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.
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.
