Wx4cast

 

Here is a question that was posted on my Facebook weather page. 

“I’ve often heard “back door cold front” as a weather term. Once a (BDCF) reaches the say, “Mid-Continent” if it does, what typically happens to the back door cold front” at this point? –like does it spawn other weather systems to come back at us under a different name”?

I thought this was an excellent question; so, since many others most likely have the same general question, I thought it would be an excellent topic of a blog post.  

Before I answer the question, I want to touch on the movement of systems over North America.

Why do most of our frontal systems and disturbances come from the west?

The easiest answer is that it is the jet stream. In the Northern Hemisphere, weather systems generally tend to move from west to east.

Image from Comet/MetEd.

Image from Weather.gov

As a result, we see storms approach out of our west, southwest or northwest. But this isn’t true of all weather systems. In the tropical Atlantic we see tropical waves move off the West African Coast; the eastern trade winds carry these waves east to west, sometimes these tropical waves become tropical Storms or hurricanes. As these tropical cyclones work their way west, they make a turn to the north, where they can make a landfall in the Caribbean or on the U.S. Coast. This is due to a process called the Coriolis Force. If you want to know more about this force, you can read about it here.

Eventually, as these tropical systems get far enough north, they leave the trade winds, then become subject to the west to east flow at which point they can recurve and travel back eastward.

Now back to the question that was asked.

Living in the Northeast, we’re no strangers to Back Door Cold Fronts (BDCF).  But what really are they? A cold front is a boundary between warm air and cold air. As I already stated, typically cold fronts approach from the north, northwest or west.

A back door cold front is a front that moves east to west rather than the normal west to east direction.  Most of our cold fronts come out of Canada, Great Lakes, and Midwest.  But a BDCF is a front that moves east to west or northeast to southwest rather than the normal general west to east direction. For instance, one that enters New England from the Canadian Maritimes, or into New York State out of Southeast Canada. BDCFs are common from the Northeast to Virginia.  BDCFs are also fairly common along the east side of the Rocky Mountains; for places like the Dakotas, Wyoming and Colorado.

Image from NY Metro Weather.

Here in the Northeast and northern Mid-Atlantic region:

During the Spring and early Summer, the Sea Surface Temperatures off the Northeast/Mid Atlantic Coast are chilly. If high pressure sits north of our region, the clockwise flow around the high, will allow that chilly Atlantic air to move ashore. With the cooler air mixing with the warmer air, Cloud’s form and showers break out. The air behind a BDCF, can fall 10-30 degrees in as little as 24 hours. The Southeast Coast is more or less protected from BDCFs due to the warm Gulf Stream just off the Coast.

The cooler temperatures and unsettled conditions can last a few hours or even a couple of days, depending on the strength of the cold airmass behind the BDCF. Here in the Northeast BDCFs can move a few miles ashore or even a couple hundred miles where they are normally stopped by the Appalachian Mountain Chain. It’s the same out west where the Rockies, make a roadblock preventing the BDCFs from pushing farther west.  

BDCFs will stall, then the boundary will reverse and move eastward as a warm front. This pushes the high pressure farther east, allowing for sunshine and dryer weather to replace the gloomy weather conditions that were in place.

Well that covers the subject of Backdoor Cold Fronts. I hope y'all found it interesting and educational. I will be posting these kinds of subjects from time to time. If there is a subject you would like to know about ask, and it too could become a blog post.