Severe convective storms are among the most challenging weather phenomenon to predict. They are inherently small scale, short-lived phenomenon, compared with cold season snowstorms, such as Nor’easters, which impact many states for up to 24-36 hours. Predicting the occurrence of thunderstorms is one matter. Whether storms will become severe takes it to another level entirely, because severe local storms encompass a panoply of hazards: dangerous lightning, torrential rain, hail, damaging wind and tornadoes. Each hazard has its particular combination of “ingredients” that must be considered.
Sunday afternoon’s storm outbreak had a lot of classic ingredients going for it — elements that were predicted days in advance. The combination of a moderately unstable atmosphere and strong wind shear is often a dead-ringer for severe storms in the Mid-Atlantic. In the spirit of complete transparency, we are presenting these important elements.
First, the instability. Severe storms are built on powerful updrafts, and we use CAPE (convective available potential energy) as an integrated measure of cloud buoyancy, fueling updrafts. CAPE changes rapidly, and we at CWG monitor it every hour that we are under a severe-storm threat. In the hour before the squall line moved through central Maryland, our CAPE rose to 2,000 J/kg (figure below).
This is in the “moderate” range. One can expect general thunderstorms (other ingredients being conducive) when CAPE rises above 1,000. And Sunday afternoon’s CAPE rose to 2,000, because breaks in the morning overcast allowed surface temperatures to climb into the mid-80s. Combined with dew point temperatures in the low 70s, the atmosphere was “juiced,” amply primed for widespread thunderstorms.
Second, the wind shear. Wind shear is an increase in wind speed and/or change in direction with altitude. We always explain that wind shear leads to longer lived, more organized thunderstorms, promoting updrafts and downdrafts that become stronger. The stronger the shear, the stronger the thunderstorms.
Wind shear values on Sunday afternoon soared to 55-60 mph over our region (figure below). That sends off alarms, because values this strong promote rotating storm complexes — including supercells (rotating updrafts) and bow echoes (“bookend vortices”).
We at CWG, and our colleagues in both the National Weather Service and Storm Prediction Center, had every reason to believe that significant severe storms were in the offing. The combination of 55-60 mph shear and 2,000 J/kg CAPE is a potent one here during the Mid-Atlantic summer.
But on Sunday afternoon, what actually happened was this: An initially intense squall line dropped east off the Blue Ridge during the mid-afternoon. Bowing segments created a swath of wind damage not far to the north of D.C., courtesy of numerous downbursts. The locus of heavy damage extended just south of, and along, the Mason-Dixon line.
Close to D.C., that same line dropped southeast in the form of mostly just showers, sans thunder (albeit with peak gusts in the 40-45 mph range), and this line did not even impact northern Virginia.
The lack of vigorous storms from the Beltway and south was surprising to some, but this is the nature of summertime convection. Rule No. 1: Severe local storms are localized, with lots of spatial variability expected. You cast a wide net for a severe thunderstorm watch, out of necessity. The reality is that not everyone experiences severe weather.
Yes, the lack of severe weather in D.C. and northern Virginia left us scratching our heads and wanting to do better. The setup was nearly perfect for severe storms, but nature did not deliver.
We don’t have a good answer for why not, at this point. One theory I have is related to the convective line’s gust front (leading edge of the cold, downdraft-driven outflow), which ran way out ahead of the storm cells. You can see this in the figure above.
Now, what does this mean, and why did this happen?
What it means: The gust front acts like a miniature cold front, scooping up moist, unstable air ahead of the convective line, supplying it with buoyant energy. You want that “scoop” to be located right along the leading edge of the convective cells. When it runs way out ahead, the unstable air is scooped up, but it does not feed directly into the cells. Bottom line: energy wasted, storm cells die, or at least “fail to thrive.”
Where the convective line was most severe — in northeastern Maryland — the gust front did not surge out ahead of the storm cells.
But why did this happen? We don’t completely know. Limited modeling studies suggest that squall lines crossing the Appalachians have complex interactions with the high terrain. At times, the cold, dense outflow surges down the mountains and outruns the parent convective line. After this initial surge, the line often regenerates, going through a second period of intense regrowth, close to D.C. This particular line did not. It could have something to do with the crossing angle or a host of other factors. At this time, much more research is needed.
The guidance we had late morning on Sunday, from the best high-resolution models, suggested that the squall line would survive the Appalachians, and indeed thrive, crossing the District. These models are our best tools for assessing future outcomes of inherently small-scale, short-fuse processes. But they also are not perfect.
Part of what we do, as meteorologists, is to learn their biases, their foibles, how best to incorporate them into our experience base. We do the absolute best we can, but we still have much learning to do.