Wednesday, June 12, 2013

Widely Feared But Not Understood

June 29th, 2012 Derecho Radar - From NWS
11:00 PM 6/11/13 
June 29th, 2012 was a day that produced weather that will not soon be forgotten by those in the Mid-Atlantic region. For those unfamiliar, that was the day of last year’s infamous “derecho”. Now, there are discussions of another possible derecho heading for the Mid-Atlantic and naturally, people are fearing a similar situation to June 29th, 2012, where people were without power for days or weeks. However, we should really try to understand what a derecho is before we start discussing any future possibilities.

DEFINITION – The first thing to understand is that a Derecho is not a special type of storm. It is fundamentally no different than a regular squall line or bow echo. However, it’s strength and duration gives it a separate classification. In order to be considered a derecho, a storm or line of storms must produce wind damage from winds in excess of 58 mph along most of length for more than 240 miles along its path.

Severe Weather reports from June 2012 Derecho - From SPC
Derecho’s are often classified as such after the fact once the damage is surveyed and analyses are done. The June 29th, 2012 event was an exceptionally strong event produced by relatively rare conditions that will be discussed below. Wind damage was sustained from the storm all the way from Chicago to the Atlantic Ocean.

CAUSES – A derecho producing storm usually starts out as a Mesoscale Convective System (MCS). An MCS is simply an organized complex of thunderstorms on a larger scale than just the individual thunderstorm cells. Sometimes these can form squall lines or bow echos which typically are the most likely candidates for derecho storms. The storm is first indicated as an MCS, but as it strengthens, may be considered a derecho.

CONDITIONS – The most common time/place we see derechos occurring is on the fringes of major heat waves. Take a look at the 700 mb level of the atmosphere on June 29th, 2012 pictured below. The instability of the heat wave combined with an anomalously strong jet creates conditions prime for derecho development. These came together perfectly last June.
The overall pattern that lead to last June's Derecho storm - from SPC

WHERE DO THE STRONG WINDS COME FROM? – Not all thunderstorms create winds like we saw a year ago. When there is enough energy to fuel these storms, we can see downbursts develop. A thunderstorm complex like a derecho is fueled by in influx of hot, humid air from in front, but then the heavy rains create a downdraft beneath the storm. These winds spread outwards when they reach the surface, often in the same direction that the storm is moving. The result is the production of a “gust front” shown in the diagram above.

These winds are what can cause the extreme destruction that the Mid-Atlantic saw a year ago, knocking out power to millions, downing trees, and causing damage to buildings.

Anatomy of a typical Derecho storm

AND THIS YEAR? – The big question on everyone’s mind is if something like last year will happen again. Taking a look now, the heat wave this year is not as intense as last, but the positioning of that jet is similar. This similar set up may spawn the development of an MCS on Wednesday or Thursday. Before the June 2012 derecho, there was an absurd amount of energy over the Mid-Atlantic with temperatures over 100ยบ and extremely high humidity.

That is not expected to occur again tomorrow or Thursday but that does not completely eliminate the chance. A derecho is absolutely not a guarantee with this kind of atmospheric set-up, but sometimes they can happen. If an MCS does develop, we will have to examine where it will track since they are very difficult to forecast before they develop, then look at the amount of energy it will have.

CONCLUSION - It’s too soon to be saying that the whole region will be slammed by a derecho once again. As with all storms like this, it is better to understand fully what is going on first, then move on to discuss the possibilities in the future. Be sure you are staying with Foot’s Forecast for updates on any severe storms in the next two days!


SailingFairSkies said...

Wonderful discussion. Thank you, as always.

SailingFairSkies said...
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Laurie said...

Very Informative. Thank you for posting.