A wind event of superlatives
The winds were generated by a severe nor’easter off the Mid-Atlantic and New England coasts. The storm met the criteria of a “bomb cyclone” because of its explosive development off the Northeast coast. Not only did the storm deepen at a rapid rate, but it also was massive. It lingered in the same region, close to the coast, for nearly a day before ejecting southeastward into the open Atlantic.
Ten states were simultaneously under a high-wind warning Friday, which was issued for sustained winds of at least 35 mph, or gusts exceeding 55 mph.
The storm was held in place by an enormous blocking high pressure system over Greenland. Its long duration was a key factor in its destructiveness
For the Washington region, the storm produced high wind warning conditions for nearly 36 hours. High wind warnings, in general, are rare here; many years go by without one issued. Many locations experienced 12 or more consecutive hours of 50-plus mph wind gusts — including Washington Dulles International and Reagan National airports. A timeline of hourly wind observations at National is shown in the following image.
For several hours in the early afternoon, the sustained wind exceeded tropical storm force (39 mph threshold).
In many locations, wind gusts were derecho-intensity (65-plus mph) including a 71 mph gust recorded at 11:36 a.m. at Dulles. In fact, this tied a 71 mph gust recorded during the infamous 2012 Ohio Valley-Mid-Atlantic derecho. A map showing the wide spatial distribution of damaging wind gusts is presented below.
Several factors contributed to the wind’s ferocity
Shortly after midnight, the winds ramped up exceptionally fast. Many people were awaked by the visceral roar and booming sound of these gusts. At this time, a low-pressure system tracking across Pennsylvania and its attendant cold front surged across our region.
Behind the front, atmospheric pressure rose very rapidly, at the rate of three to four millibars (mb) per hour. At the same time, a newly developing coastal storm was forming over the Delmarva, with pressure dropping at a rate of two millibars per hour. The difference in pressure tendency values (five to six mb per hour, total) accelerated the airflow (this is called the isallobaric wind). As shown below, this effect contributed to a swift ramp-up of wind speeds that would continue into the day.
The rapidly deepening coastal cyclone took over and dominated our region’s wind field for the next two days. The storm’s pressure dropped more than 24 millibars in 24 hours, meeting the criteria for a meteorological bomb. At the peak of its intensity, the central pressure of this massive vortex reached 974 mb (average sea level pressure is 1,013 mb). The next image shows the incredible pressure gradient (change in pressure over distance, revealed by contour lines called isobars).
But it wasn’t just the extraordinarily low pressure that impelled our strong winds. To the west, a more subtle pressure feature called a ridge was building across the Ohio Valley. The ridge pressure climbed to 1,032 mb. The total difference in pressure between the core of the ridge and the nor’easter approached 60 mb, an astounding value in its own right. But the local pressure gradient was most intense along the New England coast, where isobars were extremely crowded together, driving 40 to 50 mph sustained winds. Over Washington, the local pressure gradient was sufficient to deliver sustained winds in the 30 to 40 mph range.
The sustained wind creates a steady and stressful force on trees and structures. But it’s the buffeting of wind gusts — the suddenly high acceleration, then deceleration — that create damage. How do gusts in the 50-to-70 mph range arise from steadier winds? One process is turbulence, as the wind interacts with friction-generating surfaces and structures close to the ground. Another key factor is mix-down of momentum from higher up in the atmosphere.
What this means is that the more turbulence, the more the air layer is stirred. Wind speeds commonly increase with altitude in a large vortex such as a nor’easter. As shown in the image below, there was a pocket of 70-plus mph wind between 3,000 and 5,000 feet above the surface, arcing along the back edge of the storm. Big, deep swirls of turbulent airflow mixed down some of this high-momentum air to the ground, causing periodic bursts of higher wind speed, i.e., the peak gusts superimposed on the steady wind.
Early Friday morning, wind gusts dropped into the upper 40s beneath a thick cloud layer. But then the sun broke out during the late morning. The air layer above the ground warmed and became weakly unstable. Rising thermals helped to more vigorously stir the deeper air layer, mixing down momentum from an even higher altitude. The surface wind gusts responded by surging to nearly 60 mph from noon until sunset before rapidly diminishing through the evening. A special weather balloon launched at 1 p.m. from Dulles revealed that the depth of this unstable “mixing layer” was nearly 7,000 feet, where 70 mph winds also presided.
Below, see some of the evidence of this wind storm, which will long be remembered in our region.