Kevin Myatt is The Roanoke Times' weather columnist.

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• Moisture could get caught up in cold blast
• Forecast for Weather Journal: Partly print, with frequent Internet
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• For now, it looks like a quiet, mostly mild week ahead for SW Virginia
• Coldest morning of winter so far likely across much of Southwest Virginia; Tuesday precipitation looking doubtful
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You've always got to watch that back door.

The majority of the cold fronts that pass through Southwest Virginia approach us from the west or northwest. That would be the "front door," I suppose. It is often wide open for fronts to wander through; occasionally, it is slammed shut.

But some cold fronts slip down on us from the north or even the northeast, pressed southward by cool high pressure in Canada. These are nicknamed "backdoor cold fronts."

Our mini-heat wave of early this week was broken when a front slipped southward Wednesday and Thursday.

The cool air behind the front encountering warmth and some moisture parked ahead of it triggered a few thunderstorms Wednesday afternoon and evening.

This episode of storms was spotty and mild compared with July 19. That was the evening that part of the Virginia Museum of Transportation roof blew off during a thunderstorm microburst and there were numerous other reports of wind damage and hail across the Roanoke and New River valleys. That severe weather outbreak was caused by a backdoor cold front pushing into very hot, much more humid air.

By Thursday, a much cooler air mass had fully settled in. Our temperatures, which topped 90 on Tuesday in Roanoke, were in the 50s. Moisture overrunning that cold air from the south triggered some much-needed rainfall.

The cooler air remained over us Friday and will likely extend into the weekend, along with a few showers.

For areas east of the spine of the Appalachians, which runs roughly along the Virginia-West Virginia line, backdoor cold fronts are often more efficient in delivering cooler air than more traditional fronts.

That's because the cold air has to get over the mountains when coming from the west or northwest. We are somewhat sheltered from that in the Roanoke Valley, and sometimes it takes a day longer for the cold air to settle in when coming from the west.

But when cooler air arrives from the north or northeast, the mountains help it bank up over us. This mass of cooler air trapped east of the mountains is often called a "wedge," or in winter, it is often referred to as "cold air damming."

So this wedge will remain over us until warmer air from the south and southwest can sweep it out early next week. It does look like a few days of warmer weather, though probably not as hot as early this week, will be on tap until another front -- using either the front or back door -- can arrive.

Abnormal April ends up 'normal'

Amazingly, after some weeks when it appeared April could challenge for being one of Roanoke's coldest Aprils on record, the month ended up averaging closer to normal in temperature than any month since May 2002.

Roanoke's average temperature for April was 55.9 degrees, which is 0.2 degree below normal. Every month since May 2002 had been at least 0.3 degree off the 1971-2000 average mark, which is considered by the National Climatic Data Center to be the "climate norm" for individual recording stations.

April was a month of extremes. The first four days were 6 to 15 degrees above normal; the next seven were 10 or more degrees below normal, peaking at 20 degrees below normal on April 7. Five of the next eight days were 5 to 8 degrees below normal, and then after a couple of near-normal days, the last nine were 5 to 15 degrees above normal.

So someone looking back on April 2007 decades from now will note it as a normal month for temperature. The baking and freezing we experienced alternately tells us a much different story.