Interesting Weather Information

Saturday, March 1, 2014

Reality Check: Should The National Weather Service Issue Tornado Warnings for Brief, Weak Funnels Rated as EF0s and EF1s.

The text below is the script from a segment I researched and presented on WXIX-TV, Cincinnati, OH on Tuesday February 25, 2014.

As a TV meteorologist I realize, as do many - if not most - of my colleagues that there is a big problem with the NWS over warning tornadoes.

The national false alarm rate is about 75% and despite that their tornado data, on file with NOAA's Storm Prediction Center, clearly shows this is unnecessary they continue with no sign that this flawed policy will be changed.

In fact research indicates it could be dangerous in the long-term.

The record of the NWS is good for big tornadoes EF3, EF4 and EF5 storms but for the small, weak, brief spin up whirlwinds the record of the NWS is poor if not dangerous.

Here is the text of my report. After that there is additional information not contained in the story. It is in all caps because I cut and pasted it directly from the FOX19 system which is in all caps for the teleprompter.

VO = voice over, reporter is not on screen
GFX = graphics

----- Begin Script -----

THE FALSE ALARM PROBLEM 
[STEVE ON CAM]
TO BE CLEAR THIS REALITY CHECK IS NOT ABOUT HOW WELL NWS WILMINGTON DOES THEIR JOB …
… BUT ABOUT A TORNADO WARNING POLICY THAT TREATS WEAK SPIN UP FUNNELS THE SAME AS MONSTER KILLER TORNADOES. THAT POLICY IS SET AT HIGH LEVELS IN THE NATIONAL WEATHER SERVICE BUREAUCRACY …

LET ME SHOW YOU WHY THAT POLICY NEEDS TO BE FIXED.

ON THE HIGH END OF THE ENHANCED FUJITA SCALE --- BIG TORNADOES ---

[VO BIG TORNADO VIDEO THEN DAMAGE ]
THE EF 3s, 4s AND 5s REQUIRE BIG THUNDERSTORMS WITH STRONG ROTATION. THEY ARE EASY TO SEE ON DOPPLER RADAR AND EASY TO WARN.

THEY ARE THE HEADLINE GRABBERS - THAT CREATE NIGHTMARES LIKE THIS.

THE BIG ONES MAKE UP LESS THAN 6% OF THE TOTAL NUMBER OF TORNADOES.

[STEVE ON CAM]
BUT ALMOST 80% OF ALL TORNADOES END UP ON THE LOW END OF THE ENHANCED FUJITA SCALE RATED AS EF1s AND EF0s.

[VO GFX FULL SCREEN – LEADING EDGE RADAR IMAGES FROM OSGOOD 02.17.2014]
Link to Osgood Radar Images: 

http://stevehorstmeyer.blogspot.com/2014/02/the-ef1-near-osgood-indiana-20feb2014.html

THEY ARE SMALL VORTICES AT THE FONT EDGE OF A THUNDERSTORM … THEY SPIN UP QUICKLY … AND TOUCH DOWN BRIEFLY.

THEY ARE HARD TO SEE ON RADAR AND THEY ARE THE SOURCE OF THE MAJORITY OF THE FALSE ALARMS.

[VO VIDEO FROM OSGOOD INDIANA THURSDAY 2.20.2014]

THIS  DAMAGE FROM NEAR OSGOOD, INDIANA LAST THURSDAY IS WHAT AN EF1 OR ZERO PRODUCES  – MOSTLY MINOR AND NOT WIDESPREAD.

[STEVE ON CAM]
IN FACT GOING BACK TO 1950, IN WHAT IS NOW NWS WILMINGTON’S COUNTY WARNING AREA

[VO GFX FULL]
NO DEATHS HAVE BEEN CAUSED BY  LEVEL ZERO TORNADOES AND ONLY ONE DEATH HAS BEEN ATTRIBUTED TO A LEVEL 1.

THAT’S IN MORE THAN 63 YEARS.

THOSE STATISTICS MEAN THAT TORNADO WARNINGS FOR EF0 AND EF1 TORNADOES ARE NOT SAVING LIVES BECAUSE A FUNNEL OF THAT STRENGTH IS JUST NOT THAT DEADLY.

[VO GFX FULL SCREEN ADD ON LINES BASED ON INITIAL ISSUANCE]
AND ...  DURING THE DOPPLER RADAR ERA - SINCE 1995 – THE WILMINGTON NWS OFFICE HAS ISSUED 542 TORNADO WARNINGS.
BASED ON A NATIONAL FALSE ALARN RATE OF 75% -
 MORE THAN 400 OF THOSE WERE FALSE ALARMS


BOB RYAN, FORMER PRESIDENT OF THE AMERICAN METEOROLOGICAL SOCIETY WROTE,
[VO FULL SCREEN GFX]

  There are small whirls ….  and the real McCoy tornadoes. We all have a role in effectively communicating the real danger, beyond just yelling TORNADO WARNING…”

[STEVE ON CAM]

THE FALSE ALARM RATE IS A GLARING RED FLAG POINTING TO A POTENTIALLY DEADLY PROBLEM.

