Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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770 FXUS63 KFSD 212326 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 626 PM CDT Fri Jun 21 2024 .KEY MESSAGES... - Widespread thunderstorm activity is expected tonight, bringing risk of severe weather but also another round of very heavy rainfall. - Evening severe weather risks focus on hail up to 1.5" in diameter, brief 60 mph winds, and a tornado or two. - Already ongoing flooding may worsen with convection tonight, spreading widespread impacts further into Northwestern Iowa and extreme southeastern South Dakota. Record river flooding is likely on several basins. - Conditions will quiet down through the upcoming weekend into early next week. Temperatures remain near seasonal normals. - Pattern through next week doesn`t favor any long duration rain, but several brief risks for passing convection. && .DISCUSSION... Issued at 244 PM CDT Fri Jun 21 2024 THIS Afternoon: Pesky elevated convection continues along an 850 frontal zone just north of I-90 this afternoon. Weak mid-lvl wave is beginning to push into central South Dakota this afternoon and should help serve as a focal point for renewed convection later this afternoon evening. At the surface, a stationary boundary is meandering through northeast Nebraska into portions of northwest Iowa. It`s difficult to say if much more northward progression of the front will take place, given cool outflow from convection to the north. Further south within the warm sector, we`re gradually reaching convective temperatures spring isolated to scattered convection to form. THIS EVENING and TONIGHT: We`ll begin to see additional convection develop later this afternoon along and north of the surface front. Soundings again reveal a fairly large and tall CAPE profile, suggesting upwards of 1500-2000 J/KG MLCAPE across the MO river valley into northwest Iowa. Most of the warm sector will reside on the edge of the best area of effective shear, though with values approaching 40 knots near the front, some supercells will be possible. Hail potentially up to 1.5" may be possible, but moist soundings may limit overall wind potential. The coupling of 250-300 J/KG and 0-1km helicity approaching 100-150 m2/s2 could lead to a confined risk of a tornado or two along the front early this evening. Perhaps the greatest concern will once again be from heavy rainfall. Convection forming along the boundary will be very slow moving, likely drifting east and then southeast as cold pools develop. The environment remains very primed for heavy rainfall with PWAT values pushing 200% of normal, deep warm cloud layer, and a classic setup for training along the warm front this evening. CAMs all support high rainfall rates as high as 2-3" per hour, leading to the potential of widespread 2 to 5" rainfall totals across southeastern South Dakota and Northwestern Iowa, generally between I-90 and Highway 20 by midnight. The only caveat is if activity can become a bit more forward propagating than some CAMS suggest. Convection may be a bit more isolated to scattered further north towards Highway 14 as convection further south interrupts the low- lvl flow. The initial burst of heavy convection may linger through midnight before propagating east of the CWA. However, a secondary shortwave moving out of western South Dakota may bring another complex of storms eastwards. CAMS are more conflicted on if this system will bring a secondary risk for heavy convection within a linear band north of I-90 through daybreak. SATURDAY: Elevated convection may continue deep into the morning hours of Saturday before shortwave energy drives the baroclinic zone eastward. Cooler temperatures and dry conditions then move into the region for the rest of the day Saturday. SUNDAY: Increasing heights over the Plains leads to a warmer day on Sunday which is desperately needed after multiple rounds of rain. Temperatures rise into the 80s. MONDAY-FRIDAY: The pattern relents for much of the upcoming week, but that doesn`t mean we`re out of the woods. A quasi- zonal flow through mid-week will allow at least one progressive risk for thunderstorms in the region Monday night as a front drops in from the northwest. Thankfully with mid-lvl heights rising over the Rockies, this should drive the associated front well south of the CWA by Wednesday and Thursday. However, we just can`t shake the rain risks, as most medium range models show aggressive return flow moving back into the Plains by the end of next week. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 617 PM CDT Fri Jun 21 2024 Mostly VFR conditions will transition to MVFR and lower tonight mostly due to developing thunderstorms along with the potential for developing fog. Taking a look across the area, scattered showers and thunderstorms continue to develop mostly north of I-90 causing MVFR to IFR ceilings/visibilities. Expect developing more activity to fill in over the course of the evening with greatest chances being focused east of I-29 overnight. Otherwise, light easterly winds will become more light and variable overnight as the surface high moves overhead. With lingering moisture in the lower levels of the atmosphere especially north of I-90, could see some patchy fog develop along the Hwy-14 corridor as the surface high decouples the winds. Lastly, winds will become northwesterly by Saturday afternoon with some marginal breeziness possible to end the TAF period. && .HYDROLOGY... Issued at 244 PM CDT Fri Jun 21 2024 No changes to the ongoing flood watch for the forecast area. Models in strong agreement that a focal point for heavy rain will develop this evening south of I-90 and north of Highway 20. Potential rainfall totals of 2 to 5" may be possible, which may lead to even worsening conditions on many area rivers, and increasing the potential for widespread areal flooding. Some river basins may see record flooding over the upcoming days. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Flood Watch through Saturday morning for SDZ038>040-050- 052>071. MN...Flood Watch through Saturday morning for MNZ071-072-080-081- 089-090-097-098. IA...Flood Watch through Saturday morning for IAZ001>003-012>014- 020>022. NE...Flood Watch through Saturday morning for NEZ013. && $$ DISCUSSION...Dux AVIATION...Dux HYDROLOGY...Dux