Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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391 FXUS63 KARX 070747 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 247 AM CDT Tue May 7 2024 .KEY MESSAGES... - Line of Showers and Thunderstorms Early, Scattered Storms this Afternoon and Evening - Active pattern continues with additional thunderstorms Wednesday evening into Thursday morning. - Cooler Saturday, Potential Shower or Two Friday Night through Saturday && .DISCUSSION... Issued at 244 AM CDT Tue May 7 2024 Today: Line of Showers and Thunderstorms Early, Scattered Storms this Afternoon and Evening Line of showers and thunderstorms are making their way into the area from the west this morning and will continue to shift east through the rest of the morning. Latest RAP analysis suggests up to 700 J/kg of MUCAPE with around 40kts of bulk shear; however model soundings suggest parcels are lifting from around 5kft with nearly uniform winds through the instability - bringing the effective shear down considerably. There`s ample upper-level forcing to support convection through the rest of the overnight/morning hours across the area, but the threat for any severe storms at this time appears low when assessing the radar trends. There remains uncertainty on the chances for thunderstorms later this afternoon across the area as remnant cloud cover could hinder instability. After the line of thunderstorms wraps up by mid- morning, cloud cover will begin to decrease from the SW to NE. While low-level destabilization is less certain, there is good agreement on steepening mid-level lapse rates this afternoon across the area, which will increase fuel for thunderstorms should surface parcels get going. Mean mixed-layer CAPE values for this afternoon are forecast around 1000-1400 J/kg, but the shear decreases as instability increases. CAMs this afternoon show a mixed bag of solutions with some models not resolving any convection and others continue convective rather quickly after the first round exits. So while there`s decent chances (20 to 50%) for afternoon thunderstorms, strong to severe storm chances are rather low with hail being the primary threat. Temps today are highly dependent on exiting cloud cover. Model spread is rather large, with the 25th to 75th percentile temp spread over 10 degrees in spots. NE Iowa is most favored to see clearing skies this afternoon, with temps climbing into the low 70s. SE MN will likely see temps near 70 as well. As for SW into central WI, modeled sky coverage has a larger spread which makes temps less certain. If cloud cover remains throughout the day, could see temps closer to 60; however, if clouds diminish temperatures will climb into the 70s. Wednesday into Thursday: Another Round of Thunderstorms A remnant surface low and longwave trough from the Dakotas dives south Wednesday, bringing upper-level divergence and fgen forcing across the region. While there has been a slight shift to the south from previous runs, an inverted trough and Fgen forcing along 850 to 600mb coupled with a mid-level low shifting over IA and a skinny CAPE profile will bring the potential for more showers and thunderstorms and potentially a localized heavy rain threat. Modeled rainfall amounts among the GEFS/EPS/CMCE rain from near 0 inches of precipitation to as high as 2.50 inches across NE Iowa and SE MN, but as a whole, there`s roughly a 10-20% chance of rainfall exceeding 1 inch across the aforementioned areas Wednesday evening into Thursday morning. This threat will diminish Thursday morning as cooler air moves in behind the exiting trough. Behind the cooler air Thursday, temps will back down a bit to the upper 50s to low 60s. This weekend: Not as Warm, Some Showers Long wave trough drops south from Canada across the Upper Midwest into the weekend bringing temps into the low to mid 60s with scattered showers as the upper level low pivots overhead. The best chances (30 to 40%) for precipitation appear to be on Friday night into Saturday morning at this time with the initial shortwave vort max moving overhead. As broad cyclonic flow remains through Saturday night, expecting additional showers across the area at times - best chances east of the river. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1015 PM CDT Mon May 6 2024 An area of showers and weakening storms will develop from the west after 07.06Z tonight, spreading into Wisconsin through Tuesday morning. Timing since the last update has slowed by an hour or so. Ceilings may vary from VFR to MVFR with probabilities for MVFR between 30-60% as the rain moves through. Some reductions in visibility are possible within heavier rains/storms. Additional scattered showers/storms will remain possible into Tuesday afternoon/evening, but confidence in how showers/storms will evolve later in the day is lower, so have not included any explicit mention. With less mixing into the predawn hours tonight at KLSE, did mention LLWS at KLSE, with winds trending S/SW on Tuesday. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAW AVIATION...JM