Strong cutoff low will be the main weather story this week

A strong upper level storm system will cutoff from the main atmospheric flow/jet stream this week over the Central US, becoming the first strong cutoff low of the autumn season. These systems are typically observed in the transition seasons of Autumn and Spring, and can bring a plethora of weather impacts. One of the signature features in a cutoff low is the entrenchment of cold air, especially in northern stream systems which drop south out of the Pacific Jet and/or Canada. This one, coming from Western Canada, will dive south and east into the mean 500mb low over the Central US and briefly cutoff before being lifted northeast by a generally progressive pattern.

ECMWF forecast model showing the strong cutoff low at 500mb (mid levels of the atmosphere). Notice the upper level low is “cut off” from the main atmospheric flow.

Technical Stuff: From a meteorological standpoint, cutoff low events aren’t overly rare. Yet, they remain impressive each time they occur. The strong surface low developing in response to this system is expected to develop and curve northwest over the North/Central US, while the cold air pool dives south of it and brings below normal temperatures and mid-level height anomalies (colder than normal averages). The result will be potential severe weather on the far south and east periphery of the system, and the potential for isolated frozen precipitation on the north/northwest fringes. The 12z Euro from October 17th shows the system very well, with the mid level heights displaying the upper level low cutoff from the main height field.

What’s it mean for us?: In our area, the system will bring a chance of showers and thunderstorms beginning late Thursday Night and continuing through Friday. There is isolated potential for a strong thunderstorm or two, given the very impressive shear from the upper level cutoff low to our west. However, meager instability is expected to generally mitigate the threat. We can’t completely rule out an isolated strong wind gust or maybe a brief spin up/weak tornado if all the pieces fall into place. Either way, the weather will be unsettled from Thursday Night through Friday, and continuing into Saturday before the cold front moves through. Thereafter, the cutoff low is expected to shift North and East away from the area.

Keep reading for more information on the cutoff low, as well as a peek into the winter season…

Are these common in winter, too?: Yes and no. They can happen in the winter season, but generally the atmospheric pattern supports these types of systems in autumn and then again in late winter into spring. They can happen, though, especially if the Polar Vortex gets involved. A Polar Vortex anomaly occurs when a piece of the Polar Vortex drops south of it’s usual position, and it can often cut off from the mean atmospheric flow and provide unusually cold air. Just for the sake of comparison, a cutoff low like the one we’re seeing this week would still provide our area with a surge of warm air in the winter as well. The warm air is forced north on the eastern side of the cutoff low, just as the cold air is forced south on the west side of it. If you were hoping for snow out of this pattern in the winter, we would need the system to be farther east.

Can we expect more of these? Probably. Typically we experience a few cutoff lows per year in the autumn. Additionally, the pattern were in now features lots of blocking across the Northern Hemisphere. Which means the pattern is prone to larger systems being forced south. I’d bet my money that we see another cutoff low over the US sooner than later. Where it ends up is a whole other story.

 

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