From the ABC 7 Weather team

Archive for June 2012

D.C. Derecho: Midwest To East Coast Destructive Storms

June 30, 2012 - 04:00 PM
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Before Friday, June 29th, you may have never heard of the term "Derecho".  I'm sure by now, you're well aware of what it means.... and it's probably something you wish you never knew about or had to experience.

A large cluster of thunderstorms developed Friday afternoon on June 29th in Eastern Iowa and continued to intensify, as it marched East into Northern Illinois and Indiana.  A derecho is defined as a long lived wind storm that can travel for hundreds of miles with damage generally following a straight path.  This is where the term "straight line winds" come from.  The powerful thunderstorms travel very fast and as they accelerate, the storms sometimes tend to "bow".  That can result in wind gusts between 60 and 100 mph.  Take a look at a graphic, from the Storm Prediction Center, showing the over 600 mile radius the derecho traveled. 

The strong line of storms approached the D.C. metro area around 10:30 PM.  Many people recount the event, as all of the sudden hearing a loud gust of wind and in many cases, the highest wind gusts occurred before the actual line of heavy rain pushed through.  What was so impressive about Friday night's storms was that they occurred after an afternoon of record breaking heat.  D.C. made it to 104° - breaking the previous record of 101° set back in 1934.  Even after the sunset, temperatures were in the low 90s when the storms plowed through, so there was definitely enough heat energy to support the strong storms, even after they crossed the mountains.  Dewpoint temperatures were also near 70°, so there was a lot of moisture in the atmosphere, as well.  Here's what our Doppler looked like at 10:55 PM, as the line moved through the D.C. metro area.

Doppler at 10:55 PM Friday, June

The line of storms produced wind gusts to 70 mph at both Reagan National and Washington Dulles.  The storms lasted about an hour, but left millions without power and unfortunately a few fatalities.  Take a look at the NWS Storm Reports from Fridays event.  This MidAtlantic view totals 463 reports of wind damage!  There was one confirmed tornado in Newcomerstown, Ohio and even some reports of hail. 

The clean up continues after the devestating storms tore through the Nations Capital Friday night.  Derecho's aren't common in our area, but occur more often in the Southern Plains and Ohio Valley.  The derecho climatology map below puts the D.C. area in a category of one derecho every four years.

SPC Derecho Climatology

Looking back at weather records, D.C. hasn't experienced a significant Derecho event before the Friday occurrence.  Now Western Maryland, parts of Virginia, and West Virginia have dealt with similar weather events, but again, it's nothing that happens frequently.   The "West Virginia Derecho of 1991" originated over Arkansas and traveled Northeast through Tennessee, Kentucky, and eventually moved through West Virigina and extreme Western Maryland.  What I found even more interesting was the "July 4-5 1980 Derecho".  This line of storms traveled almost an exact track to the 2012 D.C. Derecho.  Take a look at the track below:

SPC Track of derecho July 4-5, 1980

Notice how the storm began in Iowa, tracked through Illinois, Indiana, Ohio, and then eventually shifted Southeast through West Virigina and then into Northern Viginia, Maryland, and D.C. 

The 2012 D.C. Derecho will definitely make the record books.  It was unlike something many of us have seen, but will remain infamous in our memories. 

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Live Blog: Dangerous D.C. Storms Tonight

June 29, 2012 - 08:06 PM
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1:34 PM  Bob Ryan here.  Finally made it home but widespread damage is worst I have seen since the storm of June 14, 1989  in the macrobust.  This instense  line damage and power outages more widespread.  Many days to recover and unfortunately hot and humid through weekend.  Stay safe

 

12:04 PM: The latest doppler shows that line across the Bay, but now it's all about the clean up -- numerous trees down, power outages, flooding, and more.  Even though the nastiest of the storms have moved through, we can't stress enough to be safe.

 

11:52 PM:  Local Storm report issued by the National Weather Service in Sterling, VA - 1 Fatality and 3 people injured with a tree that fell on a car in Fairfax, VA.  More updates to come.  

11:41 PM:  Worst of the storms moving across the Chesapeake Bay now and onto the Eastern Shore of Maryland. 

11:26 PM:  More and more power outages being reported.  The bulk of the lightning around Baltimore with the line of storms right over Annapolis, Dunkirk, and moving East at 35 mph.

11:14 PM:  Storms have passed through D.C., but the heaviest rain and lightning is now moving through Annapolis and Bowie - approaching the Chesapeake Bay.   

