Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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744 FXUS63 KGRB 282352 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 652 PM CDT Fri Jun 28 2024 Updated aviation portion for 00Z TAF issuance .KEY MESSAGES... - Thunderstorms are possible late this afternoon through late tonight, but thunderstorms are most likely this evening. There is a 30-50 percent chance that localized areas south of highway 10 will see over an inch of rain. Gusty winds and hail are also possible all areas, though the overall severe potential is low. - Conditions hazardous to beach-goers and mariners will continue through late tonight. - Some rivers and streams will remain above bankfull or near minor flood stage into the middle of next week. - The next period to watch for heavy rain or stronger storms will be late Monday through Tuesday. && .DISCUSSION... Issued at 219 PM CDT Fri Jun 28 2024 Short Term...Tonight and Saturday Extent of showers tonight along with severe potential is the main focus. Sharp mid-level shortwave trough is over MT while a secondary H7 wave is over eastern NE. Sfc low ahead of the mid-level trough is over northern MN with a cold front extending southwest to another wave of low pressure over eastern NE. Warm front is not well- defined, but based on sfc obs is somewhere from central MN to southwest WI. Ahead of these fronts, we have seen intervals of light showers since mid morning supported by H7 moisture advection just ahead of a 40-50kt H85 jet. No thunder thus far with NIL instability. Light showers will continue rest of the afternoon as additional moisture advection works into the area ahead of the warm front. Late this afternoon, MUCAPE increasing over 500J/kg begins to move in from the southwest which will start to support at least a chance of thunder. Greater chances for thunderstorms will occur this evening into the early overnight as mid-level trough moving to the Dakotas pushes mid-level jet into the upper mississippi valley which will act upon MUCAPES by this time of 500-1000J/kg. Yet, the strongest low-level jet will be steadily shifting east of the area before the greater instability arrives so that continues to result in only a low chance of seeing severe storms in the form of large hail and gusty winds. Cannot rule out an isolated strong storm though given the effective shear over 40 kts. Heavy rain possible as well as PWATs ahead of the approaching cold front surge to 1.8 to 2.0 which is near 200 percent of normal and well over 90 percent of climo. HREF (short range forecast ensembles) probs point to 30-50 percent chances of local amounts over an inch this evening with heavier storms especially south of highway 10. FWIW, NBM probs are not near as high, less than 10 percent. Additional showers and some storms could continue well after midnight northeast/east-central as the axis of the main mid-level trough arrives. Lows tonight not too far off from readings late this afternoon, around 60 north and upper 60s from the Fox Valley to the lakeshore. Although the initial cold front will be through the area late tonight into early Saturday morning, a secondary cold front is still expected to track through on Saturday. Low-end pops (20-30 percent) cover this scenario, with best chances northern to northeast WI. MLCAPEs of 300-600 J/kg will result in some thunder but storms should remain below severe levels. Highs on Saturday will range from around 70 across far north-central Wisconsin given the earlier fropa from the second cold front, with highs reaching the lower 80s east-central WI with the later arrival of the front. Long Term...Saturday Night Through Friday Focus of this forecast revolves around the thunderstorm and heavy rainfall potential from Monday night through Tuesday night. Saturday night...A potent shortwave trough will be passing across northeast Wisconsin in the evening. With such strong forcing, could see scattered showers lingering along the Lake Michigan shoreline early in the evening. Instability is negligible by this point, so the risk of strong thunderstorms is low. Then clearing will occur overnight. Monday night through Tuesday night...On the backside of a large high pressure system, strong warm advection will occur on Monday night thanks to a 30-40 kt low level jet, pushing precipitable water values (pwats) over 1.50 inches. Models generate widespread precipitation in the elevated moisture gradient/convergent zone from late evening into the overnight hours. Despite the strong ascent, instability is very weak at 100-200 j/kg, so severe weather chances look low. But some showers could generate a moderate rainfall with probability of 1/2 inch of rain from 25 to 50% for the Monday evening through Tuesday morning timeframe. Then, a second more focused round becomes possible late in the day Tuesday as another shortwave rounds the base of the trough, enhancing the overall heavy rainfall potential in the region. A bit more thunder could accompany this second round as overall instability and forcing will be better, but the exact details will depend on how much rain sticks around from Monday morning. Rest of the forecast...General flow then flattens out across the region. While this will mean mostly dry conditions for the middle of next week, a few scattered showers could still cross the region but will lack the moisture to create heavier precipitation in the area. && .AVIATION...for 00Z TAF Issuance Issued at 639 PM CDT Fri Jun 28 2024 An area of showers and thunderstorms will continue to shift through central and east central WI this evening. Have kept thunderstorms at the GRB/ATW/MTW TAF sites through 02z-03z, and prevailing showers for a few more hours. Additional showers and isolated storms may develop along the cold front as it moves through overnight, but not confident enough to include prevailing showers in the TAFs beyond this evening. Expect flight conditions to drop to IFR/LIFR at most locations tonight, and continue until the cold front moves through early Saturday morning. Partial clearing and rising ceilings will lead to VFR conditions regionwide by mid to late morning. A short-wave trough and secondary cold front will bring a chance of showers Saturday afternoon. There could a few thunderstorms as well, but confidence in the occurrence or location of storms is not high enough to include a mention in the TAFs at this time. LLWS will impact the eastern TAF sites this evening and into the early overnight hours. Gusty west surface winds will develop in the wake of the cold front passage, especially Saturday afternoon. && .GRB WATCHES/WARNINGS/ADVISORIES... Beach Hazards Statement through Saturday morning for WIZ022-040- 050. && $$ DISCUSSION.....JLA/Uhlmann AVIATION.......Kieckbusch