 [VO GFX]
IN THEIR PEER REVIEWED STUDY, “FALSE ALARMS, TORNADO WARNINGS AND TORNADO CASUALTIES” PUBLISHED IN 2009 KEVIN SIMMONS AND DANIEL SUTTER WROTE,

"We have found strong evidence that a higher local, recent FAR [FALSE ALARM RATIO] signi´Čücantly increases tornado fatalities and injuries ..."

ACCORDING TO THE RESEARCH OF SIMMONS AND SUTTER  -  IF THE NWS DOES NOT FIX THE FALSE ALARM PROBLEM .....

WE MAY SEE DEATHS AND INJURIES INCREASE.

MANY METEOROLOGISTS FEEL IT IS TIME TO FIX THE FALSE ALARM PROBLEM.

THAT’S REALITY CHECK.

Copyright 2014 WXIX. All rights reserved.

----- END SCRIPT ----- 

Sources:
Tornado Data: NOAA Storm Prediction Center: http://www.spc.noaa.gov/gis/svrgis/
Tornado Warning Data:  Iowa Environmental Mesonet: http://mesonet.agron.iastate.edu/
Peer Reviewed Journal Articles:
False Alarms, Tornado Warnings, and Tornado Casualties
Kevin M. Simmons, Daniel Sutter
Weather, Climate, and Society
Volume 1, Issue 1 (October 2009) pp. 38-53
Tornado Warnings, Lead Times, and Tornado Casualties: An Empirical Investigation
Kevin M. Simmons, Daniel Sutter
Weather and Forecasting
Volume 23, Issue 2 (April 2008) pp. 246-258
Bob Ryan Quote: Storm Watch 7 Weather Blog
Copyright 2014 WXIX. All rights reserved.
 
ADDITIONAL DATA, NOT USED IN THE STORY FOR PERSPECTIVE

Ohio, Kentucky and Indiana Lightning Deaths 1959 - 2012 Source: NOAA

OH   146
KY     95
IN      90
Total 331 (5.61 Deaths Per Year or 56 deaths in a decade)

Ohio, Kentucky and Indiana F/EF Zero and 1 Tornado Deaths 1959 - 2012

OH   1
KY   4
IN    7
Total 13 (.24 deaths per year or 2.4 deaths in a decade)

.006 deaths per F/EF 0/1 tornado (6 deaths per 1000 F/EF 1 and 0 tornadoes)


YOU ARE 25.5 TIMES MORE LIKELY TO DIE FROM A LIGHTNING STRIKE THAN 
AND EF 0 OR EF1 TORNADO.



So I ask the question, "Should the NWS issue tornado warnings for EF 0 and EF 1 tornadoes?

Why not just cover the small, brief, insignificant whirlwind with verbage in a severe thunderstorm warning, instead of all the chaos they create each time there is a bit of rotation in a thunderstorm?




Tuesday, February 25, 2014

The EF1 Near Osgood, Indiana, 20Feb2014

The EF1 tornado that touched down about 10:14 PM EST Thursday  20Feb2014 was a classic leading edge funnel.
Wide view of KIND NEXRAD Doppler Radar 
lowest elevation (0.51° tilt) reflectivity 03:12:00z (10:12 PM EST) Thursday 
20Feb 2014.There is no easily discernible sign here or in the close 
up of a tornado. It touched down 2 minutes after this.

Close up view of the top image.
The tornado touched down about half way between the napoleon and Osgood dots.

KIND radial velocity at 10:12 PM EST 20Feb2014.
The small bright green spot indicates air rushing towards the Indianapolis
 radar. Immediately southwest the color is red indicating air flowing 
away from the KIND radar.

Close up of the image above.

Tornado signature high-lighted.

Rotation indicated by the radial velocity display.

Saturday, February 8, 2014

And The Winner Is ...

Bill Nye, The Science Guy (left) and Creation Museum founder Ken Ham at their debate 02.04.2014.

... Everyone. 

As a scientist I had my favorite going into this debate and in a few paragraphs I will make my case for a winner based on content.

But first I would like to look at this debate another way. 

To me it is clear that we all won on this one because Bill Nye and Ken Ham treated each other with respect. They did not resort to name calling, innuendo or sophomoric humor that lowers the bar and diverts attention from the debate itself.

