Monday, July 23, 2012

Latest Model Forecast Highlights Derecho Concern


The latest 0z NAM model forecast continues to highlight the concern of another derecho in the Ohio Valley and Mid Atlantic regions tomorrow, with this latest forecast giving the two Virginias and Kentucky the brunt of this complex.
The image above is valid at 4:00 PM CDT, and highlights 3 hour precipitation in millimeters. Based on the above image, we see a bowing segment placed in West Virginia and Kentucky putting out potentially high precipitation. With any bowing segment comes the risk of damaging winds, but with a derecho this danger is greatly increased.
A derecho is a long-lived wind storm spanning a large area, while a bowing segment involves a small, weaker bow in a much smaller scale region. Considering the strength and span of this bowing segment, as well as the distance it will have travelled (origins from Chicago, IL), I believe that this has the characteristics of a derecho, and would most likely prompt damaging wind potential.

I will issue a special morning post on this matter tomorrow morning with a full model update.

For the initial derecho post, click here.



Anonymous said...

You cannot look at 3 hr precip maps to guess the shape of a bow echo. In order for a bow echo in the particular orientation to produce that rain pattern, the thing would have to be sitting still for 3 hours. This, of course, doesn't happen with now echoes. The image is simply displaying an area where the NAM model believes a heavier storm will track. That is all.

Andrew said...

It would not have to be sitting still for 3 hours. All weather is made by something in motion, whether it be uneven heating that creates wind, or building clouds that create falling rain, weather is always in motion. What would be the scenario would be the storm putting down that precipitation over a span of three hours as it moves, not showing it stays still. Reflectivity images off models tend to be on the messy side, so I prefer 3 hour precip maps when the opportunity arises.

ERN WX said...

Tstms tend to produce a lot of rain in a little time so that does make sense. Derechoes can be semi predictable because of certain parameters and simulated radar can alert meteorologists to possible formation.

Anonymous said...

Yes Andrew, I realize, however, you cannot use 3 hr precip to look for bow echos. Bow echoes are a product of reflectivity, they show an area of higher intensity rainfall in the shape of a bow. However, this rainfall rate is instantaneous, not lapsed over 3 hrs. In otherwords you are treating an area of precip that takes place over three hours that is shaped like a bow as a bow echo, which is incorrect. I don't know any better way to explain it. In simple terms, to me it seems like you are substituting 3 hr precip in for reflectivity, which you absolutely can not do.