Severe weather will be the story in the Northern Plains for the weekend and into Monday as a low pressure system ejects off the Rockies and an upper level low pressure system moves across the Northwestern portion of the United States. The storms through the weekend and into Monday, forced along by this boundary will have ample amounts of convective energy in the form of CAPE (Convective Available Potential Energy) and turning of the winds in the form of wind shear. Wind shear will be helped along each day by a strong low level jet stream that will be increasing in strength daily as the upper level low pressure system approaches from the West. Also, low level moisture will be plentiful allowing for strong moist inflow into the storms. Warmth will be strongest Sunday with surface temperatures reaching well into the 90s.
So, the warmth, moisture, and winds are all there, the last piece of the puzzle for severe weather is the need for a boundary which will break the cap on the atmosphere and cause convection. This is similar to the reaction of breaking the full soda bottle’s cap after the bottle has been shaken. Various models show the general evolution of the Saturday evening storms starting in the northern North Dakota and evolving southeast into southeastern North Dakota as a low level boundary moves southeast across the Northern Plains allowing the cap to be broken. The upper level low pressure system enters the western United States so stronger disturbances are closer the region on Sunday. Storms will be moving across the region from southwest to northeast in the average low to mid-level flow as a result. The stronger boundary on Monday will allow for more thunderstorms along with stronger wind. Temperatures won’t be as high though on Monday, but the added forcing of the cold front will add to the convective energy.
To get an idea of the possible outcomes of the daily convection, analogs of the event can be perused. The Northern Plains are within an area where at least 30% of the analogs to this event have at least 1+ severe report during the weekend and into Monday. According to the analogs, the severe weather on Saturday will be across western and southern North Dakota and into central and northern portions of South Dakota. Sunday’s analogs have the severe weather in the western Dakotas, but still a strong signal moving northeast into the eastern portion of the Dakotas. Monday’s severe weather threat according to the analogs appears within eastern North Dakota. Eastern North Dakota and into western Minnesota has the highest threat for tornadoes during this period according to the analogs.
Often, to approximate the threat of severe weather, the number of lightning strikes within an area can be used. In this case, the ensemble put out by the National Center for Atmospheric Research (NCAR) shows a low amount of lightning strikes Saturday night, but at the end of the run, by late evening, early overnight on Sunday a higher amount of lightning strikes approaches the Missouri River so that appears to be the higher severe threat. In addition, the Storm Prediction Center’s ensemble model has a tornado ingredient parameter showing the possibility of a tornado based on the right ingredients being around indicates a 30%+ chance for a tornado in the Northern Plains Saturday and Sunday nights. In addition to the threat of a tornado, hail, wind and heavy rain, especially within any thunderstorm is possible.
To approximate the strength of the system causing the weather, not only is the severe weather ahead of it a good measure, but also the strength of the cold air behind it. Often, a quick change from warmer to cooler weather creates windy weather and strong wind gusts between 40 and 60 mph are possible in the Northern Plains after the system leaves. Furthermore, with the cold air gushing in behind the system, some left over snowfall is possible in the Northwestern US.