Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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870 FXUS63 KARX 182003 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 303 PM CDT Wed Sep 18 2024 .KEY MESSAGES... - Above average temperatures continue into the weekend with some cooling through early next week. - Scattered non-severe storms possible tonight into Thursday morning, with higher chances (60-80%) for storms late Thursday afternoon/evening, potentially severe, especially near and west of the Mississippi River. - Low to medium (30-50%) rain chances continue later in the weekend into early next week. && .DISCUSSION... Issued at 303 PM CDT Wed Sep 18 2024 Tonight-Thursday night: GOES water vapor channels early this afternoon showed a trough across the northern Rockies with a plume of moisture extending north and west towards it from the plains and much drier low-level air over the Great Lakes. The axis of stronger moisture transport will shift east into the area later tonight and Thursday ahead of the trough, likely resulting in scattered showers/storms, most numerous near and west of the MS River, gradually diminishing through early afternoon as the moisture transport weakens a bit before refocusing late in the day ahead of the trough/front. Storms through the morning should be non-severe. The main focus of attention will be late in the afternoon and evening as the front approaches and ascent increases with the main shortwave passing by to the north. A narrow corridor of 2000+ J/kg of MLCAPE is indicated by the RAP along/ahead of the front with cross frontal deep layer shear profiles sufficient for supercells/updraft rotation before some upscaling/clustering occurs through the evening. Any initial supercells could pose a risk for large hail in excess of 1" with a plume of steeper mid-level lapse rates ahead of the trough, especially west of the Mississippi River. A tornado could not be completely ruled out with any supercell with modest curvature noted in low-level hodographs. These threats likely would be greatest earlier in the storm life cycle before upscale growth occurs with a wind/hail risk. Eventually, storms should weaken eastward through the evening as diurnal cooling occurs/instability wanes. Friday-Wednesday: Weak high pressure builds back into the area on Friday with temps staying above average in the mid 70s to mid 80s given only slight low-level cooling. There continues to be spread in the model suite with the evolution of upper level troughs progressing from the Desert Southwest and from northern plains/southern Canada through the weekend into early next week, especially with the amplified ridging across western Canada. With the uncertainty in the flow progression, rain chances remain in the 30 to 50% range Saturday night through early in the week. A general cooling trend is expected early next week, although global ensemble members show notably higher spread during this time. NBM 25th-75th percentile max temps are mainly in the mid 60s through the 70s during this time. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1238 PM CDT Wed Sep 18 2024 VFR conditions will prevail through the first half of the TAF period. An approaching cold front from the west will increase southerly winds Thursday morning and bring a 30% chance for showers and thunderstorms in the morning. I have included a prob30 at RST and LSE for these chances. Brief MVFR visibilities will be possible with any thunderstorms that develop, but ceilings are expected to remain VFR. Beyond this TAF period, the cold front sweeps through the area Thursday evening, bringing gusty winds and a line of showers and thunderstorms Thursday evening. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JM AVIATION...JAW