17 July 2013

The Amazing Retrograding Low

Posted by Dan Satterfield


These images are from the GOES using a wavelength that shows water vapor in the mid and upper troposphere. We meteorologists like this imagery because it allows the easy tracking of many atmospheric features.

It developed last Friday over Western Maryland, and brought 10 inches of rain to the coast of Delaware. Then it began to drift westward. This is not that unusual in the summertime, when an easterly wind can develop to the south of a strong upper ridge over North America. The easterly trade-winds form the same way, and steer tropical waves form the coast of Africa to the New World all summer long.

WV OKLAWhat is very unusual however, is for an upper level cutoff low to drift westward from such a high latitude, across much of the continent!

Below is today with the upper LOW in New Mexico.


The rain and cloud produced by the low, held temps in the 60’s across Oklahoma and West Texas on Monday. In some places it was the coolest July day since the 1940’s! The Low is now being steered by an intense upper level high pressure over the Northeast U.S. It’s this same system that is pushing the temp into the 90’s as far north as Boston this week.

The raob weather balloon at Pittsburgh Monday night had to climb to 600 decameters (6 kilometers) to reach the 500 millibar pressure level (Pressure at the surface is around 1000 millibars, so 500mb is about halfway through the atmosphere). The hotter the atmosphere, the higher you have to go to reach 500 millibars, and a 600 DM height is very rare indeed, and almost unheard of that far north.