Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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423 FXUS63 KGRB 151828 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 128 PM CDT Sat Jun 15 2024 Updated aviation portion for 18Z TAF issuance .KEY MESSAGES... - Well above average temperatures are expected from Sunday through Tuesday. Heat indices may surpass 90 degrees at times. The warmest temperatures and highest heat index values are expected across central into northeast Wisconsin. - A round of showers and thunderstorms is expected to move across the area tonight through Sunday morning. Heavy rainfall is likely, especially west and north of the Fox Valley, where amounts over an inch are possible. High rainfall rates could result in localized street flooding and ponding of water on roads. Severe thunderstorms are not expected. - Thunderstorms are possible Sunday afternoon and Sunday night. Some of these storms could become severe with damaging winds and large hail, especially over north central Wisconsin Sunday night. - Thunderstorms are possible at times next week. Due to the heat and humidity, some stronger storms and locally heavy rainfall may be possible at times from Monday through Wednesday. Confidence is low in the timing of any rounds of storms. && .DISCUSSION... Issued at 430 AM CDT Sat Jun 15 2024 Short Term...Today...Tonight...and Sunday The forecast area remained under the influence of a Canadian high pressure system, which was centered over eastern Ontario early this morning. Developing return flow and WAA over southern MN and northern IA was generating some showers, though much of this appeared to be virga or light sprinkles falling from a mid-level cloud deck. Dry weather should prevail over most of the forecast area this morning, with a few sprinkles arriving in central WI toward midday. Increasing WAA and a weak short-wave trough should generate scattered showers across mainly C/NC WI this afternoon. Despite increasing clouds, highs should still reach into the 70s, except upper 60s near Lake Michigan. Precipitation chances ramp up tonight, especially overnight as a 45 to 55 kt low-level jet develops, a potent short-wave trough moves through, elevated instability arrives, and a warm front approaches the region. The strong dynamics, combined with PWATs increasing to 1.5-1.8 inches, supports a heavy rainfall threat, with amounts of an inch or more possible west and north of the Fox Valley. Several models show two main QPF maxes during the overnight period; one affecting NC WI, and another in central WI. There is support for this, as NC WI will be under the influence of the RRQ of an upper level jet, while central WI will get the main surge of elevated instability. WPC has outlooked most of the forecast area in a Marginal risk of excessive rainfall, with parts of C/NC WI in a Slight risk. Suspect the main issue will be localized flooding of urban areas. The late arrival of elevated instability and widespread shower activity (leading to a lack of discrete cells) should preclude a significant threat of severe storms during the overnight period. Have raised min temps several degrees, as southeast winds will be ramping up through the night. Lows should be in the upper 50s to middle 60s. On Sunday, the main surge of showers and storms should depart from SW to NE during the 14z-18z period. Stabilization from the morning rainfall, plus lingering cloud cover, slight ridging aloft and an inversion/capping evident in the 850-800 mb layer, all suggest a relative lull in convective activity during the afternoon. However, with mid-level lapse rates increasing to 7.5-8 C/km and CAPE eventually building to 2000-3000 j/kg, I`m not brave enough to go with a dry forecast. Will keep slight chances over the southern part of the area, and chance pops across northern WI, where some models suggest the warm front will reside later in the day. Cannot rule out an isolated severe storm, but the best chance appears to be over north central WI Sunday night. Highs should range from the lower 80s north to the middle 80s to around 90 south. Locations along the immediate Lake Michigan shoreline should remain in the 70s. South to southwest winds will gust to 25 to 30 mph, especially in C/EC WI. Long Term...Sunday Night Through Friday 500mb ridge across the eastern United States with a trough across the Rockies will result in an active weather pattern for much of next week with periodic chances of thunderstorms. Heat and humidity will be the main story across much of the area early in the week, with Monday and Tuesday being the warmest days. A weak cold front will work across the area Wednesday, bringing temperatures closer but still slightly above normal for the end of the work week. For Sunday night, the models are showing a weak disturbance at 500mb across the northwest half of the state that some of the models are depicting convection firing during the evening. Bufkit soundings indicated CAPE values of 3,000 to 4,000 J/KG and 0-6 km shear values around 25 kts to support strong or severe storms with large hail and damaging winds. The greatest risk of severe storms would be across north-central Wisconsin closer to the upper level feature. There is a second feature at 500mb that moves into western Wisconsin shortly after 12z on Monday that could bring stronger storms to our northern counties Monday morning. On Monday, it will very warm or hot depending on your location. Temperatures across portions of central and northeast could climb into the lower 90s with heat index values as high as the middle 90s across east-central Wisconsin. With the active southwest flow pattern aloft and building ridge, much of the convection Monday night and Tuesday may remain west of the forecast area. With thunderstorm complexes to the west of the area during this time frame, it would not surprise me if a complex snuck into our northwest counties at some point. Also, several models are depicting diurnally drive convection Tuesday afternoon and evening. If storms do develop, there is plenty of CAPE to support strong or severe storms. Damaging winds would be the primary threat, but could not rule out some large hail despite the very warm temperatures at the surface. On Wednesday, the models are depicting a weak cold front moving across the area which will bring continued chances of storms. Some of the storms could be strong or severe as well depending on how much sunshine we see before the storms arrive. The 00z ECMWF is indicating continued chances of showers and storms as a frontal boundary lingers across the area. Being this far out, there is always concern that one of the thunderstorm complexes could push the front south of the area to bring a break in the chances of rain. Will resolve these concerns over the next several days. Temperatures will be well above normal early next week, then turn closer to normal but still above normal late in the week. && .AVIATION...for 18Z TAF Issuance Issued at 128 PM CDT Sat Jun 15 2024 VFR conditions are expected through at least early this evening, with a few light showers at the central WI TAF sites this afternoon. These light showers are expected to increase in coverage and lift northeast through late this evening. A few of the showers may reach the east-central WI TAF sites this evening, but will remain light. A brief lull in precip is expected from southwest to northeast as the showers lift across the region. During this time, as surface winds will remain from the southeast, LLWS will increase from the south ranging from 30 to 40 kts. Later tonight into early Sunday morning, the next wave of rain, moderate at times, will arrive from west to east, bringing MVFR conditions and occasional IFR conditions. Elevated instability will increase towards Sunday morning, leaving the small potential for a few thunderstorms across all TAF sites as well. The precip will come to an end from west to east mid Sunday morning through the end of the TAF period. && .GRB WATCHES/WARNINGS/ADVISORIES... Beach Hazards Statement from 1 AM CDT Sunday through late Sunday night for WIZ022-040-050. && $$ DISCUSSION.....Kieckbusch/Eckberg AVIATION.......Kruk