Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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096 FXUS63 KFSD 190340 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1040 PM CDT Sat May 18 2024 .KEY MESSAGES... - A dry overnight period gives way to scattered showers developing Sunday morning. Some gusty winds are possible Sunday morning at times. - Confidence is lower, but scattered showers and thunderstorms that develop late Sunday morning continue Sunday afternoon and evening, with low potential for strong convection with hail arriving late in the evening and into the overnight hours. - Confidence increasing for potential of strong convection Monday evening and early overnight along and southeast of a Yankton to Marshall line. Most likely elevated hail risks, but portions of Iowa could see all hazards. - Also growing confidence for widespread heavy rain potential Monday night through Tuesday. Probabilities for 1" of rain top 90% in most locations, with potential for 2+" of rain over 50% in the 24 hour period ending early Wednesday. We`ll need to monitor for potential flood impacts. && .DISCUSSION... Issued at 228 PM CDT Sat May 18 2024 THIS AFTERNOON: Another sunny and warm day across the region, but we continue to see occasional gusts over 30 mph at times. Weak cold air advection has actually reversed this afternoon, which should allow us to warm a bit further into the 70s, with 80s on north central Iowa. TONIGHT: Confidence remains high that we`ll see dry conditions through at least 4am. Models still support a cluster of convection developing over SW Nebraska and NW Kansas later this evening. Mean westerly to slightly northwest flow should keep this activity south of the forecast area as it reaches us shortly after daybreak. A second area of scattered elevated convection may form over central and north central SD on the nose of the LLJ. This activity should focus along or north of Highway 14. Between these areas, substancial dry air in the column should limit any northward advancement of precipitation with ceilings remaining quite high. SUNDAY-SUNDAY NIGHT: Pesky southwest flow will make for more forecast challenges for Sunday, as small perturbations in the flow are poorly handled by model data. There seems to be some consensus on a subtle wave arriving by mid-day out of central Nebraska which may begin to drive the development of scattered showers and thunderstorms. This activity will be non-severe as it moves through, but could produce scattered rainfall totals of 0.25-0.50". With a split branch of the LLJ trailing the departing wave, a SW to NE orientated band of showers may continue into the evening from SW Minnesota through NW Iowa. The ECMWF is by far the deepest solution with the passing shortwave Sunday afternoon, with higher QPF than most other models. There is still some potential for late afternoon evening development West River as a surface low and front enter the Dakotas. MLCAPE upwards of 1500-2000 J/KG does exist towards the James River, but effective may keep this activity very multi- cellular if it crosses the Dakotas late in the evening/overnight. The increasing meridional LLJ after dark may also allow additional convection to form further east, which could bring a marginal hail threat towards I-29 and western Minnesota into Monday morning. MONDAY-TUESDAY: A very brief period of shortwave ridging is possible Monday, and this may lead to mostly dry conditions into the evening hours. Temperatures may warm up into the 70s once again. The deeper southwesterly mid-lvl flow will send at least 1 or 2 additional waves into the Upper Plains Monday night into Tuesday. At the surface however, a warm front is likely to develop across central Nebraska into northwest Iowa late in the day. A fairly unstable airmass will develop either side of this boundary, with upwards of 2000-2500 J/KG MLCAPE by Monday evening. A sharp increase in mid-lvl flow late in the day may make the potential for supercells slightly more possible along the boundary, especially in eastern Nebraska into northwestern and north central Iowa. The instability gradient of most consequence seems to be focused along and southeast of a line from Yankton to Marshall, but the area for the potential of surface based activity may just be within the SE portions of the CWA. While most models are following this line of thinking, the ECMWF is a bit deeper with the wave ejecting northeast, suggesting a slower and more delayed onset to strong accent towards the overnight hours and moreso into Tuesday. Perhaps the risk with the highest confidence will be for heavy rainfall. Forecast PWAT values breaking the 1.50" mark by Tuesday morning are nearly 200 percent of normal and surpass the 95 percentile across the Tri-State area. However in this case it`s certainly possible for two areas of heavy rainfall to develop, one over Iowa/Minnesota driven by deeper convection, and a second area from south central South Dakota into northeastern South Dakota focused by stronger 850:700 convergence. ECMWF ensembles continue to support 100% probabilities of >1" over much of the area, with nearly 70% probabilities of >2". The GEFS/GEM are slightly lower with probabilities, but both suggest high probabilities for 1-3" of rain in many locations by late Tuesday. What`s noticeable is that there are some hints at a more climatologically significant rainfall within ECMWF guidance as the 24 hour forecast QPF ending 06Z Wed from central to northeast SD and western Minnesota is approaching 75% of the 100 ARI. This carries through to an EFI greater than 0.7 over a large portion of the CWA, which also suggests this could be a climatologically significant rainfall event if things come together a certain way. This bears some watching as it could send rivers upwards by Wednesday or at least prime most of the area for flooding if additional rain arrives later in the week. WEDNESDAY-FRIDAY: The deepening upper low moves into the western Great Lakes by late Wednesday, pushing the area into gusty northwest winds and much cooler temperatures. Highs likely stay in the 60s Wednesday. Ridging moves into the Plains to provide at least a second day of dry weather for Thursday, but that will be short- lived. Deterministic and ensemble guidance both indicates yet another trough enters the Plains late in the week. This trough provides a brief jump in temperatures, but also another risk for rainfall by Friday into Saturday. Depending on the track of this wave, we`ll have to monitor for severe weather risks. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 1039 PM CDT Sat May 18 2024 VFR through the period. Showers and isolated thunderstorms will be possible Sunday morning into the afternoon with some chance for stronger thunderstorms later Sunday afternoon into Sunday night. Confidence is fairly low on this activity. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...Dux AVIATION...08