A potent disturbance is forecast to eject eastwards from the Pacific Ocean this week, reaching the Western United States and eventually moving northeastwards towards the Rockies during the middle of the week ahead. Meanwhile, northern stream energy (associated with the colder, northern jet stream) will be sliding across the Southern parts of Central Canada, just north of the international border. This reinforcing cold air, combined with the disturbance ejecting from the rockies, will lead to the potential for a winter storm in the Northern Plains and Great Lakes late this week.
A low pressure system will eject from the Central Rockies into the Plains states from midweek onward — essentially from Thursday into Friday. This low pressure area is expected to strengthen as it moves through parts of the Central Plains (Nebraska and Kansas) north-eastwards towards Iowa. Forecast models have struggled to pin down exactly where the low pressure center will track, which has led to considerable forecast uncertainty.
In the past 24 hours, however, models have begun to tighten the envelope of possible solutions. Better agreement in terms of the evolution of mid level atmospheric features has led to increased confidence in the track of the developing low pressure system.
What are the key features in the setup?
A very important factor in the storms evolution is a piece of mid level energy — confluent in nature — that sinks across the North Central US during the next 24 hours. This disturbance adjusts the mid level height field ahead of a low pressure system which will be ejecting out of the Rockies and into the Plains states. Models have struggled with the strength and positioning of this feature.
As you can imagine, if this feature is stronger or deeper, the low pressure area which develops will have a tendency to move further south. A weaker or less impressive disturbance of confluent nature will give the developing low pressure system more room to amplify. The GFS and ECMWF have only recently come into better agreement on this features positioning and intensity.
Meanwhile, as the low pressure system develops during the later part of the week, the disturbance aloft also carries its own uncertainty. Models continue to fluctuate in regards to its strength, intensity and speed — all of which will impact the low pressure center and intensity of precipitation that develops.
What about the dynamics of the setup?
As a mid level disturbance moves through the Northern Plains, notable dynamics are forecast to develop in the mid and lower levels. Impressive vorticity associated with the disturbance will move through the aforementioned regions, with a closed mid level low developing during the latter part of the week. The combination of these developments support a low pressure center strengthening as it moves from the Plains states towards Iowa and eventually the Great Lakes.
Meanwhile, the system will also expand aloft and the dynamics on forecast models are impressive there as well. A 250mb jet streak is forecast to develop as the storm system begins to strengthen from Thursday into Friday. With a low pressure center developing in the mentioned regions and an expanding jet streak to its northwest, precipitation will be able to effectively expand to the northwest of the storm system.
In addition to the development of the aforementioned jet streak, and owing to the impressive dynamics in the mid and upper levels of the atmosphere, forecast models are suggesting the development of impressive snowfall rates. Nearly-convective elements to mid level forecast soundings are appearing on both the GFS and NAM model suites over the past 12 hours. With impressive lift and omega in the snow growth region, forecast models are showing impressive snowfall rates.
Where this banding develops, snowfall rates of 2″ to 3″ per hour will be possible. Forecast models currently are indicative that these snowfall rates will develop from Northern Nebraska and Southeast South Dakota into Southern Minnesota from late Thursday Night into Friday morning. The extremely heavy snow combined with gusty winds will produce blizzard conditions at times.
Wrapping it all up, what do we expect?
Gathering all of our ideas of the meteorological evolution, and factoring in forecast models as guidance moving forward, the expectation is that a moderate to significant winter storm will evolve late this week into the early weekend across the North-Central USA and parts of the northern Great Lakes.
The overall track of the storm system will favor a swath of wintry weather across areas to the north and west of the low pressure center, from Northern Nebraska and Southern South Dakota through Southern Minnesota and Northern Iowa (including Minneapolis) and northeastwards into Wisconsin.
Snowfall Map updated at 1:00pm Eastern time February 22nd, 2017.