Mike Wilhelm's Alabama Weather Blog

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Bamawx.com

I was invited, along with several ABC 33/40 Skywatchers, to attend a radar seminar at the National Weather Service in Birmingham on Saturday June 23, 2012.  Meteorologist Kevin Laws, Science Operations Officer for the NWS Birmingham, conducted the training. 

Below are my notes from the seminar.

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Radar is a very complex machine from a mechanical and engineering standpoint. Radar is an “active sensor” of energy. By “active sensor” we mean that it sends out a signal and waits for a return.


There are categories of weather radars. There is no such thing as a “best” radar, they all just have different applications.

1. S-Band (10 cm) radar. Example: (NWS Nexrad Doppler Radar)

2. C-Band (5 cm) radar. Example: (Terminal doppler radars)

3. X-Band (3 cm) radar. Example: (DOW, mobile Doppler radars).

NWS Doppler (WSR) radar objective is to survey and cover broad areas up to 250 nautical miles. WSR stands for “Weather Surveillance Radar”.

C-Band has better resolution that S-Band radars but the surveillance range is much less.

X-Band has the highest resolution but range is the lowest. These 10 mm radars can “see” targets that are 1 mm in size. Some radiation returns from the target, some scatters, some is absorbed, and some goes through the target. S-bands have less attenuation than X and C bands.

Radar equation:



is received power, is transmitted power, is the gain of the transmitting antenna, is radar wavelength, is the radar cross section of the target and is the distance from transmitter to target.

Power of the return is measured in decibels (dBZ).

Doppler radar was introduced in 1942. Wind speed is measured by comparing the angles on the wavelength that the returns are coming back from on consecutive pulses. For example, if the first pulse came back at 0 degrees and the second pulse came back at 90 degrees, the radar “sees” that the target has moved from 0 to 90. Radars can only measure angles between 0 and 180 degrees, limiting the maximum “measureable” wind speed to 58 knots at 180 degree differences. Once wind speed exceeds 58 knots, the angle measurement starts going down from 180. This maximum velocity that can be correctly displayed by a Doppler radar is known as the Nyquist velocity.

By sending out more frequent pulses, the ability to measure higher velocities is possible. However, due to the decreased “listening” time between the more frequent pulses, the effective range is reduced significantly. This is known as the “Doppler Dilemma”.

In order to suit the needs of the meteorologist during different weather conditions, NWS WSR-88 Doppler radars have nine available Volume Coverage Patterns (VCP). Each VCP has a predefined set of instructions given to the antenna that control the rotation speed, pulse and listening time, and elevation angles. The longer listening time is better for reflectivity, but worse for accuracy. Some of these VCP’s enhance the measurement of velocity (VCP 212). Clear air mode VCPs of 31 and 32 are used when velocity is not a concern.

Dual Pol. Single pol radar cannot distinguish between the size of objects within the target area. For example, a target area may contain 700 1 mm raindrops (drizzle) or one 300 mm raindrop. Since those reflectivities are equal, they appear the same on the radar display. Dual pol radar can help distinguish between these targets. Single pol radar looks horizontally only. As a result it doesn’t distinguish well between the size of the objects causing reflectivity. Dual pol, looks at a vertical cross section in addition to the horizontal. This additional data can be compared. As a result, meteorologists can obtain a good idea of the size of the objects being reflected. This, in turn, enhances the ability to distinguish between precipitation types.

One of the dual pol products is ZDR, or differential reflectivity. Hail has a differential reflectivity of around zero. This is measured by subtracting the horizontal dBZ value from the vertical dBZ. When looking at radar products, if you see an area of high dBZ (i.e. 61)on reflectivity and a corresponding low (0) ZDR, the precipitation is hail.

Correlation coefficient (CC) indicates how similarly sized the objects in the target are. This helps indicate where there are areas of mixed precipitation or hail.

