FXUS63 KARX 251034 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 534 AM CDT Thu Apr 25 2024 .KEY MESSAGES... - Active weather pattern starts on Friday lasting into Monday. Rain and some thunder moves in Friday, delayed a bit from previous forecasts. - Probabilities increase for more impacts from weather later on Saturday through Sunday night. Severe thunderstorms are possible across northeast IA into southern WI Saturday afternoon and eve, transitioning to periods of rain, possibly heavy into Sunday night. Risk of flooding has taken a small step up. && .DISCUSSION... Issued at 337 AM CDT Thu Apr 25 2024 Active Weather Begins Friday Another nice day is in store today as high pressure remains over the area. Temperatures will be in the upper 50s to mid 60s with mostly sunny skies expected. Winds will be light, 10 to 15mph, with gusts up to 25 mph. Relative humidity values will be between 20 and 30%, with the lower values east of the Mississippi. With an approaching weather system, southeast winds will increase overnight Thursday into Friday, just below Wind Advisory criteria /30mphG45mph/ in MN/IA Fri afternoon. Some adjustments were made to the onset of precipitation due to dry air in the lower levels throughout the morning hours. With this in mind, for areas west of the Mississippi, precipitation is expected to begin early to mid morning on Friday. With modest instability of hundreds of elevated CAPE, and low-level moisture transport convergence increasing and shifting east, precipitation will spread east into WI mainly in the afternoon. Forcing and instability increase on the low-level jet Friday night with widespread rainfall expected. Most probabilities suggest 0.50-1.0" is likely by Saturday morning. Severe Storms Possible Saturday East/South of La Crosse Saturday still looks to be drier than Friday or Sunday, with a fairly nice afternoon for most as shortwave ridging builds in after the departing Friday wave. Probabilities in the 24.18Z and 25.00Z ensembles are starting to suggest a cold front shifting slowly southeast Saturday, arriving in northeast IA and southwest WI by late afternoon. Instability /MLCAPE/ with rich low-level moisture grows to around 2000 J/Kg near and south of the front, with steep lapse rates in an elevated mixed layer aloft, and supercell wind shear. This would provide a threat for damaging wind, large hail, and tornadoes. Questions remain however on the frontal location 65+ hours out and also the upper level forcing/lift with shortwave ridging building through and the next low pressure area far away in western Kansas. This could limit the coverage of storms. Stay tuned as this still has plenty of time to change, and thus decided not to heavily message this quite yet. For Saturday afternoon, have decreased the rain chances northwest of the front (a nice day!). Heavy Rainfall Chances Increase Saturday night to Sunday Night The cold front is expected to stall in the vicinity of northeast Iowa and southwest Wisconsin, possibly south, in advance of the next stronger shortwave trough shifting toward the area. An increasingly moisture rich southerly flow will encounter the front, isentropically lifting unstable air and converging on the northern nose of the low-level jet. Depending on how well the ingredients come together, there could be an elevated storm large hail threat overnight and definitely a heavy rainfall threat. Precipitable water values in this incoming warm sector will be in the 1.25-1.50" range per ensemble means from the latest ECMWF/GFS ensembles...which is near or record for late April. Should MUCAPE remain ~1000 J/Kg and the low-level moisture transport convergence moderate to strong (suggested in 25.00Z NAM), rainfall rates could be 1-2" per hour, with a preference for areas south of I-90. The latest ECMWF ensemble suggests a bit further north solution and a more widespread rain solution and no so focused near the warm front. With the ensemble consensus suggesting the low-level low center track to the northwest of the area, and prolonged southwest flow of near record precipitable water values, the area could be in a Maddox frontal-type heavy rain archetype. Thus, the threat of heavy rainfall and possibly flooding (~10%) has increased some in this forecast. The WPC excessive rainfall outlook for the weekend is in the marginal category, but could grow should confidence in these ingredients change. Something to keep an eye on. Above Normal Temperatures With Continued Rain Chances The forecast for the remainder of the week looks of lower confidence with above normal temperatures likely and periodic rain chances. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 529 AM CDT Thu Apr 25 2024 VFR conditions expected. Winds will be increasing today and through the night with gusts expected as low pressure draws closer to the area. Moisture will be on the increase Friday after 12Z with increased probabilities for lower LIFR and IFR ceilings. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...Frost Advisory until 8 AM CDT this morning for WIZ053-054-061. Freeze Warning until 8 AM CDT this morning for WIZ055. MN...None. IA...None. && $$ DISCUSSION...Baumgardt/Cecava AVIATION...Baumgardt