10:27 PM:  The line with intense lightning from Montgomery county to Fredericksburg - expect widespread power outages and damaging winds over the next hour.

10:11 PM:  THE ENTIRE VIEWING AREA, with the exception of Calvert and St. Marys counties, UNDER A SEVERE THUNDERSTORM WARNING UNTIL 11:15PM.  Latest lightning image: Over 1600 strokes in our viewing area.

9:50 PM: Storms continuing to push East and still hearing of wind gusts to about 70 mph.  Most intense line of thunderstorms stretching from Martinsburg, to Winchester and Front Royal.  The storms are approaching Culpeper and Warrenton.  Also, our Live Super Doppler is indicating some rotation, so now is the time to get indoors.  About another hour and a half until storms in D.C.

9:34 PM:  FROM BOB RYAN:  Some of these storms are showing some rotation.  There may be warnings for possible small whirlwinds.  The big danger is the approaching line of thunderstorms with straight line winds up to 70 mph.  Lightning and likely power outages within the next two hours in the Washington metro area.

9:28 PM:  Winds reported up to 80 mph along I-81 near the Shenandoah Valley.  The line continues to move East at around 45 mph towards Skyline Drive.  Far Western suburbs expected to see the storms within the next hour. 

9:00 PM: Severe T'storm Warning for Frederick, Washington, Jefferson, and extreme Northern Loudoun.  Notice this one cell popping up ahead of the main line of severe storms, still West of the Blue Ridge.

Live Super Doppler 7

8:20 PM:  A powerful line of storms that barrelled through Indiana, Ohio, and West Virginia is moving into the viewing area.  This particular line of storms is called a Derecho, and fast moving line of thunderstorms that can track for hundreds of miles.  Numerous people are without power, as the storms produced damaging wind gusts and dangerous lightning.  Take a look at some of the latest storm reports and you can see they are concentrated over Indiana, Ohio, and West Virginia.

Storm Prediction Center - Latest June 29th storm reports

We continue to track the storms, as they move East.  Here's the latest Storm Scan image. 

Latest Storm Scan Capture

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Severe storms possible in the D.C. area tonight and Saturday

June 29, 2012 - 03:57 PM
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Everything is in place for strong storms with the potential for damaging wind gusts. High heat and humidity has led to very high levels of instability but it has been capped through the day with such a warm layer above the surface. One thing that can act to break that cap is a lifting mechanism, and there is a big one in the form of an MCS (Mesoscale Convective System) heading our way from the Ohio Valley.

Severe Weather Threat for Tonight from the Storm Prediction Center

Timing of the Storms:  As of 4pm, our team of meteorologists thinks this system will reach the western suburbs by 9pm and move into the D.C. Metro around 10pm. This is a dangerous line of storms, with wind gusts reported up to 70 mph common. There was even a report of a 91 mph wind gust in Fort Wayne, Indiana. The Storm Prediction Center even upgraded the Outlook for severe weather to Moderate for the Ohio Valley where widespread wind damage is possible.

What to watch out for:  The biggest threat in these storms will be damaging wind gusts. The secondary threat appears to be heavy rainfall and lightning, with an extremely minimal tornado threat. Severe thunderstorm watches are already in effect and I wouldn't be suprised if the threat continued east into our area tonight.

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D.C. excessive heat warning issued through 9 p.m.

June 29, 2012 - 10:24 AM
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An Excessive Heat Warning has been placed in effect for much of the D.C. area today as temperatures around 100 degrees and dew points in the 70s will make it feel like 110 degrees at times. This is very dangerous heat so try to stay indoors as much as possible today and drink plenty of fluids. Remember that heat is actually the number one weather-related killer in the United States so this is a very serious situation.

Excessive Heat Watch in Magenta and Heat Advisory in Orange

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Beach Forecast: From Atlantic City To The Outer Banks

June 28, 2012 - 07:00 PM
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A weekend heat wave with possible record breaking temperatures!  So what can you do to stay cool?  Well, the beach and bay are always an option.  This weekend the beaches and bay will be quite hot and water temperatures will be rather refreshing.  Also, mainly sunny skies are expected.

Along the Delaware Bay and Northern Atlantic beaches, highs will be in the low 90s!  Water temperatures are in the low 70s, so with that heat, I'm sure many people will be cooling off in the water.  Water temperatures are a bit warmer near Virginia Beach and the Outer Banks with air temperatures at the Southern beaches into the mid 90s. 

