textproduct: Aberdeen

KEY MESSAGES

- Slight Risk (2 of 5) mostly focused across far northeast South Dakota and west central Minnesota Sunday afternoon/evening. Marginal Risk for severe weather (1 of 5) includes parts of the northeast and central South Dakota. Main threats are large hail and a tornado or two.

- Colder air returns Monday through Wednesday, with high temperatures around 15 to 20 degrees below normal. Temperatures Wednesday morning may drop to near or below freezing, bringing the potential for frost.

DISCUSSION

Issued at 120 AM CDT Sun May 17 2026

Light shower activity associated with the first of several southwest flow waves moving in from western South Dakota. This activity is expected to slowly spread across the region through the morning hours, though its facing some very dry air in the low levels with a 20 degree dewpoint depression. Current radar returns are a few hours behind model onset timing. Most of this will come up through the Missouri valley, with lesser rainfall potential across the eastern CWA through mid-day. Profiles indicate skinny CAPE and weak mid level flow and that pesky dry subcloud layer. HREF precipitation with this initial round of moisture is around 1/3-1/2" west of Mobridge and a tenth of an inch or less for the rest of the CWA.

In regards to instability later this afternoon, CAMS show a decent gradient that runs up through Huron towards Sisseton. Each has a slight variation in regards to this, but are generally consistent with a peak instability around 4pm after which is a rapid shift east. About that time 0-6km shear is about 50kts, which is in line with previous discussions and is dominated by the strong mid level unidirectional flow. For Watertown, NAM/GFS BUFKIT profiles are still consistent in indicating a stable boundary layer and possible stratus. The RRFS RAP and HRRR are a little more unstable with the boundary layer. This is thanks to southeasterly low level flow in the lowest 1-2km. So the main question is whether we get surface based convection or elevated convection. Either way, storm mode continues to be fast moving/northeast moving supercells, maybe splitting storms thanks to the unidirectional shear above the BL. Surface based convection would favor right moving and increase the tornado threat for the southeast Sisseton hills (Deuel/Hamlin/Codington/Grant) and points south IF IF IF we can get low level destabilization. In that regards most of the CAMS bring in stratus with the gradient into our western Minnesota counties. An yet more evidence for a warm advection/elevated convection is the HRRR paintball >40dbz very much highlights the northeast but on the north end of the instability gradient. The one wrinkle to the convective mode is that the RRFS is a little more on a wind threat kick, and several of its members present with a blowing type system. Still not certain this should be the outcome given the lack of dry mid level air in a southwest flow regime.

Next round of moisture comes up from the southwest late in the day Monday. Profiles are more indicative of strato-form rain, maybe some weak instability. NBM mean QPF is only a tenth or two which is a drop from the previous run, in line with the GFS, however the Canadian and NAM both have a little higher QPF with a deformation band lining up across central South Dakota and its possible the NBM ensembles are smoothing this feature out.

NBM is still latching onto the frost potential for Wednesday morning. Deterministic lows are still in the low-mid 30s. 75th percentile for Aberdeen is only 33F, and 34F at Pierre/Mobridge. Will maintain frost messaging.

Low moisture chances and seasonal temperatures as the upper flow shifts back to zonal for late in the week/next weekend.

AVIATION /06Z TAFS THROUGH 06Z MONDAY/

Issued at 1217 AM CDT Sun May 17 2026

Terminals KABR,KATY,KPIR,KMBG

VFR conditions start out the period, with MVFR ceilings over most locations by around 13Z Sunday as a 60%+ chance of showers moves in. Winds will continue to gust out of the east through 13Z, before diminishing and shifting out of the north (PIR/MBG), and ABR by 21Z, and by 00Z at ATY as the surface low shifts north and resides over southern MN. Thunderstorms remain possible, especially Sunday afternoon over eastern SD.

ABR WATCHES/WARNINGS/ADVISORIES

SD...None. MN...None.