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Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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236 FXUS63 KGRB 151156 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 656 AM CDT Thu Aug 15 2024 Updated aviation portion for 12Z TAF issuance .KEY MESSAGES... - Periods of showers and thunderstorms will bring 0.5 to 2 inches of rain to much of the region from today through Saturday. The rainfall will be heavy at times, resulting in locally higher amounts, and localized flooding in urban and low-lying areas. - A few strong storms are also possible late this afternoon into this evening over mainly central Wisconsin that could produce small hail and brief gusty winds. - Conditions hazardous to small craft will occur on the Bay and Lake from late this morning through Friday morning. Dangerous swimming conditions due to high waves and strong currents are expected on Lake Michigan beaches. - Quiet weather conditions and near-normal temperatures in the mid 70s to low 80s are expected Sunday into next week. && .DISCUSSION... Issued at 419 AM CDT Thu Aug 15 2024 Short Term...Today...Tonight...and Friday Rain & Storm Chances / Heavy Rain & Severe Wx Potential: An unsettled period of weather is on tap for the region today into Friday as vertically stacked low pressure slowly tracks from eastern ND early this morning, to near Duluth Friday morning, to northeast WI Friday afternoon. Much of the initial batch of WAA/isentropic lift precip is getting eaten up by the dry air, but continued WAA, an approaching LLJ, shortwave and increasing moisture/lift will allow the shower/storm activity to our south/west to continue to expand/push northeastward this morning and exit this afternoon. Thunder chances will be low due to little to no instability. Another shortwave and LFQ of a jet push in later this afternoon/evening, with another wave of showers and better chance of some storms expected, but it will be more isolated/scattered in nature. A few lingering showers/storm are possible late tonight into early Friday, but most spots look to stay dry as we should be in between the better forcing/moisture and in a pseudo dry slot. On Friday, look for shower/storm coverage to increase through the morning, peaking in the afternoon as we max out instability/moisture/forcing as the low pushes overhead. The widespread flood risk appears to be low due to the progressive nature of most of the shower/storm activity, plus there will be breaks in between each wave, limiting the threat to reach flash flood guidance. But localized flooding will remain possible where storms occur as soundings still showing deep saturated warm cloud depths to 4 km, along with PWATs of 1.5-2.0" (150-200% of normal) today and 1.4-1.8" on Friday, leading to efficient rainfall production, and very heavy downpours. Most spots look to see up to a half inch of rain with the first wave, then the showers/storms later today into this evening could drop locally heavy rain (over an inch), especially in central WI. Any training storms or if/where storms pivot over the same location on Friday will lead to localized heavier rain totals (over an inch) with some flooding possible. especially if it coincides with heavy rain today. The threat for organized severe weather remains low due to the lack of sufficient shear and much surface heating/ instability. Could be a small window very late this afternoon/early evening, especially if we can get a little clearing, mainly across central WI, where some decent elevated instability (up to ~1700 J/kg) advects in. Some small hail and gusty winds (to around 40 mph) would be the main hazards with the strongest storms. Winds / Marine & Beach Hazards: Pressure gradient will tighten up as low pressure approaches from the west, allowing southeast winds to increase through the morning hours. Gusts of 20-30 mph are expected by late morning and into the afternoon, highest over eastern WI where some locally higher gusts are possible. The persistent southeast winds will create large waves and dangerous swimming conditions on the beaches of Door, Kewaunee and Manitowoc counties. A Small Craft Advisory and Beach Hazards Statement remain in effect. As the low moves closer/overhead, winds will decrease later tonight into Friday. Temperatures / Humidity: It will be cooler today with highs in the upper 60s to mid 70s for most locations. A little more humidity will be in the air as dewpoints climb into the low to mid 60s. It will be a relatively warm and muggy night across the region, with lows only dropping into the 60s. Guidance came in a good 5 degrees warmer in many spots on Friday, but with the expected cloud cover and precip around, temps will be hampered a bit, especially over the north. Will hold most highs in the 70s, but a couple spots could touch 80 if the rain hold off a little longer and especially if we can get some peaks of sun in the south. Dewpoints will be in the mid and upper 60s for most spots on Friday, with a couple spots hitting 70 degrees possible, making for a humid day. Long Term...Friday Night Through Wednesday Rainfall trends through Saturday continue to be the focus of this forecast. Thereafter, typical late summer weather is forecast as the upper flow turns northwest which is conducive for quiet conditions. Rainfall trends: The forward motion of the vertically stacked low pressure has slowed over the past 24 hours, which doesn`t shift east across northern Lower Michigan until Saturday. Deep saturation within the cyclonic circulation will bring scattered shower activity to the region at times through Saturday afternoon. The most widespread activity should reside over eastern WI. While instability will be very low for most of the period, perhaps minor destabilization could take place with diurnal heating on Saturday that could lead to a slight thunderstorm chance. Otherwise, the chance of storms is very low. There is potential for an additional quarter to a half inch of rain across northeast WI. Rest of the forecast: Northwest flow aloft and high pressure at the surface will bring seasonable late summer weather to the area from Sunday through Tuesday. There are some indications of a cold front diving southeast through the region during the Wednesday to Thursday time frame, but predictability is low this far out. && .AVIATION...for 12Z TAF Issuance Issued at 656 AM CDT Thu Aug 15 2024 Flying conditions will deteriorate to IFR/LIFR from southwest to northeast this morning and early afternoon as showers spread across the region ahead of low pressure system. Another round of isolated to scattered showers are expected this afternoon and early evening. Most of the shower activity will come to an end later this evening, but the low ceilings, along with some mist/fog, will continue into Friday morning. Thunderstorms will be possible this afternoon and early evening as well, mainly across central WI and possibly the Fox Valley. Will include a TEMPO group for AUW/CWA as this is where the best chances for thunder (30-50%) reside. Southeast winds will gust up to 25 kts later this morning and afternoon. Winds will decrease and slowly shift to the south and southwest tonight. && .GRB WATCHES/WARNINGS/ADVISORIES... Beach Hazards Statement from 1 PM CDT this afternoon through Friday morning for WIZ022. Beach Hazards Statement from 10 AM CDT this morning through Friday morning for WIZ040-050. && $$ DISCUSSION.....Bersch/MPC AVIATION.......Bersch