“Three body scattered spike” or 3BSS. This refers to an “artifact” that can occur on radar that is caused by large hail. When the wave intersects hail, it is reflected back and also reflected down to the ground. If the ground is wet it is sometimes reflected back to the hail and back again to the radar. This is displayed as a weak echo region that resembles a “spike” protruding from the part of the thunderstorm producing large hail. While many “artifacts” or anomalies on radar are problematic, some, such as the 3BSS can actually be beneficial to the meteorologist.

Future of radar will be phased array radar. The phased array radar is able to scan a variety of pre determined targets which will provide a real time “movie” of a storm system.

GR data: Level 2 vs. Level 3 – Level 2 data is raw data (reflectivity, velocity, spectrum width).

Level 3 data includes processed data from the NWS. In addition to the raw data, it includes SRV, VIL, Mesos, and TVS. Level two radar software from weather vendors may include processed data, it is not the same processed data as the Level 3 NWS data.

Velocity aliasing and folding can be a problem in data provided by level two images provided by vendors. Since the max velocity is 58, the doppler display sometimes shows “couplets” that are nor “real” It also may mask real couplets. Kevin Laws showed examples of various broadcast meteorologists using bad images from their vendor data and compared it to the valid NWS data.

Changes to Gibson Ridge GR3 version 2…

There is a button to pres too “de-alias” velocity data but the three main changes were the Google Earth map background option, Dual pol data (ZDR, CC, HCA, and KDP), and changes to the color scale.

An excellent site to get additional training online is the Warning Decision Training Branch website… http://www.wdtb.noaa.gov/courses/dualpol/

The NCDC archives level 2 data from previous weather events.

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Today, I am going to talk a little about the weather this weekend out on the Plains of Oklahoma. Six people were killed in Woodward, Oklahoma Saturday night during a tornado. This was one of 135 tornado reports in the Plains Saturday. I witnessed a few of those tornadoes myself.

My long-time storm chase partner and friend, John Brown and I left Alabama Friday night and drove to Oklahoma. We actually stopped just past Little Rock and caught an hour or two of sleep in John’s truck. Our plan was to target northern Oklahoma or southern Kansas. Both of those areas were included in a “High Risk” of severe weather by the Storm Prediction Center (SPC). This was only the second time a “High Risk” was issued for an event in the “Day 2” outlook by the SPC. While tornadoes were being reported to our north in Kansas, John and I were hanging back in northwestern Oklahoma. Storms were firing along the dryline in western Oklahoma and moving north-northeast into Kansas. Although we witnessed a wall cloud and a funnel cloud north of Woodward (a town which was later had six fatalities in an overnight tornado), the afternoon storms in our location were not producing tornadoes until they moved into Kansas. This was really testing our patience. But we kept reminding each other of two very important things. One was that the primary window for tornadoes in this area, according to model data, wasn’t until around 7 p.m. and later. The other factor was that often the southern supercell (“tail-end Charlie”) becomes the dominant cell. We were counting on this, because as the afternoon wore on, we were running out of daylight and no more new cells were forming to our southwest.

Video from the storms that rolled through southern Morgan County, Thursday March 15, 2012. This video includes still shots of the lightning that I took as well as actual audio video clips.



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These pictures were taken at the intersection of Interstate 65 and County Road 55 in southern Morgan County near Falkville, Alabama.




























This is the only picture not taken at Falkville. This was on the Madison - Limestone County Line beside the Huntsville International Airport, looking south.

A record crowd of over 250 people were trained tonight by the National Weather Service Huntsville in basic storm spotting at the U.S. Space and Rocket Center in Huntsville, Alabama. Here are a few photos from the event.

















