Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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947 FXUS63 KARX 212345 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 645 PM CDT Tue May 21 2024 .KEY MESSAGES... - Significant severe weather event with a MODERATE RISK (Threat level 4 of 5) for severe storms late this afternoon/evening. Another rounds of storms expected late this afternoon into evening. The highest severe threat appears to begin late afternoon and into the evening with damaging wind gusts and tornadoes being the primary threats, large hail is a secondary threat. - With additional 1 to 2 inches of QPF through tonight. This could cause rises on area rivers and streams. With increase rainfall rates in the stronger showers/storms, ponding of water on roadways can be expected. - Shower/storm chances increase for Thursday night into Friday. Severe potential is still to early to discern due to timing of front. && .MESOSCALE... Issued at 235 PM CDT Tue May 21 2024 At 19Z, surface analysis showed the main low pressure center undergoing cyclogenesis in southwestern IA with 5mb/3hr pressure falls over nwrn WI. A maritime tropical airmass warm front was located roughly along I-80 with gusting southerly winds to 25-30kt. Secondary frontal boundary extends as an inverted trough from the low to about Duluth...another area of convergence and destabilization occurring south of it. Mainly monitoring the return flow and destabilization over the next hours as the wind field is in place. KDMX wind profile has 70+ 0-6km bulk shear, and 37 kts in the 0-1km layer with a potent 300 m2/s2 of 0-500m SRH. A very serious wind profile for supercell tornadoes, and severe weather in general. Using WoFS guidance over the area, values of 5+ surge northward to about I-94 (remarkably!) but just south of the Twin Cities by 6 pm, so some serious destabilization with big dynamical lift coming in. There seems to be a slight preference for southeast MN into northcentral WI for a higher probability footprint for 0-2km vertical vorticity swaths over nc IA into sern MN, with more sporadic signals further south, meaning there may be a slight preference for that area through 6 pm for tornadoes. Current forecast is on track, with generally a 5pm to 9 pm window from I-35 to I-39 in WI. Also, there is signal that an initial line will move in to the western areas just east of I-35, but storms will develop out ahead and possibly a discrete phase of storms along the Miss river in concert. This is verified in the HRRR and WOFS runs over the past many hours. So, be careful with assuming steady state motion of the initial storms and no downstream development. && .DISCUSSION... Issued at 336 PM CDT Tue May 21 2024 This Evening into Thursday Latest water vapor satellite imagery indicates vigorous shortwave trough over eastern Nebraska and producing showers/storms across western Iowa/southern Minnesota. This shortwave trough will lift northeastward over the forecast area late this afternoon and evening and spread showers/storms into the forecast area. Severe potential with the storms is expected with these storms especially this evening and the details of the parameters are in the above Mesoscale section. Main severe threat are damaging winds and tornadoes. With the recent 1 to 3 inches of rain falling over the southern half of the forecast area...along with the potential of another inch this afternoon/evening. This will cause rises on area rivers and ponding of area roadways. In addition...precipitable water values of around 1.50 inches and decent moisture transport/deformation band setting up over the northern half of the forecast area...rainfall amounts of 1 to 2 inches is expected this evening/tonight...especially over north central Wisconsin. Cold front associated with shortwave trough moves east of the area tonight. Subsidence behind cold front and tight pressure gradient will allow for wind speeds to be 20 to 25 knots with gusts 30 to 35 knots across much of the forecast area. Cooler airmass advects into the forecast area Wednesday. Much of the forecast area will remain dry. However...north central Wisconsin will continue to be under the influence of the shortwave trough. With daytime heating...bufkit soundings showing instability and breaking the cap over this area...scattered showers and few storms will develop during the afternoon hours. These showers/storms will diminish near sunset. Then...weak surface ridge builds into the Upper Mississippi River Valley Thursday and area should remain dry for much of the day. Focus turns to Thursday night into Friday. Another shortwave trough moves across the Northern Plains/Upper Great Lakes Region. There is decent moisture convergence/vertical motion with the shortwave trough. Showers/storms are expected and will move across the forecast area west to east during the day Friday. Timing of showers/storms continue to be an issue with the deterministic GFS/NAM/ECMWF. Severe threat with the storms is still too early to discern...based on timing of surface features/showers/storms. Main forecast concerns from Friday night through Tuesday are shower/storm chances through the forecast period. Flow pattern aloft becomes west to east zonal flow across the Upper Great Lakes Region early in the forecast period Saturday into Sunday. Then...upper level trough develops over the Upper Great Lakes Region late in the forecast period. Weak pieces of energy embedded in the west to east flow aloft may produce some showers/storms Saturday into Sunday...however these are low chances less than 40 percent. With upper level trough digging over the Upper Great Lakes Region shortwave troughs are expected to track over the Upper Mississippi River Memorial Day into Tuesday. The main issue Memorial Day into Tuesday is the placement/track of the shortwave troughs associated with the upper level trough. This will impact where convection develops over the forecast area and will continue chances of scattered showers/storms for both days. High temperatures Saturday into Tuesday are expected to be near to slightly below normal with highs into the middle 60s to middle 70s. && .AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 644 PM CDT Tue May 21 2024 A line of strong to severe storms is approaching KLSE. It will affect this TAF site through 22.00z. Carried wind gusts up to 50 knots. Showers and storms will continue at KRST through 22.02z and KLSE through 22.03z. In the wake of these storms, there will be IFR/MVFR ceilings. These will last much of the night and then become VFR between 22.13z and 22.15z. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...Flood Watch until 9 PM CDT this evening for MNZ079-086>088- 094>096. IA...Flood Watch until 9 PM CDT this evening for IAZ008>011-018-019- 029-030. && $$ MESOSCALE...Baumgardt DISCUSSION...DTJ AVIATION...Boyne