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Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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470 FXUS63 KARX 141147 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 647 AM CDT Wed Aug 14 2024 .KEY MESSAGES... - Wednesday night into Thursday morning will feature showers and thunderstorms bringing potential for heavy rain, with a 40 to 70 percent chance for an inch of rain across the southwestern two thirds of the forecast area. - Thursday afternoon and evening will likely feature an additional round of showers and thunderstorms. While rain rates look to take a small step downward, a severe thunderstorm could occur in the southern portions of the CWA. - The remainder of the seven day forecast features seasonable temperatures with the potential for an afternoon shower or thunderstorm. && .DISCUSSION... Issued at 413 AM CDT Wed Aug 14 2024 Wednesday night into Thursday morning: Potential heavy rain This afternoon, upper shortwave, over western Nebraska as of 08z, will eject eastward over the forecast area. Cooling aloft associated with this feature may lead to a few showers and perhaps a rumble of thunder. Progged instability is very low - 14.00z GFS suggests MUCAPE will be less than 100 J/kg - so think severe thunderstorms are not in the cards this afternoon and early evening. Additionally, PWAT values at this time remain under 1.75" so am not too concerned with heavy rain potential with this shortwave. That assessment changes as we move into the nighttime hours. While severe chances continue to appear minimal given available instability, an assessment reflected in the scant (<5%) potential for severe storms in post-processed 14.06z HRRR output, the threat for localized heavy rain increases as robust low level jet causes PWAT values to increase to around or greater than 2", around the 99th percentile of GEFS/ENS climatology. With deep warm cloud depths also present, rain rates should be highly efficient as showers and thunderstorms develop across the area as the low level jet increases as another shortwave approaches from the west. Given the convective nature of the rain, while most areas should receive at least some precip, narrow pockets of higher amounts look to be the norm. In other words, rain distributions have a long tail on the high side. Indeed, NBM probabilities suggest a 40 to 70 percent chance for at least an inch of rainfall and a 10-25 percent chance for at least three inches. Saving grace will be the progressive nature of the pattern and progged storm motions near 40 mph, limiting flood risk to only those unlucky enough to experience the heaviest thunderstorms of the bunch. Thursday afternoon and evening: Additional rain, potential severe Thursday afternoon, next upper trough approaches from the west, likely kicking off another round of convection. A marked reduction in low level moisture transport will mean PWAT values back down to around or below 1.75", limiting the risk for additional heavy rain. However, the chance for a severe storm may re-enter the picture as MLCAPE values build to around 1500 J/kg with 0-6km bulk shear of around 35 knots. Given some turning seen in progged hodographs and steep mid-level lapse rates, severe hail would be possible if that amount of instability is indeed realized. Additionally, sfc-500m SRH of 50-100 m2/s2 suggests a low but greater than zero risk for a tornado, primarily in SW WI. Will need to monitor Thursday closely, particularly if any daytime clearing of skies occurs in SW WI. Friday through Tuesday: Seasonable, low potential for afternoon showers/storms Northwesterly flow aloft looks to dominate the remainder of the seven day forecast, a pattern that usually yields a stray afternoon shower/storm or two if an upper shortwave arrives coincident with peak surface heating. With an upper low present nearby to the east Friday and Saturday, expect temperatures to run a bit under normal and for the probability for that stray afternoon convection potential to be realized to approach 30 percent given the cold air aloft. Aside from potential lightning risk to those outside during the mid to late afternoon, am not concerned with either heavy rain or severe thunderstorms Friday through Tuesday. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 618 AM CDT Wed Aug 14 2024 Precipitation chances increase from southwest to northeast this afternoon. Initial impacts will remain in southeast Minnesota as dry air challenges overall saturation farther east. Therefore, initial impacts at KRST TAF site expected this afternoon (~19Z) before spreading east by tonight at KLSE (~15.00Z). Even so, initial saturation at KRST may present a challenge as well, potentially delaying any impacts closer to 21Z. Have kept the earlier solution (19Z) in 14.12Z TAF for now but may need to be adjusted before 14.18Z TAF issuance. Given the slight delay in overall saturation, lowest ceilings stay west of the local forecast area until tonight. Eventually, near the end of 14.12Z TAF period, IFR/LIFR ceilings become more likely as they shift east into southeast Minnesota and northeast Iowa. Have reduced ceilings to low MVFR at KRST given current forecast hour but will be subsequent forecast detail to iron out. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Ferguson AVIATION...JAR