Both debaters delivered their message and stayed on point, Ham as a true believer and Nye as a dedicated scientist. In the end we got a clear picture of the thought processes which lead both Ham and Nye to the passionate defenses of their sides of the argument.

Two thumbs up for the audience too.  Watching the web video stream and based on the accounts of reporters who were there the audience did what audiences were supposed to do - observe.

In this day and age of screaming radio talk show hosts, world views that would fit on a bumper sticker and utter disrespect of anyone who does not think as you do this debate was a refreshing break.


Now On To the Substance of the Debate


I will not feign neutrality here. I am a scientist, not because I am a TV meteorologist but because I think like a scientist.

I use the scientific method in problem solving, I have seen how it works, how it self corrects and how it got humans to the moon with much less computer power than my iPhone.

I do not believe in the scientific method. I have observed it in action and the evidence is ample and conclusive - the scientific method works!

A scientist observes, organizes those observations into data, analyzes the data, develops a hypothesis and then subjects the work to the unrelenting criticism of her peers.

Science is always a work in progress. Sometimes new work throws the hypothesis out the window and other times the hypothesis is merely tweaked.

Eventually the evidence is so overwhelmingly supportive of the hypothesis that it is elevated to a theory, and that is where much of the difficulty between the brainiacs of science and the remainder of the world begins.

The public will quip, "After all evolution is only a theory." A scientist at the same time says "... yes it is a THEORY."

To the scientist a theory is almost as sure as it gets. A theory may need to be tweaked in the future but a theory is just about a done deal. The only thing more sure is a Universal Law. 

The Law of Universal Gravitation is an example.  

No one in the universe can tell you what gravity is but we know how it works so well that when an astronomer using a powerful telescope sees a barely visible far-distant star wobble, the astronomer can predict that the star is orbited by a planet.


For example the most distant known exoplanet is OGLE-2005-BLG-390Lb. It is near the center of the Milky Way galaxy at a distance of 21,500 +/- 3,300 light years from Earth. OGLE stands for the Optical Gravitational Lensing Experiment.

Artist's depiction of the exoplanet OGLE-2005-BLG-390Lb. Courtesy NASA

When an astronomer is viewing this exoplanet she is seeing it as the planet was about 21,500 years ago. It took light, travelling at 186,000 miles per second, that long to get here.

We also know the planet is about 5.5 times the mass of Earth, takes about 10 Earth years to orbit its sun (star OGLE-2005-BLG-390L)  and would fit in our solar system between the orbits of Mars and Jupiter. 

Because the output of its sun is less than the output of our sun we know a planet orbiting that far out is unlikely to support life.

The point is that we cannot see the planet and even if we could we would be looking into the distant past. Yet the scientific method allows us to make predictions because we know the laws that govern the universe. Those laws apply everywhere in the universe past, present and future.

In a heady, mind-blowing kind of way this example summarizes Bill Nye's major argument. We make observations, apply the laws of physics that govern the universe and can predict the existance of a planet that will likely never be seen by human eyes.

We apply one scientific method and one set of laws that extend far into the past, because now is the past at the far edge of our ability to observe.

While scientists observe and analyse persons of faith believe. Faith requires no proof and a case can be made that a person who seeks proof cannot be a person of faith.

To me the first weakness of Ken Ham's argument is that he is debating at all.  Mr. Ham has a deep, profound belief that defines his journey through life. In a faith-based world view it is enough to say, "This is what I believe."  Period. Proof belongs in a world view based on observation.

Ken Ham's biggest problem with his approach is that so many of his points require special circumstances. Historical science vs. observational science is the prime example. You did not see it so you cannot prove it, Ham contends.

An accident investigator called to the scene did not see the fatal car crash. But when she observes skid marks crossing the double yellow line ending in two mangled vehicles she concludes which vehicle is at fault.  Evidence, the skid marks, allows the investigator to see into the past and envision the sequence of events. 

The trained accident investigator does not invoke a special circumstance explaining that the double yellow line moved because the laws that govern those things were different in the past.

In the end this is not like debating whether the 1927 New York Yankees were better than the 1975 Cincinnati Reds. Both teams played the same game and played by the same basic rules and individual and team statistics can be invoked as evidence to support an argument.

The science vs. faith debate is like debating whether the Seattle Sea Hawks are a better football team than Manchester United. The name of the game is the same but the rules are not comparable.

In the end Nye and Ham summed it up best. When asked what could change his mind and move him to the other side of the argument about the age of Earth Ham said nothing could. For Nye all you have to do is show him the evidence.

Faith vs. Science. In my view science won this one.














Friday, February 7, 2014

January 2014 vs. January 1977 in Cincinnati - A Clear "Winner"

Those of us who have been around a while longer then the average American love to tell stories of the brutal winters that visited Cincinnati in 1976-1977 and the following winter of 1977-1978.

