Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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450 FXUS63 KGRB 190347 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1047 PM CDT Wed Sep 18 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Near-record high temperatures are expected on Thursday. - Locally dense fog is possible across mainly eastern Wisconsin late tonight into Thursday morning. - There is a marginal risk of severe storms in north-central and central Wisconsin from late Thursday afternoon into Thursday evening. Most locations are forecast to see a quarter to a half inch of rain from Thursday afternoon through Friday afternoon. - 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 243 PM CDT Wed Sep 18 2024 Short Term...Tonight and Thursday The latest RAP analysis and satellite radar imagery show a vertically stacked low pressure system spinning over eastern Montana early this afternoon. An arcing band of clouds is moving east and well ahead of the system into the northern Mississippi Valley. Cirrus along the leading edge of this cloud band is dissipating as it spreads east across Wisconsin. Otherwise, it is another warm September day with temps in the upper 70s to middle 80s across the region. As this upper low slowly moves east, fog potential early Thursday morning and thunderstorm potential late Thursday afternoon are the focus of this forecast. Fog potential: While cirrus will continue to invade the region from the west, conditions will be favorable for fog development over northeast WI late tonight into Thursday morning. Models continue to show light onshore flow off the warm waters of the Bay and Lake combined with mostly clear skies and temps falling well below their cross-over temps. The highest probabilities will be just west of the Lake Michigan and Green Bay shoreline over inland Oconto, Marinette, Manitowoc, and Kewaunee counties. Locally dense fog will be possible starting around or shortly after midnight through about 8-9 am Thursday morning. Thunderstorm potential: A very dry airmass remains present over eastern WI this afternoon where precipitable water values are estimated at about 60% of normal. This dry airmass will shift east as the moisture axis ahead of low pressure over the northern plains slides into the state on Thursday afternoon. By 4 pm Thursday, precipitable water values are expected to climb to 175% of normal over north-central WI. The increasing moisture will lead to a more unstable airmass invading the region on Thursday. Thunderstorms are expected to develop over eastern Minnesota at peak heating in the afternoon. Some guidance develops additional thunderstorm activity further east into central WI after 5 pm where instability will reach over 1000 j/kg. If storms can move into central WI before nocturnal cooling sets in, a few strong storms appear possible due to deep layer shear over 30 kts and dcapes over 1000 j/kg. Temperatures: A little warmer on Thursday with highs in the low to middle 80s. Lows tonight will fall into the upper 40s to middle 50s. Long Term...Thursday Night Through Wednesday Tha main forecast concern is severe thunderstorm potential for the western part of the forecast area Thursday evening, and precipitation chances and amounts for the entire forecast area Thursday night into Friday. Cooler temperatures are also anticipated later this weekend and early next week. The blocking upper level ridge will break down, with a negatively- tilted short-wave moving through the western Great Lakes later Thursday into Friday. An associated cold front will push through the region, with PWATs increasing to near 1.5 inches along and ahead of the boundary. The main severe threat will occur west of the forecast area Thursday afternoon, but lingering daytime instability and deep layer shear of 35-40 kts may allow a few strong to marginally severe storms to impact NC/C WI during the evening. Convection should weaken as it shifts farther east during the overnight/Friday morning period. Probabilities are fairly high for most of the region to receive a tenth to a quarter inch of rain with this system, though there is a 30 percent chance of a half inch or more in NC/C WI. Most of the rain will be east of the forecast area by midday Friday. Generally dry conditions are anticipated Friday night into Saturday, but another frontal boundary could bring a chance of showers or storms Saturday night into Sunday. As was mentioned previously, the details of the forecast become muddled later in the weekend into next week, with low confidence in timing/location/precipitation amounts. There is higher confidence in temperature trends, with the much above normal temperatures coming to an end later this weekend/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.....MPC/Kieckbusch AVIATION.......RDM