Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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202 FXUS63 KFSD 201746 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1246 PM CDT Thu Jun 20 2024 .KEY MESSAGES... - Showers and isolated weak storms spread through southeast SD and southwest MN this morning through early this afternoon. - High confidence in moderate to heavy rain tonight into early Friday morning. Highest rainfall amounts are most likely to remain near/north of a line from Yankton to Beresford to Worthington in a broad swath of 1-2" with isolated 3+ inch amounts. Isolated severe storms are possible this evening in south central SD with large hail and damaging winds as the main threats. - Another round of storms comes Friday night with threat of severe weather. This is expected to bring another swath of 0.50-2.00" of rain with isolated 3+ inch amounts possible. All hazards of large hail, damaging winds, a few tornadoes, and flash flooding are possible so be sure to monitor the forecast. - Cooler temperatures and dry conditions are expected Saturday night into early next week, though rain chances return Monday night. && .DISCUSSION... Issued at 330 AM CDT Thu Jun 20 2024 TODAY: Early this morning, scattered showers and isolated storms continue to develop and stream northeast into areas west of I-29, but much of this rainfall is not reaching the surface yet. As expected, dry low level air has greatly limited rainfall (to <0.10") so far. Broad ascent from 850-700mb warm air advection coupled with an upper jet streak and weaker low level jet nosing in will gradually bring more expansive showers with occasional rumbles of thunder across south central and southeast SD this morning. By mid day, much of the region west of Highway 18 in eastern SD should see 0.25-0.75" of rain. This initial wave of rain will shift further north through the afternoon, bringing the most rain (0.25-0.75") to areas near and north of Hwy 14. Further south, expect breaks in the rain for much of the region near and south of I-90 late this afternoon through early this evening. This evening, attention turns to developing storms in the NE Panhandle and WY. CAMs suggest this activity will merge into an MCS tracking east down the instability/moisture gradient into central and eastern SD/NE overnight. Cannot rule out isolated strong to severe storms tonight with this activity when MUCAPE of 1000-1500 J/kg will nose into the MO River Valley, especially 8 PM to midnight. Large hail to half dollar size and wind gusts up to 65 mph will be threats with the strongest storms, most likely in south central SD. The greatest impact will be locally heavy downpours overnight into early Friday morning, prompting issuance of a Flood Watch. More details are available in the Hydrology section. Showers and storms training in overnight will bring a broad swath of 1-2" of rain north of a Yankton to Beresford to Worthington line with pockets of 3-4" of rain possible for a few locations per HREF PMM guidance. The further south the heaviest rain occurs, the more likely flash flooding is to occur given recently saturated soil and heavy rains. FRIDAY: After Thursday night`s round of storms moves out Friday morning, a drier period is favored for at least the morning hours. Low confidence in current moderate to high (40-70%) rain chances for Friday given poor agreement in CAMs. As the sfc front continues to lift north and puts us deeper into the warm sector Friday, we`re in for a hot muggy day with dew points in the 70s and highs in the 80s to near 90 degrees and a south or southeast breeze. On Friday night, attention turns to another round of threats for severe weather and flash flooding. Low to moderate confidence in where and when storms will initiate (central/eastern SD vs NE) at this stage, but 2-3 kJ/kg MUCAPE coupled with 25-35 kts 0-6km deep layer shear, an attendant inverted sfc trough and encroaching upper level trough are likely to bring scattered strong to severe Tstorms Friday night. With even more anomalous deep layer moisture (top 1% of ensemble guidance) and a model QPF signal for another swath of 0.5-2.0 inches of rain and isolated 3+ inch amounts, encourage those with outdoor plans Friday night to monitor this period for details regarding all hazards of severe weather (hail, damaging winds, and tornadoes) and flash flooding. SATURDAY AND BEYOND: Deterministic models mostly put us on the north/dry side of the sfc front by Saturday afternoon, favoring drier conditions and near seasonal conditions Saturday night through Sunday. An early look into next week favors a warmer and wetter than normal pattern continuing. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1245 PM CDT Thu Jun 20 2024 Scattered showers and thunderstorms continue across the area early this afternoon. Ceilings are a mix of VFR all the way down to IFR with one or two locations showing LIFR ceilings. Visibilities are also erratic as visibilities down to IFR levels is reported in falling rain while VFR in rain free areas. The showers and thunderstorms are moving northeastwards which will push the heaviest rain away from most TAF sites over the next several hours. However, isolated development is possible through the rest of the afternoon hours. Additional thunderstorms are expected to move in from north central Nebraska and central South Dakota during the evening hours. These storms may produce strong winds gusts with them. The storms look to stay mainly along and north of I-90 through the evening and overnight hours. However, MVFR to IFR ceilings will continue through the overnight hours and into tomorrow morning. Light showers, low ceilings, and southeasterly flow turning southerly will finish out the TAF period. && .HYDROLOGY... Issued at 330 AM CDT Thu Jun 20 2024 A Flood Watch for localized flash flooding has been issued for a broad swath of the region tonight through early Friday morning. Anomalous moisture content develops tonight via precipitable water at the top 10th percentile of GEFS guidance for this time of year thanks to return flow from the Southwest. Additionally an atmospheric pattern with a coupled low level and upper level jet aided by a mid level wave primes the atmosphere for flash flood threat as thunderstorms track in overnight. Low to moderate confidence in where the heaviest rainfall axis will occur tonight. The sfc front positioned near the I-70 corridor early this morning has already begun a northward trek, bringing low level moisture already further north than expected by most deterministic models aside from the RAP. This may shift the axis of heaviest rainfall further northwest, centered closer to a De Smet to Chamberlain to Mission SD line. If this occurs, flash flooding would be less likely because locations such as Huron to Mitchell have seen below to near normal rainfall for June thus far (respectively). However a shift a county or two further south would greatly increase flash flood threat given recent heavy rain and saturated soil, and this could occur as far south as Yankton to Sioux Falls to Slayton. With another round of storms expected to track through the region Friday night, flash flood threat increases considerably Friday night into Saturday so will need to monitor this period closely as well. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Flood Watch from 8 PM CDT this evening through Friday morning for SDZ039-040-050-053>070. MN...Flood Watch from 8 PM CDT this evening through Friday morning for MNZ071-072-080-089-097-098. IA...Flood Watch from 8 PM CDT this evening through Friday morning for IAZ001. NE...None. && $$ DISCUSSION...BP AVIATION...Meyers HYDROLOGY...BP