I swore I would not do it, that is tell stories about the hardships of my younger days and how soft the younger generations have become. My dad did it and I rolled my eyes and shrugged it off as another fish story. No, I would never do it.

The moral of this story is never say never. So here we go.

It is important to remember that it has been about 20 years since we have had a very cold January in Cincinnati.

January 1994 was a nondescript month  until an arctic air mass arrived on the 14th.  By the morning of the 18th the temperature was -20°F (-28.8°C) and the next day the low temperature was tied for the second coldest ever at -24°F (-31.1°C).

January 1994 had cold episodes,  87 hours that month were 0°F or colder and there was a streak during the arctic outbreak of 48 consecutive hours zero or colder (18th Midnight - 20th 11PM).

The arctic air went its way and by the 23rd the low was 32°F (0°C).
The graph shows a typical January interrupted by a brief, but impressive record-setting arctic outbreak.


What we have to remember is that this is the coldest air in Cincinnati during the life of anyone 20 years of age or younger. So January 2014 is the yardstick (oops, meter stick) by which they measure winter severity.

With ice slabs floating down the Ohio River everyone wanted to know if the river would freeze solid from shore to shore again. But it was not to be because when it comes to temperature 2014 was a minor league month compared to January 1977, the coldest month ever in Cincinnati.

Here is a great source of photos of the frozen Ohio:

FROZEN OHIO RIVER PHOTOS


The next graph shows the hourly temperatures for both January 2014 and 1977 and following that is a graph of the difference, January 1977 was colder than January 2014 when the bars are below zero.




In short there is no comparison. January 1977 is the coldest month in Cincinnati weather history and january 2014 is way down the list.

Here are some numbers that will convince you. Notice only 2 hours the entire month of January 1977 were warmer than freezing.


HOURS 0° OR COLDER
NUMBER
% OF MONTH
1977
138
18.5
2014
67
9.0
HOURS > 32°
1977
2
0.3
2014
261
35.1
AVERAGE OF HOURLY TEMPS
1977
12.8

2014
23.5

CONSECUTIVE HOURS <= 0°
1977
71
9.5
2014
27
3.6
CONSECUTIVE HOURS <= 32°
1977
615
82.7
126
16.9
2014
50
6.7
84
11.3
110
14.8
107
14.4
21
2.8
89
12.0

Thursday, July 18, 2013

It's Not The Heat ... It's the Humidity: Some Things You May Not Know About Atmospheric Moisture

It's not the heat, it's the humidity, a bit of folk wisdom I have heard countless times from as far back as I can remember.  And of course, within limits it's true.

The human body sheds heat mainly by the evaporation of sweat. While you may perspire, I sweat and though it may not be a pleasant sight a sweat drenched athlete's body is doing what it evolved to do when rising internal temperature triggers the sweat glands.

Each drop of sweat (a salty solution) that evaporates takes with it excess heat. The drier the air the faster the evaporation and the greater the heat loss.  So a heat index of 100° when the temperature is 93° means your body loses heat as if the temperature is 100° not 93°. High humidity hinders heat loss because it slows evaporation. As a result the "feels like" temperature, in this case heat index, communicates how much the heat loss is hindered.

Four Ways Humidity May Affect Your Daily Life?

1. High humidity can accelerate mold growth and some molds release spores when the humidity increases. Most allergenic molds release spores when humidity is low. Do you know why? A dry spore can travel farther than a spore moistened by a humid atmosphere that weighs more. However if you are sensitive to mold spores released in humid weather as humidity increases you may get the sniffles.

Mold spores. Image courtesy www.bioideas.net


2. Your hair lengthens as humidity increases. In fact for years most instruments that measure humidity used human hair. A bundle of human hair is linked to a mechanical arm and levers scale up and down motion so it can draw a graph on a rotating drum in the hygrometer.

But there is more to this than than you probably may have imagined.

According to   The Atlas of Human Hair Microscopic Characteristics, straight hair generally has a round cross section while curly hair is flatter in cross section. The flatter the cross section the curlier the hair.



So when humidity increases round, straight hair strands lengthen and the effect of the humidity may make your hair go limp and lose body. If you have curly locks, that is strands with a flat cross section, humidity may give you the frizzies as your hair curls even more.



3. Did you ever reach for a potato chip that has been outside for a while on a hot, humid, summer picnic afternoon?  You anticipated that satisfying "crunch" but your chips had lost their chomp and instead you reeled back startled by the salty, spongy silence.

