Explaining the cutoff low: Washington’s gloomy weathermaker this week

It’s been accused of missing the bus, but that’s okay, because it’s seldom welcome; in fact, it’s annoying. Some compare it to a drunken relative who won’t leave the dinner table. No matter the description, cutoff lows – like the one Washington, D.C. encounters this week – meander through the jet stream, plaguing those in their path with days upon days of cloudy, unsettled weather.

Indeed, the scene will be murky and dismal through midweek, though it could be worse. Cutoff lows sometimes move more slowly and develop more strongly as they approach our area, spreading monsoon-like rains up along the Eastern Seaboard.

Expected rainfall totals of 1 to 1.5 inches locally and a slate-gray sky will make for challenging commutes to work and may evoke a “May swoon” feeling. If this cutoff provokes such crestfallen reactions, shouldn’t we at least get to know a little more about why it does what it does?

A number of stages define the life cycle of a cutoff low (as described in this study: Climatological Features of Cutoff Low Systems in the Northern Hemisphere). First, in the upper levels of the atmosphere, strong wind currents blow a greater than normal amount of cold air into a dip in the jet stream, otherwise known as a trough of low pressure. The low pressure wave, or trough if you will, intensifies as a result of the wind blowing in colder and colder air (this increases the temperature difference within the wave, leading to a clash between warm and cold air and increasing instability, from which the pressure lowers even further).

upperlow-3

Evolution of the cutoff low pressure system, starting last Thursday morning and ending 48 hours later on Saturday. System drops in over the Plains as an open wave, or trough (first image). Note the westerly wind barbs cutting across the red, dashed lines – known as isotherms, or lines of equal temperature. This perpendicular movement of colder air from one area – the Dakotas and Nebraska – to another point downstream via the wind flow (called “cold air advection” in meteorology) enables the trough to intensify. The wave begins to detach from the jet stream as the normal poleward and equatorward transport of warm and cold air, respectively, shut off (second image). Low pressure fully cuts off over the Ozarks at the southern end of the trough, the northern end of which splintered off into Canada (third image).

At some point, the wave starts to separate from the jet stream. The normal interplay of cold air sinking south from the polar region and warm air rising northward out of the subtropics begins to shut off, leaving a cold area of low pressure behind. That low takes on a closed circulation where the southern part of the trough – since detached from now-independent jet stream – had formerly been located. After moving sluggishly for days, the cutoff low merges with the main steering flow and picks up speed as it chugs away from the area in which it caused so much angst.

The cutoff low will make its closest approach to Washington on Wednesday (left image) before speeding up and kicking out to sea into Friday (right). At midweek, the low remains an independent entity, fully separated from the jet stream over Canada. New energy diving into the Canadian trough begins to “reel in” the cutoff system over the Northeast in the late week period.

The cutoff low will make its closest approach to Washington on Wednesday (first image) before speeding up and kicking out to sea into Friday (second image). At midweek, the low remains an independent entity, fully separated from the jet stream over Canada. New energy diving into the Canadian trough begins to “reel in” the cutoff system over the Northeast in the late week period.

This week’s storm epitomizes the aforementioned life cycle, having germinated as an open low pressure wave (or again, as a trough) over the central U.S. four days ago before maturing into a closed circulation center and, finally, cutoff low over the Ozarks last Saturday. The system – currently spinning along the Tennessee-Georgia line – will translate across the Carolinas, passing just to our southeast on Thursday. Thanks to a moisture-laden easterly flow from off the Atlantic, any part of our area could see heavy rain bands at any time from late Tuesday through early Thursday.

2-lows

As of this writing, the cutoff low is spinning just east of Tennessee (water vapor imagery at left depicts the telltale circulation center, along with an arc of enhanced moisture and heavy rains from Illinois to off the Carolina coast, where thunderstorms rumble). Also note the brown stream – denoting the dry slot – extending from the western Gulf of Mexico to northern Florida). By Wednesday morning, a heavy rain band or two could align over the area (GFS forecast graphic at right).

The cutoff will also live up to its slow reputation, covering a distance of roughly 925 miles from near Memphis to the Virginia coast in 96 hours from Sunday to Thursday morning. That’s an average speed of 9.64 miles per hour. Hurricanes move a bit more quickly on average, while a large raindrop falls from the sky about twice as quickly as the cutoff low is moving.

Four days – starting today – may seem like an eternity in what Matt Rogers refers to as “Operation Slow Down Summer.” However, the dryness of recent months will keep the flood threat tamped down, and we won’t be dealing with the record cold and snow from the system that beleaguered the Central and Southern states. Consider that a small consolation in this spring that can’t shed the wrath of winter.

Also on Capital Weather Gang

D.C. area forecast: Sun vanishes, clouds and showers take control