Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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189 FXUS63 KGRB 051918 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 218 PM CDT Wed Jun 5 2024 Forecast discussion for routine afternoon forecast issuance .KEY MESSAGES... - Thunderstorms are expected to impact the region from late this afternoon through late this evening. The strongest storms are expected mainly between 5 pm and 9 pm and may produce isolated damaging wind gusts. But the threat of most storms will be strong, gusty winds in excess of 40 mph and small hail. - Conditions hazardous to small craft will be possible at times from Thursday through Friday. - There will be a chance of showers at times from Thursday through the weekend. While low chances of thunderstorms may develop, the severe weather risk will be low. && .DISCUSSION... Issued at 218 PM CDT Wed Jun 5 2024 Short Term...Tonight and Thursday The latest RAP analysis and satellite imagery show a cold front moving across northern Lake Michigan early this afternoon. Precipitation along the front and associated with a shortwave aloft has exited northeast Wisconsin. Strong isolation behind the back edge of the cloud line continues to take place over western and central Wisconsin where 0-3km surface based capes (sb cape) are rapidly rising to around 100 j/kg. Will be monitoring western Wisconsin for storm intensification over the next 1-2 hours which resides within the left exit region of a 100 kt jet streak and just ahead of a potent shortwave trough. As storms push east, potential for strong to severe thunderstorms is the main forecast concern. Severe weather chances: Thunderstorms are expected to grow upscale from mid to late afternoon before entering north-central and central Wisconsin around the 5 pm hour. Despite modest instability (500-800 j/kg sb cape), wind fields aloft are forecast to significantly strengthen in the 700-500mb layer into the 60-70 kt range. Both the HRRR and RAP indicate that mid-level winds at 15 kft could approach around 80 kts over central WI by around 22z. This is upwards of 99th percentile in the NAEFS climatology. With low level lapse rates around 9 C/km and downdraft capes around 700 j/kg, potential is certainly there for damaging wind gusts out of the strongest thunderstorms. Think the highest potential will be across central and east-central Wisconsin from about 5 pm to 9 pm when the overlap of max instability and jet stream winds will occur. While a lesser concern, the strongest updrafts could also produce hail. Considering the low cape in the hail growth zone, it will likely take a rotating updraft for severe hail to develop. Effective shear is 25 to 30 kts so severe hail remains more marginal and the focus will be on damaging straight line winds. Late tonight through Thursday: A lull in the precipitation is expected overnight. Deeper low and mid-level moisture will return towards sunrise and will likely see light rain showers spread from northwest to southeast over the course of the morning across the region. That doesn`t bode well for thunderstorm chances and will remove the mention. Will also lower high temps due to cloud cover and light precip. Gusts to 30 mph will also be possible. Long Term...Thursday Night Through Wednesday Extended period will primarily be dominated by a near stationary cut- off low that is forecast settle over the eastern Great Lakes/SE Ontario. Cyclonic flow around the low will lead to low to moderate end PoPs (20-30%) at times this weekend. In addition, to the broad rain chances there will be a chance for a few embedded thunderstorms, mainly Saturday. Thursday night through Sunday... A few showers may linger into early Thursday night as the initial trough pushes off to the east. Expect dry conditions overnight through Friday as drier air filters into the mid-levels. Winds may become breezy Friday afternoon as the pressure gradient tightens, northwest winds may gusts 20-25 mph. Chance for precipitation returns Saturday as a second low pressure system moves across the MN Arrowhead and phases with the stalled low off to the east. There is a low end chance (15-25%) for a few embedded thunderstorms to develop late Saturday morning through the afternoon. With only marginal instability (MUCAPE 300- 500 J/kg) and weak shear expected thunderstorms to be weak and pulsy. One caveat here is if clouds clear out for Saturday morning and and low-level lapse rates steepen enough to get a few accelerating updrafts. In terms of rainfall amounts Saturday QPF looks low, with most locations expected to only see up to a tenth or two. Another chance for rain showers is forecast for Sunday as a piece of jet energy is progged to phase with the stalled low. Don`t expect more than a few rumbles of thunder with these showers as instability will be lacking. Rest of the extended...The stubborn low is forecast to finally release control of the region Monday as ensembles show a quick round of ridging returning drier conditions to the region. There are some signals that the next chance for showers and storms may come Tuesday as the ridge departs the region. && .AVIATION...for 18Z TAF Issuance Issued at 1144 AM CDT Wed Jun 5 2024 Light rain is exiting northeast Wisconsin along with the associated cold front. MVFR cigs hang back into the Fox Valley and west of the Bay, but should see improving cigs by 18-19z. A period of clearing skies is then expected until late afternoon. Scattered showers and storms are expected to spread west to east across much of the area after 4-5 pm this afternoon, then end in the late evening or early overnight. These thunderstorms will have high bases (generally above 5-6 kft), with dry air below. Conditions will be favorable for locally strong downdrafts, with winds in excess of 40 kts in the strongest of storms. Small hail is also possible, but not as high of a concern as winds. A period of LLWS is expected as winds at 2000 ft increase to 35-40 kts overnight. MVFR cigs in scattered showers will push southeast across the area on Thursday morning. Think the MVFR cigs will remain northwest of the Fox Valley. Gusty winds to 25 kts will also be possible on Thursday. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....MPC/GK AVIATION.......MPC