Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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168 FXUS63 KFSD 011743 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 1243 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Rivers across the area have crested, with a slow downward trend in river levels as dry conditions continue. - Below normal temperatures continue today and through most of the week with high temperatures on the order of near seasonable to 5 to 10 degrees below average. - Strong to severe storms along with the potential for heavy rain remain on track for this evening and night. Locations along and southeast of a Slayton, MN to Sioux Falls, SD to Tyndall, SD line are expected to see 1-2 inches of rain. Isolated higher amounts up to 3-4 inches is possible under the heaviest storms. Large hail up to the size of ping pong balls and damaging winds to 60 mph are the primary severe hazards. - An active pattern aloft will continue rain chances for the rest of the week and potentially into the weekend. && .DISCUSSION... Issued at 329 AM CDT Mon Jul 1 2024 Quiet conditions begin the day early this morning but this will change as a mid level wave currently over western Nebraska pushes east and encounters an advancing 850 mb boundary. Scattered showers and thunderstorms look to develop along this boundary as soundings show instability on the order of a few hundred J/kg lifting from 850 mb. Given the weak instability, severe weather is not expected but the showers are expected to produce between a few hundredths to up to a quarter of an inch of rain. Should see the showers track eastwards into the afternoon hours as the showers and storms follow the 850 mb boundary. However, the showers will outrun the minor instability and weaken. Lots of cloud cover will persist through the day today which will continue the trend of cooler high temperatures in the upper 60s and 70s. With a stronger upper level wave ejecting into the Plains this afternoon, the low level jet (LLJ) will strengthen in response to the approaching wave with speeds up to 40- 50 knots. Strong warm air advection (WAA) will result from the LLJ but also stronger southeasterly winds with gusts up to 30-35 mph. The WAA could lead to some isolated elevated showers throughout the afternoon hours but little it any rainfall is expected. The LLJ will also continue to advect in higher moisture aloft which will contribute to building elevated instability into the area. The highest surface based instability will reside in Nebraska where dew points into the 70s will reside. As the previously mentioned upper level wave finally approaches the forecast area, convection looks to develop on the western side of the warm sector. Storms look to fire in central South Dakota late this afternoon but should encounter drier mid level air and weaken if not dissipate with eastward extent. However, storms that fire in central and southern Nebraska will have much better moisture to work with. These storms will push eastwards along the surface front but an elevated portion of these storms looks to develop along the 800 mb front which will extend to locations along and southeast of a Slayton, MN to Sioux Falls, SD to Tyndall, SD line. As these storms develop, they will encounter much better elevated instability on the order of 2,000 J/kg of CAPE and effective shear values up to about 40 knots. With mean wind flow mostly parallel to the 800 mb front, convection should remain along the boundary as it moves east. This will result in upscale growth into a line that may be capable of damaging wind gusts up to 60 mph. Large hail up to the side of ping pong balls is also possible but is most likely as convection initially develops before congealing into a line. The developed thunderstorms will also poses the possibility of heavy rainfall as PWATS moisten to over 2 inches along with strong moisture transport taking place between 925-700 mb. The heaviest rainfall looks to fall along and southeast of the previously mentioned line between Slayton, MN to Sioux Falls, SD to Tyndall, SD where 1-2 inches of rain is expected with isolated higher amounts to 3-4 inches possible. This will result in rises to the rivers along with the potential for flash flooding given wet antecedent conditions. However, a few of the hi-res models are showing an alternate scenario where convection that develops in Nebraska takes up most of the available moisture which could result in lower rainfall totals then currently expected. Think that this possibility is not as likely given the strong moisture transport from the strong LLJ advecting sufficient moisture northwards. Thunderstorms look to exit the area around 3 am but light showers could persist through the night. A cold front will slide through the area on Tuesday as the previously mentioned upper level wave continues to push into the Northern Plains. 850 mb temperatures in the mid teens will lead to warmer highs, closer to seasonable up to the upper 70s and low 80s. Uncertainty remains in how fast the cold front pushes through the area. If the front pushes southeast of the area, then another round of strong to severe storms will be harder to come by. If the front slows down and does not push past the forecast area, then better moisture will pool ahead of the front contributing to sufficient instability across part of northwest Iowa. The right entrance region of the upper level wave will be parked over the area contributing to enhanced lift and shear over the front. Still too early to say which scenario will play out but do think the possibility of the front pushing through the area faster may be the most likely scenario as the direct thermal circulation from the entrance region of the upper level jet may result in a bit better momentum within the post frontal northwest flow to push the cold front through at a faster rate. Any chance for rain will come to an end Tuesday evening leaving mainly dry conditions for the overnight hours. Low temperatures will fall to the 60s overnight. Surface ridging slides through the area on Wednesday which will keep dry conditions going through most of the day. 850 mb temperatures will warm to the upper teens which will result in seasonable high temperatures up to the mid 80s. Southerly return flow will quickly set up in the wake of the passing surface ridge which will set up another chance for showers and thunderstorms. Ensembles support this as they show high probabilities between near 100% for exceeding a tenth of an inch of rain. However, their probabilities drop to about 30-50% for exceeding a half an inch of rain. The higher probabilities lie across south central South Dakota. Have stuck with model blended PoPs as the highest probabilities also lie across south central South Dakota. Chances for rain continue through the rest of the work week as an active pattern continues aloft. Deterministic guidance hints at the possibility of a closed low developing aloft which could result in a slightly longer stretch of cooler conditions. High temperatures look to be on the cooler side regardless if a closed low develops though with highs only making it up to the mid to upper 70s. Given uncertainty regarding precipitation potential, have again stuck with model blended PoPs. Drier conditions could make a return this weekend as upper level ridging builds into the western CONUS but too early to say if any ridge riding waves will bring continued chances for rain to the area. Highs look to remain near to below average though in the upper 70s and 80s. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1243 PM CDT Mon Jul 1 2024 Scattered showers and storms continue across the region resulting in MVFR to VFR levels. Should see a brief lull in activity this afternoon, with additional development possible this evening. As alluded to in the previous discussion, confidence is highest in locations along and southeast of a line from Tyndall/Sioux Falls/Slayton. A few storms could become strong to severe, with hail and damaging winds being the primary hazards. For now, have only included mention of thunder at KSUX. Will continue to monitor trends and adjust other sites as needed. Cigs during this time look to fall to IFR levels, though brief period of LIFR may still occur. Otherwise, expect breezy southeasterly winds to weaken after sunset. Cold front dives across the region Tuesday, causing winds to shift to the northwest. Another round of precipitation will also be possible, though this will largely depend on how quickly the front advances across our area. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Flood Watch from 7 PM CDT this evening through Tuesday morning for SDZ070-071. MN...None. IA...Flood Watch from 7 PM CDT this evening through Tuesday morning for IAZ012>014-020>022-031-032. NE...Flood Watch from 7 PM CDT this evening through Tuesday morning for NEZ013-014. && $$ DISCUSSION...Meyers AVIATION...SST