Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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260 FXUS63 KARX 271907 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 207 PM CDT Sat Apr 27 2024 .KEY MESSAGES... - Isolated severe storm chances in the 4-9 pm window today south of Highway 18 in swrn WI and Clayton/Fayette counties in IA. Large hail and damaging wind are the main threats. - Heavy rain axis has shifted south of the area. A more widespread rain of 0.75-1.25" is expected through Monday, with most falling Sunday and Sunday evening. Storms will be around but the risk of those being severe is quite low after this evening. - An active week is ahead with periodic rain and storm chances, with severe storm probabilities looking low at this time, but stay tuned. Later Tuesday is the next opportunity. && .DISCUSSION... Issued at 206 PM CDT Sat Apr 27 2024 Severe Thunderstorm Potential This Afternoon/Evening: At 1 PM, surface observations show southwesterly winds persisting area wide. The hi-res models are in very good agreement for winds to slowly begin turning to the west throughout the afternoon for much of the area except far southwest Wisconsin and parts of eastern Iowa. This will create a weak convergence zone along an axis from the Strawberry Point, Iowa area towards Lone Rock, Wisconsin. With a lack of synoptic- scale forcing due to weak shortwave ridging aloft, this will be the primary forcing mechanism for ascent. South of the aforementioned convergence axis, a reservoir of 700-1200 J/kg of mixed layer CAPE is progged to continue to increase with daytime heating. HRRR model soundings support the CAPE axis growing to 1500- 1800 J/kg in the southern parts of Clayton and Grant Counties by late afternoon. In previous forecasts and model soundings, this instability was capped by an elevated mixed layer 2-3 km off the surface, but recent HRRR soundings at Dubuque have reduced the strength of the warm nose aloft leading to less capping. Day Cloud Phase satellite imagery shows hints of this as well, with bubbly cumulus cloud tops along and south of Highway 151, and some glaciation and orphan anvils further south. The lack of a stable layer aloft could lead to airmass thunderstorms developing as daytime heating continues to fuel the environment. Thunderstorms that form before the weak convergence zone sets up would be rather pulsy in nature and may decrease the potential of an organized severe weather threat this afternoon/evening. If the cap does develop this far north this afternoon and inhibit convection until convergence is present, isolated to scattered severe storms are possible in southern Clayton and Grant Counties. The seasonably strong CAPE mentioned above combined with 35-40 knots of deep-layer shear supports supercells capable of producing large hail and damaging wind. A tornado or two cannot be ruled out with around 20 knots of 0-1 km shear present and a very brief window of helicity greater than 150 m^2/s^2 in model soundings, but generally speaking hodographs are straighter in nature for much of the event making large hail the primary threat. With the lack of strong forcing this evening, thunderstorms will succumb to a stabilizing atmosphere after sunset this evening. The forecast for tonight is much drier than previous forecasts with all of the stronger forcing remaining south of the forecast area until early tomorrow morning. Rain and Storms for Sunday into early Monday As the current four-corners deep low ejects out of the southwest and tracks northeast across the region, lift increases Sunday on a brush-by of QVector convergence /strongest northwest/ and the Highway 20 warm front trying to shift north as the surface low tracks through Minnesota. Low-level moisture advection shifts north during the morning hours and continues to supply the area abundant moisture /precipitable water values at the 95th percentile to near record by late day/. One change is the moist air stream is bifurcated, keeping the very juicy air in the northeastward stream from IL->MI /precipitable water 1.50"+/. While the surge in moisture occurs, the model consensus surface warm front remains near the WI/IL border and west into IA as the low occludes, limiting CAPES to 500J/Kg, reducing severe storm potential. This triple point will evolve northward overnight Sunday ahead of the cold front, with some surface-based instability /500 J/Kg/ dragging northeastward into WI per the latest RAP forecasts. Wind shear is still pretty impressive for this time frame with long,straight hodographs. Right now, this RAP migration of the triple point and surface instability is more of an outlier compared to the HREF CAMS. It is something to monitor Sunday night. The current SPC marginal severe storm risk appears appropriate with such reduced instability. So, waves of showers and storms continue until Monday where cold and dry air advection stabilize the atmosphere from southwest to northeast post- low. Rainfall amounts look to be in the 0.75-1.25" range from Sunday to Monday, with the 27.12Z HREF having about 20-35% chances for 1"+. Active Weather Next Week, Monitoring for Severe Storm Potential Tuesday: Increasing mid-level winds ahead of a deeper longwave trough evolving over the northcentral/northwest CONUS Tuesday will lead to the monitoring for our next chance of severe storms. An open Gulf of Mexico flow will provide a moisture tongue into IA/MN ahead of a north- south oriented cold front Tuesday afternoon. By evening, the 27.00Z Grand Ensemble mean places the cold front near I-35 with 70-80% probabilities of 40 kt 0-6km bulk wind shear. Forecast soundings from the 27.12Z GFS/NAM for 7 pm Tuesday show some moderate capping in place and the area being on the northern extent of the instability tongue. The ensemble probability of SBCAPE over 500 J/KG is only 15-20% just west of the Miss river. So, the kinematics are in place for severe storms, but instability looks to be a limiting factor at this time. The 27.00Z CSU severe probabilities from machine learning indicate as axis centered on I-35 from DSM-DLH of 5-15% late Tuesday. So, will continue to keep an eye on this potential. Mid-Late Week: Swift zonal flow continues across the CONUS with various solutions and large spread on the shortwave troughs moving through with a frontal zone and instability in the region. Forecast confidence is quite low on any details beyond Wednesday but big picture says periodic rain/storm chances and breezy at times into the weekend with near normal temperatures. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1233 PM CDT Sat Apr 27 2024 MVFR ceilings at the TAF sites to start the period. Model guidance continues to support ceilings lifting to VFR later this afternoon, before returning to MVFR late this evening. Showers and storms form across parts of northeast Iowa and southwest Wisconsin later today, slowly spreading northward overnight. Impacts at the TAF sites are not likely until Tomorrow morning. Gusty southeast winds gradually weaken from west to east this afternoon, turning to the west/northwest as the day progresses. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...KAA/Baumgardt AVIATION...KAA