Too Close for Comfort - Twice! (Part 2 of 4)

Two days after our unplanned tornado shelter test, mother nature decided that it would be good to have another go at it.  The severe weather outlook for North Texas was pretty similar to what it had been on Friday, April 24 - in that for Sunday the 26th we were once again in the enhanced risk area. By the 11:30 AM update, however, the SPC had issued a upgraded parts of North Texas to a MODERATE RISK.  Areas included Dallas, Tarrant and Ellis Counties.

From the SPC:

EXPECT STORMS TO GROW UPSCALE INTO ONE OR MULTIPLE MCSS OVERNIGHT WITH THE RISK FOR POTENTIALLY MORE WIDESPREAD DAMAGING WINDS INCREASING ACROSS PORTIONS OF CNTRL INTO ERN TX.  LARGE HAIL AND A FEW TORNADOES WILL ALSO REMAIN A CONCERN. 

At 4:15 PM (2115Z), the first tornado warning was issued for Erath county - about 95 miles away as the crow flies.  Normally, this wouldn't be a big deal, but due to the directions the storms were heading that day, if the storms DID hold together, they'd be knocking on our door step in just a few hours. 

 

 

Just before 5 PM that evening, but we were put under a tornado watch.

 

Continuing its initial northeasterly track for about 2 hours, the leading storm which had prompted the first tornado warning started to fizzle out while another large supercell had formed in along the same track as the first.  Another tornado warning (#2) was issued at 6:18 PM (2318Z) for nearly the same spot.

 

 

 The new leading storm continued northeastward for a short while, but then began to take a right turn - an indication that the storm was strengthening.  Another tornado warning was issued at 6:39 PM (2339Z)

 

 

30 minutes into this new warning, the storm changes direction back northward and the 4th tornado warning was issued at 7:08 PM (0008Z).

 

 

Strangely enough, this leading cell soon began to die out and be replaced by a strengthening storm further to its southwest.  And just like its predecessors, a tornado warning was called for it at 8:15PM (0115Z).

 

 

After the hook portion of this storm moved out of Earth county and into Somervell county, it began to take and easterly (or right-moving) direction - possibly indicating that it was strengthening.  The 6th tornado warning of the evening was issued at 8:25 PM (0125Z) for the large storm complex to the northwest of this main supercell.  However, my main focus continued to be this monster. 

 

Over the next hour (roughly 8:40 PM to 9:40 PM), this huge storm made quite a bit of progress toward us. Tornado warning after tornado warning literally started to pave a way to our door.  No joke!  Here's an image of the outlines tornado warnings 7 though 11 - note the time stamps for each new warning. 

 

 

Notice how the last 3 warnings seem to get stacked right on top of each other?  That's because the storm pretty much became stationary around the Granbury area!  Reports of a mile-wide wedge tornado began to filter in on Twitter and broadcast media.  Luckily, these reports were grossly overestimated due to it being after dark.  Although radar velocity signatures were measuring a very wide rotation area, not all of that rotation was making its way to the ground.   Numerous tornadoes were confirmed, but all were EF-0.  Thank goodness!

 

The whole time this monster supercell was sitting in place, a separate complex of storms had been forming to the northwest. They grew much larger and appeared to move southeast -not east - like the leading storm until around 10:15 PM when what appeared to be 3 supercell structures were nearly in a due east-west line.  

 

 

Another 2 hours pass as I watch these storms begin to congeal into the biggest mass of colors I had seen on radar in quite some time.  Then, at 12:12 AM (a full 8 hours after the initial warning was called 95 miles away) a tornado warning was issued for parts of our city.  Our house wasn't directly in the warned area, but due to it being so late at night, we decided it would be safer to head for the shelter.  Due to the erratic motions these storms had performed all night we surmised that anything could happen - and we wanted to be ready.

 

12:20 AM - Safely with the family in the bunker, I catch my first glimpse on the radar and finally make sense of what I'm seeing - a distinct pattern known as an MCV (Mesoscale Convective Vortex).  An MCV is a low pressure center within a larger MCS (Mesoscale Convective System).  An MCV pulls winds into a counter-clockwise (cyclonic) rotating pattern.  The once individual supercells had merged and mutated ending up with the northern tip of the storm taking on a distinct comma shape.  This comma shape was set directly over my house! 

 

 

For about the next hour the MCV slowly rotated counterclockwise as it crawled off to our east - dumping copious amounts of rain during the process.  When the tornado warning for our city was let to expire, we decided to leave the shelter.  3" of standing water in the yard made the dash back into the house all the more exciting.  By 1:30 AM the storm complex had pretty much moved out of the county and with work looming just a few hours in the future - it was time to go to bed.

