Tree down, April 2, 2016, D St., NE D.C. (Phil Yabut)

Aside from Snowzilla and perhaps the Presidents’ Day snow and ice storm, the severe wind storm that hammered D.C. Saturday night-Sunday morning was the most disruptive weather event in the past six months.

With a sudden roar, winds gusted to 60-70 mph, accompanied by brief heavy rains and plunging temperatures. Downed trees and broken limbs were common around the region, and during the event’s peak, more than 40,000 customers lost power.

The cause was not a potent line of thunderstorms, nor a powerful Nor’easter, rather a cold front attended by strong winds in the lowermost atmosphere. In this article we examine the processes behind the frightening wind storm.

A strong cold front sweeps through

We recently showed that March is our region’s windiest month and last year I wrote a piece describing the meteorology behind big wind storms.

The figure below reveals the approach of a potent cold front from the Ohio Valley, early Saturday evening. The low pressure it’s attached to (red “L” in western New York state) was not particularly intense (986 mb) nor was the high pressure center dropping southeast out of Canada particularly strong. Between them, the pressure difference between them was modest, and only a contributing factor to the extreme northwesterly winds behind the front.

(National Weather Service)

What helped moved the air was an unusually strong pressure surge, or rise in surface pressure, behind the cold front, approaching two millibars per hour. Additionally, a pocket of fast moving air over the Mid-Atlantic was strengthening aloft.

Winds at the 5,000 foot level increased to 70 mph. With very cold air moving in aloft, this made the atmosphere unstable, such that air began overturning, or mixing, from the surface to 5,000 feet. This brought down high momentum air in periodic blasts or “eddies”, leading to some exceptional gusts.

These gusts are captured in the graph below, showing hourly observations of sustained wind and peak gust at Washington Reagan airport.

(Jeff Halverson)

But the one defining characteristic of this wind event was its hammer-like blow upon arrival, around 10 p.m. This is clearly seen in the figure above, with wind gusts surging to 60-65 mph at area airports. This characteristic required a different type of process.

The line of convective showers

The general area of showers that crossed the Ohio Valley blew up into an intense line of shallow convective cells, once the front raced down the eastern slopes of the Appalachians. A radar snapshot at 10:30 p.m. is shown below. This was a squall line, albeit a bit disjointed, with a separate band of storms over Baltimore.

The instability which energized the storms was miniscule, but the wind shear (increase in winds with altitude) was significant, a combination that can still breed strong convective storms, particularly during the cool months. The storm cells did not become tall enough to produce lightning activity. But they contained strong downdrafts.

(RadarScope, adapted by Jeff Halverson)

The downdrafts acted to pump all that high momentum air aloft, straight to the surface, where it struck the ground and blasted toward the east-southeast. The right panel (above) is a Doppler snapshot, showing air velocity. Pink and red colors represent winds flowing away from the NWS Sterling Doppler radar. The white ellipse encloses a region of 70+ mph winds. This type of violent outflow, emanating from convective cells, is called a downburst, and it etched a swath of tree and structural damage from Loudon to Fairfax Counties.

Mapping the strongest winds

The figure below, created by Jordan Tessler, shows the geographical location of peak wind gusts reported to the National Weather Service. The pattern of gust intensities shows no systematic pattern. The exception is a small clustering of severe (greater than 58 mph) gusts over Alexandria and central Prince George’s County – along the track of the aforementioned downburst.

(Jordan Tessler)

Comparing recent high wind events

The widespread and intense nature of Saturday night’s winds reminded many of the June 29, 2012 derecho – which also struck the D.C. region between 10-11 pm. CWG’s Ian Livingston came up with some comparisons between recent, high impact wind storms across the D.C. region.

(Ian Livingston)

Shown are official, peak gusts at the region’s major airports, comparing Saturday night’s front with Superstorm Sandy (2012) and the Ohio Valley-Mid Atlantic derecho of 2012. All three storms led to widespread tree damage and utility outages, but the derecho takes the prize; it created 9 million power outages from Chicago to the Chesapeake Bay. The derecho was a violent line of thunderstorms, tapping an explosively unstable air mass. Trees were in full foliage and the wind speeds at the airport sites likely under-sampled smaller regions of much higher wind gusts, in the 80-90+ mph range.

Severe, widespread wind storms rake the D.C. region throughout the year, with many causes (cold fronts and arctic fronts, Nor’easters, hurricanes, derechos). Our utilities remain vulnerable, and with increasing population density and expanding infrastructure, that vulnerability will only increase in coming years.

Storm damage pictures

Below is a sample of wind damage pictures submitted by our readers (thank you!) from around the region…

Tree down in McLean, Va., April 3, 2016. (Won Kim)

(Jason Samenow contributed to this post.)