Saturday and Sunday Atlantic Beach Forecast

So are you thinking of heading to the Chesapeake bay waters?  Well, again, temperatures will once again be sizzling!  Highs climbing into the mid to upper 90s with a light West wind. 

Although a general ridge of high pressure will deliver us a good bit of sunshine, thunderstorms aren't out of the question with the heating of the day and instability.  Keep an eye to the sky during the afternoons, but also some storms may develop late in the evening and with the warm nights could last for quite a few hours.  The StormWatch7 weather team will follow storm developments and have all the updates on our homepage.

Saturday and Sunday Chesapeake Bay Forecast

Overall, stay cool and hydrated if you plan to be outdoors.  Have a great weekend!

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Fires From Space

June 28, 2012 - 05:37 PM
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The heat is building over us and tomorrow may be a record breaker.  But dryness is also increasing and with it the fire danger.  Nothing like the terrible fires in the west, especially Colorado, but a fire in Shenandoah National Park and westerly winds have brought the small of smoke to the DC area.  Look at this image from 400 miles above us today from the NASA AQUA Satellite. .

ZZZZZ

 MODIS Today-SSEC/University of Wisconsin-Madison

And a very tight view.  A bit of smoke from a fire on the Massanutten also .  

ZZZZZ

Be careful with the heat that will be with us and with it, always a fire danger.

 

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Hottest Ever: All time records

June 28, 2012 - 05:00 AM
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What heat in the Great Plains!  Cities like McCook, Nebraska on Tuesday, June 26 recorded their all time hottest day. The temperature soared to 115°.  Well not hottest ever, but hottest in weather records. A careful difference.  Look at the many records and high temperatures Tuesday, June 26 and again Wednesday June 27. 

High Temperatures Tuesday, June 26, 2012
High Temperatures Wednesday, June 27, 2012

Denver tied their "all time" record of 105° 2 days in a row on Tuesday.  And we have seen headlines such as "June 2011-May 2012 Hottest 12 months every in United States".  But be a bit careful about what all this means.  Is the climate slowly warming? Yes. Does this mean more heat waves next summer like this, and that from  June 2012 through next May 2013 again it will be the "hottest 12 months every in the United States"?  Want to make a bet?  Look at this  graph for the 12 month period, June-May temperature, over the last 100+ years.

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Courtesy National Climatic Data Center/NOAA
This last 12 month period was sure a record breaker with U.S. temperature more than 3º above the long term average.  But remember, at best most spots in U.S. weather records only go back about 150 years. So "all time" is not really ALL time, but within weather records time.  Plus, look at the long term trend (Diagram above).  Sure, warming at a significant rate of 1.3º/100 years, but plenty of year to year fluctuations.  The last 12 months across the U.S. the temperature was so much above average, that the probability of this coming June - May 2013 being warmer is very low.  Ready for a wager?  On the other hand, I'll bet that there might be some headlines next year such as  "June 2012 to May 2013 much cooler than same period last year-There is no warming".   Be careful about reading just the headlines.  Read between the lines.  Better yet, look at the actual data and long term trends.  Of course keep cool and hydrated this weekend, records or not. Here they are with our forecast highs from Wednesday.

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Record challenging heat is on the way...again

June 27, 2012 - 08:26 AM
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As a rule of thumb, in order to tie or break a record high at DCA in late June and early July, the temperature has to hit the triple digits.  That kind of heat is not out of the question by this weekend, but upper 90s seems more likely at this time.  The hottest days of the near future will be Friday, Saturday and Sunday.  All of which hold record highs at or above 100°.

Here's a comparison showing our forecast (as of Wednesday) compared to the record high for each day:

 


June 29 - July 1

So far DCA has recorded ten 90° days this year with the warmest temperature being 99° on June 21st. In comparison, last year at this time there were eleven 90° days. Also, 2011 featured a total of 49 90° days. 

Records fell earlier this month, which was noted here.

Ways to Beat the Heat:

Visit the Ocean

Visit the Pool

 

 

 

 

 

 

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National Mall Sky: A beautiful classic sky of D.C.

June 26, 2012 - 04:30 PM
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From our vantage point high above Rosslyn, Virginia we have a perfect view down the National Mall.  This day the sky was brilliant blue and with northwest winds the air very dry and a field of cumulus clouds formed.  This is how the sky looked over an hour and a half during the middle of the afternoon.  Enjoy.

