Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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944 FXUS63 KGRB 120346 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1046 PM CDT Thu Jul 11 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Small chances (30% or less) for showers and storms this afternoon and evening, and again Friday afternoon. Severe weather is not expected, but storms could produce locally heavy downpours. - Warmer and more humid conditions arrive this weekend through early next week. Heat index values in the 90s are expected over central, east central and parts of far NE WI Sunday into Monday. Heat indices could approach advisory criteria over the southern CWA on Sunday, but current probabilities of heat indices exceeding 100 F are only around 10 percent. Thunderstorm development could also provide relief from the heat at times. - Increasing moisture and instability, combined with a couple short-waves and a cold frontal passage on Monday, will bring potential for a few rounds of strong to severe storms this weekend into early next week. && .DISCUSSION... Issued at 232 PM CDT Thu Jul 11 2024 Building heat, humidity and severe thunderstorm potential for the weekend into early next week, are the main forecast concerns. Even though there is little in the way of dynamic forcing, MLCAPE of 500-1000 j/kg and weak MLCIN over the southwest half of the forecast area should result in at least widely scattered showers and storms this afternoon and early evening. Not expecting any severe weather, as deep layer shear and winds aloft are very weak. Slow movement of any storms could result in very localized 0.25 to 0.50 inch rainfall amounts, just like the past couple days. Later tonight, partly cloudy skies, light winds and high pressure drifting just south of the forecast area should allow for at least patchy fog development to occur. Models highlight the greatest potential over central WI and the western Fox Valley. Similar setup on Friday, with weak dynamics and developing afternoon instability across the region. There is weak mid and upper level ridging, which is certainly a negative factor, and a weak cap may also be in place. The degree of instability and the hint of a weak boundary across N WI support the idea of keeping slight chance pops in place across much of the forecast area during the afternoon. Heat/Humidity: Southerly winds will bring an increasingly warm, humid and unstable air mass into the region. Heat indices in our southern CWA will approach 90 on Saturday, mid to upper 90s on Sunday, and low to mid 90s on Monday, though the heat will likely get beaten back by a few rounds of thunderstorms. The hottest day should be on Sunday, as a capped atmosphere may hold off storm development in C/EC during the day. However, ensemble probabilities of heat indices >100 F (Advisory criteria) are only around 10 percent in C/EC WI. Thunderstorm/Severe Potential for the Weekend/Early Next Week: Strong instability will be in place across the region through this period, and models show several weak short-waves moving through zonal flow, and a fairly strong cold frontal passage Monday afternoon and evening. On Saturday, a short-wave trough is expected to move through, but timing varys a bit between the models. With decent instability (CAPE 1-2 K j/kg), but deep layer shear of only around 20 kts, convective organization is in question. Agree with the current Marginal Risk from SPC. On Sunday, strong instability (2-3 K j/kg) and increasing deep layer shear of 30-40 kts will be more supportive of severe storms. However, a potent cap will likely limit development over C/EC, at least until the evening. Farther north, on the edge of the cap, short-wave energy may kick off some severe storms. A fairly strong cold, at least by summer standards, will move through the region Monday afternoon and evening, and be accompanied by strong instability, increasing mid/upper level wind fields, and fairly strong deep layer shear. Could see potential for a more widespread severe weather event Monday afternoon and evening. A secondary cold front and associated short-wave trough may keep some showers in the forecast Tuesday into Tuesday night, but Canadian high pressure will bring dry weather, cooler temperatures and low humidity for the mid to late part of the week. && .AVIATION...for 06Z TAF Issuance Issued at 1045 PM CDT Thu Jul 11 2024 Outside of some areas of ground fog that may form late at night, VFR conditions are expected through Saturday with scattered Cumulus clouds in the afternoon and early evening Friday and Saturday. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Kieckbusch AVIATION.......RDM