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

 

 

 





The Dangers of Downbursts from Pulse Severe Thunderstorms

On Saturday, August 16, 2014, the Storm Prediction Center put all of North Texas into it's lowest probability category for severe weather. 

Our area didn't even make it into a SEE TEXT region, yet with our moderately unstable air and weak wind shear environment, conditions were favorable for pulse thunderstorms which were capable of producing a phenomenon called DOWNBURSTS.

A pulse thunderstorm is a single-cell thunderstorm forms and dissipates relatively quickly and typically doesn't yield any severe weather.  However, some pulse storms grow fast enough and drop enough rain to produce a downburst.  A downburst occurs when rain-cooled air falling out of a thunderstorm hits the ground and begins to spread out in all directions.  This expansion of winds (called outflow winds) can sometimes reach 100 MPH and cause quite a bit of damage!  The easiest way to visualize this is to imagine pouring water out of a glass and onto the ground.  As the water slams into the ground, it immediately fans out in all directions, pushing air in front of it.  Is this forward motion that can drive winds as far as 50 miles away from the original storm.  Here is a good visual representation, shown in cross-section. (h/t Wikipedia)

There are actually 2 varieties of downbursts - wet and dry.  A wet downburst happens when rain falling out of the thunderstorm reaches the ground.  A dry downburst happens when rain falling from a storm evaporates before hitting the ground.  In either case, it's the outward-spreading winds from this cooler, sinking air that is responsible for causing damage.  Downbursts smaller than 2.5 miles in diameter are called micro downbursts or microbursts.  Downbursts greater than 2.5 miles in diameter are called macro downbursts or macrobursts.

Let's examine a real-world example of downbursts.  This example begins northwest of Fort Worth, TX.  At 3:02 PM, the National Weather Service issued a severe thunderstorm warning for northwest Tarrant and northeast Parker county.

 

Although the radar returns look unimpressive, there were some interesting things that were about to happen.  One of the dangers with downbursts and the storms associated with them is the speed in which they form, drop their rain-cooled air, and eventually collapse.  The following 5 snapshots of radar reflectivity and radar velocity start at 3:12 PM (about 10 minutes after the initial warning was issued) and ends at 3:31 PM - a span of only 19 minutes!! 

 

TIME 3:12 PM

The first thunderstorm core is in the process of growing. 

 

 

 

TIME 3:17 PM

The first thunderstorm core is still in the process of growing, while a new core is forming to the northwest.

 

TIME 3:22 PM

The first thunderstorm core has matured fully and has begun to produce a downburst (visible on the right side of the image).  The velocity view clearly shows that winds directly underneath the thunderstorm are hitting the ground and are spreading out.  The red colors indicate winds that are blowing away from the radar, while the green colors show winds blowing toward the radar.  The second thunderstorm core continues to grow.

 

 

 

TIME 3:27 PM

The first thunderstorm core is now in the dissipating (or dying) stage.  Reflectivity values (intensity of rain) has dramatically dropped off.  The outflow winds from the original downburst have spread further and further apart.  The second thunderstorm core continues to build up.

 

 

 

TIME 3:31 PM

The first thunderstorm core is now simply an area of heavy rain, while the second thunderstorm core has finally reached its mature stage.  It too has begin to form a downburst beneath itself as indicated by the diverging velocity signatures.

 

 

Let me reiterate that these 2 thunderstorm cores formed, matured, created downbursts, and began to dissipate in about 30 minutes total.  When damage can occur from these storms in such a small amount of time, it's crucial to be aware of the weather in your area.  Boaters and others enjoying open waters are likelier to get caught off guard as the there is little in the way to shield outflow winds that blow across these bodies of water.  Luckily, the downbursts in northwest Tarrant county did not produce any damage to my knowledge.  However, numerous power outages along with downed power lines and trees were reported just north of the downtown Dallas area due so similar conditions.

 

-Andrew