Blame the salt! It is what scientists call hygroscopic or water attracting. The sodium and chlorine of table salt carry positive and negative electrical charges while one end of a water molecule is slightly positive and the other slightly negative. Opposites attract and that satisfying salty sprinkle captures humidity, free wheeling molecules drifting through the space surrounding the chip. The water molecules are absorbed by the starch of the potato chip. The result is a mealy mouthful.




4. Humidity, also known as Earth's most abundant greenhouse gas is not only important as an element of global warming but locally it drastically effects night time temperatures.

The high deserts of California may drop to near freezing overnight even in summer and soar to 100° during the day.  It cools so much at night because of a lack of humidity.  The same factor that makes the dry heat of the day tolerable brings a shiver at sunrise. Earth heat escapes easily to space through an atmosphere that lacks water vapor because of a weak greenhouse effect.



Thursday, July 11, 2013

Microburst near Hilliard, OH 10 July 2013

A National Weather Service damage survey completed July 11, 2013 concluded a microburst caused straight-line winds estimated at 95 mph based on their damage survey.Six individuals were injured during this event.

A microburst occurs when a thunderstorm collapses. The collapse can be caused by a variety of  factors that cause the warm, moist inflow that drives the storm to be cut off or weaken dramatically. With the updraft weakened or stopped there is no upward directed force to hold the cloud mass aloft.

The entire cloud mass crashes to the ground over several minutes. As the water drops and everything else within the cloud accelerate towards the ground under the influence of gravity a rapid burst of air is pushed ahead of all the cloud material.

When the air hits the ground it fans out and surface winds can exceed 100mph. Damage can be extensive at ground level and even worse is that the change of wind direction and speed can be a great hazard to aviation.

Artist's depiction of a microburst over an airport.  As the air crashed to the ground it fans out and wind speeds can exceed 100 mph. Aircraft that approach the microburst experience a sudden and violent change of vertical wind. On approach or takeoff the aircraft is close to the ground an may not have time to adjust flaps and air speed to avoid a crash. Courtesy: NASA.

A 3D (volume render) view of the thunderstorm that caused the Hilliard micoburst rendered in GRLevel2 Analyst. Notice how the volume of heavy rain (yellow and red) collapses, and, in approximately 18 minutes dissappears.  


Text of the NWS ILN Local Storm Report on the Hilliard, OH microburst:


PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE WILMINGTON OH
625 PM EDT THU JUL 11 2013

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0400 PM     TSTM WND GST     4 SSE HILLIARD          39.99N 83.12W
07/10/2013  E95.00 MPH       FRANKLIN           OH   NWS STORM SURVEY

            *** 6 INJ *** A NWS STORM SURVEY CONFIRMED A MICROBURST
            ABOUT 3 OR 4 MILES SOUTH-SOUTHEAST OF HILLIARD. BASED ON
            OBSERVED DAMAGE...STRAIGHT LINE WIND SPEEDS OF UP TO 95
            MPH WERE ESTIMATED.

Saturday, June 22, 2013

Occluding Mesocyclones - Evidence from the El Reno , OK Tornado - Part II Radar

Updated 06.22.2013 20:45z
Introduction

In Part I of this post I covered the basic meteorological setup of the El Reno, OK EF5 tornado of May 31, 2013. It was the widest tornado ever measured in the U.S. with a maximum width of 2.6 miles (4.2 km).The former record holder was the Wilber - Hallum, NE tornado of  May 22, 2004 which was 2.5 miles (4.02 km) wide.

In Part II I take a look at radar data, more of it than you would normally see on a TV weather program. I hoped to post this a couple days after the first post but this is so much going on with this tornado that I had to go back and review a number of journal articles to clarify this complicated tornado.


Incredible Storm Chasing Video

If you think the only dangerous part of a tornado is the funnel,  KFOR Meteorologist Emily Sutton probably has some advice for you. Take a look at what she and her crew got into while chasing the El Reno tornado.

Here is the video link:

http://www.youtube.com/watch?v=UjZDY142Y-Q


In an email about her close call Emily wrote "I'm blessed to be alive".  After staring at the radar data for hours and hours, I think that is an understatement.  Emily and Kevin Josefy got about as close as a chase team can get and live to tell the story.  Her encounter was on the same road and a very short distance from where professional storm chasers Tim Samaras, his son Paul Samaras and Carl Young were killed and not too far from where the tornado ripped the hood from the armored tornado attack vehicle called The Dominator.


Meteorologist Emily Sutton, KFOR TV, Oklahoma City.
Courtesy KFOR.com

 In Emily's opinion she got into an intense rear flank downdraft (RFD) and the air sinking behind the tornado was screeching towards and into the tornado circulation.

