From the ABC 7 Weather team

Archive for category D.C.-centric July 2014

Severe Storms Likely Monday and Tuesday

July 13, 2014 - 06:39 PM
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We've reached the climatological hottest time of the year in D.C. with our average high now 89°.  Some big changes on the horizon, as a strong cold front approaches the area.  This cold front will bring a welcome drop in the heat and humidity by Wednesday, but it will come with the price of severe weather for Monday and Tuesday.

Ahead of the front, our airmass remains very hot and humid.  Forecast high temperatures Monday are in the low to mid 90s.  Factor in the humidity and it will feel closer to the 100 degree mark.

The cold front will slide farther east Monday increasing the atmospheric dynamics for severe weather.  Take a look at the surface features, as of Sunday night.

The viewing area will remain under the very warm and moisture laden air until the cold front passes late Tuesday night.  That will help fuel the fire for the strong and severe storms.  The Storm Prediction Center has the entire vewing area (shaded in yellow in the image below) under a 'Slight' risk for severe storms Monday.   This means damaging winds (60+mph), large hail, and isolated tornadoes will be possible with any storms that develop.

Storm Prediction Center

A big dip in the jet stream will allow temperatures and humidity to drop off significantly by Wednesday and the rest of the week.  The clash of airmasses; however, is what is causing this elevated risk for severe weather.  Check out an upper level map (500mb level) that clearly shows the large trough digging into the area.  The brighter colors indicate higher wind speeds, which leads to increased wind shear.  Wind shear can cause damaging wind gusts and tornadoes.

WeatherBell - European Model 500mb heights and wind speed/direction

As mentioned, behind the front much cooler and drier air.  Take a look at the difference in forecast dewpoint temperatures on Monday and Wednesday.  Dewpoints will go from the mid 70s Monday afternoon to the low 50s Wednesday.  You will certainly feel the difference by midweek.

WeatherBell ECMWF Dewpoint Temperature Monday
WeatherBell ECMWF Dewpoint Temperature Wednesday

Remember to stay updated with the StormWatch7 weather team for the latest weather updates.  We also encourage you to download the free StormWatch7 weather app.  You'll have access to Live Super Doppler within the app, so you can track storms on the go.

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Cooler Air, but Not Cold Air Infiltrating the D.C. Area Next Week

July 12, 2014 - 05:10 PM
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Once we get through the possibility of severe storms on Monday and Tuesday, cooler air will move right into our region for the end of next week.


Classification. Yes, we are a society that likes to name things: Snowmaggedon, Commutergedon, SnowGuinness, etc. Where a squall line is a “derecho” and where a spill of cold air from the north is coined the “polar vortex.” I get it. I really do. Classification of tornadoes or hurricanes is absolutely necessary, however, to call every snowstorm a “blizzard” just makes me shake my head-but then move on with my life.


Before 2012, we had NEVER heard of the term “derecho” and now it seems every storm that comes through during the summer is classified as that. Before last winter, we NEVER EVER heard the term “polar vortex” used before and now, several months later, we gravitate right to it.


It is all in the eye of the beholder and left up to interpretation amongst professionals in the meteorology field. Although I didn’t want to touch this subject with a ten foot pole, I feel that we are forced to say something. The storm on Tuesday night left so many people wondering whether a “derecho” in fact hit the area that we had to reach out to the Storm Prediction Center to include their thoughts. So now onto the “polar vortex” or let’s just say the “unusual pattern setting up for Mid-July.”


All in all, we do know that next week unseasonably cold air will migrate out of the north and into the United States, spreading out into the Mid Atlantic. There is so much misuse of several weather terms throughout the year that I thought we should revisit the term “polar vortex”.


Our friends at the National Weather Service in New York made this graphic after the hysteria in social media yesterday about “the return of the polar vortex.” They reminded everybody on their Facebook page that “the most basic definition for “polar vortex” is that it is a piece of energy that comes from the polar region. It is generated by the large difference in temperature between the tropics and the poles. The temperature gradient is strongest during the winter; therefore it is more likely to affect mid-latitudes (our area) in the winter. However, it can affect us in the summer, but it is not nearly as strong as it is in the winter.”

NWS

There are differing opinions in the meteorology field on whether this is a “true polar vortex’ and that is why you will see so many different articles ALL OVER the internet on whether this is or it is not. This is what the Weather Prediction Center put out yesterday:\


“A DEEP UPPER LOW... NOT THE POLAR VORTEX AS ITS ORIGINS ARE FROM THE NE PACIFIC... WILL SWING THROUGH THE GREAT LAKES EARLY NEXT WEEK WITH AN IMPRESSIVE COLD SHOT OF AIR INTO THE CENTRAL AND THEN SOUTHERN PLAINS AND THE MIDWEST.”


Great. So in the grand scheme of things I ask myself “does it really matter to the general public what we coin this?” The answer I give myself is “no”.
So here is what we know. We have some unseasonably cold air coming down from the north and spilling into the region. The Midwest will get the brunt of the cold weather where temperatures could be as much as 20-25 degrees below normal! That is very impressive for summertime climate. Our normal temperature in the D.C. area for this time of year is a warm 89 degrees. The Climate Prediction Center has given us about a 40%-50 of seeing below normal temperatures next week.

Climate Prediction Center

Now, I got to tell you, if you are expecting temperatures in the 50s/60s then you have to travel to the north and west of here. Temperatures next week (starting Wednesday) are going to fall below normal but we are still talking 70s/80s. Here is a look at our 7-day forecast:

However, before we even get to that cooler air moving in, as you notice on the 7-day we are heating up! Temperatures on Sunday, Monday and Tuesday will have no problem reaching into the 90s. With the added July humidity, we are talking about heat indicies approaching 100 degrees all with the added threat of severe weather each day. The Storm Prediction Center has ALREADY outlined us in an area for a shot at severe weather on Monday into Tuesday:

Storm Prediction Center

So all in all, are we cooling down slightly for the end of next week? Yes. Are we going to have to deal with heat and humidity and severe storms at the beginning of next week? Yes. It is going to be a roller coaster week and we will certainly welcome the cooler air by the end of next week, not matter what we want to call it.

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Arthur Update and Fireworks Forecast

July 4, 2014 - 09:05 AM
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Hurricane Arthur has put a damper on numerous firework's displays with many locations having to postpone or cancel.  Fortunately for us, the fireworks should be able to get off without a hitch in the Nation's Capital.  Here's a list of cities where fireworks have been rescheduled.

Hurricane Arthur made landfall near Beaufort, N.C. around 11:15 p.m. Thursday night.  Arthur was downgraded to a category 1 hurricane with winds at 90 mph as of the 9 a.m. update from the National Hurricane Center.  The storm has moved off the coast of the Outer Banks, NC and is quickly moving northeast.
(National Hurricane Center)
As a cold front slides through our area and Arthur moves farther out to sea, skies will clear from west to east, as drier air filters in.  It'll also be quite breezy, with winds gusting to around 30 mph through the day.
Winds will subside by the time the fireworks are set to go off downtown at 9:10pm.  And we couldn't ask for a more pleasant fireworks forecast.  It will be very comfortable, with low humidity and still just a bit breezy.  Click here for a full list of Fourth of July parades, events, and fireworks' displays.
The rest of the holiday weekend looks fantastic!  The humidity will remain low with plenty of sunshine and seasonable temperatures.

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