From the ABC 7 Weather team

Closed lows can bring weather woes

September 20, 2011 - 09:22 PM
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Two weeks ago it was a former tropical storm (Lee) that dropped record amounts of rain in the D.C. region. This time around it's a similar system that could create even more headaches if bouts of heavy rain come back into the area.

Remember the flash floods that started off September? I'm sure you do. That was all courtesy of a fairly infrequent weather occurrence: a combination of a former tropical system, tropical moisture, and a stagnant weather flow. This time around it's a similarly stagnant weather flow, but instead of the leftovers from a tropical system, it's the evolution of a regular ol' storm system that gets cut off from the rest of the general weather flow and becomes a "cutoff low."

What we refer to in the weather world as storm systems/disturbances/upper-level storms are all just areas of low pressure. These areas of low pressure usually skirt along in the atmosphere getting pushed around by the winds of the jet stream. In fact, it's really the interaction between these lows and surrounding high pressure that actually create the jet stream, but that's another topic for another time. Sometimes these lows deviate from the course and get isolated from the rest of the general jet stream flow. That's when these storms can just spin around over the same location for up to several days and drop huge amounts of rain in some cases. Take a look at this example; it is an actual forecast for us in D.C. by 1 p.m. Monday, Sept. 26th.



Note the big storm just west of D.C. that's cut off from the rest of the jet stream in Canada.

This is a classic example of a closed low. Take note of how this storm is a pretty big monster by terms of size and strength. As winds flow counterclockwise around areas of low pressure in the northern hemishpere, we are directly in a southerly wind pattern which will feed in moisture from the Gulf of Mexico and Atlantic. Exactly how much moisture will be the key to our heavy rain potential by the end of the week.

For a complete picture of these pretty unique weather features click on my video explainer below. We'll also keep you updated throughout the next seven days with the latest outlooks on this forecast and beyond right here on WJLA.com.

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