Severe Weather Threatens Southeast Today...

Above is the latest severe weather outlook from the Storm Prediction Center (SPC) in Norman, OK, valid for today through tonight. Severe storms are forecast within the yellow shaded area on the image.

Strong to severe storms are already ongoing across portions of the Deep South this morning, as shown by a recent radar image from Jackson, MS:

Damaging wind gusts and large hail will be the primary severe weather threats across the severe weather outlook area today.  Isolated tornadoes cannot be ruled out, particularly from any storm that is able to remain isolated and become well organized.

The ongoing activity will re-organize and/or reintensify this afternoon, and new development will also take place along and near several surface boundaries across the region.

The highest risk of large hail this afternoon will take place within the red shaded areas on the image below, which includes the Tuscaloosa, Birmingham, Chattanooga and Atlanta areas:

If you live within or near the severe weather threat areas outlined above, please remain alert today.  Listen to NOAA Weather Radio, local media or another trusted source for later statements, watches and possible warnings.  Remember, a Severe Weather Watch means to "watch out" for possible severe weather.  A Severe Weather Warning means that severe weather is imminent, and you should seek shelter immediately.

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Severe Storms Possible Mississippi / Tennessee Valley Region Today...

The same storm system responsible for severe weather across the Southern Plains yesterday is moving slowly East and will bring a threat of active weather across portions of the Mississippi and Tennessee Valley regions today.

Above is the latest severe weather outlook for today from the Storm Prediction Center (SPC) in Norman, OK.  Severe storms are forecast within the yellow shaded areas, with large hail and damaging wind gusts the primary threats.  Isolated tornadoes cannot be ruled out from any particularly organized storm that is able to remain isolated from the larger overall precipitation area.

The cool front of Pacific origin that moved across the Southern Plains yesterday is approaching the Western edge of the severe weather outlook area this morning.  This and other boundaries left over from thunderstorm activity of yesterday will serve to focus for new development and/or reorganization of the existing activity later in the day.

Along the Mid-Atlantic Coast and adjacent portions of inland North Carolina and Virginia, a few strong to severe storms can be expected in the afternoon heating today.  Damaging wind gusts and some hail are the primary threats.

Storms may also approach severe limits, mainly with respect to hail, during the peak heating this afternoon across portions of Oklahoma and along the Kansas/Oklahoma border region (as indicated in brown on the above image).  Thunderstorms will pop-up during the heat of the day beneath a cold pocket of air aloft.  This may allow some of the storms to produce hail near severe limits and/or gusty winds before sunset.

Folks living across the severe weather threat areas should remain alert today.  Listen for later statements and possible warnings, and be sure that you've identified the safest place to go for shelter in the event that severe weather threatens your area.

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New Technology Coming "Soon" to a Radar Site Near You...

The National Weather Service radar in New Braunfels, TX (which serves the Austin and San Antonio Metro areas) was just put back online about an hour ago after the "dual polarization upgrade" was complete.  It will be interesting to see how the radar performs during the severe weather event that is expected to take place this afternoon.

With this and many other sites coming online recently, I thought it would be appropriate to put the following post regarding Dual Polarization Radar back up to the top of the blog.  The original post from July 9, 2011 follows below:

The first major upgrade to the National Weather Service's WSR-88D radar network in over 20 years is getting underway across the country. The latest technology being rolled out is called "Dual Polarization".

"What the heck is that???" and perhaps more importantly, "what's in it for me???" - you ask?

In a nutshell, the conventional WSR-88D radar sends out a single pulse of radar energy into a storm, which captures a generally horizontal snapshot of raindrops, hailstones, snow flakes, etc. (as shown on the left half of the illustration below): 

The right half of the same illustration shows what Dual Polarization technology will do.  The radar will not only send out a horizontal pulse of energy (red) but also a vertical pulse (blue), which will give us a far greater depiction of the size, shape and density of raindrops, hailstones, snow flakes and other targets.

Dual Polarization (also known as Dual-Pol for short) radar technology has been installed and tested for a number of years at the NSSL in Norman, OK.  Within the last few months the rollout to operational NWS radar sites has begun.  The first site to be installed was at Vance AFB, OK, followed by Phoenix, AZ and Morehead City, NC.  As this is being written, Dual-Pol is currently being installed at radar sites near Pittsburgh, PA and Wichita, KS.  A complete schedule of the rollout, which will continue into early 2013, is located here.  Each installation takes about 2 weeks to complete, during which time the radar unit will be out of service to allow for both the upgrade and training at the local NWS office.

