How to Read Weather Radar

but this tornado warning is something of course is much more serious radar has indicated from the National Weather Service they tornado is likely to form or may have already formed here in northeastern Macomb County a very dangerous signature the hook see the red here's Joplin there's a hook and Oh still seeing some rotation with the storm around grape Grove and and Shady Grove he in the mow yeah that is a violent tornado or the little bulge there right over I 44 is where the tornado is sitting coming across the Grady and McClain counties there's definitely there's two if there's not a tornado on the ground there's about to be on this area here I've never seen radar signatures this absolute defines tornadoes on the ground is what we're seeing tonight radar is a big part of understanding the current weather situation especially when it comes to severe weather but what do those different colored blobs actually mean and how can somebody look at what looks like an abstract piece of art and say things like oh there's a tornado over there or they're getting some monster hail right now or it's about to get really windy how about when a tornado warning is issued and the warning says radar indicated rotation or radar confirm tornado how can they tell in this video we're gonna look at radar what it is how it works some different things that you can see using radar and how it's used from forecasters and meteorologists at the National Weather Service or your local news station to chasers like me out in the field we'll also go over some basic radar signatures and what different types of severe weather look like on radar so let's first look at how radar works and how it came to be used for weather purposes I'm really going to oversimplify it but radar basically works like this the radar site sends a beam of energy out and then listens to see if any of that energy is reflected back if there's nothing out there the beam of energy travels off never to return but if a reflection happens it's called an echo and it means that there's something out there that reflected it the radar can then calculate where that something is the word radar is actually an acronym which stands for radio detection and ranging and it saw a lot of use during World War two specifically to detect enemy aircraft but during that military use radar operator started the notice that rain and snow would also create echoes on their screens and actually block their ability to see what they were supposed to be watching for after the war was over scientists started looking at radars ability to detect precipitation and that was the beginning of weather radar as we know it today as of the time of this video the national weather service here in the US operates a network of 159 weather radars known collectively as NEXRAD each radar in the NEXRAD network is a wsr-88d which is a weather surveillance radar developed in 1988 with Doppler capabilities they're usually located at or near local weather service forecast offices as an example here's the Detroit Pontiac weather forecast office which is the one closest to where I live and up front here is the actual office where the meteorologists work and out in the backyard is the Detroit wsr-88d radar dome this network of radars is where the majority of weather radar information in the u.s. comes from there is no such thing as a satellite based radar that can see the entire country at once when you look at a nationwide radar image what you're actually looking at is a mosaic or a picture made up of all the local radar images stitched together like a big radar quilt all right so let's dive into what we're seeing when we look at a radar image most of the time this is what we think of when we think of weather radar some green yellow orange and maybe red blobs laid over a map this is called reflectivity and it's one of several things that are a dark an OS probably the most common one and what it's showing us is just that how much of the radar beam is reflecting off the precipitation in the air the heavier the precipitation the more of the radar beam will bounce off and return to the radar site areas of heavier precipitation are shown by the orange and red colors whereas lighter precipitation lets more of that radar being passed through it it only reflects a little bit these areas are shown with blues greens and yellows sometimes though a thunderstorm will produce hail and a hail shaft in a thunderstorm reflects even more of the radar beam hail cores are often shown in purple gray and even white anyway most of us get that we know that it might be raining buckets in the red areas where the blues and greens might be more drizzle or garden-variety rain if we see purple in a storm coming our way we tend to get the animals to shelter and cover our cars some people think that red means severe weather though and that's not exactly true sure severe thunderstorms often bring heavy rain with them but heavy rain and red radar echoes by themselves do not necessarily mean that the weather is severe in fact I've been in some very heavy rain that showed as red on a radar without any lightning or thunder at all it wasn't even a thunderstorm it was really just a heavy shower I've also chased low precipitation supercells complete with rotating updrafts and funnel clouds but very little rain only showing up as yellow and maybe a little orange on reflectivity radar the National Weather Service has very specific criteria on what constitutes a severe thunderstorm to be severe a thunderstorm has to produce either wind gusts greater than 58 miles per hour or hailstones one inch or larger in diameter or a tornado notice that heavy rain isn't on that list so if rain isn't part of the equation then how do meteorologists use radar to determine if a thunderstorm is severe