 

Trying to summarize 13 hours of weather is meaningless unless you have the whole picture.  To that end, I created the following animation (281 frames) which spans the whole duration of time I just described above.  (Watch on YouTube for a larger experience)

 

 

 

Thankfully on this day (and night) we were never directly under a tornado warning or threatened with any major storm.  But after parsing through all of the day's events to write this article, I'm still in awe of the power and complexity of mother nature - which is one reason why I love studying the weather.

 

-Andrew

 

 

 





Too Close for Comfort - Twice! (Part 1 of 4)

Spring 2015 will certainly be a memorable storm season for a number of reasons. As I begin to write this article on the evening of May 30, 2015, North Texas has already seen about 85% of the rain that we would normally get throughout the year! Our normal yearly rainfall is about 36.1" and we have received 30.74" as of May 29. In fact, this is the first time since 2012 that all of North Texas is completely out of the "Severe Drought" category. These stunning facts hit both sides of the good news/bad news spectrum. Obviously the good news is that much of this great state is no longer under drought conditions. The rain for which we have prayed earnestly has shown up - nearly all at the same time. The bad news is that this has created catastrophic flooding throughout most of the state. The next few days is forecast to be rain free - which should hopefully give the swollen rivers and streams time to calm down a bit.

Apart from the record rainfall, we have seen a very high number of tornadoes as well! As of May 28, the Fort Worth CWA (County Warning Area) has had 57 confirmed tornadoes. A map showing the CWA as well as the breakdown in tornado strength is shown below (Image is courtesy of the Fort Worth National Weather Service office). The majority of these tornadoes were determined to be EF-0. People think that the EF (Enhanced Fujita) scale of rating tornadoes is based on wind speed, but the scale is actually based on the damage done by a tornado. The idea is that the higher the damage, the greater the likelihood of higher wind speeds.

The 4/24 listing from the above graphic is pretty significant for me. This was the first tornado for which we officially got to use our tornado shelter. You see, we moved to North Texas in 2006 and moved into our current house in 2007. Coming from Michigan, we always had friends and family inquire as to why we wanted to live in tornado alley. At the time I really didn't have an answer other than the fact that the economy down here was much better than up north. However, now that I chase storms for a hobby, the answer is blatantly clear!! Be that as it may, we were always cognizant of the fact that we did live in a tornado prone area. After going back and forth for a number of years and having a few close calls, we finally took action and in 2014 decided to pull the trigger and install an in-ground tornado shelter. Here's a picture of the "bunker" shortly after the install.

This behemoth weights in at just under 30,000 lbs., and is comprised of 5 1/2" thick steel reinforced concrete. It's supposedly F5 rated and I truly hope that I never have to test that out personally. 

Fast forward just over a year from the install date - to April 24, 2015:

The setup on this day was actually quite complicated. Storms that occurred the previous night were allowing for some uncertainty in the forecast. Still, much of north and central Texas were placed in the SPC's Enhanced Risk for severe weather.

The consistent theme throughout the day was the prediction that isolated supercells would form west of the DFW Metroplex and have the ability to "... grow upscale into multiple line segments/bows. Thus...large hail and damaging winds can be expected...along with tornadoes where cellular convective mode can be maintained..." (text lifted from the 4/24 SPC convective outlook page)

At 11:12 AM, the SPC put out Mesoscale Discussion #433 stating that there was an 80% chance that a tornado watch would soon be issued.

And sure enough at 2:10 PM, Tornado Watch #100 was issued for a large portion of north and central Texas.

This happened to be a Friday, so I left work early knowing that we may be in for some potentially crazy weather. After arriving home and trying to have a normal dinner time with the wife and kids, the first warnings of the area started to be issued. This image was from about 5:40 PM and shows the initial warnings near our area.

 

These warnings were 2 or 3 counties to our west, but they were tracking eastward very rapidly. The image above shows a local storm report (LSR) of a wind gust measuring 75 MPH along with 2" radar-indicated hail. These storms were nothing to mess with! Shortly after the complex moved into the DFW Metroplex, a tornado warning was issued for Tarrant County - which included the southern portion of the city of Fort Worth. By this time, the storms had done just what the SPC had predicted earlier - discrete cells had formed into a fast-moving line of storms.

