When A Tornado Debris Signature (Apparently) Isn’t A Tornado

 

This is a mind bender. Imagine you’re nearly certain something is happening and is a threat to someone’s life…then after an additional investigation, nothing happened.

There was a rotating thunderstorm in northern Brown County Sunday night around 1am. It needed a Tornado Warning, and it got one. Shortly after the warning was issued, there was a tornado debris signature, which essentially confirmed a damaging tornado was occurring (based on radar).

I put my own credibility on the line to highlight what I saw on Facebook:

I did the same on Twitter…once to highlight the original warning:

…and the second to emphasize the radar confirmation of a tornado:

This is the “usual” shower and thunderstorm mode of radar – formally called reflectivity – during the warning:

nov6-blog-z

If you look carefully and near Buford (north of Mount Orab), you might see a hook-like feature, signaling a possible tornado. This isn’t easy to see, so let’s look at the Doppler part of Doppler radar, which will tell us how winds are moving relative to the radar:

nov6-blog-bv

In pink, I’ve circled what is called a couplet, or an area where winds are moving towards and area from the radar in close proximity. Red here means winds are moving away from the radar, and green colors means winds are going towards the radar. The environment around this storms has to be supportive of tornadoes in order for us to see this as a tornado; in this case, there was support for strong, severe, and tornadic storms. Let’s subtract out the overall motion (speed and direction) of the storm so we can see the storm relative motion. If you’re still confused, think of it like this: it’s easier to assess the rotation of a toy top on a table if it’s nearly stationary versus moving rapidly across a table. Here’s the storm relative velocity animation:

nov6-blog-srm

Again, I’ve highlighted a couplet with a pink circle. That’s strong rotation slowly weakening as it goes east.

Modern, dual-polarization radar can tell a meteorologist about the size and the shape of objects it scans, relatively speaking. If a radar scans a “bin” of the atmosphere and finds objects of varying shapes and sizes, that bin’s targets will have a low correlation. If a radar scans a “bin” of the atmosphere and finds objects of similar shapes and sizes, that bin’s targets will have a high correlation. To quantify this, we’ll turn this correlation into a correlation coefficient. Here’s the animation of this correlation coefficient just after 1am Sunday night:

nov6-blog-cc

I’ve circled a consistently low correlation area moving east. This area is basically the same area with strong rotation and a strong thunderstorm. As a meteorologist, this area low correlation coefficient within the area of a strongly rotating thunderstorm signals that a damaging tornado has occurred. The higher up in the atmosphere this low correlation coefficient goes, the more likely there is a damaging tornado and the more likely the tornado is to be strong. That low correlation coefficient area went up to about 5,200 above radar level:

cross

The actual altitude is greater when you consider the height of the radar and the curvature of the Earth.

This signature (the combination of the storm, strong rotation, and low correlation coefficient) is called a tornado debris signature. Now you see the reasoning for my Facebook post and tweets above. This is clearly a dangerous situation.

The Tornado Warning ends in about 30 minutes. The storm weakens, and order is restored.

Let’s fast forward to this morning. The National Weather Service plans to survey the area for damage. The damage is surveyed, and the NWS has a verdict:

4

What? What happened here? We had a tornado debris signature last night.

I asked a radar expert for a second opinion:

Professor Matt Kumjian of Penn State makes a good point: it’s a blurry area. A low-level circulation with leaves in it is not a tornado. Another broadcast meteorologist – Ryan Hanrahan – suggested a drone may be helpful in a damage survey. Joey Picca of the Storm Prediction Center also has some thoughts:

I don’t know how “exhaustive” the search was by the NWS, but apparently they didn’t see anything of interest. Maybe it is a false positive. But the radar in this case is like a camera witnessing a crime where police later find nothing at the scene. Or the radar shows a series of facts but the jury says not guilty.

What do you think? I think I’m frustrated.

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Let’s Talk About The Severe Weather Threat Late In The Weekend

It’s November in the Tri-State, which means we are entering a secondary severe weather season. 11 tornadoes have been confirmed in the Tri-State during November since 1950. On average, 7 Severe Thunderstorm Warnings and 2 Tornado Warnings are issued in the Tri-State each November. The jet stream is getting stronger this time of the year, and the amount of instability in the air is getting lower as temperatures and dewpoints drop.

There’s a threat for severe storms late in the weekend, specifically Sunday night and early Monday, as a cold front sweeps into and through the Ohio Valley. While the Storm Prediction Center does not issue severe weather categories (marginal, slight, etc.) 4 or more days out, they have highlighted an elevated risk for severe storms in the Ohio Valley Sunday night:

nov2-blog-spc

Think of the yellow area as a “slight risk” of severe storms and the orange area as an “enhanced risk” of severe storms. Cincinnati is – more or less – in a slight risk for severe storms Sunday night and early Monday.