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All week, computer models pointed to a couple of potential severe weather threats, one on Wednesday February 29 and a greater threat for Friday March 2, 2012. Wednesday’s “event” produced no severe weather in North Alabama. By Thursday, the Storm Prediction Center included North Alabama in a “Moderate Risk” for severe weather. All parameters seemed to be coming together for the potential for all forms of severe weather, including tornadoes. Thursday evening I poured over models and maps looking for as much insight as I could get for the chase. I communicated with several of my weather friends, including Jason Simpson, Chief Meteorologist at WHNT Channel 19. I spoke with chase partner John Brown about his plans and what he was seeing in the model output. I also spoke to Meteorologist Barry Britnell, Jennifer New, and Carrie Rumbo about posting on my Facebook weather page and communication while I was in the field. Earlier in the week I had spoken with Sarah Vines about chasing with me. It became apparent that there could be two waves of severe weather, one in the morning and one in the afternoon. I actually only got one or two hours of sleep overnight Thursday.

I stepped out of the house before sunrise and it was obvious that a warm front had drifted north and moved over my location in Huntsville. Temperatures were already warming before daybreak. Scattered showers were breaking out. These were mostly elevated and not very strong. By 7 a.m. I was loading and preparing my vehicle for the chase. I was expecting a call from Jason, but my phone never rang. I found out later that my Droid X was malfunctioning. More on that later. Showers and storms by this time were mostly training from southwest to northeast from Franklin and Marion counties, across Lawrence, Morgan, and Limestone, west of Huntsville. By 8 a.m. I was getting interested in a cell back in West Alabama. We were not under a watch and there were no warnings. What intrigued me about that cell was its shape on radar. It wasn’t large, but it seemed to be encountering some shear and I thought that if it developed more, it may become dangerous. I decided to drive toward Decatur to intercept it. It was probably over an hour away from Decatur when I decided to leave Huntsville.

I left home around 8 a.m., believing I had plenty of time to purchase gasoline and snacks at Kroger. Never get behind me in line at the grocery store. I waited for what seemed like 20 minutes for a lady with multiple WIC vouchers and not enough money to purchase the amount of food she loaded in her buggy. I am usually pretty patient, but my patience was really being tested then! Finally I got out to the vehicle and fired up my live video stream. This time I used a headset microphone for the audio. I got some feedback from Barry that it was much better than Wednesday. I texted Jason. He said he had tried calling me but that it had gone straight to voice mail. I had several phone issues through the day. It froze, wouldn’t accept calls, wouldn’t allow me to send outgoing texts, and randomly opened apps. It also got to a point where it would not charge, even though it “said” it was charging. This proved very frustrating, especially during the morning. I was receiving multiple incoming texts and calls. Unfortunately, it is impossible to respond to most of those when chasing, even when the phone is working.

I drove to the marina on the Tennessee River, on the north side of Decatur. The storm I had earlier thought showed potential did, in fact, turn severe to my northwest. I decided to watch it from a distance at the river. It was initially warned for a severe thunderstorm. The cell split. The lead cell started rotating and the NWS Huntsville upgraded the warning to a Tornado Warning. That was a very good call. The storm produced a tornado that touched down on the south side of Athens. That tornado stayed on the ground across Limestone and Northern Madison County. It produced low-end EF3 damage. I decided to drive north on U.S. 31 and follow the rear storm. As I drove north toward that storm, it was being warned for a Severe Thunderstorm Warning. I pulled off U.S. 31 in the southern part of Athens, close to Athens Country Club to watch the storm. I didn’t know that I was within a few hundreds of yards of where the first storm dropped a tornado. As I started videoing this storm, the NWS Huntsville issued a Tornado Warning for it. I saw rotation and very strong updraft winds under what appeared to be a developing wall cloud. I was sending live video of this back to WHNT 19, which I believe they used on the air. Unfortunately I did not save any of their coverage on my DVR between 9 and 10 a.m. This storm produced the second tornado, an EF2, in Madison County. I hope that the video and reports being relayed from my location helped people “upstream” from the storm take it seriously. I was on the phone with Jennifer New, a trained storm spotter and chaser from South Georgia. She was providing radar support and helped me relay reports to the National Weather Service.