Then, at 6:43 PM CDT, it happened - a tornado warning was issued for our area. We gathered the kiddos and let them know that we would have to go into the shelter - assuring them that this was safest place in the entire neighborhood! We gathered our stuff and headed into the bunker. It was very hot and stuffy in there and in the midst of trying to calm scared kids and listen to the weather radio I didn't realize that we had lost cell service - and in turn - lost radar visibility. Only after the tornado warning was cancelled and we were able to regroup inside our house did I find how crazy things looked.

 The following image, from around 6:50 PM, shows a split view of radar reflectivity (on left) and velocity (on right). The reflectivity image depicts areas of precipitation - with reds/oranges/pinks being the highest values. The velocity image depicts winds moving towards (green) or away (red) from the radar. The red outlined area is the tornado warning (a severe thunderstorm warning had already been issued) and my house is depicted by the red plus sign just north of the city of Waxahachie, in Ellis County. What's notable in this image is that a small inflow notch had formed on the leading edge of the storm. An inflow notch is an area that due to the mechanics of interacting winds, allows for circulation to take place. This notch was designated with a TVS (tornadic vortex signature) icon. While these icons are nothing more than algorithms generated by the radar and usually don't denote an actual tornado, it does highlight areas of enhanced inbound/outbound velocity. In this case, the TVS icon was right over the inflow notch and thus was the area of concern. If you look at the velocity portion in the area of the TVS, you can almost visualize a rotating section of the storm. The reds are moving left to right and the greens are moving right to left in a counter-clockwise (cyclonic) motion. (click the image for a larger view)

 



Over the next 2 frames of radar, the area of circulation seems to be heading in a quasi-northeast direction. Then something very interesting happens - the circulation changes direction and begins to head southeast - right towards our house! This change of motion does happen in discrete supercells as well. As they begin to intensify in overall strength, the storm changes its mean motion and begins to turn right - hence the term right-moving storm. With this line of storms too, that seemed to be what happened too. These next images show two velocity radar images - one at 6:58 PM and one at 7:00 PM. The brighter reds (the higher outbound winds) can clearly be seen moving to the southeast.

 

 

It is important to note that although a tornado warning was in effect, this was for radar-indicated rotation only. This means that although a tornado was not visually confirmed, the people at the National Weather Service felt that the radar showed enough persistent rotation to warrant issuing the warning.. Visual confirmation or not, any tornado warning is cause enough to seek shelter - which is exactly what we were doing at the time.
This image shows the circulation's closest approach to the house - a very close call indeed!

Finally, when the storms had passed and the outdoor sirens were turned off we all headed back inside. After settling everyone down I took my father, who had been with us throughout this whole event, around the neighborhood to check and see if there was any damage. Luckily the only thing we found was an overturned porta-john in the section of our neighborhood that is still under construction.

The setting sun made for an extremely high rainbow and the storms still racing to our east set the scene for this beautiful juxtaposition.

The next day, the NWS sent out survey crews and did confirm a brief EF-0 tornado south-southeast of Midlothian. It seems that the tornado spun up west of us but had dissipated and became a part of the larger straight-line winds before reaching us. Admittedly I was a bit nervous when all of this was going on, I did have good piece of mind knowing that we had our shelter in place. Here was the official release from the NWS:

ELLIS COUNTY TORNADO

RATING: EF-0
ESTIMATED PEAK WIND: 80 MPH
PATH LENGTH /STATUTE/: 0.87 MILES
PATH WIDTH /MAXIMUM/: 60 YARDS
FATALITIES: 0
INJURIES: 0

START DATE: APR 24 2015
START TIME: 652 PM CDT
START LOCATION: 3SSE MIDLOTHIAN
START LAT/LON: 32.45/-96.96

END DATE: APR 24 2015
END TIME: 654 PM CDT
END LOCATION: 3.8SSE MIDLOTHIAN
END LAT/LON: 32.45/-96.95

SURVEY_SUMMARY: THE EF-0 TORNADO WAS A SMALL PART OF A MUCH LARGER SWATH
OF DAMAGING WINDS TO THE SOUTH OF THE CITY OF MIDLOTHIAN.

EF SCALE: THE ENHANCED FUJITA SCALE CLASSIFIES
TORNADOES INTO THE FOLLOWING CATEGORIES.

EF0...WEAK......65 TO 85 MPH
EF1...WEAK......86 TO 110 MPH
EF2...STRONG....111 TO 135 MPH
EF3...STRONG....136 TO 165 MPH
EF4...VIOLENT...166 TO 200 MPH
EF5...VIOLENT...>200 MPH*
 
The weather setup for the next few days was much of the same - but little did we know at the time that we'd have to use the shelter again so soon!

-Andrew