The environment that these storms develop and evolve in is important. It will be warm Sunday with high temperatures in the 70s:

nov2-blog-highsunday

Low temperatures Monday will be around 60°, and it appears we’ll hit our low temperature at least a couple of hours after sunrise Monday:

nov2-blog-lowmonday

Temperatures will likely fall through the 70s and 60s Sunday night. This is warm enough to support thunderstorms.

Moisture is another important ingredient for storms. Dewpoints will be in the 60s ahead of Sunday’s front:

nov2-blog-dewpoint

This is sufficient moisture for thunderstorms, including severe storms. Wind shear, the change in the direction and speed of the wind with increasing altitude, supports organized storms and the threat of severe storms. Here’s what the Thursday morning’s NAM model thinks for effective speed shear Sunday evening:

nov2-blog-effshear

Numbers over 40 (knots) here support thunderstorms and severe storms. How likely are storms to rotate? Here’s what the same run of the NAM thinks for helicity (storm relative, indicating the likelihood for storms to rotate):

nov2-blog-effheli

Number of 200 (m2/s2) here are significant, but we need other ingredients present. We need bubbles of air near the ground to rise rapidly if severe storms are to form; one way to do this is having a high low-level lapse rate, or a fast drop in the temperature going from the ground to a few thousand feet above the ground. Here’s what the NAM model thinks of that for Sunday night:

nov2-blog-lllr

Those are low values, working against the thunderstorm and severe threat. How are the mid-level lapse rates looking?

nov2-blog-mllr

These higher numbers are supportive of storms and severe storms if bubbles of air close to the ground are able to get higher up in the atmosphere. 

How about instability and layers of stable air aloft? Here’s what the NAM model thinks:

nov2-blog-cape

The warm colors are instability, and the blue colors are stability. If severe storms are to form we want a lot of the former and less of the latter. Instability is modest, and stability is generous here. This works against the likelihood of storms and severe storms, but these ingredients are less important in the colder months of the year and more important in the warmer months of the year. 

For lower-instability, higher-shear scenarios, I review what is called the SHERB parameter. As I discussed back in 2015:

While instability can often have a big influence on the chance for thunderstorms, it isn’t as important this time of the year. If thunderstorms are likely […], the SHERB parameter or index can be very helpful to a meteorologist in the colder months when looking a threat for severe weather. The SHERB parameter is helpful for getting a handle on a severe weather threat in the colder months because it focuses on temperature changes near the ground, lift in the atmosphere, and wind shear instead of instability (instability tends to be low in the winter even when we get severe weather).

Why is SHERB important? Unlike summer severe weather events which are driven by high instability and less of everything else, cold season events are driven by everything else and not often by instability. SHERB is a special blend of “everything else” that is important when gauging a severe weather threat…which makes it valuable when we don’t have summer-like heat and humidity. When SHERB values are high and the chance for rain and storms is high, severe weather is often a concern.

So what does the NAM model think of SHERB Sunday evening?

nov2-blog-effsherb

I’m looking for values of 1 or higher, which are focused northwest of Cincinnati. The 0.5- 1.0 values west of Cincinnati at this will likely drop slowly and translate east later Sunday night.

Wind speeds 5,000 feet or so above the ground are important, too; heavy rain can drag these winds aloft down to the ground. What does the NAM model think of these wind speeds?

nov2-blog-llj

These are significant, but not off the charts. If you’re of the math variety, these values are about 2-3 standard deviations above normal. This is enough wind to support storms and severe storms.

The positioning and strength of the jet stream is very important in the colder months of the year. Where does the NAM have the jet stream Sunday evening?

nov2-blog-jetnam

This is not ideal for thunderstorms and severe storms. The highest wind speeds are to the west (but still moving east) at 8pm Sunday night. Divergence (rising air) is in purple and positioned west and north if Cincinnati. What does Thursday’s morning’s GFS model say?

nov2-blog-jetgfs

It has a lot more lift Sunday evening, and it has it stronger compared to the NAM model as it progresses east. There are clearly some strength and timing differences to resolve.

In summary, here’s what I’m thinking for Sunday night and early Monday:

nov2-blog-severeimpact

This is still a wishy-washy threat at this point. We have good mid-level cooling, plenty of wind shear, and sufficient lift…but the lift timing and positioning are uncertain, instability and stability forecasts work against the storm threat, and we’ll need sunshine (even if filtered) to get the storm threat maximized.

