A Long Drive Pays Off (Part 4 of 4)

In a perfect world of storm chasing, the entire day would be spent focusing on the day's events to come.  Early mornings would be spent pouring over model data and narrowing down a general target area. Late morning/early afternoons would be spent driving to the target area and refining a narrow area to wait.  Late afternoons would be spent at the target area waiting for storms to initiate.

I do not live in this world.

My hierarchy to see if storm chasing is going to happen seems to follow a very logical - almost programmatic - flow which goes something like this: 

IF storms are expected to arrive on Saturday or Sunday
AND IF storms are forecast to be within a county or two
AND IF the kids are cooperating
AND IF they have something to do in the car
AND IF there are no other obligations
THEN perhaps, maybe possibly we'll see about going storm chasing. 

In other words far, far from perfect.

Therefore, looking at the SPC outlooks throughout the day on Saturday, May 16, it looked like a great for storm chasing - just not near me.  Sure, we were in the slight risk category, but the action area looked to be well to our northwest.

As I write this I can't even think of what we did after church that day, so I'm assuming it was anything but spectacular.  Regardless, much like the previous week, we make a last-minute decision to drive northwest towards the Wichita Falls area.  I'm really not exaggerating when I say 'last minute' either.  We ended up leaving the house about 4:30PM.  If you figure a 2 1/2 hour drive, sure that would get us somewhere into the enhanced risk area, but being so late in the day, we'd need to have some pretty good luck to see anything before dark.  The drive out there was pretty uneventful.  The kids, mesmerized by their Leapster games, seemed not to notice the long drive out there - which was certainly a benefit.  As we got closer, the once distant clouds began to grow taller and obscure the blue sky.  It was almost go time.

At 6:50 PM, the first tornado warning of the area was issued for a storm located northwest of Wichita Falls, TX.  The problem was that we were still nearly a half hour away in the next county!

This storm looked awesome on radar, but I knew that due to the late hour, we'd only have time to pick a single target and stick with it before we lost daylight.  Yes, the storm was heading northeast, but could we make it through Wichita Falls in time to catch up to it?  I had my doubts. 

By the time we made it to the outskirts of Wichita Falls it was 7:10 PM and my hopes were dwindling even further.  However, it was also at that time when another storm - to the southwest - became tornado warned!  YES!! This was the new target storm!!

We jumped on US-82 to head southwest toward the storm.  By 7:30 PM, we had made it as far as Holiday, TX - which was inside the warned area.  We continued our way along some back roads trying to catch up with the storm..  Much to my surprise, we still had daylight left!

 
10 minutes later, we pull off the road to admire this astonishing sight....

WOWSA!  Seeing this made the entire trip totally worth it!!  The structure on this storm was simply incredible.  The striations of the mesocyclone were clearly visible - indicating that this storm was indeed rotating.  Looking to the right of the storm I noticed something that I hadn't seen before in person - an inflow band, also known as a beaver's tail.

This section of the storm is the boundary between the warmer air flowing into the storm (inflow) with the rain-cooled air (outflow) rushing out of the storm.

We continued to follow this beauty for the next 20 minutes but with it being so late in the day, the lack of light soon prevented my iPhone camera from doing any more good. We officially broke off the chase and headed back towards Wichita Falls to grab a late dinner.

The long drive home gave me plenty of time to reflect on the amazing sights I had seen that evening as well as come to the realization that the spring chase season was probably coming to an abrupt halt.  The long, hot days of summer were soon going to eclipse the exceptionally wet and active spring we had.  All in all, Spring 2015 was fantastic and I can't wait for more chases ahead.

-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