WHNT 19 Tornado Coverage 3/2/12 10-10:15 a.m.

During this part of the chase I was alone in the car. That makes it much more difficult. I decided to drive back toward Madison County. I was hoping to send WHNT some live video of the damage but I was getting conflicting reports of where the damage occurred and never found it. Complicating matters, the power was out in Athens and other points east on U.S. 72. This is a high traffic area and since street lights were out, it was very slow going. I made it back to Huntsville and stopped at the Krispy Kreme on North Memorial Parkway. Sarah was on her way north from Jefferson County to join me for the remainder of the day. While I waited on her, I made an executive decision to order three delicious and delectable glazed doughnuts and a large coffee. I had not eaten breakfast and it was getting close to midday. I had planned on having breakfast on my way to Decatur, but the lady in front of me at the grocery store used up any spare time I might have otherwise had! The doughnuts were awesome! When Sarah arrived, she had to get her equipment loaded. She is an amateur radio operator. She has a magnetic mount roof antenna. She also had her own laptop with radar.

We decided to drive to Buckhorn High School to look at the damage there. We parked there and walked around. There was significant damage to two homes near the school, numerous large trees uprooted and snapped, and a large concrete power pole was blown over. Amazingly, a mobile home was virtually untouched despite the fact it was surrounded by all of this other damage. After walking around the area, we got back in the car to prepare for the rest of the day. I posted some damage photos on my blog and sent live video out of the damage. I saw Andy Kula, Meteorologist with the NWS Huntsville arrive on the scene. He and another NWS employee were already out doing a storm survey. Those folks at the NWS Huntsville are really on top of things! We also had a chance to speak with WHNT 19 Chief Meteorologist Jason Simpson about the afternoon weather and chase plans. At this time there were only a few showers developing in West Alabama. Temperatures had risen to near 80 degrees. Our thoughts at that time were to head back toward the area where I-65 and I-565 intersect. I also spoke to John Brown who was now on I-65 driving north. He decided to stop in Cullman. After spending a little more time at Buckhorn High School, we drove west on Winchester Road back toward U.S. 231 (Memorial Parkway).

By the time we got to the Parkway, storms were rapidly intensifying in Northwest Alabama. Instead of driving southwest toward I-65 and I-565, I decided that we should drive north on US 231 toward the Alabama – Tennessee state line so that we could intercept storms that would be moving in that direction. That was a good call, because we would see three separate tornado-warned storms between 2 and 4 p.m. in southern Lincoln County Tennessee and northern Madison County. The first storm we witnessed from U.S. 231, just north of the state line in Tennessee. This storm produced some amazing features, including a rotating wall cloud, cloud to ground lightning from the wall cloud, a possible funnel cloud, and a very large and well-defined “beaver tail”. As we followed this storm east on highway 275 in southern Lincoln County, we were able to see a second tornado-warned storm follow on the heels of the first one. The wall cloud was not as impressive from our vantage point on the second one. Sarah did an excellent job relaying these reports via ham radio and to various sources back at WHNT. WHNT meteorologists Jason Simpson, Ben Smith, and Brandon Chambers provided extensive coverage of these storms. They frequently used our live stream along with radar to describe what was happening in the storms. I also did a phone interview with Jason, describing what we were seeing.

While we were in Lincoln County, we were stalked. Yes, stalked! Back when we were parked at Buckhorn High School, 30 miles and 45 minutes away, a lady came up to our vehicle and asked whether we were storm chasers. She thanked us for what we do and then proceeded to ask several questions, including if we gave paid tours! Well, little did we know, she followed us all the way up to Lincoln County. When we were parked and looking at the second wall cloud, Sarah told me to look behind us. Sure enough, it was the lady we saw back at Buckhorn. Fortunately after we pulled up a little further, we didn’t see them again.


WHNT 19 Tornado Coverage 3/2/12 2-4 p.m.

