Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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003 FXUS63 KARX 192005 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 305 PM CDT Sun May 19 2024 .KEY MESSAGES... - Another round of showers and thunderstorms late tonight through Monday afternoon. A few stronger storms possible. - Confidence continues to increase for severe thunderstorm potential Tuesday across the region. Severe risk level 3 out of 5 for most. - Another system moves in Friday into Saturday. Best chances (60%) on Friday evening across the area. && .DISCUSSION... Issued at 303 PM CDT Sun May 19 2024 Overview: Active period of weather this week. Increasing chance of severe thunderstorms Tuesday. Broad troughing across the western CONUS continues to dig south into the Four Corners this evening through Monday. Meanwhile, a low off the southern CA coast is progged to phase with the digging trough as it moves onshore into the Baja region. As it does, SW flow takes hold across the CONUS with a downstream elongated ridge across the Appalachians. This SW flow will allow PWATs to climb above 1.5 inches on both Monday and Tuesday, which remains near the climatological max of DVNs sounding. The aforementioned trough and associated shortwave trough will eject from the Rockies Tuesday morning as a leeside Colorado Low develops and lifts NE through the Central Plains. This trough takes on a negative tilt as the surface low is expected to deepen quickly into a seasonably strong low (into the 990s hPa). Tuesday remains the day to watch for severe weather as we have a deepening surface low passing through the region, bringing ample forcing for ascent with strong, deep layer shear all while peak daytime heating is occurring. All the ingredients remain in place for a convective severe weather event, but the details remain uncertain with an expected MCV on Monday that may perturb the environment. After Tuesday, the upper-level pattern becomes temporarily more zonal and progressive with another upstream trough dropping south along the West Coast. This trough moves inland across the northern Rockies Thursday and through the Northern Plains Friday through Saturday bringing another surface low and chance for widespread precipitation. Variability among the medium range guidance will bring low chance PoPs Friday through Saturday, but the best timing looks to be on Friday evening through Saturday morning. Tonight through Monday: Thunderstorms Tonight into Monday. Severe Risk Level 1 out 5. Warm air advection and isentropic ascent continue to increase this afternoon with clouds increasing from the west all day. A shortwave trough tracks through the area after midnight tonight, with increasing PoPs after 06Z across the area. These precipitation chances appear more likely than specific details on any severe weather threat. Evolution on that threat depends on how the upstream MCS evolves through Central Plains this afternoon. It`s possible this MCS perturbs the warm front enough that it doesn`t lift far enough into our region, which would decrease any severe threat considerably. Large scale shear outside of the upstream convection is on the lower end, which decreases threat as well. By Monday afternoon, better large scale forcing appears to be just east of our local area across SE WI where a 40-50kt LLJ lifts into the region with 40-45kts bulk shear. So overall, higher chances for showers and thunderstorms tonight into Monday, but uncertainty remains on severe evolution into the afternoon with convective cloud debris and better shear/instability trending east. Tuesday: Strong Low Pressure with Increasing Confidence for Severe Thunderstorms. Severe risk level 3 out of 5. There are two separate chances for storms on Tuesday, with the first one occurring as a warm front lifts into southern MN/WI. The next, potentially more significant, round of thunderstorms will occur as a strong low pressure system and associated fronts move into the area Tuesday afternoon/evening. A potent negatively tilted trough and seasonably strong surface low pressure system are expected to lift NE from the Central Plains into the Upper Midwest Tuesday, bringing severe thunderstorm chances along with it. There is ample moisture (PWATs over 1.5 inches), large scale ascent, and instability for severe thunderstorms to occur, but will need to wait for more specific details. After the potential morning convection, models favor the surface low passing to the west of our local area. This puts our area firmly in the warm sector of the low pressure system with dewpoints in the 60s expected. As the low lifts north, steep mid-level lapse rates and surface destabilization will bring increased instability as sufficient shear moves in. At this time it seems like wind would be the primary hazard as the threat evolves with time with upscale growth, but initial development of isolated storms could pose a tornado or hail threat. With the upscale growth in the afternoon and tightly wound low pressure system, embedded QLCS tornados would also be possible. Continue to monitor this forecast as this is the highest severe weather threat of the season so far. Wednesday and Beyond: Remaining Active There could be some lingering showers and thunderstorms on Wednesday, but overall a drier trend is favored Wed into Thursday. The next wave advancing from the Rockies across the Northern Plains Friday, bringing a widespread chance (50-60%) of showers and thunderstorms Friday afternoon into Saturday morning. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1229 PM CDT Sun May 19 2024 VFR conditions will persist into tonight with increasing mid/high clouds and perhaps some light showers (20-30% chance) impacting terminals from weakening storms farther south and west late this afternoon/evening. More widespread showers and storms will develop after 20.06Z with the highest thunder chances between 06 and 12Z. Reduced visibility will accompany any heavier rain while some guidance brings MVFR/IFR ceilings (40-70% chance) into the region as low-levels saturate Monday morning, including the higher IFR probabilities (30-50%) at KRST. There is lower confidence in how quickly rain exits Monday morning with some guidance clearing the more widespread showers/storms from KLSE by 12Z. However, these details will be refined in later updates. Southeast winds will trend west to southwest near the end of the TAF period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...JAW AVIATION...JM