2PM Day 3 Convective Outlook for Saturday, June 28. THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SATURDAY AFTERNOON INTO SATURDAY NIGHT ACROSS PARTS OF CENTRAL AND EASTERN SOUTH DAKOTA NORTHEASTERN NEBRASKA MUCH OF SOUTHERN AND CENTRAL MINNESOTA ADJACENT NORTHWESTERN WISCONSIN
SUMMARY
Scattered strong thunderstorms, posing a risk for severe hail and wind, are possible across parts of the middle Missouri Valley into Upper Midwest Saturday afternoon into Saturday night.
Discussion
Little change in the large-scale mid/upper flow is forecast from Friday into Saturday, with modest westerlies remaining generally zonal and confined to the northern tier of the U.S. and adjacent portions of southern Canada. Within this regime, one notable short wave trough is forecast to progress east of the Great Lakes into New England Saturday through Saturday night, as broad upstream troughing (with at least a couple of embedded smaller-scale perturbations) progresses eastward along the north central Canadian/U.S. border vicinity.
In lower-levels, cooler and drier boundary-layer air is likely to be maintained across parts of northern New England, with another influx of cooler air spreading southeast of the Great Lakes through the upper Ohio Valley, northern Mid Atlantic coast and southern New England by late Saturday night. A trailing cold front is likely to continue slowly advancing east/southeast of the northern Rockies, through the northern Great Plains. Elsewhere, east of the high plains, seasonably moist boundary air may become characterized by moderate to strong potential instability with daytime heating.
Northern Great Plains
The extent of convective development and its evolution remain unclear for this period due to a number of lingering uncertainties. Focus for renewed thunderstorm development could be impacted by outflow from convection developing Friday night into early Saturday. Additionally, strongest potential instability by Saturday afternoon is likely to become focused beneath the nose of the warmer and more strongly capping elevated mixed-layer air, roughly across the northern/eastern South Dakota vicinity, before slowly becoming suppressed southeastward. However, this might tend to become displaced to the south of the stronger and more favorably sheared westerlies.
Even so, given thermodynamic profiles becoming characterized by steep lower/mid-tropospheric lapse rates and large CAPE, scattered strong to severe thunderstorm development appears possible ahead of the southeastward advancing cold front by late afternoon. Aided by forcing for ascent supported by low-level warm advection beneath broadly difluent upper flow, activity may consolidate into one or two eastward/southeastward propagating clusters by Saturday evening.
Northern Mid Atlantic into Champlain Valley
A corridor of moderate to large developing mixed-layer CAPE, ahead of the southeastward advancing cold front, and within preceding deepening surface troughing, seems likely to provide the focus for vigorous thunderstorm development by late Saturday afternoon. Although lapse rates may be modest, and stronger potential instability across the northern Mid Atlantic may be displaced to the south of the better flow/shear across Upstate New York into New England, the environment may still become conducive to scattered strong thunderstorms accompanied by a risk for potentially damaging wind gusts. If/when lingering uncertainties become more clear, it is possible that severe probabilities could still be increased a bit further in later outlooks for this period.