Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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453 FXUS63 KFSD 210853 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 353 AM CDT Sat Sep 21 2024 .KEY MESSAGES... - Rainfall amounts continue to trend down for today through Sunday, with only low chances (20-30%) for a few hundredths of an inch of rain through the Woodbury to Clay counties corridor. - Northwesterly winds will increase today behind a cold front, gusting 30 to 40 mph into this evening. - With the passage of a cold front, temperatures trend downward to below normal levels for Sunday and Monday. - Temperatures trend back to near normal levels for next week, with a limited chance of precipitation through the period. && .DISCUSSION... Issued at 350 AM CDT Sat Sep 21 2024 In a strong southerly flow with high pressure to our east and a surface trough to our west, it has been a very mild night across the area with current temperatures in the mid 60s to lower 70s. Not expecting temperatures to fall a whole lot more through the night - bottoming out in the 60s near daybreak. The main story for today continues to be the passage of a cold front through the area - presently located over central SD very early this morning. As has been the trend over the past few model runs, the impacts from this front continue to lessen - at least in terms of precipitation chances. Latest models have the front entering the western portions of our CWA between 09Z to 12Z, then sliding eastward through morning and afternoon, exiting our area around 20Z. Current hi-res models produce very little precipitation with the front - with the better forcing well to our north in association with an upper level trough moving across ND during the day. Related to this, current forecasted precipitation amounts during the day are only a hundredth or two of an inch in the Spencer to Storm Lake IA corridor. Latest HRRR and RAP soundings indicate capping up to around 650 mb over that area in the afternoon, which lend support to storm chances being very low. Even so, SPC continues to hold onto a marginal risk (level 1 out of 5) for severe storms (large hail/damaging wind) - mainly affecting Buena Vista and Ida counties, though again this looks to be an unlikely scenario. The bigger impact may be the winds behind the front. With strong cold air advection, northwesterly winds are expected to increase with the passage of the front, with winds gusting 30 to 40 mph at times into the evening. 850 mb temperatures will be decreasing from northwest to southeast through the day, and we are looking at highs in the lower 70s from east central SD to south central SD, to the lower to mid 80s through northwestern IA. Slightly better - albeit still low (20-30%) - chances may come tonight over our far southeastern IA counties (Woodbury to Buena Vista) with possible shower activity along an elevated boundary as an upper level shortwave tracks through NE into southern IA. Even so, deterministic models and ensembles support only a few hundredths of an inch of precipitation at most. With cold air advection continuing tonight, temperatures will be at or just below seasonal normals, dropping into the mid 40s to lower 50s. Strong surface high pressure then builds into the Northern and Central Plains on Sunday. Although we currently carry a low chance (20%) for precipitation south of Highway 20 as an upper level low lifts into NE during the day, with the exception of a couple hi-res models, most keep it dry. This is supported by ensembles which indicate only around a 10% probability of receiving a hundredth of an inch of rain over that area. With an 850 mb thermal trough (7-10 degrees C) overhead on Sunday, it will be cooler with highs in the mid to upper 60s. High pressure drifts to the east on Monday, and with warm air advection in a returning southerly flow, temperatures will warm slightly - into the upper 60s to lower 70s. For the remainder of next week, temperatures will slowly warm back to normal to just above normal levels through the week. This as upper level troughing gives way to some ridging by the end of the week. Unfortunately, current guidance gives little hope for any significant rainfall, with current ensembles suggesting a less than 20% probability of our area receiving .05" on any one day for Monday through Friday. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1018 PM CDT Fri Sep 20 2024 Mid-lvl moisture continues to slowly arrive late this evening. As the LLJ and a weak wave approach from the southwest, some isolated elevated convection will be possible over eastern SODAK after 3am. This aforementioned LLJ may also bring a period of LLWS through daybreak. At the same time, a surface front will plunge southeast in the morning, likely reaching KSUX/KOTG by mid-day, and then exiting the CWA by 3pm. Behind the front, lingering mid-lvl clouds may be possible, but more impactful is the gusty northwest winds over 25 mph at times. Most of the area will stay dry on Saturday, with some elevated post frontal shower development near Highway 20 after dark. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...JM AVIATION...Dux