Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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122 FXUS63 KARX 241706 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1206 PM CDT Tue Sep 24 2024 .KEY MESSAGES... - After today, mild temps settle in for the rest of the week with highs mostly mid to upper 70s (5 to 10 degrees above normal) - Next shot for measurable rain holds off until Friday, through the weekend. Uncertainties with movement/development of a tropical system (and its interaction with a low pressure system south of the region) lowers confidence on areal coverage/potential amounts for the local area. Should get some clarity over the next few days. && .DISCUSSION... Issued at 215 AM CDT Tue Sep 24 2024 > Overview: a shortwave trough is dropping southeast across the northern plains/western MN early this morning, per latest water vapor imagery. Areas of light showers were resulting. Meanwhile, another weaker ripple in the flow was moving northeast out of MO and across IL. Both features have been well handled by the models over the past few days, and both are expected to continue on their same track today, more-or-less skirting to the west and east of the local forecast area. The northern shortwave will continue to dive south/southeast with medium/long range guidance all developing a cutoff low over the lower mississippi river valley by 06z Thu. An upper level ridge over the plains "bends" northeast across the region, becoming the primary influence on the region through Thu. Moving into the weekend the GEFS and EPS both take the developing tropical system tracking into the gulf northward, pinwheeling counter-clockwise around the cutoff low, eventually merging - lifting northeast - weakening as it slides over the eastern great lakes for the start of next week. A whole lot of uncertainty here with strength of the tropical storm/likely hurricane, its ultimate track, and just how it interacts with the closed low. Most members of the GEFS and EPS ensemble suites suggest at least southern portions of the forecast area will be brushed by the northern extent of the merged low pressure system. Current favored timing is from Friday night through the weekend. 24 hour probabilities for over 1/4" are fairly low in the GEFS and EPS at this time, mostly 10 to 30% through the weekend (of note the Canadian ensemble is more enthusiastic with chances and amounts, skewing the grand ensemble of models with higher chcs/qpf). The tropical system muddies the water quite a bit and lowers confidence in how the weekend will ultimately play out. It will probably be a few more days before this evolution gets some clarity. > Rain Chances: while the showers to the east/southeast look to continue to shift northeast, away from the local area, the light showers to the west could reach portions of the southeast MN/northeast IA. Trends in the radar imagery show some eastward push - although the parent shortwave has more of a southeast tack. Rain is coming out of mid level clouds (10kft) and Bufkit soundings point to ample dry air sub cloud. Sfc obs show that pcpn is reaching the ground, but light with trace to maybe a couple hundreths for amounts. CAMS models aren`t enthused that even small chances will reach the forecast area. Will continue to keep an eye on trends, but for now will lean into a "dry" forecast, adjust if it looks more likely that a few sprinkles will be realized. Any rainfall would be confined to the early morning hours. Moving into the weekend, as mentioned in the "overview", a lot of uncertainty with how the cutoff upper level low/remains of the tropical system interact and move. Significant impacts on the rain chances and potential amounts for the local area. The bulk of the members of both the EPS and GEFS paint at least light QPF for the southern half of the forecast area and the NBM covers this with 20- 40% for rain chances. Given the uncertainties, this look reasonable. > Temperatures: a very steady signal in the long range guidance with a return of milder air as the region comes under the influence of the upper level ridge. The upper 75% of the EPS and GEFS members continue to favor above normal temperatures for Wed through the weekend - but generally just by 5 to 10 degrees. Confidence remains high with mid to upper 70s for highs (a few locations could push 80). && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1206 PM CDT Tue Sep 24 2024 Light winds and mainly VFR conditions are likely through the TAF period. River valley fog once again could impact KLSE for a few hours early Wednesday morning. Surface winds will be light, but winds aloft may increase somewhat approaching sunrise. Did trend the TAF towards at least some impact at KLSE, but fine-tuning likely will be needed through the evening as trends become more clear. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Rieck AVIATION...JM