 

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Tropical Storm Debby expected to move into Florida

June 26, 2012 - 10:19 AM
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Check out the rainfall totals from Tropical Storm Debby so far. In some areas, over 20 inches of rain has fallen. This includes areas in the Panhandle of Florida just south of Tallahassee. Know that these are radar estimated rainfall totals, so they aren't perfect, but they are close. Check out a list of rainfall totals from the past 72 hours.

The storm itself should finally begin to slide to the east but unfortunately will continue to bring it over the same areas which have been hammered by heavy rainfall already. There is also a storm surge threat, as the Tampa area had a surge already this morning and will again in the coming high tides until the storm moves ashore. Here is the forecast path as of 11am this morning.

Tropical Storm Debby Forecast Track as of 2pm Tuesday

Also check out the briefing page put together by the National Weather Service office in Jacksonville. Very detailed and great information. The main concern in that area will be more widespread flooding. The severe threat doesn't appear to be quite as bad as a few days ago, but it is still there for some spin up tornadoes through much of Florida. Here's a picture of a boat washed ashore from Peter Acker Photography.

Photo of a boat washed up on shore in Sarasota, FL from Peter Acker Photography

There isn't much of a concern for the D.C. area, though the beaches this weekend may be hit with some rough surf put out by the storm. That could be an issue, as it is going to be particularly hot this weekend and a lot of people will want to take to the water. Just know there will be a rip current threat so keep that in mind if you or your kids decide to get in. Look at the satellite picture showing the cold front from yesterday helping keep this system south and steering it over Florida and into the Atlantic. Good news for us.

GOES Project Satellite Picture of Debby

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Space Station visible tonight

June 25, 2012 - 02:47 PM
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The International Space Station will be flying high over the DC area tonight.  Our skies are clearing and with nice dry air and clear skies a perfect summer evening to see the space station now almost 1 million pounds and flying about 200 miles above us at about 18,000 miles/hr.  Here's where to look courtesy of Heavens Above 

ZZZZZ

  Low in the northwest sky at 9:47 PM then very bright and 45° high in the sky at 9:50 and then at 9:52 as it moves south it enters the earth's shadow and disappears.  Here's where to look in the sky.  Tonight it will be very near the moon at about 9:48 PM 

ZZZZZ

 

Enjoy and share

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Downburst or Tornado: What happened around Bladensburg?

June 23, 2012 - 06:04 PM
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Friday night there were numerous warnings for severe thunderstorms, but the only major damage was from a storm that produced a powerful "downburst" that did some terrible damage in the Bladensburg area (as you can see in this photo gallery).

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Courtesy Hieu H. Nguyen/University of Texas

Three weeks ago, on June 1, there was an equal number of warnings for tornadoes during a dangerous afternoon and evening, and there were a number of small, short lived tornadoes later confirmed by the National Weather Service. 

ZZZZZ

But here in the Washington area, severe thunderstorms, downbursts and straight line winds have done more damage over the years than any tornado.  Terrific advances in radar meteorology over the last 20 years (now with dual-pol)  have allowed us to see storm structure in small detail.  But while Doppler can "see" rotation in storms and even very small whirlwinds in the air above us, which technically might produce a small tornado and trigger a TORNADO WARNING and result in some minor damage, downbursts can produce winds of 100mph or higher and cause major damage. More damage than a small tornado if they hit a metro area as we saw last night.  Downbursts happen as the rain and hail generated in a thunderstorm cell begin falling and drag the air toward the ground, cooling the air as the precipitation melts and evaporates. As a result, a falling bubble of cold air that can reach speeds of 100 miles per hour is generated.  Then BANG, the racing cold bubble of air hits the ground and  causes the damage we saw.  Remember, the severe storm that generated a big downburst, or macroburst, that caused major damage in Great Falls, Potomac, Bethesda, Chevy Chase, NW D.C. and in Prince George's County on June 14, 1989? 

