Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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274 FXUS63 KGRB 211808 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 108 PM CDT Tue May 21 2024 Updated aviation portion for 18Z TAF issuance .KEY MESSAGES... - Impactful strong to severe storms are anticipated across portions of the area today into this evening. The areas most likely to see severe weather will be central to east-central Wisconsin. Damaging winds, large hail, and a few tornadoes will be possible with the stronger storms. - Several rounds of heavy rainfall today and this evening may also pose a flooding concern. - Chance of showers and thunderstorms across northern Wisconsin Thursday afternoon into Thursday evening. - Better chance for showers and thunderstorms over all of northeast Wisconsin Friday afternoon into Saturday morning. && .DISCUSSION... Issued at 232 AM CDT Tue May 21 2024 Short Term...Today...Tonight...and Wednesday Despite cloud cover, areas of dense fog have formed across much of the area early this morning with abundant low level moisture and light winds across the region. Although some areas may not receive dense fog this morning, overall the dense fog should be ubiquitous enough to continue the Dense Fog Advisory through this morning across the entire region. The dense fog should lift this morning with daytime heating as the boundary layer winds mix to the surface. Attention then turns to the convection expected today and this evening. Hi-res models indicate there will be two rounds of convection; one moving from west to east from late morning through early this afternoon and another moving through this evening. The first round of showers and thunderstorms will track through later this morning into the early afternoon north of a warm front lifting north across the western Great Lakes region. These initial storms are not expected to be severe as they will mainly be elevated with limited instability. The bigger concern are the storms that are anticipated this evening. This activity will be driven by a pronounced negatively tilted mid level trough and attendant shortwave along with a surface cold front. Although some supercells are possible ahead of the main activity, the convection is expected to quickly develop into a QLCS that will track through the area from west to east this evening. Although instability isn`t overly impressive with MUCAPEs around 1000 J/kg, bulk shear values are off the charts at 40 to 60 knots. Hodographs indicate quite a bit of low level shear and helicity, especially across central Wisconsin, which will bring increased probabilities for tornadoes (10-14%) across this area. North and east of this area for enhanced tornado risk the probabilities are lower but are certainly not zero. The most prevalent severe weather type is expected to be straight line winds given the linear nature of these storms and very strong winds, 60-70 knots, just above the surface. The probabilities for wind damage are rather high given these factors with 30-44% across central and east-central and 15-29% across north-central and northeast Wisconsin. Large hail will also be a threat as there is some cooling and enhanced CAPE in the hail growth zone from these deep updrafts. The main area of concern will be central Wisconsin for all threats as supercells will have a better chance of developing or tracking through this region before conglomerating into a QLCS. Central and east-central will be the primary areas of concern for the wind threat and possible QLCS tornadoes where instability and shear will be maximized with the best chance to be in the warm sector for surface based storms. Further north, although the severe weather threat is lower, this area could also see strong to severe storms. In addition to the severe potential, flooding could be an issue from these storms with two rounds of heavy rainfall expected today and this evening. Despite the fast moving nature of the storms, PWATs of 1.4 to 1.6 inches are expected, representing the climatological maximum for this time of year, along with long skinny CAPE profiles. Behind the cold front, cooler temperatures and lower dew points are expected on Wednesday. Some showers could linger, especially across north-central Wisconsin, under cyclonic flow as a shortwave tracks through the western Great Lakes region. Long Term...Wednesday Night Through Monday Models indicate a progressive pattern to exist through early next week as a series of Pacific upper lows/shortwave troughs move across the northern tier of states. This would bring periodic precipitation chances to northeast WI. Timing of these systems will be the main forecast challenge. Temperatures for the most part to average near normal. Wednesday night and Thursday... Dry conditions are expected Wednesday night as the large system responsible for the stormy weather continues to lift north over western Ontario and weaken. A prevailing southwest wind at 5 to 10 mph should keep fog at bay through the night despite mostly clear skies. Min temperatures to range from the middle 40s north, upper 40s to lower 50s south. There is a concern that a weak frontal boundary will drop south and potentially stall over northern sections of the Great Lakes on Thursday. There may be enough convergence along this boundary to generate a chance of showers and a few thunderstorms across northern WI by the afternoon. Have added a pop to the forecast along with more cloud cover. Otherwise, partly cloudy skies can be expected over the rest of the forecast area with max temperatures in the upper 60s to around 70 degrees north, lower 70s near Lake MI and middle to upper 70s south. Thursday night and Friday... This boundary is parallel to the mean flow aloft which means it is not going anywhere fast. May need to carry a small pop over the north Thursday night, but any precipitation would be spotty and light. The rest of the area should remain partly cloudy with min temperatures near 40 degrees far north, upper 40s to around 50 degrees south. A better chance for showers will arrive on Friday as a mid-level shortwave trough moves into the Upper MS Valley, preceded by a cold front that is forecast to reach western WI late in the day. Marginal instability may allow for some thunderstorms to develop, but too early yet to discern severe storms potential. Max temperatures on Friday to be in the middle 60s near Lake MI, mainly lower to middle 70s inland. Friday night and Saturday... Showers/storms will continue through at least Friday evening as the shortwave trough moves northeast toward Lake Superior and the cold front sweeps east across the rest of WI. The far north and far eastern part of the forecast area may see this precipitation linger past midnight before ending. Early indications show that most of Saturday appears to be dry with weak high pressure in the vicinity. Models are waffling a bit with this high pressure, thus confidence for a completely dry Saturday still has some doubt. Little change in air mass will bring max temperatures on Saturday into the middle to upper 60s near Lake MI, around 70 degrees north and lower to middle 70s south. Saturday night and Sunday... Model differences continue into Saturday night with the possible existence of a fast-moving, weak shortwave trough. If this trough comes to fruition, it would be a chance of showers to eastern WI so have kept a small pop in the forecast for now. This weak system would exit the region by Sunday morning and with little synoptic forcing evident, it appears that most of Sunday would be dry. Max temperatures on Sunday appear to mirror Saturday which would be close to normal. Sunday night and Memorial Day... The next prominent upper low/shortwave trough is progged to move from the northern Plains into the Upper Midwest on Memorial Day. Latest timing among the models would keep any morning festivities precipitation-free, but expect precipitation chances to increase over central WI in the afternoon. There is potential for this system to slide south enough to keep any showers away, so stay tuned. Max temperatures to remain near normal with upper 60s lakeside, around 70 degrees north and lower to middle 70s south. && .AVIATION...for 18Z TAF Issuance Issued at 108 PM CDT Tue May 21 2024 VFR to MVFR conditions will be in place for the afternoon and evenin, but IFR/LIFR conditions are possible at times as showers and thunderstorms cross the area. Initial convection early this afternoon is not likely to be severe, but will have thunder and the potential for small hail or a brief wind gust. As we get into the late afternoon and evening hours, storm strength and potential increases, with a strong round of storms expected to cross Wisconsin in the 23Z-05Z timeframe. Timed this thunderstorm round with a set of TEMPO groups for the area TAF sites for now. Damaging winds, large hail, and a few tornadoes are possible with these storms. After the storms pass, active weather should return for the late overnight but winds will likely remain gusty. The passing system will also bring a period of LLWS that will likely last through the overnight hours. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Kurimski/Kallas AVIATION.......Uhlmann