Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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808 FXUS63 KGRB 191949 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 249 PM CDT Thu Sep 19 2024 Forecast discussion for routine afternoon forecast issuance .KEY MESSAGES... - There is a marginal risk of severe storms in north-central and central Wisconsin this evening. Damaging winds are the main threat, with small hail and locally heavy rain also possible. - A pattern shift is expected this weekend, with highs dropping closer to normal Sunday through the middle of next week. Confidence is low on the timing/location and rainfall amounts Sunday into early next week. && .DISCUSSION... Issued at 249 PM CDT Thu Sep 19 2024 Short Term...Tonight and Friday Forecast concerns include precipitation trends and severe thunderstorm potential tonight, and possible fog late tonight into early Friday. Scattered showers and thunderstorms continue to percolate over north central WI, on the leading edge of the moisture and instability gradient. This activity has been slow to advance eastward due to lingering dry air in the mid-levels. The main show was firing up along the MN/IA state line, where strong convection was developing along the cold front, in an area of strong instability (CAPE around 2000 j/kg) and deep layer shear (40 kts). In general, the CAMs have been showing a slightly later arrival time for this main band of covection; around 00z-01z in our far western counties. This narrows the window of opportunity for severe weather over NC/C WI, as MUCAPE around 1000 j/kg quickly wanes after sunset. CAMs do suggest that convection will organize into a bowing line, and there should be enough instability and DCAPE (currently 1000-1200 j/kg just upstream of the CWA) to support a short period of strong to marginally severe wind gusts in our far western counties between 7 pm and 10 pm. Much of the forecast area should receive a quarter to half inch of rain, with locally higher amounts up to an inch possible in NC/C WI. The convection is expected to weaken as it shifts east during the late evening and overnight hours. A few models suggest a weaker secondary area of convection developing overnight as the cold front front arrives, but confidence is low, so did not go any higher than chance pops with this feature. If this occurs, any lingering precipitation chances should end as the cold front shifts east Friday morning. Dry conditions are anticipated in the late morning and afternoon, along with at least partial clearing. Lows tonight should be in the lower to middle 60s. Highs on Friday should reach the middle 70s to lower 80s. Fog may develop across the northwest half of the forecast area late tonight into early Friday morning, but there should be enough wind to prevent dense fog from occurring. Long Term...Friday Night Through Thursday Focus of the long-term forecast mainly revolves around shower and storm chances later Saturday afternoon through Sunday as a cold front sweeps across the state. There is more uncertainty with the forecast early next week as ensembles struggle to resolve a pattern featuring several short-waves. Friday night through Sunday...Anticipate the week ending on a dry note as a weak ridge moves over the region late Friday into Saturday. The well above normal temperature trend will continue into Saturday with LREF probs for highs greater than 80 degrees in the 50- 70% range across much of the area. Ensembles show an upper-level trough riding along the US/Canada border arriving to the western Great Lakes late Saturday. WAA ahead of this trough may drive scatter rain showers late Saturday afternoon and evening, with higher chances for precipitation (40- 60%) arriving along an attendant cold front late Saturday evening into Sunday. Given the expected overnight frontal passage the severe weather threat is low, however, forecast soundings do show a rather dynamic wind field, 0-6km wind shear around 35-40 kts, MUCAPE values around 1000 J/kg, and PWATs between 1-1.4". With this environment storms may be able to produce briefly gusty winds, small hail, locally heavy rain, and frequent lightning. Sunday`s rainfall totals are currently expected to range from around 0.25-0.75" with only a 20-30% at most locations to see greater than 1". However, a few ensemble members do point to about a 10% chance for greater than 1.5" at locations that see the heaviest rain. Even the lower end of these expected totals will be welcome as parts northeast WI were placed in a D-1, moderate drought, recently due to lack of rainfall over the past month. Monday through the end of the extended...The feature to watch for the early part of next week is a low pressure system that is expected to eject out of the 4-corners region sometime Sunday and arrive in the Upper-Midwest Monday. The majority of LREF ensemble members have the thunderstorms and precipitation associated with this low staying south of the region, however, the more aggressive members show about a 10% for greater than an inch of rain in central WI Monday. The range in the potential rainfall is likely a function of how far north the low allows a plume of Gulf moisture to make it. The generally unsettled pattern will likely persist through Tuesday and into Wednesday as another pair of disturbances are forecast to cross the western Great Lakes. Temperatures are forecast to become more seasonal during this period with highs ranging from the middle 60s to low 70s each day, and a less than 10% chance for highs above 75 degrees anywhere in the CWA. && .AVIATION...for 18Z TAF Issuance Issued at 1241 PM CDT Thu Sep 19 2024 Scattered showers and a few thunderstorms were trying to edge into NC WI mid-day, but were not making much progress due to lingering dry air. Although a few showers may brush past RHI this afternoon, VFR conditions are expected to prevail through the afternoon. The main band of showers and thunderstorms will arrive in NC/C WI after sunset, then track east, reaching the eastern TAF sites during the late evening and overnight hours. Models are hinting at a secondary period of showers and storms along the cold front later tonight, but overall confidence is greatest in the first band. Expect some fog and low clouds to form across the northwest half of the forecast area late tonight into early Friday, with possible IFR/LIFR conditions at RHI/AUW/CWA. Showers should taper off by early Friday, with at least partial clearing and a return to VFR conditions occurring from west to east during the mid to late morning. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Kieckbusch/GK AVIATION.......Kieckbusch