Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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894 FXUS63 KGRB 280352 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1052 PM CDT Thu Jun 27 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Showers and thunderstorms may bring periods of moderate rainfall to the forecast area Friday evening and overnight. Small hail is possible with some storms Friday night, but the overall severe weather threat is low. - Increasing winds and waves will create hazards for mariners and beach-goes on Friday. A Small Craft Advisory and Beach Hazard Statement have been issued. - Some rivers and streams will remain above bankfull or near minor flood stage into the middle of the week. - The next period to watch for heavy rain or stronger storms will be late Monday through Tuesday. && .DISCUSSION... Issued at 351 PM CDT Thu Jun 27 2024 Short Term...Tonight and Friday Cooler weather and partly sunny skies will persist through the late afternoon before cloud cover builds back in from the southwest this evening. Low temperatures for tonight are expected to be slightly more mild than previous nights given overcast skies, ranging from the low 50s in far northeast Wisconsin near the Upper Peninsula border to the upper 50s in the Fox Valley. Precip chances will move through the forecast area in two waves, with the first round expected to arrive late tonight into tomorrow morning as a surface low treks across the upper Mississippi Valley into Ontario. Light to moderate rain during this time will be driven by a combination of weak shortwave energy and isentropic ascent from warm air advection ahead of an approaching warm front. QPF continues to look unimpressive due to meager forcing and the lingering influence of low-level dry air from departing high pressure to the east. Probabilities for exceeding a half inch of rainfall remain on the low end, with at most a 10 to 20 percent chance across central Wisconsin through Friday evening. Some elevated storms will be possible mainly tomorrow afternoon during peak heating, although the axis of strongest instability will remain off to our west along the cold front. This being said, best potential for any strong to severe storms won`t be until Friday evening and into the overnight when the cold front provides a more robust forcing mechanism. Southerly winds will begin to ramp up Friday afternoon as a nocturnal LLJ propagates across Wisconsin and mixes down to the lower levels, creating conditions hazardous to small craft and beach-goers on the shore of Lake Michigan. Long Term...Friday Night Through Thursday The main focus is on severe thunderstorm potential Friday night, followed by rain and thunderstorm chances early next week. Friday night...A cold front will be located across eastern MN at 00Z/Sat with convection expected to be ongoing along and ahead of this front. Additional support from a mid-level short wave is coming in behind the front. Instability along the front in MN is anywhere from 1700 to 2300 J/kg, but as the front moves east instability wanes with only 600 to 900 J/kg in central Wisconsin at 06Z/Sat. Thunderstorms will remain possible area wide Friday night, but the severe threat will be decreasing by this point. SPC maintains a small section of the GRB forecast area in a marginal for this time window, which is reasonable. With storms also becoming elevated, hail looks like the main threat. Small chance for some winds, but inversion around 850mb will make it more challenging for severe level winds to reach the surface. Meanwhile, pwats rise to near 2.0 inches, but due to the relative quick movement of this system event total QPF values only range from around 0.25 to 0.50 inches, with a less than 10 percent chance of exceeding 1.0 inch. Therefore, excessive rain/flooding impacts should be minimal. However, with a number of rivers in or near bankfull any rain will maintain this near-flood state a bit longer. Saturday...A break in precipitation for the rest of Saturday morning, with some widely scattered instability showers or thunderstorms on Saturday afternoon and early evening. Severe potential is low. High pressure should keep conditions dry and quiet for Sunday and most of Monday. Early next week...Another system organizes in the Plains with frontal boundaries and coinciding shortwaves impacting the area in the Monday night to Tuesday night time frame. Pwats climb close to 2.0 inches, with convective parameters climbing as well. Parts of the GRB forecast area are already highlighted in the Day 5 ERO. Will need to monitor this time frame for potential heavy rain and thunderstorms. && .AVIATION...for 06Z TAF Issuance Issued at 1052 PM CDT Thu Jun 27 2024 VFR conditions will continue late this evening and much of the overnight as mid and high clouds continue to lower and thicken. A few light rain showers or sprinkles will be possible overnight, but most/all of what is on radar in central and west-central WI is virga and not reaching the ground as there will be some dry air to overcome. On Friday, look for clouds to lower and flying conditions to deteriorate into the MVFR/IFR categories as rain spreads across the area. Thunder threat is very low with this first area of showers. A secondary area of showers is forecast to spread in Friday evening, with better (mainly elevated) instability. But there is still some uncertainty on if/how widespread the storms will be, so will hold off on a mention of thunder. Some LIFR conditions will be possible in central and north-central WI. Some low-end LLWS will be possible on Friday as a LLJ of ~50 kts pushes over the area. && .GRB WATCHES/WARNINGS/ADVISORIES... Beach Hazards Statement from 10 AM CDT Friday through Saturday morning for WIZ022-040-050. && $$ DISCUSSION.....Goodin/KLJ AVIATION.......Bersch