or if it's going to produce at or and the answer is that they look at several different types of radar images radar can do more than just tell us how much precipitation is currently in the air it can show us many other things as well each different thing that the radar can show us is called a product so let's look at another radar product velocity velocity is maybe one of the most useful radar products there is especially for detecting severe weather if you've watched any of my videos where I'm out in the field chasing you might notice that the radar displayed in my truck is usually set to a split screen with reflectivity on the left and a different display with red and green blobs on the right that right side display is showing me velocity thanks to the miracles of modern technology not only can weather radar detect precipitation in the air but it can also detect which way that precipitation is moving and how fast it's moving because the precipitation is being moved around by the wind this is how wind speeds can be estimated from radar echoes it does all this using the Doppler effect which might sound familiar from science class it's the same principle that causes emergency sirens to seemingly change pitch when the vehicle drives past you it's also by so many weather radars are called Doppler radars if a radar as a Doppler radar it can detect motion in precipitation which means it can give us a velocity product in order for velocity to work though there needs to be precipitation in the air Doppler radar can't see the air itself so the only way it can determine what the winds are doing is if there's precipitation that it can see being blown around but because of how the Doppler effect works the radar can only determine if something is moving toward or away from it and how fast something is moving toward or away from it here's a live radar screen shot from a chase that I was on earlier this year don't worry about the extra dots lines and icons on the screen again this is an actual screen grab from a real chase and I turn on a few extra options and overlays when I'm out in the field for the purposes of this video I just want you to focus on the radar blobs I normally set up my display so that I can see two different radar products at once in this case reflectivity is on the left and velocity is on the right velocity radar only uses two primary colors red and green these colors do not correspond to precipitation intensity like in reflectivity products green also doesn't mean slower motion nor does red mean faster motion with velocity red and green just represent which direction the precipitation is moving red means that it's moving away from the radar while green means that it's moving toward the radar in these images the radar site is just off the screen in the upper right corner that said the brighter colors the faster the motion which usually means higher winds sometimes you'll see purple in a velocity image the purple color means that the radar can't determine velocity in that area due to a phenomenon called range folding so we're not a hundred percent sure what's going on under the purple blob though we can look at the areas around it and get an idea I try to remember velocity colors like this I pretend that the radar is lonely it's it's up at the top of its tower all by itself and it likes to have company if precipitation is moving toward it the radar gets happy and it gives it a green light whereas if it's moving away then the radar gets sad and it tries to stop it by giving it a red light again that's just my way of remembering it but if you have another maybe less cheesy way of remembering it I'd love to hear it leave it in the comments take a look at this radar loop from a severe thunderstorm earlier this year in Indianapolis the reflectivity image on the left shows some decently heavy rain but look at that gust front on the velocity side look at the bright colors of the front indicating very strong winds along the leading edge and notice how it changes from green to red as it passes the radar site because it was moving toward the radar but once it passes it moves away so the colors change I was stuck in traffic trying to get to a tornado warn storm when I was clipped by the western edge of that thing and it certainly got a little windy where I was it was severe warned and it caused extensive tree damage in the crater Indianapolis area the Indianapolis Storm is a good example of a severe storm feature that's usually pretty easy to identify on radar the bowing line segment or bow echo for short when a lot of storms starts to form that forward bulge winds are often very strong especially at the front of the bow even without a velocity radar product these bow echoes usually show up very well on reflectivity radar because the wind gusts are literally pushing the rain forward along that leading edge this is the kind of wind that produces straight-line wind damage take a look at this bow echo from a chase on May 23rd of this year in Illinois see the well-defined bow shape and look at those winds on the velocity side winds over 80 miles per hour were reported at the front of the bow it moved east through central Illinois and into west central Indiana overnight causing significant widespread wind damage and even spawned a few tornadoes along the way if a Boeing line segment is large enough strong enough and stays together long enough it might be classified as a duration is like a Boeing line segment on steroids they usually hold together for hours or days across several states and pack severe level winds of 58 miles per hour or more along their entire path the May 31st 1998 two ratio here in Lower Michigan packed winds of 130 miles per hour which is the same as a high-end ef-2 tornado and because those severe winds occur all along the duration