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Courtesy Chevy Chase Historical Society

More than 250,000 residents lost power. It was days before the damage was cleaned up. A number of homes were destroyed.  Year after year, severe thunderstorms and downbursts cause more damage and take more lives due to flash flooding, lightning and wind than tornadoes in our area. ZZZZZ
NWS Doppler image of tornado near Yazoo City, MS April 2010

Doppler radar can detect spin and horizontal winds, but really can't measure vertical winds or rapidly descending cold air that will become the downburst when it hits the ground.  Many thunderstorms or individual "cells" within a line of thunderstorms may last for only 30 minutes.  They rapidly grow with rising warm, humid air, produce rain and rise to 5-7 miles high, where the temperature may be 20 to 30 BELOW ZERO in a few minutes.  I still like a Snickers bar every once in a while.  OK I admit it.  A Snickers has about 120 calories.  Imagine 1 TRILLION snickers.  That is how many calories or energy an average thunderstorm can generate in its 30 minute average life.  Surprised that this atmospheric monster with all those Snickers can cause damage like the folks in Bladensburg experienced?  About 50 years ago we didn't have the knowledge and tools to be able to accurately predict things like floods, snow storms, hurricanes and tornadoes.

 

ZZZZZ
 

 Mike Smith/WeatherData

Predicting downbursts and microbursts is still a challenge but I predict that 50 years from now we will not only be able to better see these terrible winds, but give 20-30 minute advance warning for a local town and even neighborhood, such as Bladensburg. While we can't stop the damage, people can be well protected and ready for what some described as a bomb blast.  In the future, I predict we may hear a 'DOWNBURST WARNING" and know here in our area it might be more serious than "TORNADO WARNING"

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D.C. area record round up

June 22, 2012 - 08:14 AM
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With highs nearing the 100 degree mark Wednesday and Thursday, surely some records had to be broken right? The answer was yes, though it was very close. Record highs were broken and tied at both Reagan National and BWI Marshall but were not seen at Dulles Airport. Check out the list below.

Records broken Thursday

Reagan National set a record high on Thursday of 99 degrees. This broke the record of 98 set back in 1988.

BWI Marshall tied a record high of 100 degrees on Thursday which was last seen in 1923. Nothing like tying a 89 year old record!

That was it for record high temperatures. There were other records though but in the form of record high minimum temperatures. This seems to be happening more often and I think could be attributed to the urbanization in bigger cities. The asphalt and concrete in the roads and buildings in the cities can retain heat much better than the forests that used to be in its place. Check out the record high minimums met.

Reagan National set a record high minimum of 78 degrees Thursday which broke the record of 77 degrees set back in that terribly hot summer of 2010.

Friday morning, the low temperature at Reagan National was 82 degrees. The old record is 76 degrees set back in 1988. The low will all depend on what occurs before midnight tonight, as temperatures may cool below 76 by then with the storms and passing cold front.

Think about this though... also confusing but worth a mention. On Monday in the SAME WEEK of this record heat, Dulles Airport set a record low maximum temperature of 68 degrees. The old record was 71 degrees set back in 1973. Nothing like being 30 degrees warmer just two days later!

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Beach Forecast: From Atlantic City, NJ To The Outer Banks, NC

June 21, 2012 - 07:45 PM
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After record breaking temperatures Thursday, you may be thinking of hitting the beaches for the upcoming weekend.  A cool front will move through Friday and will bring some relief from the sweltering heat and humidity.  The cool front will not bring a significant drop in temperatures, but it will drop highs back to more seasonable levels and dewpoint temperatures will be back into the upper 50s to low 60s.

Heading to the Delaware and Maryland beaches?  It looks like highs will be in the mid 80s with a light Northerly breeze.  Plenty of sunshine is expected, as the front will be well offshore.  The map below shows the forecast for both the Northern and Southern beaches with temperatures and cloud cover for both Saturday and Sunday.  Notice water temperatures are in the low 70s along the Delaware Bay down to Ocean City.  Once you get to Virgina Beach, water temperatures start to rise in the upper 70s to low 80s.  Here's where to go to find local surf conditions

Beach Forecast for Saturday & Sunday

Beaches farther South, from Virginia to the Outer Banks, will deal with a few lingering showers and thunderstorms on Saturday, as the cold front stalls over the area.  Temperatures will be in the mid to upper 80s on Saturday, but then the front will push offshore and bring more sunshine, lower humidity, and slightly lower temperatures (low 80s) for Sunday.  As for the Chesapeake Bay beaches the weather looks delightful.

Chesapeake Bay Forecast

The Chesapeake Bay beaches will have pleasant summer weather. Highs will climb into the mid to upper 80s both days with light Northerly winds.  Also, enjoy comfortable humidity.  Expect a good bit of sunshine on Saturday, but with an advancing weather system, more clouds are possible late Sunday. 