Now that we know what Dual-Pol is, what will it do for us?  Perhaps the greatest initial impact will come in the form of vastly improved precipitation estimates.  With the current WSR-88D technology, a thunderstorm must be in existence for about 1 hour before the radar is able to estimate how much rain is being produced.  With Dual-Pol technology, we'll receive a nearly instantaneous read on rainfall rates.  This will obviously have enormous potential not only in regard to flash flooding & other severe rainfall situations, but also in dealing with snowfall, ice accumulations, etc.

Dual-Pol rainfall estimate (left) vs. WSR-88D estimate (right)

Current technology on the WSR-88D sometimes makes it difficult for the radar to differentiate between hailstones and raindrops, particularly at greater distances from the radar site.  As a result, the 88D often over estimates the rainfall rate with severe storms (mis-identifying hail as heavy rain).  The Dual-Pol technology will make the estimates more accurate in these situations (see the above image for just one example).

Dual-Pol technology will also aid in the tornado warning process.  Right now, it takes a fairly large debris field (which usually means a fairly large tornado) to produce a debris signature on the WSR-88D.  A Tornadic Debris Signature (or TDS) can be detected by Dual-Pol radar in association with a much smaller debris field (which means a smaller tornado).  Please note that I am talking about debris being detected by the radar here, not rotation.  In the database that was used to develop some of the algorithms for the new Dual-Pol radar system, debris signatures were observed with tornadoes as weak as EF2 intensity (winds of 111-135 mph).  Today we would normally only see a debris signature on the WSR-88D in association with a tornado of high-end EF3 to EF4 intensity or greater (roughly 150+ mph surface wind speeds). 

The above image was taken from the Dual-Pol radar at the NSSL radar testbed in Norman, OK.  The white and pink colorations within the white circled area on the image shows where the Dual-Pol is detecting tornadic debris.  This debris field was not as easily identified on conventional radar.   Of course, you must also keep in mind that the tornado, regardless of strength, would have to be in an area where debris could be picked up before the radar would be able to see it.

Dual-Pol technology is likely to be of particular help in the tornado warning process in 2 specific areas:  (1).  when the circulation is rain wrapped and not easily visible by storm spotters and (2).  when the circulation is occurring at night and not easily visible to storm spotters.

The system will also be a big help with non-precipitation events as well.  Because it takes a more detailed "slice" through the atmosphere, Dual-Pol technology will be able to forewarn of local hazards near the radar site, like dust storms for example.  Below is an image taken by the Dual-Pol radar near Phoenix, Arizona this past Tuesday, July 5th:

The red dashed line notes the leading edge of a wall of dust that was advancing toward the city from the South.  Here is what that advancing dust cloud looked like outside the National Weather Service office:

The enhanced scanning of the atmosphere very near the earth's surface by the Dual-Pol radar will allow it to detect very small particles, like the dust particles above, as well as insects, birds, etc., close to the radar site.  The WSR-88D is already able to do this to some extent, but the resolution and detail of such scans will be even greater with the new Dual-Pol technology.

Now that we have a few Dual-Pol radars online and others being added about every 2 weeks, I'll be posting some images of the new technology in action, and comparing it to the old where possible.  Stay tuned!

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Severe Weather Update - Southcentral Texas...

The above visible satellite image was taken a short time ago. I have drawn two boundaries on the image, the first, in yellow, is a surface dryline. The second, in red, represents the leading edge of cool air that was blown out of nighttime thunderstorm activity along the Red River, and is now moving Southward.

Thunderstorms are already trying to develop along the dryline to the West of San Antonio, as shown on the radar image below:

It may take another couple of hours for this part of the dryline to become active, but plenty of sunshine to the East of the boundary over southcentral Texas is making the atmosphere very unstable.  Once thunderstorms persist and break through a capping inversion that is present across the region, they will rapidly become severe.  This process will be aided by the arrival of energy from an upper-level disturbance to the West by early afternoon.

Based on present trends, thunderstorms may threaten the I-35 corridor from San Antonio through Kyle and Buda to Austin anytime after 2pm.  In particular, the time period of 3-5pm appears vulnerable.  If the dryline were to make a move Eastward more rapidly than currently expected, this timing may be earlier.

Folks living across southcentral Texas, including the Austin-San Antonio corridor along I-35, should remain alert this afternoon.  Take a few moments to review severe weather safety tips now, and have a sheltering location in mind should severe weather threaten your area later today.

