From the ABC 7 Weather team

Archive for category Just For Fun September 2014

Only A Few Days of Summer Left Before Autumn Begins

September 19, 2014 - 02:19 PM
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Enjoy this last weekend of summer - because it is going to feel more like summer here in the Washington D.C. area! Temperatures throughout this last week of summer have been running below normal giving us a little taste of Fall.  However, by the time we head into this weekend, temperatures will top out in the 80s for much of the region thanks to high pressure clearing out cloud cover and helping us boost into the 80s with a light southeasterly flow.

A cold front will cross the area Sunday which will give way to some clouds and about a 30% chance of showers in the afternoon and evening hours moving from west to east. This cold front will also sweep away any of the summer-like weather we have left and leave us with another spell of Autumn weather around the region.

This is happening right in time because the autumnal equinox arrives at 10:29 p.m. on Monday! People celebrate this moment across the world and really the precise moment is when the sun passes directly over the equator! This happens twice a year as you may recall (the "spring equinox" and the "autumnal equinox"). 

 

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In latin, the word "equinox" means "equal night" --"equi" meaning "equal" and "nox" meaning "night". This signifies the equal parts of daylight and darkness.

However, this is not always so "matter-of-fact." The sunrise on Monday is 6:55 a.m. and the sunset is 7:07 p.m. This isn't exactly equal. In fact, we have to wait until Tuesday, the first full day of Autumn to get the full equal light-equal night. The sunrise on Tuesday is 6:56 a.m. and the sunset is 7:06 p.m.

My colleague Eileen Whelan summed it up correctly when she wrote:

"We have the atmosphere to thank for this oddity. Also, the definition of sunrise and sunset. Sunrise occurs the moment the tip of the sun can be seen on the horizon and sunset is the last minute the sun can be seen before it dips below the horizon. Also, keep in mind our atmosphere refracts, or bends, light, which makes it appear as if the sun is rising or setting earlier.

The true equinox occurs when the center of the sun's disk crosses the celestial equator and this occurs at 10:29 p.m. EDT on Monday, September 22. At the same time the equinox occurs in D.C., it occurs across the globe."

Either way, you won't notice a different from Monday to Tuesday, from Summer to Autumn. However, you will notice that the hours of daylight start getting shorter until the Winter Solstice which begins on December 21, 6:03 P.M. EST (which is also the darkest day of the year meaning the day with the least amount of daylight.

However, as we make the transition into Fall, it may seem a little cooler according the outlook by the Climate Prediction Center:

 

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But, also according to the Climate Prediction Center for the 8-14 day outlook, temperatures may be a little on the warmer side!

 

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So don't pack away those flip-flops yet for those boots. Fall is here to stay but bouts of summer may stick around until early October!

 

 

 

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Shelf Clouds Common Prior to DC Storms Lately

September 7, 2014 - 07:58 PM
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Many social media sites have spotlighted these saw-like storm clouds. Just this past weekend, a north to south line of storms produced a long shelf cloud as it hurled from the Cumberland Valley to the nation’s capital.

What does it look like? The image below shows the low, horizontal wedge-shaped cloud. It forms along the leading edge of a thunderstorm’s gust front.

shelf cloud

To understand a gust front, consider the mechanisms in play in a storm. In a thunderstorm downdraft, heavy rain forces the cool air in the atmosphere’s upper-levels to spread to the ground. The air then fans out in all directions once it reaches the surface (since air can’t exactly dig into the ground). The leading edge of this cool air outflow ahead of the storm is called the storm’s gust front.

Shelf cloud 2

Where the gust front meets the warm air ahead of the storm, the air rises and forms shelf clouds. Immediately in the shelf cloud’s wake is where the heaviest rain and strongest winds can be found (such as shown in the image below from the storm that moved through Hagerstown, Md., on Saturday). Shelf clouds are found a few miles ahead of the actual storm.

Shelf cloud 2

Now, shelf clouds don’t precede every thunderstorm. Typically, they are found ahead of thunderstorms that form ahead of a vigorous cold front (like Saturday’s front), an upper-level low pressure or along the leading edge of a derecho. The more powerful the cold front or upper-level disturbance triggering the storm, the better opportunity to see this cloud feature.

So, the next time you see a shelf cloud, remember that often the worst weather is just in its wake. The milky color behind the shelf feature is the heavy rain shaft accompanying the thunderstorm.

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