Original post from early afternoon
The widespread nature of Tuesday’s storms and their intensity were unexpected, taking seasoned meteorologists by surprise. Microburst winds even downed numerous trees in a few neighborhoods.
Today’s weather setup is similar, so we expect another round of potentially strong storms between 3 and 9 p.m., with a few severe storms possible. Not everyone will experience storms, but for those that do, prepare for locally torrential rain, intense lightning and the prospect of strong to damaging wind gusts.
What happened Tuesday?
Meteorologists knew that the cooler, drier air mass of the weekend and on Monday would begin to transition to a warmer and more humid setup. This was courtesy of high pressure moving off the East Coast, allowing a northerly wind to become southerly.
The greatest potential for storms appeared to be to the north and west of D.C., over elevated terrain. There, warm air rising up mountain slopes offered the best prospects for triggering convective cells.
A few of these cells amalgamated into a fast-moving cluster that ripped through Montgomery County, causing pockets of tree damage there. This first round came during late afternoon, and this cluster pushed through to the Chesapeake Bay, triggering a number of severe thunderstorm warnings.
This cluster depleted some of the energy that had built up during the afternoon, but not enough to impede a second, more widespread line of storms that developed during the early evening, sweeping across the greater D.C. region, again triggering warnings.
The extent of wind damage left behind by these two systems was impressive, given a morning forecast that portended only a slight chance of storms, with the best chances north of Washington.
The wind damage occurred in a number of pockets in the immediate area from localized downburst winds or microbursts. These are blasts of winds that originate in the clouds and slam to the ground, fanning out in all directions with sometimes violent gusts.
Downed trees were reported in eastern Fairfax, central Montgomery and northern Prince George’s counties. The Fairfax County zone, from Fairfax to Annandale and Falls Church, was hardest hit, with many reports of downed trees and power poles, including some large tree limbs on homes.
During the morning forecast for late afternoon, we examined many parameters, including morning surface and upper-air observations, model guidance that specifically resolves convective (small-scale) motions, and we tap into technical discussions from the National Weather Service and the Storm Prediction Center.
What should have been an afternoon with a few, isolated “pop-up” storms rapidly transitioned “upscale” into clusters of storms, which then coalesced into a large, linear system. How can this happen?
At times, scattered cells initiating in the same general, small area can “self-organize” by virtue of their cool pools — the chilly pockets of downdraft air — which merge and begin to behave like a small version of a cold front. This leads to sustained growth and amalgamation of cells into a larger storm system.
A north-south-oriented, elongated zone of low pressure east of the mountains, over the Piedmont and coastal plain (termed a lee trough), may have also helped organize scattered cells and clusters into a line, which took on similar orientation.
Finally, there are hints that a weak, upper-level disturbance slipped across our region — one that was not captured by all the models and that evaded detection by our network of weather balloon stations.
The sum total of these effects may have created an uncommon “perfect storm,” and a day that overproduced compared to forecasts.
What will later today bring?
The atmosphere is in the process of “reloading” after having much of its buoyant energy depleted Tuesday. Sunshine, warm air arriving from the south and increasing humidity will all conspire to strongly destabilize the atmosphere late today.
Once again, there is a ripple of energy in the upper levels that is expected to help initiate storms.
Compared to Tuesday, a lee trough is present; however, its location has shifted farther east, now along the Atlantic coastline. The surface winds over the District are now more from the west-northwest. If this pattern persists through the afternoon, it could limit triggering and coverage of storms; as air descends the eastern slopes (lee side), it dries, and this tends to suppress convection over our region.
The winds aloft are weaker than Tuesday, and the wind shear — the increase in speed with altitude — is also quite less. Storms will tend to drift slower. Wind shear helps to organize individual cells into larger, stronger clusters. Lacking the shear, today’s storms will be what we term “pulse” type: Short-lived, singular entities.
But given a very unstable atmosphere, even pulse storms can pack a punch. A lot of the instability concentrated near the ground raises the chances of damaging downdraft winds. High-moisture content and sluggish storm motion raise the prospects of locally very heavy rain. The lightning in a pulse storm can be briefly very intense.
The Storm Prediction Center has placed our region in a Marginal Risk, a Level 1 out of 5 in terms of overall severe threat. This translates into a 5 percent risk of damaging wind within 25 miles of a location.
Photos from Tuesday’s storms