October 22, 2021


Watch Likely


EASTERN, SOUTH DAKOTA - The Storm Prediction Center (SPC) advised that the probability of a watch being issued is likely across the eastern half of the state.  Thunderstorm development has increased including a few supercell storms with large hail being the main threat.

The SPC stated the considerable convective development, with increasing embedded thunderstorm activity, is ongoing in across central, South Dakota. This is near the nose of a strong southwesterly mid/upper jet, still mostly over the southern Great Basin, but with one significant emerging speed maximum now progressing through the Nebraska Panhandle vicinity.

Forcing with this speed maximum appears to be contributing to recently increasing thunderstorms across and northeast of the Pine Ridge and Rosebud Reservation vicinities, where surface observations suggest that there may also be a developing surface low. Models indicate that increasingly divergent upper flow will contribute to lower/mid tropospheric cyclogenesis today through tonight, with more rapid deepening forecast east/north of the Huron and Mitchell vicinities into the upper Red River Valley this evening.

In the nearer term, southeasterly near-surface winds are contributing to gradual boundary-layer moistening to the east/northeast of the developing low, across southeastern into central South Dakota, where dew points are now rising into the lower 60s. It is not clear that moisture will increase much further, but, coupled with mid-level cooling accompanying the emerging speed maximum, thermodynamic profiles may become increasing conducive to severe hail. Based on recent model output and observational data, it appears that this may become focused across and northeast of the Huron/Mitchell vicinity as early as this afternoon.

Thereafter, a continuing gradual increase in forcing for ascent and southeasterly inflow of destabilizing boundary-layer air probably will support further upscale growth, intensification and organization in the presence of strong deep-layer shear. This may include a couple of supercells, with perhaps low-level hodographs becoming supportive of a risk for tornadoes, though stronger south/southeasterly 850 mb jet strengthening is generally not forecast until after dark across western Minnesota.