Meanwhile, storms continued to develop in some of the same areas in Northwest Alabama. A severe storm moved out of Colbert and Lauderdale counties and was moving into Limestone. This storm showed signs of rotation and large hail. We decided to drive a little south on U.S. 231 near Hazel Green to cover this storm. I think I was initially a little too far north. We had a good view, though, of a wall cloud. As it approached, it had the appearance that it was possibly producing a rain-wrapped tornado. We reported it as such; “possible” being the key word. I decided we needed to play it safe and drive south. As we were doing that we experienced quarter-sized hail. It was a good decision to drive south. If the storm had produced a tornado, our original position would have not been safe at all. That is why it is always good to keep watching the sky and always have an escape route. Of course, ideally, it would have been better if we had been further south all along. After the storm passed we drove up and down 231 and saw no damage. Other than snacks and doughnuts, we still had not had a chance to eat. We had even stopped at the McDonald’s drive thru twice in Hazel Green. Both times we gave up on them because the line was long and the service was slow.


Chase Highlights Video

Radar seemed to indicate that there was now a lull in the storms in the areas that we could access in time to see storms. We considered going further north to intercept a storm in Tennessee. Then we considered intercepting a storm near Arab. Stopping to eat, traffic, and a train in Huntsville made that impossible. We finally decided to drive to Falkville in Morgan County. There we saw several strong thunderstorms move through which produced an amazing lightning show after the sunset. Photographing lightning after dark is always a great way to end a successful day of chasing. I drove back to Huntsville to take Sarah to her car.

It was an interesting day. I saw four storms that produced confirmed tornadoes along their path. Both cells I saw during the morning in Limestone County produced tornadoes. One of the cells in Tennessee produced a brief tornado and the cell I saw in Hazel Green had produced a brief tornado back in Limestone County. We saw some amazing cloud structure, and most importantly we were able to communicate storm reports to the NWS Huntsville and provide live video coverage for WHNT 19. I sincerely hope that the efforts we made helped heighten awareness and aided in the overall warning process. Even though the area experienced four tornadoes (EF0, EF1, EF2, and EF3) it was great news that there were no fatalities in the area.


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Tornado damage from US Hwy 231, North of Huntsville, 3/2/12

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Photos from damage on Maysville Rd, Just off Winchester, near Buckhorn HS in Madison Co.

Today was a case study in why storm chasing in real life is nothing like storm chasing "as seen on TV". The Tennessee Valley of North Alabama was under a "Moderate Risk" of severe weather and a Tornado Watch was issued which included the following text:

TORNADOES...HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND
GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
AREAS.

I am a huge fan of the Storm Prediction Center. They are amazingly accurate considering what they are expected to do. And this moderate risk may have verified for other parts of the risk area. But for my region here in North Alabama, storm chasers would call it a bust. Not only were there no tornadoes in the Tennessee Valley of North Alabama, there were no severe wind or hail reports. Frankly, I heard of no hail at all. I was in one of the stronger storms and the peak gust I recorded was 25 mph. There were not even any tornado or severe thunderstorm warnings.

I am not complaining. We in Alabama need a break from severe weather. And I am not criticizing the SPC. They are awesome and I continue to rely heavily on their products. I would recommend that everyone do the same.

Today was a case study on why storm chasing is not for adrenaline junkies. Even on the most eventful days, there is much more time spent in "hurry up and wait mode" than in experiencing nature at its worst. I took leave from work at 1:00 and put 130 miles on my vehicle. For what? To see a lot of light rain, a 25 mph wind gust (in the strongest storm in the Tennessee Valley), and a few cloud to ground lightning bolts that I couldn't photograph. Do I regret it? Not really. I actually like weather most folks consider boring. I enjoyed the "company" of my weather friends in the online world. Of course if I knew that literally nothing would have happened in advance, I would have stayed at work and saved my leave. But this goes with the territory. In order to provide this service to the public, you have to be available and especially during days where the experts consider the risk to be elevated. Much more often than not they are correct.

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