damage is also much more widespread than it would be with a tornado recently a large Boeing line-segment moved through my part of the country after having originated all the way out in South Dakota this was eventually classified as a de ratio one story that made the news was in Croswell Michigan where an outdoor parade was ruined and people were sent scrambling for shelter as severe straight-line winds blew into town [Music] [Music] [Music] so let's look at the radar data from this storm for those who are curious I'm using the gr level 3 application to show the NEXRAD archive data here it's going to look a little bit different than the other examples I've used so far which are from radar scope Pro which is the primary radar application that I use when I'm chasing anyway you can see that Croswell was under a severe thunderstorm warning at the time that that video was shot but looking at reflectivity radar the precipitation doesn't look too bad for some people this might be the only radar that they look at and looking at this it may not appear that it's worth canceling the event not a lot of reds and oranges right let's add in velocity now again because I'm using a different program the red and green color scales are going to be a little bit different than what I've been showing so far but if you look you can see that a little Bowl just started to form within the larger bulge that is the de ratio and that smaller bulge as you might expect show some pretty strong winds right at the front headed straight for Croswell so that's one type of severe weather let's look at another specific type of severe weather that radar can help us identify one that a lot of people take interest in and one that I specifically use radar to maneuver around out in the field tornadoes tornadoes can form from a few different types of thunderstorms but for now let's start with the granddaddy of them all the supercell a supercell is defined as a thunderstorm with a mesocyclone or a deep rotating updraft rotation and thunderstorm shows up in a very specific way on velocity radar remember that when it comes to velocity radar can only measure things moving directly toward or directly away from it so it can only show us the parts of the rotation that are moving toward and away from it this shows up as an area of red and green right next to each other with red on the right and green on the left from the radars perspective this is how cyclonic rotation looks on velocity radar in the northern hemisphere supercell thunderstorm updrafts and most tornadoes rotate cyclonically or counterclockwise so this is the velocity signature that we're looking for to identify rotation specifically this is called a velocity couplet this one's kind of loose but here's an example of a tighter couplet notice that in both cases Green is on the left and red is on the right from the radars point of view and that's a good transition to the next point if you think you see a velocity couplet it's important that you evaluate it from the radars perspective or from the radars point of view to do that imagine yourself at the radar site and then look toward the area where the potential couplet exists if the green is still on the left and the Reds on the right then it's possible that you are looking at cyclonic rotation for example this couplet is cyclonic so is this one even though the colors look reversed at first glance from the radars position Green is on the left and red us on the right for both I remember it by reminding myself that from the radars perspective red goes on the right for cyclonic rotation if the colors are reversed with green on the right and red on the left from the radars perspective than the rotation if there is any would be anti cyclonic or clockwise while anticyclonic tornadoes do occur they often occur in the presence of larger cyclonic tornadoes or in air current eddies formed when air flows intersect near large storms what I'm trying to say is they're not very common if the red and green spots are stacked on top of each other from the radars perspective then either convergence or divergence is occurring but this is not the same thing as rotation convergence is motion toward a center point whereas divergence is motion moving out from a center point if green is stacked on top of red from the radars perspective then convergence is happening which might signal a non rotating updraft if the colors are flipped with red on top and green on the bottom from the radars perspective then divergence is happening divergence is often seen with microbursts and downbursts and depending on how strong the winds are can be an indicator of potential straight-line wind damage but let's go back to cyclonic velocity couplets for just a minute because that's what a lot of people are going to be interested in an important thing to remember when using radar to look at a storm is how far away the radar site is from that storm because the radar beam is emitted at a very slight angle and to a lesser degree because of the curvature of the earth the further out from the radar site the beam goes the higher up it's actually scanning in fact a full circle radar scan is just a very wide short cone shaped cross-section of the atmosphere because tornadoes are surfaced based phenomena we often want to know what's going on at the ground level which radar isn't that great at telling us especially if the nearest radar to a storm is still pretty far away from it just seeing cyclonic rotation on radar does not necessarily mean that there's a tornado there some cyclonic velocity couplets that show up on a radar are actually a rotating thunderstorms updraft because the radar beam even at its lowest tilt is still high enough off the ground in some cases that it's looking higher up into the storm cloud rotating thunderstorms are sir only