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Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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676 FXUS63 KGRB 161751 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1251 PM CDT Fri Aug 16 2024 Updated aviation portion for 18Z TAF issuance .KEY MESSAGES... - Locally heavy rainfall in excess of one inch will be possible this afternoon into this evening. The heavy rain could produce localized flooding in urban, low-lying, and poor drainage areas. - Strong storms, and perhaps a few severe storms, are possible over central, east-central, and far northeast Wisconsin this afternoon into early this evening. Damaging winds, hail and torrential rainfall are the main hazards. - Quiet weather conditions and near-normal temperatures in the mid 70s to low 80s are expected Sunday into much of next week. && .DISCUSSION... Issued at 419 AM CDT Fri Aug 16 2024 Short Term...Today...Tonight...and Saturday Shower/storm/fog trends, and heavy rainfall and severe thunderstorm potential, are the the main forecast concerns today into Saturday. Rain & Storm Chances / Heavy Rain & Severe Wx Potential: The unsettled period of weather will continue into Saturday as vertically stacked low pressure slowly tracks from near Duluth early this morning to northeast WI afternoon/evening to Lower MI Saturday afternoon. Latest wave of showers and a few storms formed a little after midnight along/ahead of some moisture convergence and shortwave energy, with 1200-1600 J/kg of MUCAPE to work with. Heavy rain, gusts to ~30 mph and small hail (but high wet-bulb zero heights will limit hail production) look to be the main hazards with this activity as it spreads northeast through the early morning hours, with some other spotty/lighter showers possible. Then as low pressure and next shortwave pushes into the area, another round of showers and storms is expected to develop across the region. SB/MUCAPEs should remain/climb to 1500-2000 J/kg by late morning, with shear remaining fairly weak (up to 25 kts). While widespread severe storms are not expected, can`t rule out an isolated severe storm or two with damaging winds being the main threat, with some small hail possible in the strongest cores. Best chance for a severe storm looks to be across central and eastern WI from 1-7pm, where the higher dewpoints, better shear/instability resides. SPC has placed most of this area in a Marginal Risk for severe weather. While we won`t highlight as the better threat appears to be just to our west, will need to monitor for the threat of some funnels or non-supercell tornadoes as the upper low moves in. Some issues about how much low-level instability we can get due to cloud cover, especially over the north. Better low-level vorticity also looks to remain our west (closer to the surface low), and by the time it gets here we should lose the better low level lapse rates /CAPE. Heavy rain will be the other main hazard from the showers/storms today into this evening. PWATs of 1.3-1.7" will remain in place, along with warm/deep cloud depths and tall/skinny CAPE, will provide for very efficient rain makers. Localized flooding will be possible, especially in flood prone and urban areas (due to high rain rates), and where any storms move over the same location. Slowest moving storms will be across northern WI. WPC has extended the Marginal Risk for excessive rain back into all of northeast WI. As the low slowly spins into Lower Michigan, cyclonic flow will bring wrap-around showers, clouds and possibly a few storms across the area later tonight into Saturday, with the greatest coverage across eastern WI. Locally heavy rain and localized flooding will be the main threat. Fog Trends: Dewpoints in the 60s to around 70, recent rains and relatively light winds have allowed fog/mist to develop across much of the area overnight. This will continue into the morning hours, with visibilities improving after sunrise. Some dense fog is expected, especially where any breaks in the clouds occur and where the heavier rain fell yesterday. But think the threat for widespread dense fog will remain on the low side due to the low stratus deck and showers pushing through to keep things stirred up. But will need to watch for the stratus deck lowering toward sunrise. Dewpoints won`t be as high as tonight, but patchy fog is expected again, especially where the heavier rain falls today. Temperatures / Humidity: After a warm and muggy night, temps don`t look to climb much today due the clouds/precip expected. However, with such a warm start, it won`t take much warming to get highs in the mid 70s to low 80s across most of the area. Warmest temps will be in east central WI where the best chances for breaks in the clouds later this morning will be, with some mid 80s possible if the breaks are more persistent. Lows tonight should range from the upper 50s in north central WI to the mid 60s across eastern WI. CAA, clouds and precip will hold highs on Saturday mainly in the 70s, with a few upper 60s possible in far north central WI. It will humid today, especially in eastern WI where dewpoints will be around 70. Slightly less humid conditions arrive tonight into Saturday, but dewpoints will remain the 60s, keeping a touch of humidity in the air. Long Term...Saturday Night Through Thursday Relatively low impact weather remains expected late this weekend and for much of next week. With the large upper low continuing to spin across the central Great Lakes, deep cyclonic flow combined with ample moisture in the column will lead light showers continuing over at least eastern WI on Saturday night. The shower chance will finally depart on Sunday morning as the system slides further east. Thereafter, northwest flow will be present over the western Great Lakes through the middle of next week before the upstream ridge moves overhead around next Thursday and Friday. Models had been advertising a front moving into the region during the Wednesday- Thursday time frame, but confidence in this scenario has lowered over the past 24 hours. In general, precip chances look low next week. In terms of hazards, north flow on the backside of the departing low pressure system could lead to 25 kt winds on the Bay and Lake from Saturday night through Sunday night. Confidence is only medium that a small craft advisory will be needed. && .AVIATION...for 18Z TAF Issuance Issued at 1237 PM CDT Fri Aug 16 2024 Showers and thunderstorms were developing over the region due to increasing daytime instability (CAPE of 1500-2500 j/kg) and the approach of a well-defined short-wave trough over western WI. The first line of storms is poised to impact AUW/CWA at TAF issuance, with potential for torrential rainfall and gusts to 35-40 knots. The storms will continue to shift east through early evening, so have framed out TEMPO groups for the most likely period of storms at each TAF site. Flight conditions will vary between VFR and MVFR this afternoon and early evening, except for local IFR/LIFR vsbys in heavy rainfall during storms. Expect shower activity to wind down early this evening, though showers and isolated storms may continue to impact eastern WI, and especially Door county, through the rest of the TAF period. Look for MVFR ceilings to gradually lower to IFR/LIFR later tonight, and vsbys to drop to MVFR/IFR in fog. Not really expecting dense fog at this time, as N-NW winds will be picking up a bit later tonight into Saturday morning. The very low ceilings should start to rise during by late morning, with most locations improving to MVFR toward the end of the TAF period. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Bersch/MPC AVIATION.......Kieckbusch