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Severe Weather Southern Plains / Midwest / Ohio Valley Today into Tonight...

Above is the severe weather outlook for today via the SPC in Norman, OK. Severe storms are forecast within the yellow shaded areas on the image.  Large hail and damaging winds will be the primary threats today, although a few tornadoes cannot be completely ruled out with any well organized storms.

In the Southern Plains...a surface front/dryline feature is located roughly near the Western edge of the severe weather outlook area this morning.  The boundary will move Eastward during the day, with thunderstorm development likely along and ahead of the boundary.

Thunderstorms could form and/or existing storms intensify along this boundary  anytime today, however the best chance of severe weather will come from midday into the afternoon as the atmosphere becomes more unstable, and a disturbance moves in from the West.

Very large hail is possible in this region, perhaps on the order of 2 inches in diameter or larger with the strongest storms, as shown within the yellow and black hatched area on the image below:

Across the Midwest and Ohio Valley region... thunderstorm development is expected this afternoon into this evening along a North-bound warm front and ahead of a Southeast-bound cold front, both of which will be traversing the severe weather outlook area in that region.

Large hail and damaging wind gusts are the primary threats in this region, and one or more clusters of organized thunderstorms could form by late evening and move East/Southeastward across the Ohio Valley overnight tonight.

If you live in or near either of the severe weather threat areas for today, please remain alert to the possibility of severe weather during the indicated time periods.  Listen to NOAA Weather Radio, local media or another trusted source for later statements and possible warnings.  Also, please make sure that you've identified the best sheltering option for your location as well.

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Severe Thunderstorms Likely Central and Southern Plains Today / Tonight...

Above is the latest severe weather outlook for this afternoon through tonight from the Storm Prediction Center (SPC) in Norman, OK. Severe storms are forecast within the yellow shaded area on the image.

There are two basic areas on which to focus within the overall severe weather outlook area.  The first is associated with a weak disturbance moving toward the North/Northeast along the middle Texas coast.  This disturbance is currently causing strong thunderstorm activity in the Houston area, and a threat for strong to severe storms will continue to spread North/Northeast into southeast Texas and adjacent portions of Louisiana during the day today.

Hail and strong, gusty winds will be the primary severe threats in this area.

Further West, a surface front and dryline are forecast to extend near the Western edge of the severe weather outlook area, from central and western Kansas in to western Oklahoma and west Texas by late afternoon.

Thunderstorms are forecast to develop along these boundaries by late afternoon, with large hail, damaging winds and isolated tornadoes possible.  Very large hail (2 inches in diameter or greater) is possible within the red and black hatched area on the image below:

Once developed, this activity may organize into one or more thunderstorm complexes that will move Eastward overnight and into the pre-dawn hours of Tuesday.  It is unclear at this time exactly where those complexes may develop and/or threaten overnight...stay tuned for updates today, especially if you live toward the East of the initial severe weather outlook areas.

If you live in the severe weather threat areas for today, please remain alert to the possibility of severe weather during the indicated time periods.  Listen to NOAA Weather Radio, local media or another trusted source for later statements and possible warnings.  Also, please make sure that you've identified the best sheltering option for your location as well.

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Severe Storms Possible Midwest/OH Valley Again Today...

Severe thunderstorms are forecast within the yellow shaded areas on the above image for today. The greatest threat will be this afternoon and evening over that portion of the Midwest and Ohio Valley region.

Large hail and strong, gusty winds will be the primary severe weather threats across that region this afternoon and evening.  An isolated tornado or two cannot be completely ruled out.

Elsewhere, there are a couple of other areas where strong to locally severe thunderstorm development is possible this afternoon and evening.  The first is over southern Texas (near the "See Text" wording on the image above in that part of the state).  A weak disturbance along the middle Texas coast is drifting West/Northwestward, and could spawn isolated to scattered thunderstorms during the heat of the afternoon.  A few of these could become strong to locally severe, with hail being the greatest threat.

The second area to watch for storms approaching severe limits is over the northern Mississippi Valley/western Great Lakes region.  This will mainly be a risk for tonight after Midnight, with a few storms containing hail near severe limits possible near the "See Text" wording in this region on the map.

If you live in the severe weather threat areas for today, please remain alert to the possibility of severe weather during the indicated time periods.  Listen to NOAA Weather Radio, local media or another trusted source for later statements and possible warnings.  Also, please make sure that you've identified the best sheltering option for your location as well.

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