capable of producing tornadoes though that's where train spotters come in somebody with a visual on the storm can relay to the National Weather Service what's happening below the radar beam and if a storm actually produces a rotating wall cloud funnel cloud or full-blown tornado like I said at the beginning of the video radar is definitely an important part of the severe weather puzzle but it's not the only part classic and high precipitation or HP supercells will often show cyclonic rotation on velocity radar but they also have a unique signature on reflectivity radar they usually taper from a large broad precipitation area into a narrower end located near the mezzo cyclonic updraft this narrow part make curl into a hook shape which is a result of a tightly rotating updraft pulling warm moist air called inflow into it and also wrapping some of the precipitation around it which it does in conjunction with the rear flank downdraft or RFD this creates a swirl commonly referred to as a hook echo if there's a tornado it's usually in the dot at the end of the hook supercells can be discrete and isolated or they can form within lines of storms can you see the supercell embedded in this line it's right here and it produced a tornado if you've watched my chase video from May 20th 2019 in Oklahoma you might remember the part where I was trying to get away from a rotating supercell with the hook headed right toward me so we are trying to get out of here because that wall cloud with the rotation is moving right over top of us and this is what a chaser convergence will do we can't get out of here because of all the chase was heading south out of the storm and that storm with the rotation is moving right on top of us which does not make me very happy I took a screen shot off the radar in my truck at that time which I had forgotten about until just recently it shows my position as the Blue Cross hair along with the reflectivity and velocity images of that storm can you see why I was trying to get out of there let's look at one last radar product that can help us identify a potential tornado in addition to showing us precipitation reflectivity and velocity radar can also look at the particles that are reflecting its beam and determine if they're all uniform in size and shape or not this radar product is called correlation coefficient in a normal rain shower all the raindrops are well the size of raindrops sure some might be larger or smaller than others but roughly they're all the same we would say that the sizes are all correlated or that they would all have a high correlation coefficient in the thunderstorm you might have some really huge raindrops and maybe some small hail but even with that it's still pretty uniform the correlation coefficient would still be pretty high if the hail gets large enough though the size of the particles causing the radar reflection starts to vary and not be as uniform we see this as slightly lower correlation coefficient values in the vicinity of the hale shaft because the size and the shape of all the particles isn't highly correlated anymore the radar starting to see stuff of all different sizes and shapes in there so it lowers the correlation values accordingly but when a tornado touches down it starts ripping things apart and chucking debris everywhere some of this debris gets lofted up into the air so high that the radar beam hits it and while many tornadic storms bring heavy rain and hail strong enough tornadoes can start lifting other stuff up there things like tree branches whole trees two-by-fours roof trusses metal sheeting and miss Gulch when this happens the stuff that's up in the air is not uniform at all the correlation in size and shape is very low the correlation coefficient plummets dramatically in the area of all that debris we see this as a well-defined hole in an otherwise correlated environment or a hole in the correlation coefficient this is sometimes referred to as a debris signature or a debris ball and if it's located at the same location as a strong velocity couplet it very strongly suggests that a tornado is on the ground lofting debris up into the air you can also look at reflectivity to confirm debris in the air hail normally occurs in the main precipitation core of a supercell thunderstorm in the area between the updraft and the forward flank downdraft essentially it's located with all the heavy rain the hook of a storm where a tornado would normally be is part of the updraft and while the rear flank downdraft or RFD can slingshot some large hail into that area hail doesn't fall there naturally so if we see reflectivity values that normally indicate hail in an area where there isn't normally hail but where we expect that there may be a tornado chances are it's not hail that we're seeing it's debris let's take a minute and walk through a real tornadic scenario on Sunday October 20th 2019 as darkness fell on Dallas Texas people were getting ready for bed so they could get up early for school and work the following day as they did a pair of supercell thunderstorms started creeping closer to the city approaching from the West at 9:00 p.m. on the dot the National Weather Service issued a tornado warning for a radar indicated tornado by that time the tornado had already been on the ground for nearly two minutes over the next 30 minutes it would carve a 16 mile path through the northern part of the city ultimately causing EF three damage before finally lifting let's look at the radar data from a storm and see how the tornado showed itself to meteorologists remember this tornado struck after dark in the city of Dallas most chasers won't chase near large metro areas and for the volunteer spotters that live there darkness makes it very difficult to see any storm structure that would give them clues that a