There is plenty of time for conditions to change. Stay tuned!

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Why NWS Wilmington’s Radar Being Down For 2 Weeks Is A Big Deal

Bad news. The radar operated by the National Weather Service in Wilmington – used as a primary weather radar for the protection of life and property in northern Kentucky, southeastern Indiana, and the majority of Ohio – is going to be down for 2 weeks:

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This is a significant change in the time of completion compared to what was expected Monday:

aug2-outage1

In simple terms, the radar was supposed to be down through Thursday but now will be down for maintenance through August 16th.

The bull gear, the gear that drives the radar antenna, has failed. Thousands of pounds of equipment and a team of radar meteorologists will be imported to fix the radar and get it operational again.

This is a big problem. This was a scheduled upgrade meant to extend the life of the radar. Instead, the technicians have uncovered a major problem with it.

Why is this outage such a big problem? Because we still get thunderstorms in August! It is worth noting, however, that the severity of storms typically drops during the summer. The average number of Severe Thunderstorm Warnings issued in the Tri-State drops from 25 to 21 to 12 from June to July to August, respectively. The trend in the average Flash Flood and Tornado Warnings from June to July to August, respectively, goes from 7 to 5 to 4 and from 2 to 1 to 0. You may say that with the Severe Thunderstorm and Tornado Warning risk dropping that there’s no concern, but flash flooding and flooding has been a more deadly and damaging concern recently.

Here’s the NWS Doppler radar network when all of the radars are working:

aug2-radars

And here the network when the Wilmington radar is down:

aug2-radars2

That’s a large lack of radar coverage over the Tri-State! So are meteorologists blind? Not entirely, but kind of.

There are still Terminal Doppler Weather Radars near Dayton, Columbus, and Cincinnati. The one covering Cincinnati is in southern Kenton County:

aug2-tdwr1

aug2-tdwr2

So no problem, right? The TDWRs are higher resolution than the NWS radars, so we’re good to go, right? Not so fast.

The TDWRs have a wavelength that is basically the same as that of a raindrop. This means that if heavy rain is falling near the radar, storms farther away from the radar at the same angle from the radar will appear weaker. The TDWR is also a single polarization radar, not a dual-polarization radar like the NWS’. The biggest benefit that comes from dual-polarization is for rainfall estimation. For the wavelength issues highlighted above, TDWR rainfall estimates are basically garbage. Distant dual-polarization radars can give rainfall estimates, but because these distant radars don’t sample as close to the ground as Wilmington’s radar, radar estimates from distant radars are not that great.

So the spring outage problem continues into August. One has to hope we avoid severe storms in the next two weeks and those who represent us can invest in brand new technology and stop putting Band-Aids on dated technology.

 

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Severe Storm, Flooding Threat Thursday

Thanks largely to warm, steamy air, there will be an elevated risk for flooding and flash flooding in the Tri-State Thursday and Thursday night.

First, most of the Tri-State is in a Flash Flood Watch from 4am Thursday through 4am Friday for the potential of heavy rain leading to flooding:

jul12-ffw

How much rain will it take to get a Flash Flood Warning? Here’s 1 hour flash flood guidance:

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Here’s 3 hour flash flood guidance:

jul12-ffg2

6, 12, and 24 flash flood guidance is not worth showing. The summary here is that it must rain quickly and heavily to get flash flooding. Thunderstorms will be needed to get flash flooding tomorrow. 1.5″ of rain per hour will likely trigger warnings.

There is also a slight risk for severe storms over most of the Tri-State tomorrow:

jul12-spc1

As I see it, there is a lot of instability to fuel storms, but other ingredients for severe storms are surprisingly absent. Here are my forecast severe weather impacts for Thursday:

jul12-impacts

Notice the most likely time for severe storms Thursday centers between 3pm and 8pm.

As of 3pm, it’s warm outside:

jul12-temps

It’s also disgustingly humid outside, with dewpoints well into the 70s:

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This means heat indices are over 90° for many:

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There are showers and storms developing over central Indiana at this time:

jul12-215pmclouds

Current thinking is as isolated showers and storms will brush into communities northwest of Cincinnati early this evening before the sky becomes mostly clear tonight:

jul12-evening

After isolated showers and storms in the morning, showers and storms will develop and increase in coverage Thursday afternoon:

jul12-thursday

Showers and storms will diminish Thursday evening. They will be isolated overnight Thursday night and scattered Friday. The weekend will be quiet and peaceful but also humid. More heat is coming next week:

jul12-temptrends

Be alert for heavy rain and strong to severe storms Thursday, especially during the afternoon and evening!

