For days, rumors of derechos and not-derechos were swirling around throughout much of the country. A fairly well-modeled system was expected to ignite intense thunderstorms from the Plains to the East Coast.
And then it happened. Or it didn’t. In the immediate area, at least, location mattered — as it often does with even our higher end severe weather events. The D.C. region ended up eluding widespread destructive storms due to some last minute shifts, thanks in large part to the interplay between two separate but interconnected small scale (mesoscale) events.
— Chris Mergerson (@LSUHealthChris) June 13, 2013
Let’s start with a simple and important fact, one which guides CWG’s forecasting philosophy: pinning down thunderstorm specifics is extraordinarily tricky, perhaps the most tricky of all weather events. This is amplified strongly the further out in time one is from an event and frequently remains problematic even up until thunderstorms form. (To use an analogy, think of turning up the heat of a pot of boiling water, and trying to predict where the big and small bubbles will surface)
Uncertainty multiplies against uncertainty, and outside general ideas, a lot of the rest is, well… fluff. We get that, but still strive to inform our audience about the most likely outcomes and their impacts the best we can without sounding what we feel are unnecessary alarm bells.
In the lead-up to the event, even while striving to stay away from derecho talk (largely due to comparisons to last year), we warned of dangerous storms and of the potential for a widespread and high-end event unlike our more typical thunderstorm events.
Since that fateful night almost a year ago, most storm events have been compared to the derecho. None have come close. This set of events did not come close. Even with relatively widespread zones of high winds and wind damage both to our northwest and to our south, winds were generally on the low end of what we consider “severe” and the coverage of the highest winds was small.
Given the extremely high impact June 29, 2012 had in our area and a massive area surrounding it, we chose to describe expected impacts and potential rather than quibble over a word when we were inherently uncertain about the evolution of said storms in the first place.
While it’s best to not always compare events to the derecho of June 29, 2012, given its historic widespread nature, the events of the day after the derecho last year parallel yesterday’s.
The forecast for the day after last year’s derecho stressed the potential for another serious thunderstorm outbreak. It didn’t happen – and it didn’t happen for similar reasons to yesterday.
A large line of storms, and the cold pool of air that keeps them going, often saps the atmosphere of available energy for future thunderstorm development. The energy sapping can also happen because in the wake of a large squall line, cloud debris, shifts in wind, and changes in moisture levels can occur.
The June 29, 2012 derecho – the first event – helped “mess up” the second severe weather outbreak expected last year. This same dynamic – to an extent – occurred yesterday.
Behind the mostly sub-severe storms yesterday morning, via the much-weakened Ohio Valley storm complex (or “low-end derecho”), clouds were slow to clear, moisture levels dropped, and a convergence zone (or outflow boundary) left in the dead storm’s wake headed off to our south.Those features combined to push the main risk zone from northern Virginia, central Maryland, southeast Pennsylvania and southern New Jersey into central and southern Virginia, and eastern North Carolina.
Nevertheless, our atmosphere remained fairly favorable for some storms, but not nearly as primed as it could have been, and was expected to be in the days leading up. As such, new storms fired during the heating of the day and they largely focused to the south.
What we did have yesterday that we did not have during the derecho (and during many derechos that make it to this area) was an anomalously powerful disturbance at high altitudes dropping into the region during the afternoon. This helped sustain a moderately strong and strengthening low pressure area to the north which later became something of a summer nor’easter.
A low pressure area of this intensity passing nearby at this time of year can cause some serious trouble if it does not face too many obstacles. It will also strive to perform under rough circumstances.
The high altitude disturbance and the associated low pressure area were able to touch off storms even in the less-primed air over our region. Comparatively, this was isolated when viewed against widespread activity further south, with mainly one storm complex that caused damage locally albeit in a high population area (Rockville).
Even given the atmosphere that had been much depleted compared to anticipated due to morning activity, the storms that fired locally were quite intense in many locations they hit, causing not just spotty but fairly regular tree and associated damage in certain neighborhoods.
Several reports of tornadoes also came across. To my knowledge, none have been confirmed yet, though there were 8 tornado warnings issued by the National Weather Service, including one described as confirmed at the time. Funnel clouds were seen near Fredericksburg and around BWI Airport, as well as other spots.
The density of storm reports from the June 13, 2013 event is very high for the East Coast especially once you get deeper into Virginia and to the south. Had the last minute and largely unforeseen shifts not occurred, that zone would likely have been further north over the D.C. area.
Given that fact, my take away is that while there was some hype, much of it from the usual suspects, the event on the 13th and the preceding event on the 12th both mostly lived up to their potential.
At the same time, it was probably not the type of event to be cancelling indoor activities or sending workers home early for. As with most severe weather situations around the D.C. area, inconvenience is usually the biggest result for a majority of those impacted, and being indoors is highly preferable to traveling during scarier situations.
Summer is thunderstorm complex/derecho season. Most derechos don’t impact the D.C. area in any notable fashion. Saving the harshest language for the harshest of expected situations is generally a good policy to avoid complacency.
Understanding the potential, and as importantly, how a storm event can miss potential, is also a worthy endeavor. On that note, forecast errors often become learning tools for the next event.