Enjoy the weekend wherever you are.  Have fun and be safe!

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Summer Solstice 2012

June 20, 2012 - 07:10 PM
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We welcome the summer solstice with a mini heat wave.  The solstice occurs at 7:09PM DST Wednesday, June 20, 2012 and is the time the sun is perpendicular to the Tropic of Cancer.  It is the longest day of daylight and marks the celestial start to summer.  Sunrise Wednesday was at 5:43 AM and sunset was 8:37 PM.  That's a total of 14 hours 54 minutes.  Plenty of time to enjoy any and all things outdoors... that is, if you can stand the heat and humidity on some summer days. 

To help visualize what happens on the solstice, take a look at the diagram below.  The earth revolves around the sun along an imaginary axis.  Notice the earth is tilted (23.5°).  The solstice occurs at the time the sun is directly over the Tropic of Cancer, which is at 23.5°.  For every place North of the Tropic of Cancer, the sun is at its highest elevation - directly overhead at high noon.

Solstices & Equinoxes - Courtesty NOAA

For the Northern Hemisphere, the summer solstice means the start of summer, whereas in the Southern Hemisphere the solstice marks the start of the winter season (the Earth is titled away from the sun, so less daylight, which leads to cooler temperatures).  The reason we have seasons is because of the tilt of the Earth's axis.  It also has to do with the amount of daylight and the atmospheric path length. 

NASA

You may wonder, if the sun is at it's highest elevation at noon on the solstice and we have the longest hours of daylight, why it isn't the hottest day of the year, too. The main reason is because it takes much longer for the sun to warm the ocean, which absorbs and radiates the sun's energy over time. Typically, the hottest days occur in July and August.  The deep, ocean water takes much longer to heat compared to the land.  You also have to remember the Earth is made up of about 70% water.  With all that water, it's going to take longer to warm up and, in turn, give us our hottest temperatures of the year based on the moderating effects of the ocean waters.

The amount of daylight will now slowly decrease until the winter solstice.  The sunrise/sunset times surrounding the solstice, though, only fluctuate by a few seconds, so enjoy the added daylight and time outdoors!

Happy summer solstice!

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Spectacular Sunset : Washington DC (VIDEO)

June 20, 2012 - 05:11 PM
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The heat is on here in Washington as in most of the east.  But before the heat really started blasting, photographer Ben Rice recorded these beautiful scenes and sunset Tuesday June 19.   No it's not some place in ancient Greece or sunset on the Acropolis where is it?

 A hint it is one of the most beautiful spots in the District of Columbia.  This was at a fundraiser and part was at a herb garden.  But  these columns are part of ancient history . . .well ancient American History.  But where and what?  I'll have the answer on ABC7 News at 11.  And for folks out of DC area I promise to update tomorrow with where this great video was taken.  Enjoy

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D.C. braces for a mini heat wave

June 19, 2012 - 10:46 AM
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Temperatures only hit 71 degrees at Reagan National Airport on Monday. 71 degrees! That's 14 degrees lower than the normal high temperature! High heat and humidity will move into the D.C. area over the next few days. This is considered dangerous heat because temperatures in the mid to upper 90s and dew points around 70 will make the heat index around 100 degrees or greater. Be sure to take care and keep a watch of your family to make sure they stay safe through this period of hot weather. One last thing, don’t forget your pets! If you can, bring them inside or at least give them shelter and plenty of water. Check out some of the forecast high temperatures across the country today. Keep in mind tomorrow and Thursday will be the warmest for the D.C. area.

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Dangerous Heat: Heat Advisories & Excessive Heat Warnings

June 18, 2012 - 09:07 PM
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You might be aware by now that we’re expecting some potentially dangerous heat in the D.C. region by the middle of this week. But what exactly is dangerous heat, or more specifically, how hot does it need to get in order to be considered “dangerous”? 

The answer to that question is: it all depends upon where you live.

Initially, that might make perfect sense to you. Where it’s generally hot for more of the year in cities like Phoenix, Las Vegas, and Dallas we have slightly different dangerous heat criteria than for cities where it’s generally cooler for more of the year like Detroit, Boston, and Buffalo.

So let’s start at the beginning. There are two types of official heat alerts issued by the National Weather Service: A heat advisory, and an excessive heat warning. The only difference between the heat advisory and warning is the heat index (H.I.) value. The heat index is the combination of heat and humidity that makes it feel even warmer than the ambient air temperature. 

