Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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144 FXUS63 KFSD 211653 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1153 AM CDT Tue May 21 2024 .KEY MESSAGES... - A conditional risk for renewed convection exists south of I-90 by mid day. If instability can redevelop, all hazards will be possible with afternoon storms, especially over Northwest Iowa. - Strong north to northwest winds will quickly develop Tuesday afternoon. Some potential for gusts to 50 to 55 mph through the I-29 corridor and then across Southwest Minnesota. - After a brief break from rain, additional thunderstorms are expected Thursday night through Friday. Isolated strong to severe thunderstorms are possible. && .DISCUSSION... Issued at 245 AM CDT Tue May 21 2024 THIS MORNING: After a busy evening with multiple elevated supercells and hail up to 3" in diameter, the early overnight hours have quieted somewhat as the airmass overhead has generally become worked over and mid-lvl lapse rates have lessened over the CWA. Most of the strong convection has been tied closer to the surface front along the I-80 corridor where persistent overrunning is leading to flash flooding. Further north/northwest, we`re continuing to see widespread showers and thunderstorm development as deep synoptic lift overspreads the Dakotas. Rainfall rates however are much lower than other areas, as convection to the south in intercepting that deep westward moisture trajectory. Our eyes early this morning continue to focus on a cluster of severe storms over southwestern Nebraska, that is already showing signs of turning into a progressive MCS. Latest CAMS continue to hint that as the ejecting upper trough begins to lift northward, it will pull the surface front a bit further northward, and also lift the deeper pool of instability along with it. Current projections take the developing MCS towards Sioux City by 5-6am, though it`s quite uncertain if we`ll be too stable in the boundary layer north of the effective front to produce any strong winds as it moves into northwest Iowa. The heavy rain axis may drift just into the Highway 20 corridor, but at this point not envisioning it lifting much further north. TODAY: A secondary threat this morning could be the development of a wake low in association with the MCS that moves through the area. Several CAMs are suggesting the potential for 40 to 50 knot winds moving through areas west of I-29 through mid-morning as the MCS falls apart. Then the real questions develop. A lull in precipitation is expected through mid-late morning as a dry slot rotates into the Tri- State area. With the extensive low-stratus and variable winds could see some drizzle at times. A secondary lobe of vorticity will then begin to lift rapidly northeast across Nebraska late this morning. Surface cyclogenesis should begin to develop near Hastings and then track northeast into southwest Minnesota by early afternoon. Latest guidance continues to suggest a corridor of moderate instability upwards 2500 J/KG ahead of the front, but most of this instability is above a stable layer around 1KM. If we can somehow tap into the boundary layer briefly, then rapid surface based updraft growth can develop over portions of northwest Iowa early in the afternoon. However if we remain stuck with stratus and too much stability below 1KM, then it`s more likely we see rapid growth of elevated convection. All that said, this event will unfold very quickly this afternoon with storms beginning as early as 11am-12pm and exiting the eastern zones by 3pm. Given the large scale shear within the profile and 8-9C/KM mid-lvl lapse rates, 2" hail will be possible. The wind and tornadic threat remains conditional to the boundary layer, but given 0-1KM SRH approaching 70-100 m2/s2, very low LCLs, and some concentration of MLCAPE in the lowest 1-2KM a tornadic risk is certainly possible. The greatest tornadic threat will be along and southeast of a line from Sioux City to Spencer. As convection quickly moves northeast of the area by mid-afternoon, the surface low will continue to deepen, with a pronounced low-lvl wind field rotating from north to south around the surface low through late afternoon and early evening. Surface wind gusts exceeding 45 knots are likely to develop over eastern South Dakota and extend into Southwest Minnesota into early evening. Some of the highest gusts in model data approach 50 to 55 knots at times. Will go ahead and issue a wind advisory. WEDNESDAY-SUNDAY: Temperatures cool towards normal for Wednesday and our dry spell will last one full day. Another deepening upper trough moves into the Northern Plains Thursday with a warm front developing over the eastern Dakotas and western Minnesota Thursday afternoon. We`ll see how much moisture can return northward, but as the upper trough and associated wind max aloft moves into the area late Thursday, it could bring a risk for severe convection. The CSU Learning Model suggests at least 15% probabilities for hail and wind into the overnight hours, and this fits the latest SPC Day 3 thoughts. Additional rainfall at the end of the week likely averages between 0.25"-0.75". && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1150 AM CDT Tue May 21 2024 Showers and thunderstorms, with MVFR/IFR ceilings, are expected to redevelop in coverage through the afternoon. Some of these storms may become severe, with large hail, strong erratic winds, and even an isolated tornado over northwestern IA. Storms are then expected to push off to the east between 22Z and 00Z. On the backside of the system, synoptic winds will gust at 40-50 mph through the evening, tapering downward through the early nighttime hours. Ceilings will lift tonight, with VFR conditions expected through the remainder of the TAF period. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Wind Advisory until 10 PM CDT this evening for SDZ039-040-055- 056-062-067. MN...Wind Advisory until 10 PM CDT this evening for MNZ071-072-080- 081-089-090-097-098. Flood Watch until 7 PM CDT this evening for MNZ081-090. IA...Wind Advisory until 10 PM CDT this evening for IAZ001>003- 012>014. Flood Watch until 7 PM CDT this evening for IAZ003-013-014-021- 022-031-032. NE...Flood Watch until 7 PM CDT this evening for NEZ014. && $$ DISCUSSION...Dux AVIATION...JM