RFDs are known to at the very least help intensify mesocyclone tornadoes and there is mounting evidence that they may be an essential element of tornado formation by supplying air to the surface beneath the cloud base and updraft.  RFD air wraps around and confines funnel rotation, concentrates the spin and helps bring rotation to the surface. As the updraft stretches the rotating air vertically the funnel narrows and the rotation rate increases. Think ice skater.





Top: Meteorologist Reed Timmer and  and
an early version of The Dominator. Below:
Current version of The Dominator .
Courtesy: Discovery Channel
When discussing the storm with Reed Timmer meterologist, storm chaser, tornado attack vehicle designer and Discovery Channel star who also chases for KFOR TV, Reed told Emily it was probably an inflow jet that smashed the back windshield of her chase SUV as her team were in a backwards mad dash to avoid the El Reno tornado.

An inflow jet is just what it sounds like it is.  Because of the extremely low pressure in the tornado air rushes in from all directions. When tornado-bound air is confined to a narrow channel, for example by the gust fronts of the  RFD  or the forward flank downdraft (FFD) of a newly forming upwind supercell  (in this case to the west-southwest) air rushes into the vortex.


Inflow jets can be fed by RFD air and driven to impressive velocities. Inflow jets can also race parallel to the RFD gust front independent of the RFD itself. In fact inflow jets can be found almost anywhere around a tornado as air screaming into the incredible pressure falls of a tornado vortex interact with terrain, other inflow jets and neighboring storms.

Doppler radar images from the El Reno - Yukon tornado tell an incredible story. And it looks like both Emily Sutton and Reed Timmer are right about the presence of RFD air and inflow jets. Ultimately data indicates Emily was just on the edge of the tornado circulation when it was at it widest and strongest. Luckily far enough to sustain only a broken back windshield.

Radar data shows the El Reno - Yukon tornado was complicated and there is ample evidence of a strong RFD which wrapped around the tornado vortex as a strong inflow jet.  In addition it is possible that a newly formed supercell may have helped channel and intensive the RFD inflow.


Supercell Fundamentals

The diagrams below show what you will find in most tornadic supercell environments. The specific geometric arrangement varies from storm to storm and not every feature that you see in these generalized schematics may be present in each storm. You have to think in 3D to get a full understanding of what is going on.

A supercell is a thunderstorm with, by definition, a tilted, rotating updraft. Because the updraft is tilted most of the rain falls outside the updraft. In a thunderstorm if the updraft is vertical rain falling  through the updraft works against the rising air. A thunderstorm with a vertical updraft has a short lifetime while a supercell can last 12 hours and travel hundreds of miles.

The tilt is the result of vertical wind shear, that is, wind speeds increase with increasing elevation so the top of the updraft is farther downwind (farther in the direction of storm motion) than the updraft base.

Supercell rotation is largely a matter of the storm importing rotation from the environment, although there are mechanisms internal to the supercell that can create rotation also.  In general meteorologists use the term vorticity to measure rotation and for qualitative discussions vorticity and rotation can be considered to be the same thing.

For more on rotation imported into thunderstorms use this link to a post about the March 2, 2012 tornadoes south of Cincinnati, OH:

http://insidetheforecast.fox19.com/

Heavy rain ahead of the updraft core creates the Forward Flank Downdraft or FFD. The cool rush of air you feel just before the heavy rain and the arrival of the thunderstorm is the Forward Flank Downdraft.

There are two mechanisms that account for the FFD Downdraft:
  1. Each raindrop of the billions upon billions of drops pushes a bit of air towards the surface as it falls. The combined total can create and impressive wind gust when the descending air reaches the ground.
  2. Some of the raindrops evaporate on their journey to the surface cooling the descending air. Cooler air is more dense and adds to the velocity of the downdraft.
The abrupt change from warm muggy air to the gusty, cool air marks the leading edge of the FFD which we call the Forward Flank Gust Front. Just like the fronts on a weather map it is the boundary between contrasting air masses only smaller. The FFD Gust Front is also called an outflow boundary.  As the cool air undercuts the less dense warm and humid air it helps to increase the lift on the air flowing into the thunderstorm. The FFD Gust Front can:
  1. Cause the supercell to become more intense due to the increased lift.
  2. Help generate rotation when the air is lifted along the gust front.
  3. Cause new cells or a squall line to form along the gust front. 
The Rear Flank Downdraft or RFD is still a bit of a mystery with several candidates as possible mechanisms for formation of the RFD. The RFD is also the cause of several features that may contribute to tornado formation. The RFD is dry, sinking air behind the storm (relative to storm movement). It may be seen as a clear slot behind a tornado. It is likely the cause of the hook echo as the sinking dry air spirals into the mesocyclone. Radar sees the rain curtain as a spiral of high reflectivity and the air originating in the RFD a spiral lower or no reflectivity.

