Severe Weather & Northwest Flow

Warm and dry conditions returned to the Front Range Tuesday,  following an active period of severe weather last week and weekend. Several tornadoes touched down over eastern Colorado, along with numerous reports of large hail and heavy rainfall. 

Photo courtesy of 9News

This photo shows a UPS truck damaged by a tornado near Byers last Friday. The driver of the truck was reportedly taken to an area hospital, but luckily did not sustain major injuries. 

Severe weather can strike Colorado’s Front Range any time during the spring, summer, and fall periods. According to NOAA’s Storm Prediction Center (SPC), Denver’s severe weather season spans May 3rd-September 5th. Severe weather occurrences typically peak during the second-half of May through the first-half of June. 

Although thunderstorms are a relatively-small, short-lived, and local phenomenon, large-scale weather patterns certainly contribute to a higher likelihood of severe storm occurrence. Last week/weekend’s storms were largely the result of high pressure positioned W of Colorado, and an upper-level disturbance arriving under northwest flow.

The synoptic pattern depicted above can be a classic setup for severe weather outbreaks over the northern and central plains. So, what is it about northwest flow that stirs the atmosphere so convincingly? 

Instability

Instability is at the core of any discussion concerning thunderstorms, severe weather, or even cloud formation. Very simply, stability is the resistance of an air parcel to rise vertically into the atmosphere. Under favorable conditions, vertically-rising air expands, cools, and condenses into clouds, thunderstorms, and eventually precipitation. However, stability can vary significantly in the atmosphere. 

We know that warm air rises.

During the warm season, the air at the surface is typically warmer than the air above it. This is why we see more surface-based convection and thunderstorms this time of year. Warmer temperatures also yield higher moisture levels, which further contributes to storm development. The vertical profile of the atmosphere (temperature and moisture) dictates instability on any given day. Sometimes, there is resistance to rising air which can suppress storm formation. A good example of this would be a layer of warm air aloft, or a temperature inversion at the surface or mid-levels. 

Similarly, unusually-cold air above the surface can increase instability, leading to faster ascent, expansion, and condensation. 

Colder air aloft typically accompanies atmospheric disturbances – like centers of low pressure, troughs, and shortwave troughs. These upper-air disturbances are an effort to equalize differential heating from the equator to the poles. Some disturbances are stronger and colder than others. Disturbances arriving from the north or northwest typically feature colder air aloft, which can contribute to higher instability at the surface. 

Image result for unstable atmosphere

Figure courtesy of North Carolina Climate Office, NCSU

In the case of last weekend, an upper-level trough dropped out of the Northern Rockies into Colorado, which helped destabilize the vertical profile of the atmosphere. Furthermore, colder, sub-freezing air closer to the surface also helped contribute to hail growth in the thunderstorms that did form. 

Wind Shear

The second ingredient accompanying northwest flow is enhanced vertical wind shear. Vertical wind shear is either defined by a turning in wind direction with height (directional shear), or a change in wind speed with height (speed shear). Both are important considerations for severe thunderstorm development.

Directional wind shear can be common with upper-level disturbances, especially with moist SE surface winds in place. With the arrival of the disturbance from the northwest, the result is a clockwise-turning of wind direction with height – known as “veering.” In contrast, counterclockwise-turning of wind with height is known as “backing.”

Figure courtesy of Learn Weather

Directional shear helps thunderstorm cells develop a rotating updraft, which is the first stage in formation of a mesocyclone and eventual funnel cloud or tornado. Severe thunderstorms can develop in the absence of directional shear, but true tornadoes are not possible without this detail. 

Speed shear is of equal importance for severe thunderstorm formation. Increasing wind speed with height can produce a tilted updraft. This detail effectively keeps different parts of the thunderstorm separate  – e.g. the updraft vs. the downdraft. This allows for stronger, longer-lived, inflow-dominant thunderstorms that can mature to supercell status. 

Image result for directional wind shear

Image courtesy of the National Weather Service 

Inflow-dominant thunderstorms with strong, tilted updrafts will produce the largest hail. These storms also have the best chance of persisting for long periods of time and impacting large areas. 

In the case of last week, vertical speed shear was strong over Colorado due to a passing upper-level wind maximum (jet streak). The result was long-lived, severe thunderstorms over a wide swath of the northern and central plains. 

Enhanced instability and wind shear can push garden-variety storm cells into severe, supercellular status. The result can be impressive, destructive, and downright scary weather phenomenon.

Severe weather can result from a variety of large-scale weather patterns, and we didn’t discuss other required factors for strong thunderstorm formation – e.g. lifting/trigger mechanisms and moisture. However, northwest flow can simultaneously put several favorable-ingredients on the table, quickly increasing the threat for high-impact severe weather along the Front Range.  

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