Multiple sources – and credible ones (Greg Forbes, The Weather Channel; Henry Margusity, AccuWeather) at that – have mentioned there is the possibility of a derecho Wednesday, cruising from the Midwest during the day into the Mid-Atlantic at night.
Severe weather risk across much of Corn Belt, Mid-Atlantic region Tuesday night – Thursday. Derecho or MCS threat twitpic.com/cwi8j0
— LarryCosgrove (@LarryCosgrove) June 10, 2013
— Alex Sosnowski (@asoswx) June 10, 2013
Admittedly, examining the weather models and pattern setting up, I see what they’re seeing, but am unconvinced there’s reasonable confidence D.C. will experience derecho conditions, and I cringe at sounding the alarm so soon.
Models show a reasonably well-defined disturbance zipping along the southern periphery of the jet stream (river of fast high altitiude winds) Wednesday helping to initiate an area of thunderstorms that could well turn severe as heat bubbles up from the south.
But consider derechos are rare events in the Mid-Atlantic, happening on average once every 3 or 4 years.
They occur more frequently in the Ohio Valley and Midwest, and often dissipate over the Appalachians before reaching us.
They’re difficult to predict, and have strict criteria to be considered legitimate:
* A derecho must produce damaging winds (58 mph or higher) along most of its length
* Its length must be at least 240 miles.
While I don’t doubt some strong to severe storms will fire Wednesday and generally head from the Midwest towards the East by evening, it’s not clear to me how well they’ll hold together and if/where they’ll meet derecho criteria.
“It’s hard to predict if the cluster of storms actually becomes a derecho or not,” AccuWeather’s Henry Margusity correctly points out.
If had to guess (and it’s somewhat premature to do so this far out), I would favor the strongest storms late Wednesday a good deal south of D.C., in perhaps central or southern Virginia, if they survive the trip across the mountains.
We certainly don’t have nearly as much heat energy and instability this year as compared to June 29 of last year to feed the storms.
(And, truth be told, I’m more concerned about the severe weather set up on Thursday than Wednesday night. More on that tomorrow.)
As derechos are a sensitive subject in the wake of last year’s destructive storms in the D.C. area, my plan was to wait until confidence was higher to discuss what’s right now a really iffy scenario.
On Twitter, I came down hard on forecasters for yelling derecho so soon…not because I don’t see some potential in the pattern, but because it seems like some are throwing the word around too casually since last year when they don’t happen that often and are difficult to predict at long lead times.
— Capital Weather Gang (@capitalweather) June 10, 2013
My fear is that if there are too many false derecho alarms, some may not take the real deal seriously enough on that rare day the next one occurs.
I welcome your thoughts…