Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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245 FXUS63 KGRB 190849 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 349 AM CDT Thu Sep 19 2024 Forecast discussion for routine morning forecast issuance .KEY MESSAGES... - Well above normal high temperatures in the 80s continue today. - Locally dense fog across mainly eastern Wisconsin early this morning. The fog should burn off by 9 am. - There is a marginal risk of severe storms in north-central and central Wisconsin late this afternoon and evening. Damaging winds are the main risk. - The recent dry spell over the last month has led to worsening drought conditions across much of the area. Rainfall totals of a quarter to half inch are expected, especially north and west of the Fox Valley from tonight into Friday afternoon. Localized rainfall amounts around an inch are possible across north- central Wisconsin, but confidence is not high in seeing these higher amounts. - A pattern shift is expected this weekend, with highs dropping closer to normal Sunday through the middle of next week. Confidence is low on the timing/location and rainfall amounts for early next week. && .DISCUSSION... Issued at 349 AM CDT Thu Sep 19 2024 Short Term...Today...Tonight...and Friday The stretch of summer-like temps continue late this week. Main forecast concerns will be the chances for strong to severe storms this afternoon/evening, fog potential/trends, and how warm we will get today/Friday. Thunderstorm & Heavy Rain Potential: We will start off dry today with some high clouds around as weak high pressure pushes east of the region. Then attention turns to the vertically stacked low pressure moving from far southeast Alberta early today to central Manitoba Friday morning and the associated negatively tilted shortwave trough and cold front. These features are producing some showers and storms across MN/IA early this morning. Expect this activity to percolate through the morning, arriving in central and north-central WI later this morning and/or early afternoon. This activity will be outpacing the instability and is expected to weaken as it reaches the area. The main event will get going this afternoon across eastern MN and far western WI as 2000-3000 SBCAPE is forecast to develop ahead of the front. The greatest threat for severe weather will be within the first few hours of initiation west of the area, then as the area congeals into one or more clusters, it will begin to encounter more dry/stable air, as nocturnal cooling begins and a low-level inversion developing, so a weakening trend is expected. Still some uncertainty on just how fast this occurs as we could get MUCAPE of 1000-1500 J/kg, along with decent bulk shear of ~40 kts, into parts of central and north central WI late afternoon toward sunset. If this occurs, isolated severe storms would be possible before the boundary layer begins to cool off. Gusty winds and small hail will be the main threats. The cluster of showers/storms will spread east across the area in the late evening and into Friday morning. Some much needed rainfall is expected across the area, but not all spots will see the heavier (0.50+") amounts as the activity will be pretty progressive. Spots that do see repeated storm activity could see 1+ inch totals as PWATs climb to ~1.5" (150-200% of normal). While brief heavy downpours are expected (1-2" per hour rates) in any storm, the flooding threat will be very low due to the movement of the storms and the recent dry spell. Fog Potential/Trends: Webcams and surface obs showing dense fog once again across far eastern WI near Lake Michigan and Green Bay, as a healthy low- level inversion, light onshore flow off the warm waters, and mostly clear skies remain in place. It is a little difficult to assess how widespread the fog is due to the high clouds obscuring the view to the surface. Expect the fog to continue across Oconto, Marinette, Manitowoc, and Kewaunee counties early this morning. The fog will be dense fog at times and is expected to lift/mix out by 8-9 AM. May need another SPS to cover the dense fog. Patchy ground fog is possible further inland early this morning. Some fog will be possible late tonight into early Friday across central and north- central WI as the rain/clouds exit. Temperatures: Well above normal temps will continue today and Friday. Impressive 925mb temps of 20-23C and 850mb temps of 15-17C remain across the area today, which support highs in the low to mid 80s for most of the area. Could see a couple spots in the upper 80s in/west of the Fox Valley. With dewpoints in the 50s to low 60s, heat index values will remain close to the actual air temps. It will be a relatively warm night, with lows only dropping into the upper 50s to mid 60s. 850/925mb temps cool off slightly on Friday as weak CAA arrives, but highs still look to remain in the upper 70s to the lower half of the 80s. Some higher dewpoints in the mid 60s are expected to linger into the late morning or early afternoon over the east, making it feel a little humid, but heat index values will remain in the low to mid 80s. Records still look to be safe, but we will be close to a few of the lower ones. Long Term...Friday Night Through Wednesday The long stretch of 80 degree weather in September is about to come to an end this weekend as 500mb ridging across the western Great Lakes flattens as a system passes north of the Canadian border. However, temperatures early next week will still be at or possibly a few degrees above normal. For Friday night, weak high pressure will move across the area. On Saturday, the last day with high temperatures in the 80s for this stretch are expected as southerly winds increase ahead of a cold front. It will be warm, but still at least several degrees away from the record high for the date. On Saturday night, a cold front will move across the area, bringing a chance of showers and thunderstorms. Some significant difference in the bufkit soundings, but the GFS sounding at Green Bay was suggest a better chance of storms with elevated CAPE and stronger shear values. Some showers could linger across the area Sunday morning. A surface low could bring some much needed Sunday night into Tuesday. Rainfall over the last month to month and a half has been very dry across much of the area. Green Bay has only received 0.36 inches of rain this month. The GFS as a has 500mb trough swinging across the area on Wednesday, bringing a continued chance of rain. The latest GFS mos guidance for early next week has warmed by at least a few degrees between Monday and Wednesday, which is not matching up well with the ECMWF MOS guidance for this time period. Temperatures could still end up at or a few degrees above normal early next week. && .AVIATION...for 06Z TAF Issuance Issued at 1047 PM CDT Wed Sep 18 2024 A weak upper level disturbance will produce high clouds overnight. Moist air near the lake and bay will likely produce areas of dense ground fog at the airports immediately west of the bay and Lake Michigan like MTW, OCQ, SUE and MNM could have dense ground fog late tonight through 13z Thursday. Middle and high clouds will increase Thursday, as a weak cold front approaches from the west. Scattered showers or thunderstorms are possible after 2100 UTC west of a EGV to ISW line. VFR conditions are expected outside of any convective activity. Scattered showers and thunderstorms will move across the area Thursday night, with IFR conditions likely west of a IMT to STE line after midnight and MVFR to the east. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Bersch/Eckberg AVIATION.......RDM