Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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631 FXUS63 KARX 241000 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 500 AM CDT Fri May 24 2024 .KEY MESSAGES... - One round of showers/storms moves through this morning, with additional storms possible into this afternoon. Although confidence is low, a conditional risk for severe storms exists, particularly this afternoon, but cannot be ruled out with the morning storms. Localized heavier rain/flooding also possible. - Additional showers/storms are increasingly likely on Sunday (60-90% chance) with unsettled conditions potentially persisting into early next week. Higher severe weather chances for now remain farther south, but heavier rain could occur. && .DISCUSSION... Issued at 303 AM CDT Fri May 24 2024 Today-Tonight: Periods of showers/storms - conditionally strong/severe Early this morning, GOES water vapor imagery showed a strong shortwave trough ejecting across the Dakotas atop its associated occluding surface low. A line of storms has formed ahead of the attendant cold front fed by a strong low-level jet, approaching western Iowa as of 2 am. More scattered showers/storms were developing in the warm advection regime ahead of the line. The line will quickly move east into the area through sunrise. With only weak mixed layer CAPE extending into northeast Iowa, expect some weakening of the storms as they move in. However, will have to keep an eye on trends, with stronger wind gusts possible as storms move into northeast Iowa/southeast Minnesota and hail possible in any storms ahead of the line. There is higher uncertainty how this morning line of storms evolves and how this impacts later day storm potential with the cold frontal passage. Redevelopment could occur near the front with broad synoptic ascent/height falls south of the occluding cyclone. However, owing to uncertainty in morning storms, confidence is lower with the magnitude of destabilization northward into Wisconsin with CAMS generally showing a narrow instability axis (500-1000 J/kg SBCAPE from the HREF). Mesoscale influences, such as outflow boundaries associated with the morning storms, could also impact redevelopment and timing of storms throughout the day. Shear profiles become favorable for severe storms, with RAP hodographs exhibiting pronounced curvature with 20-30 kts of 0-1 km bulk shear. Thus, a conditional threat for severe storms exists this afternoon, highly dependent on instability and evolution of morning storms. With greater destabilization, scattered severe storms, including hail, strong winds, and a few tornadoes would be possible this afternoon, with the HREF/CSU machine-learning probabilities showing the greatest signal east of the Mississippi River, highest into southern WI into IL. Mesoscale updates throughout the day will be needed to refine this threat, as confidence remains low on whether a more favorable environment for severe storms will develop. Finally, given the multiple rounds of rain/storms in recent days, heavy rains remain a concern. Overall, the morning storms look progressive, but if focused re-development occurs, heavy rainfall/localized flooding could develop, especially south. See the hydrology section for more info. Saturday-Monday: Increasing rain/storm potential through the weekend Although Saturday looks mostly dry, shortwave energy will eject from the Rockies into the Mid/Upper MS Valley Sunday/Monday. Overall, confidence has risen with NBM precip chances increasing to 60 to 90% on Sunday as the lead wave works north and east. Embedded shortwaves within a larger upper level trough across the Great Lakes will keep some shower potential in place through Memorial Day (20 to 50%). The higher CAPE is forecast to be to the south across the Mid-MS Valley with the surface low forecast to pass south of the area, but there is some spread in northward placement of the low on Sunday, which could impact destabilization, especially for northeast IA into southwest WI. At this time of year, we always have to carefully monitor any storms near surface lows, where low-level shear is enhanced. Global ensembles show a modest footprint (20 to 30%) for 1"+ of rain Sunday, especially for northeast IA and southwest WI, but amounts will depend on degree of instability and low placement, which we will have a better idea about heading into the weekend. With repeated rounds of storms recently, we will have to monitor rainfall potential closely. Tuesday-Thursday: Seasonable and drier Generally a fairly quiet, seasonable period expected through the middle of the week as mid-level ridging spreads in from the west. By late in the period, guidance differs in how quickly the ridge breaks down/moves east. Right now, only low rain chances (~20%) are warranted with highs in the 60s/70s. && .AVIATION /12Z TAFS THROUGH 12Z SATURDAY/... Issued at 500 AM CDT Fri May 24 2024 A complex aviation forecast expected through the day with a band of showers/storms moving through this morning. Additional storms may develop in advance in their awake of a cold front. However, confidence in storm evolution is lower today, so limited thunder mention to the initial round of storms and further refinements will be needed. There is some potential (50-80%) for a period of MVFR conditions this afternoon, with VFR likely behind the cold front as southerly winds shift west to northwest. Marginal low- level wind shear is possible for a few hours this morning, but omitted for now. && .HYDROLOGY... Issued at 303 AM CDT Fri May 24 2024 Collaborated with WFOs Des Moines and Davenport to issue a flood watch for portions of northeast Iowa given the potential for repeated rounds of storms today. Some of the high resolution models indicate potential for localized amounts of 2-3" of rain, and given recent rainfall and 3-hr flash flood guidance of 1.75 to 2.00", flooding could develop if the higher amounts are realized. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...Flood Watch until 4 PM CDT this afternoon for IAZ018-019-029- 030. && $$ DISCUSSION...JM AVIATION...JM HYDROLOGY...JM