tornado was imminent with minimal trained eyes on the ground meteorologists relied heavily on radar to identify rotation and to get the warning out here's the path of the storms as they move through the city you can see hook shaped appendages forming in the southwest parts of the storms and velocity shows a pair of cyclonic velocity couplets that coincide with the updraft areas in each storm the northern storm exhibits a tighter stronger couplet as it moves through the northern part of the city and that's the one that created the ef3 tornado let's look a bit closer at that northern storm this reflectivity still-frame shows a cyclonic Lee shaped hook inflow would be entering the storms rotating updraft here while the rear flank downdraft or RFD would be focusing the rotation and wrapping some of the precipitation around it like this we can even see a small reflectivity da starting the form at the end of the hook which is where we would expect the tornado to be if there is one let's look at velocity now this shows a very tight cyclonic couplet with strong inbound winds immediately next to strong outbound winds this is called gate to gate shear and it looks significant coupled with the hook echo on reflectivity this would have certainly warranted a tornado warning if one hadn't already been issued this radar scan was taken less than five minutes after the warning had gone out let's look at correlation coefficient now to see if there's potentially any debris in the air and there it is a well-defined hole meaning that there is almost certainly a tornado on the ground ripping up trees houses and cars and lofting the debris high enough into the air that the radar can see it at this point the evidence is clear there is a tornado on the ground in northern Dallas if we zoom in on the reflectivity scan we see the dot at the end of the hook becoming more well-defined and if we advanced a few frames we see high reflectivity values indicating hail but unfortunately we know it's not hail this is what's referred to as a debris ball fortunately nobody was killed by this tornado and only minor injuries were reported while we're on the topic of debris it should be noted that a tornado debris signature or TDS is different from a tornado vortex signature or t vs t v-- asses are generated by the radar itself usually based on storm relative velocity which is a velocity scan that subtracts the actual motion of the storm itself from the velocity numbers again I'm oversimplifying it but if the storm relative velocity shows fast when moving toward the radar immediately next to fast when moving away from the radar again that gate to gate shear we talked about that exceeds a certain value then it triggers a T V s as a warning to the radar operator to look at that area a little bit closer it doesn't necessarily mean that a tornado was occurring but is rather a way for the radar to say to the operator hey if you haven't noticed this yet you should probably take a look at it like all computer algorithms it's not perfect so if you see a TV s icon pop up on your radar you shouldn't assume that there's a guaranteed tornado occurring there though it is possible also many weak tornadoes waterspouts Gus Nadeau's and land spouts do not generate a TBS other types of storms can form tornadoes as well though supercell generated tornadoes are often the strongest all thunderstorms take in warm moist air to keep themselves going and anytime this inflow gets focused and squeezed it can form Eddie's which can then form a brief spin up tornado this is common in linear storm modes or squall lines and it's different than squall lines with embedded super cells because remember super cells come straight from the factory with their own mesocyclone built-in rotation and squall lines often comes from the pinching or focusing of storm inflow here's an image showing an inflow notch as they're called in a squall line and here's what it looked like from down below for the record this video clip is being shown in real time it's not being sped up at all the storm did not produce a tornado but it certainly could have in general tornadoes tend to form along squall lines either in embedded super cells in the area of inflow notches or at the top of a line segment that protrudes from the squall line look for kinks on reflectivity radar which signal inflow notches then switch to velocity and look for a couplet if you suspect the tornado look at correlation coefficient to see if there's a hole at the location of the couplet indicating lofted debris and by doing that you're tying together everything you learned in this video so I think that just about covers the basics I hope you found something in this video useful also if you're watching this video because you're interested in storm-chasing that's awesome but getting in your car and driving toward a velocity couplet hoping that you'll see a tornado is generally a bad idea don't do that before you head out on a chase there's a lot of other stuff you should know in addition to reading radar that said if you have an idea for a future offseason videos leave them in the comments and I'll see what I can do until then thanks for watching and stay safe

Channel: Watch Chris Chase

Views: 864,117

Keywords: weather, severe weather, storm, chaser, chasing, tornado, hail, wind, bow echo, derecho, tornado alley, spotter, skywarn, tornado watch, tornado warning, severe thunderstorm warning, dallas tornado, nexrad, wsr-88d, national weather service, noaa, nws, reflectivity, velocity, correlation coefficient, supercell, squall line, microburst, rotation, rotating, mesocyclone, updraft, rfd, gust front, doppler, doppler radar, couplet, debris ball, tvs, tds, funnel cloud, radar, severe, thunderstorm

Id: AeX2lMUfddQ

Length: 30min 18sec (1818 seconds)

Published: Fri Oct 25 2019