Courtesy: NOAA/NWS


For most of the country, a heat advisory is issued when the heat index is expected to rise at or above 105°. Again, for a majority of the country an excessive heat warning is issued when the heat index is expected to be above 110°. These are the general guidelines, but for particular regions each local NWS office has its own specific criteria. Some local regions can meet heat advisory/excessive heat warning criteria too if low temperatures don’t dip below certain thresholds. Here are two examples of regional differences in heat advisory criteria.

Courtesy: NOAA/NWS


Note how you have distinct differences between the Upper-Midwest/Great Lakes/Northern Plains forecast areas, the Midwest/Central Plains regions, and the Rocky Mountain/Front Range region. For the mountains, a heat advisory is issued when the H.I. is forecast to reach above 95°! For the plains areas leading up to the mountains (known as the Front Range) the H.I. needs to reach over 100° before a heat advisory is issued. For the yellow shaded areas (Northern Plains/Upper-Midwest/Great Lakes) the heat index needs to reach 100° for a heat advisory, and for the red shaded areas (Central Plains/Midwest) a H.I. of 105° or greater needs to occur.

Courtesy: NOAA/NWS


Here’s a look at the Mid-Atlantic/Northeast region heat advisory criteria. Note the Philadelphia metro area. We’ll get to that. For the areas shaded in green, a H.I. of 100° is needed before a heat advisory will be issued. For the D.C. Metro and much of the Mid-Atlantic, a H.I. of 105° is required before our NWS office in Sterling, VA will issue a heat advisory. And for one little area of South Carolina and Georgia, the H.I. can get as high as 114° to only warrant a heat advisory! 

I’m sure you’ve noticed the Philly area denoted by the shading that refers to “Kalkstein Procedures”. Dr. Laurence Kalkstein is a professor that has developed specific procedures for certain cities all over the world that are particularly susceptible to heat related fatalities. Philadelphia is one major metro that has its own heat procedures/alerts/warnings researched by Dr. Kalkstein, that’s why you see more alerts issued for Philadelphia in relation to the D.C. region and the Northeast. 

That covers some of the regional variations for heat advisory criteria. Take the same maps and change the heat index values, and you’ve got the same areas defined yet for excessive heat warnings. Here’s the map of our region with the respective heat index values needed for a heat warning: 

Courtesy: NOAA/NWS

 

As you can see, we need a H.I. value of 110° in order for an excessive heat warning to be issued.
Our local NWS office has a webpage that defines certain severe criteria for our specific region:

Regional D.C. Severe Definitions 

Remember to be careful when we’re expecting a bout of extreme heat. If working outdoors, take frequent breaks, try to stay in the shade as much as possible, wear breathable hats that block the sun but let you expel heat from your head, wear light colored/loose fitting clothes, and drink plenty of water.

We’ll keep you up to date on the expected heat right here on WJLA.com, along with our team of ABC7 meteorologists on WTOP radio, News Channel 8, and of course ABC7 News!

 

 

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June 1, 2012 tornadoes: More details from the outbreak

June 18, 2012 - 12:02 PM
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The severe weather outbreak that took place 17 days ago remains fresh in most of our minds.

Luckily in this region, we rarely have to face widespread severe weather that produces large long track tornadoes like they see in the Midwest.

However, June 1 was a stark reminder that we are not exempt from tornadoes and even widespread tornado outbreaks. This outbreak, while not one of worst that area has seen, was still significant, producing eleven confirmed tornadoes in total. Out of those seven were rated EF0 producing winds from 65 to 80mph and four were confirmed EF1 with winds up to 100mph.

The EF1 tornado in Hartford County hit the Pleasant Hills area, two miles southwest of Bel Air, Md., and not only was one of the strongest twisters but also spent longest time on the ground, tracking a total of 1.25 miles/300YDS. Additionally, that tornado produced two injuries but no fatalities. The National Weather Service out of Sterling recently released more detailed information for each of the eleven twisters.

One the same page you will see the image below  that is also interactive with local storm reports of funnel clouds, flooding, hail and more.

Credit: NOAA/NWS

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Monday morning weather timelapse (VIDEO)

June 18, 2012 - 08:44 AM
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As this timelapse video shows, you could briefly see the sun this morning before more rain passed through Washington.  By the way, Monday through Saturday is the stretch of days with the most amount of daylight all year: 14 hrs 54 mins.

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