Making The Call

A review of our derecho / flooding event of the past few days, together with some thoughts about managing risk and messaging with a concerned public.

Let's start this all off with a question/scenario:

...You have been talking about the potential for severe weather for a few days now, and so has everyone else. You then see one of the following scenarios on the radar screen:

Scenario 1 Scenario 2

As the meteorologist, it's now up to you to make a call concerning severe weather coverage in the next 12-24 hours. It takes time to get staff in place, time to set up what angles are going to be looked for and covered, and what potential risks are involved with getting people to where they need to be. At the same time, one of the biggest pitfalls for the meteorologist is the bust: Making a forecast for a dangerous weather event, and nothing ends up happening.

The above event in Scenario 1 is what Tony Cavalier faced in the midnight hour of June 13th, keeping watch on Doppler radar. NBC was showing the first game of the Stanley Cup Finals, and it was already into overtime. Fortunately, this meant a lot of others on staff were staying late at the same time in addition to Tony. Around the same time, the Storm Prediction Center issues what is called a "PDS" or "Particularly Dangerous Situation" for parts of the Ohio Valley. This was confirmation for our thinking, and our storm coverage plan was put into action despite most of the principals people who were going to be marshalling it were asleep at the time.

The image in Scenario 2 was the notorious June 29th, 2012 Derecho as it appeared on radar with a similar lead-time. Pretty impressive how these things can pop up, eh?

Let's fast forward these same scenarios a little bit:

June 13th, 2013 Derecho - 2:30am June 29th, 2012 Derecho - 5:30pm

Both events were already at "derecho" status before hitting our area, having met the criteria of multiple wind gust reports >58mph from the same organized cluster of storms across a path greater than 250 miles. In retrospect with the events of June 29th firmly in mind, it's pretty easy to see how this could have been a 'particularly dangerous situation'.

Here's the 'tale of the tape' for both derecho events in terms of storm reports:

June 12-13, 2013 Derecho Event

June 12th Local Storm Reports June 13th Local Storm Reports

June 29th, 2012 Derecho Event

June 29th Local Storm Reports

It should be clear to everyone that both events were indeed significant, and worthy of respect, particularly in light of what was known upstream before the actual storm made it to our tri-state area. Regarding the forecast, it was always understood (and projected as such) that this event was not going to be as bad as the June 29th derecho, which was why we made the decision to not be so overt in explicitly declaring the imminence of a derecho. Since our forecast area really only had one derecho experience in memory, it was clear that the 'freakout factor' was going to be high which is dangerous for the forecaster if the storm never materialized. Derechoes however, are actually more common than you might think, and are on average more akin to what we just went through rather than the ridiculous storm of June 29th, 2012. By the way, if you are interested, check out a report on another derecho event that hit the tri-state area, back in 1991.

The good news regarding derechoes is that for our area, we can see them coming. Similar to a hurricane in the ocean, we do not need to declare an imminent landfalling storm for a specific area before it even develops in the Atlantic-- it has a high potential to be wrong. But, just like a tropical system, even if it weakens it must still be respected. Consider the case of this derecho as well as the occasion of Tropical Storm Allison in June of 2001. The internal organization of a storm system can remain intact (and re-fire) even if it isn't as strong as it was before. In the prime heating hours of the day on the 13th, our derecho was causing almost as much havok in the Carolinas as it was in the Ohio Valley the night before.

Derechoes and Flooding

This is another aspect to discuss with regard to our severe weather event. The primary threat from a derecho is massive and prolonged power outages due to widespread high winds. Often times a derecho event sweeps across a region in mere minutes, with little time for flooding and little impetus for tornado formation (with the exception of the edges of the bow echo radar signature). Should a derecho start to weaken, like in the case of an early morning event that features a loss of daytime solar heating, you can have a scenario in which the forward bowing part of a derecho continues to plow on while the edges (particularly the southern edge) languishes behind with less motivation. You still have the strong thunderstorms, but now they're either not moving, or they are training over the same location.

This is what be-fell our region from about McArthur, Ohio through the mountains of Rt 19 in Nicholas and Braxton counties of West Virginia. In the multi-update blog post during the event, the area highlighted for concern was along US-33 in Ohio and West Virginia.

Precipitation Totals - 24hrs Ending 7am June 13th, 2013 Precipitation Totals - 24hrs Ending 7am June 14th, 2013

The heaviest rains were in line with the southern curl edge of the derecho, and was enhanced when the storm became less 'derecho-y' in the pre-dawn hours. [I recall some outlets proclaiming a 'derech-no' when the winds weren't as bad in their area]. In my professional opinion, this was a qualifying derecho with a classic bow structure and requisite wind damage reports-- even extending to a second day and bow line on the other side of the Appalachians. Perhaps this is all chalked up to an expectant public's overly fearful experience of what might have been their only memory of such an event leading them to declare it a dud if it didn't come as strong as the last one (which might well have been the worst of recorded weather history).

And while we're recalling past storms, the flooding on the southern edge of a depleted derecho that strikes in the pre-dawn hours with the loss of daytime solar heating reminds me of another devastating flooding event close to our general area: The Johnstown (PA) Flood of 1977. Here's a link to an academic article on the subject for those interested in the innards :-) They didn't have advanced radar back then (which obviously was a problem), but here's some enhanced satellite imagery of what they were looking at that night:

This system had a similar beginning in smaller form, and then morphed into a Mesoscale Convective Complex, or MCS. MCSs and Derechoes have a lot of similar properties in that meteorologists often look to the formation of either given the initial atmospheric conditions we were anticipating a day or two prior. The similarity I want to highlight here is the enhanced flooding threat on the virtually non-propagating edge of the storm complex in the morning hours. Now, this particular MCS example might as well be a similar 'worst-ever' of its kind to that of the June 29th, 2012 derecho. We've had many other MCSs since then, so the weather-savvy public has a much healthier concern for these storms instead of abject panic-- something that we're hoping this present derecho event will inspire so that the "d-word" can come out into the open more accurately.

So, in the end, Tony made the right call late that night to go into full 'severe weather mode', and even if you are not cleaning up your backyard from tree damage and looking at 13-days without power, A derecho still occurred-- even if it took a break for some severe flooding in the early morning hours.

It's a great thing that following such storms is often some real nice weather; a peaceful time to catch up on lost sleep.

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