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Friday Afternoon Severe Weather & Forecast Update

Thanks to ample sunshine, more supportive temperature profiles in the atmosphere, and an increase in the amount of wind shear over the last few hours, the threat for severe storms remains elevated despite a weak round of storms northeast of Cincinnati now. Radar shows those storms and stronger clusters of storms east and northwest of Cincinnati at this hour:

jul7-3pmradar

Clusters of storms over central and northern Indiana are the ones to watch. Those will expand in coverage and move southeast this evening.

Through midnight, the Tri-State remains in an ENHANCED risk for severe storms:

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Damaging straight-line is still the main threat with large hail a close secondary threat. Tornadoes and flash flooding are the lowest of the severe threats at this time:

jul7-impactsafternoon

It’s warm outside. Temperatures are in the mid to upper 80s for most as of 3pm:

jul7-3pmtemps

 

It’s also steamy with dewpoints in the mid 70s:

jul7-3pmdewpoints

Showers and storms will develop late this afternoon and early this evening. Showers and storms will gradually end nearing midnight. Notice the large drop in temperatures due to showers and thunderstorms:

jul7-eveningafternoon

Cooler, less humid air is coming tomorrow. After areas of fog in the morning, a mix of sun and clouds is forecast during the afternoon:

jul7-saturdayafternoon

Enjoy the break from heat and humidity this weekend, because both are coming back next week!

jul7-temptrend

You should have multiple ways to get watch and warning information this afternoon and evening. Be alert!

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Today’s Severe Weather Threat Update

The Storm Prediction Center has placed the entire Tri-State in an ENHANCED risk for severe storms through this evening:

jul7-spc

The primary threats this afternoon and evening are damaging straight-line wind and large hail. Flash flooding and tornadoes are secondary threats. All of these threats are in play between 3pm and 8pm today:

jul7-impacts

There is already a Severe Thunderstorm WATCH in effect for parts of Ohio and Indiana just north of the Tri-State:

jul7-watch

Showers and storms have already developed over northeastern Indiana, and these clusters of showers and thunderstorms will expand in coverage and move southeast over the next several hours:

jul7-radar

Ahead of these showers and storms, there is plenty of sunshine:

jul7-clouds

Sunshine will boost instability levels available to storms this afternoon and evening. In addition to sunshine supporting storms, it’s also warm. Temperatures are already in the 70s and 80s, and these numbers are rising:

jul7-temps

It’s also steamy. Dewpoints are in the low to mid 70s:

jul7-dewpoint

The necessary ingredients for severe storms are already in place. Temperatures will rise through the 80s this afternoon with showers and storms developing:

jul7-afternoon

Temperatures will crash through the 80s and 70s this evening underneath the weight of showers and storms:

jul7-evening

Have more than one way to get watch and warning information this afternoon and evening!

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Forecast & Severe Weather Update

As of 3:15pm, the Storm Prediction Center is considering the issuance of a Tornado or Severe Thunderstorm WATCH for parts of the Ohio Valley including the western Tri-State:

mcd0577

Temperatures are rising through the 60s and 70s as of 3pm, increasing the amount of instability and likelihood for strong and severe storms:

temps

Dewpoints have also been rising this afternoon, and this is an impressive rise since this morning when dewpoints were in the the 40s:

dewpoints

A 3pm radar snapshot shows showers and thunderstorms developing to the west of Cincinnati and Tri-State:

radar

Current thinking is that showers and storms will rapidly develop and increase in coverage late this afternoon and early this evening and continue in waves through the second half of the evening and overnight:

evening

The coverage of showers and storms will gradually drop late in the overnight, but flash flooding is the main concern from late evening through sunrise:

overnight

Showers and thunderstorms will be favored early Saturday, then partial clearing and rapid warmup comes during the afternoon:

saturday

I have increased the threat for tornadoes, flash flooding, and large hail slightly from my last update this morning:

impacts

The tornado threat is focused along and south of the Ohio River through early evening. The other threats are distributed around the entire Tri-State.

The Storm Prediction Center still has the entire Tri-State in a slight to enhanced risk for severe storms through sunrise Saturday:

impacts

The threat for severe storms Saturday and Saturday night will be focused northwest of Cincinnati:

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This is forecast the threat of strong storms very early Saturday and also late Saturday night/early Sunday.

If you’re tired of this warm and stormy pattern, longer-range guidance has us much cooler next week:

temptrend

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