The leading edge of the RFD outflow is the RFD Gust Front and new cells that form along it are called the flanking line.

Possible causes of the RFD (all may operate together):
  1. Stagnation of mid-level air as it flows around the back side of the storm's updraft chimney. Pressure increases aloft forcing air to sink at the rear of the storm.
  2. Cooling of air flowing out of the storm aloft by evaporation or melting hail.
  3. Cooling of drier air moving into the storm from the rear also by evaporation or melting hail
  4. Dynamic effects that are beyond the scope of this post. Just think of it as what goes up must come down - an action/reaction pair.
Possible effects of the RFD:
  1. RFD outflow supplies air that wraps around the developing tornado vortex and confines the air to a narrower column helping to increase spin and velocity of the updraft. The updraft stretches the spinning column vertically and as the column narrows the spin rate increases.
  2. RFD air may transport rotation from aloft to the surface that aids in spinup of the tornado. Evidence for this is descending reflectivity cores (DRCs), areas of heavy rain that sink towards the base of the supercell rotating updraft.
  3. RFD air helps increase low level spin and may be crucial in bringing mesocyclone rotation to the surface or alternately in creating surface rotation that grows upward via the updraft joining the mesocyclone.
  4. RFD air sinks and is often warm and can arrive at the updraft as lower density air that is easily lifted.
  5.  If there is insufficient air flowing into the updraft of a supercell a tornado cannot form.The RFD probably supplies air that helps maintain a tornado. In other words if the updraft can remove more air than can be pulled into the storm there will not be a large, long-track tornado. Meteorologists refer to the swirl ratio of the storm. In simple terms it is the ratio of inflow volume  to updraft volume. As the swirl ration grows (> 1) the inflow air cannot be evacuated and the funnel becomes fatter eventually breaking dow to a multiple vortex tornado.
  6. New low-level mesocyclones may form on the RFD Gust Front and as the old mesocyclone occludes, that is it is absorbed by the supercell the new mesocyclone may generate a new tornado. These are called cyclic supercells and are the source of tornado families.

If you are not yet completely comfortable with stormglish ( aka chase speak), refer to these diagrams for reference during the following discussions.



A typical supercell  and surrounding environment from above.  The inflow jet shown is a "classic" situation. Because of the strength of tornado circulations inflow jets can develop from any direction.  The inflow jet along the RFD gust front is most common. the RFD flow itself can accelerate into the tornado circulation also forming an inflow jet and interaction with terrain can confine inflowing air enough to cause a jet from any direction. For more investigate the "Venturi Effect".

A typical supercell and surrounding environment from the side.

Radar Loops 
Reflectivity, Radial Velocity and Normalized Rotation
May 31, 2013 - KTLX Radar (Norman, OK NWS Office)

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Reflectivity Display. Animation of the El Reno tornado hook echo from  KTLX the Norman, OK NWS Radar. The area enclosed by the yellow path is the damage area from the tornado as surveyed by NWS meteorologists in Norman. The small yellow square south-southeast of El Reno, is the approximate location of meteorologist Emily Sutton and her crew as the tornado crossed U.S. Route 81 the road from El Reno to Union City, and the point where the tornado turned left  (i.e. headed to the northeast).  The yellow arrow shows the direction in which Sutton and crew backed up to get away from the tornado. How close were they to disaster? Look at the entire series of animations below for a better idea. Notice how the hook seems to aquire reversed curvature (anticyclonic curvature) at 23:19:11z due to strong inflow from the RFD/inflow jet from the west-northwest.


KTLX reflectivity radar loop, 2246z 31May -0037z  01 Jun 2013 (5:46 PM - 8:37 PM CDT). The down-pointing triangle is where the MDA (Mesocyclone Detection Algorithm) of GRLevel II Analyst computes enough rotation for a tornado. The animation below is a closer view. Later in this post using a series of stills I will point out some of the features you can see in these animations and discuss what those features mean.
Same as above but at the second tilt elevation. In both this view and the view above a well defined RFD can be seen developing as a low reflectivity notch on the west side of the hook. 
Two of 7 tornadoes in the Grand Island, NE area on June 3, 1980. the right funnel is a mesocyclone funnel and the debris on the left is from an anticyclonic (?) gustnado. See diagram below.

Ted Fujita's diagram of a cyclonic/anticyclonic tornado pair during the Grand Island, NE tornadoes of  3Jun1980 is similar to the radar view of the El Reno - Yukon tornado  hook echo as it intensified and turned to the northeast with the end of the hook developing anticyclonic curvature. In the Grand Island event a small anticyclonic tornado developed. There was no such development in the El Reno - Yukon tornado. It is possible that the anticyclonic tornado developed on the RFD gust front as it occluded (curled into the center of the circulation), making the anticyclonic tornado a "gustnado".  In the El Reno - Yukon case it looks like simultaneous strengthening of the RFD gust front and the FFD (forward flank downdraft) of the second cell created an intense inflow jet and the shear induced the anticyclonic curvature and rotation. See the diagram below.


