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Forecast Discussion for Aberdeen, SD
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292 FXUS63 KABR 170707 AFDABR Area Forecast Discussion National Weather Service Aberdeen SD 207 AM CDT Mon Aug 17 2026 .KEY MESSAGES... - There is a Slight Risk, level 2 of 5, for isolated to scattered severe storms this afternoon and evening along and east of a line from Sisseton to Murdo. Main threats are large hail, 1 to 2" in diameter, and wind gusts of 60 to 70 mph. A tornado or two is possible south of a line from Murdo to Watertown. Brief heavy rainfall is also possible. - There is a Marginal Risk, level 1 out 5, for isolated severe storms along and south of a line from Pierre to Brookings Tuesday afternoon through Tuesday evening. Main threats include 1 inch diameter hail and wind gusts of 60 mph. Brief heavy rainfall is also possible. && .DISCUSSION... Issued at 206 AM CDT Mon Aug 17 2026 As of 2am, a decaying storm is exiting far south central SD with additional convection moving in over north central SD. Current temps range in the upper 50s to the upper 60s and winds light. By 12Z models have a good handle on a positive tilted trough over ND extending northeastward to a stronger surface low, and its upper low, over the Hudson Bay area. On the southern extent of this trough, the weaker surface low is forecast to be over Canada/ND border with its weak warm front extending through central SD as the high pressure system that was overhead continues to shift off towards the southeast. By this afternoon and into early this evening, the center of the low will track southeast and over ND/SD/MN border (00Z) with the warm front turned surface trough extending southwestward through northeastern SD into western NE (to a weak surface low here) and its cold front draped across ND/SD border. Late afternoon through this evening is when we start to really see an increase in shortwave activity and an increase in the 500mb winds, between 40-55kts, with the CWA on the southern extent of this UL system in Canada. Latest run of the HREF ensemble mean indicates the potential for spotty convection over portions of north central through northeastern SD/MN into ND this morning through the early afternoon, however, there are differences in timing and exact location of this with some of the CAMs not showing much at all due to a slight cap till about 15Z. So low confidence on outcome and therefore stuck mainly with the NBM. We will see the potential in increased activity around ~20Z where scattered showers and thunderstorms are forecast to fire up (northeast to southwest over the CWA) with the convection increasing in coverage (and potential strength) along and near the surface trough from south central through northeastern SD/MN through the early evening. Both HREF/NBM locks into the bulk of the precip and pops over northeastern SD/MN during this time. Beyond ~02Z, the trough will be more to our southeast with the bulk of the storms over southern to southeastern SD into MN. Individual CAMs do differ on storm mode whether its more of cells merging into clusters (FV3/HRRR/NAMNEST) or more of a line of clusters (HRW NSSL). HRRR and NAMNest also have cells developing or moving in over south central SD from the Black Hills. So overall low confidence in exact storm type and location and which scenario will play out so again went with the NBM for now but added in 20% pops over central SD where NBM had none. Closer to time, HREF or Conshort can be added to the pop grids. Surface dewpoints will range in the 60s to potentially lower 70s over the eastern CWA with highs overall in the 80s creating warm and moist conditions at the lower levels. The greatest instability of 1000-2000j/kg of CAPE will be over the eastern half of SD into MN with the highest values over southeastern SD into NE by the evening time with potential max values above 3000 j/kg per HREF. RAP is a bit higher on the instability between 2000-3000j/kg of ML/MUCAPE with CAPE decreasing with sunset. Wind shear will range between 30- 40kts with an increase in shear to 50kts+ after 00Z. Mid level lapse rates will also range between 6-7C/km. Therefore, any storm has the potential to become severe especially over the eastern half of SD where the higher shear and instability lies, especially along the northeast to southwest oriented trough. With the bulk shear and steep lapse rates supercells will be able to form along with the clusters as mentioned. If they do form, splitting supercells are possible given the long and straight hodographs (especially over NESD/MN where the better overlap of Cape/Shear). SPC did add a 2% tornado risk south of a line from Murdo to Watertown, however, as mentioned hodographs seem to lack low level curvature with 0-1KM SRH around 100m2s2 or lower, 0-1km shear below 15kts, and STP below 1.The greatest potential looks to be more south of the CWA. However, you cannot rule it out especially when the atmosphere has the potential to produce supercells along the boundary and lower LCLs. So due to the severe threat, the SPC expanded the Slight Risk (level 2/5) and now is along and east of a line from Murdo to Sisseton where the potential for larger hail (1-2") is possible and wind gusts of 60-70 mph. There is a Marginal Risk (level 1/5) along and west of a line from Sisseton to Murdo to a line from Eagle Butte to Eureka with the potential of quarter size hail and wind gusts of 60mph. Brief heavy downpours are also possible in both the marginal and slight risk areas. The cold front will track a bit southward overnight and extend horizontally across central through east central SD by 12Z before stalling out. Additional chances of showers and thunderstorms are possible tonight/overnight as CAMs suggest more isolated or clustery storms passing over the CWA during this time. A lack of a LLJ will keep any storms long lived, but as mentioned, storms will have 50kts of bulk shear to work with so an isolated severe threat may continue. Additional spotty convection is possible again with daytime heating and instability Tuesday afternoon and evening. Isolated severe storms are possible near/ahead the front, as SPC highlights a Marginal Risk (level 1/5) along and south of a line from Pierre to Brookings. Main threats include quarter size hail and wind gusts of 60 mph. Another shortwave moves in from the northwest interacting with a surface boundaries bringing additional chances of showers and storms Wednesday. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1223 AM CDT Mon Aug 17 2026 Terminals KABR,KATY,KPIR,KMBG VFR conditions are expected to prevail at all terminals through this TAF forecast cycle. KABR could see a passing shower later this morning after sunrise. Light and variable winds through the early morning will eventually be followed by southwest to northwest winds during the daytime hours today. && .ABR WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. && $$ DISCUSSION...MMM AVIATION...Vipond |
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