wisconsin. As the low moves away on Memorial Day, instability showers are expected to continue at times into the afternoon and early evening. A northwest flow will continue on Tuesday, bringing additional chances of showers and perhaps a few storms. Dry conditions are expected on Wednesday with only a small chance of rain on Thursday. During the period, temperatures should average at or slightly below normal. && .AVIATION...for 06Z TAF Issuance Issued at 1106 PM CDT Thu May 23 2024 VFR conditions are expected late this evening into Friday morning. High clouds will spread in from the west overnight. Conditions will deteriorate from west to east late Friday morning and Friday afternoon as a cold front sweeps across the state. Models a little more split now regarding if/how long a break will be in between two areas/lines of showers/storms. Some models also just wash out the first wave of activity and focus more on the afternoon round. So uncertainty exists on exactly how things will play out, but will continue to lean toward the idea of a weakening line/area of showers and isolated storms crossing the area mainly between 15-21z, likely impacting AUW/CWA, but lower chances the further north/east you go. A second line/area is forecast to arrive in central and north-central WI in the afternoon and then exit eastern WI by around 03-04z Sat. Winds will be light late this evening then shift to the east and slowly increase toward dawn. Could approach LLWS criteria in central WI (AUW/CWA) late tonight, but will hold off including as winds aloft look to stay under 25-30 kts. Another chance at LLWS Friday afternoon as winds at 2000 ft pick up out of the southeast, but if we remain gusty at the surface, LLWS will be limited/reduced. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....JLA/Eckberg AVIATION.......Bersch