Base velocity display. Red is air movement away from the radar (+ in polar coordinates), green is towards the radar (- in polar coordinates). A doppler radar can only measure movement directly along a radial so the display does not show true wind velocity or direction but the component of the wind along a radial. Because of this any wind that crosses a radial will have a doppler measured velocity slower than the true wind. 

El - Reno - Yukon tornado. Background: base reflectivity, antenna angle 0.5° at 23:19:14z as the tornado was crossing U.S. Route 81, reaching maximum size and intensity and turning to the northeast. Red and Green outlines: location of  GRLevel 2 Analyst derived positive (red) and negative (green) rotation. The red outline is for NROT approximately >= +1.0, the green outline is for NROT approximately <= -0.90. The area of anticyclonic (negative) rotation is shear induced as indicated by translucent white arrows. Inflow jet is shown. It is probably a combination of RFD inflow and the influence of the developing second supercell.  The greatest inbound radial velocity is located at the small square (-140 kts/72.1 m/s).
Normalized Rotation ( Range +5 to -5, positive is cyclonic, negative anticyclonic, valuesbeyond +/-2.5 are extreme).  This product is unique to GRLevel 2 Analyst and was developed by Gibson Ridge Software. It differs from rotation algorithms used in NWS radars.  Because of the polar coordinate geometry native to radar systems data bins become larger as distance increases from the radar. A target of given size will occupy a decreasing proportion of the volume as distance increases.  This means that velocity measurements slow as range increases which changes rotation calculations.  In addition position within a volume becomes increasingly important with increasing distance from the radar due to increasing volume of data bins.

The Gibson Ridge normalized rotation algorithm takes all this into account and from this example preforms well with both placement of the center of rotation and the change of intensity when compared to ground surveys. The strange behavior of the TVS symbol in the last couple frames is likely due to the multi-vortex nature and great width of the vortex/mesocyclone as it decayed.

Same as above but with a graph of positive (cyclonic) and negative (anticyclonic) normalized rotation at an antenna elevation of 0.5°. Left click for a larger version. On the graph any value  greater than 2.5 or less than -2.5 (the yellow lines) is considered to be extreme. 


Same as above but the map is for an antenna elevation angle of 0.9°. The left graph is for the 0.9° tilt and the right graph is the next tilt up 1.4°. Note the map scale is different from the map above to acomodate two graphs. 
Emily Sutton and Kevin Josefy were close,  too close for comfort. Essentially they were within the outer fringe of the tornado's circulation when it was the widest and strongest. 

Contrast what Emily and Kevin did  when faced with life a threatening situation to try and stay safe with  what the Weather Channel crew did.  Which crew acted the smart way?


Radar Cross Section
Along a west (left) to east (right) line from
98.36° West Longitude to 97.65° West Longitude
Through the Center of Normalized Rotation @ Antenna Tile of 0.5°
Vertical reflectivity  cross section, west to east (left >> right) through the center of normalized rotation.  The BWER  is squeezed as the mesocyclone occludes and moves northeast into the main body of the supercell. The RFD flow, wraps completely around the mesocyclone and  is also squeezed as the next supercell grows and starts to merge with the supecell that caused the El Reno - Yukon tornado. In the last couple frames that have the RFD labeled  there is also evidence that the RFD is undercutting the vortex.




High angle view of the supercell's 40 dBz reflectivity from the southwest beginning 23:37:48z and ending 23:37:37z 31May 2013. Notice the following: 1. Areas of high reflectivity merging with the mesocyclone, 2. the well developed RFD (low or no reflectivity) as the hook reaches  US Route 81 and the anticyclonic curvature develops and 3. the next cell trying to spin up.


Medium angle view of 40 dBz reflectivity from the south-southeast beginning 23:56:14z and ending 23:37:37z. Careful observation reveals what are most likely descending reflectivity cores before the large curtain of rain wrapping around and into the circulation develops. Before the anticyclonic curvature develops the RFD is evident.

Medium angle view of 50 dBz reflectivity from the south-southeast beginning 22:37:49z and ending 23:37:37. What appear to be numerous descending reflectivity cores (DRCs) before and during the strengthening of the tornado descend and spiral into the mesocyclone circulation. The same thing happens with the development of the second supercell.

TDWR TOKC view of the evolution of the tornado, 1-minute time resolution. Antenna elevation angle = 0.5°