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Archive for May, 2010

What Is Base Reflectivity?

May 21st, 2010 3 comments

The other evening I was checking out the NWS page for Tucson, and I clicked on the local Radar image. This is what I saw:

Tucson Base Reflectivity

Tucson Base Reflectivity

At first glance, it would appear that it was raining outside in the areas indicated in blue. But, having spent the evening outside coaching a softball game, and having checked the skies before heading in for the evening, I knew that, if anything, the only thing in the sky was some high clouds. So, why would the radar indicate that it was raining? I decided that it was time to get a better understanding of what Base Reflectivity was all about.

Turning to Google, I found a link to the National Weather Service page on Base Reflectivity. I started reading the FAQs on weather radar, and it became very apparent that there is more than meets the eye when reading weather radar output.

To begin, we need to understand how weather radar works. Basically, the Next Generation Radar (NEXRAD) obtains weather information (precipitation and wind) by measuring returned energy. The radar sends out a burst of energy (green), and if the energy strikes an object like rain drops, bugs, birds, etc., the energy is scattered in all directions (blue). A small fraction of that energy gets directed back to the radar. How Radar Works The radar has a listening period, in which it collects and analyzes the signals that it receives. The whole process to analyze the signal is super fast, and occurs around 1300 times per second! In an average hour, the radar spends about 7 minutes sending signals, and 53 minutes listening for them. Based on some geeky physics stuff, the analysis can tell the “phase shift” of the signals it receives, which lets it know in what direction, and how fast the object it got bounced off of is going. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to “see” the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice.

Base Reflectivity, which is what’s on the map above, is a display of echo intensity (reflectivity) measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). “Reflectivity” is the amount of transmitted power returned to the radar receiver. Base Reflectivity images are available at several different elevation angles (tilts) of the antenna and are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential.

Base Reflectivity ScaleWhen you look at the Base Reflectivity map, you’ll see various colors on it, and one of the scales that you see to the left of this text. If the radar is operating in “clear aid” mode, then the values range from -28 to +28 dBZ. If the radar is operating in “precipitation mode,” then the values range from 5 to 75 dBZ. Turns out, the map I was viewing was operating in clear air mode, which was something I had never heard of. Typically, light rain is falling when the values reach approximately 20 dBZ. As you can see from my map, I was nowhere near that. I suppose that the high clouds or other particulates in the air could have accounted for the return that I saw on the map, but it certainly wasn’t raid. Had I known about the two scales, and the 20 dBZ threshold, I wouldn’t have been confused!

There’s quite a bit to learn about weather radars and how they are used to predict the weather. I would highly encourage you to visit the NWS Radar Image WSR-88D Radar FAQs to learn more. By understanding the concepts, scales and technologies used to predict the weather, you can get a better understanding of the weather potential for your area. And, it never hurts to learn some geeky science!

Monsoon 2010 Forecast for Southwest United States

May 21st, 2010 1 comment

The National Weather Service has issued their Monsoon 2010 Forecast for the southwestern portion of the United States. The overall forecast is neutral while we wait for El Nino/La Nina patterns to stabilize, and while we wait to see how wet the central plains will be. If you track this stuff like I do, you’ll find the forecast to be both interesting and informative.

Hook Echo

May 12th, 2010 No comments

I’ve been doing some reading lately on mesocyclones and tornado development, and one of the hallmark signs of a potential tornado forming is a “hook echo” being seen on the weather radar. A hook echo is produced by rain, hail or even debris being wrapped around a supercell, giving the impression of a hook on the radar. Meteorologists consider the presence of a hook echo enough justification to issue a tornado warning for an area. The hook echo has been recognized as a sign of tornado development for most of the history of weather radar. The first hook echo was detected in 1953 by the Illinois State Water Survey during their test to use radar to measure precipitation rates. In the southern US states, hook echos are not always obvious due to the heavier rainfall from the supercell. Instead, the echo will take on a more kidney shape. Here is an example of a classic hook echo – if you see this while checking out the radar, either seek shelter, or head out with your camera!

Classic Hook Echo

Bad 2010 Hurricane Season Possible According to Accuweather.com

May 12th, 2010 No comments

Back in March, Accuweather.com’s chief hurricane forecaster, Meteorologist Joe Bastardi, predicted that the 2010 hurricane season was shaping up to be huge. Today he reiterated his prediction, predicting 16-18 storms during the June 1-Nov. 30 season. He noted during only eight years in the 160 years of records have 16 or more storms formed in a season.

Bastardi also predicts an early start to the 2010 hurricane season, with one or two hurricanes forming by early July, and additional threats extending well in to October. He predicts that at least six of the storms will impact the United States.

From the standpoint of number of storm threats from the tropics to the U.S. coastline, we will at least rival 2008, and in the extreme case, this season could end up in a category only exceeded by 2005.

Citing a rapid warming of the Gulf of Mexico and a collapsing El Nino as reasons for the heightened forecast, Bastardi also feels that the Atlantic basic is “textbook perfect” for major hurricane activity.

TwisterData.com : Super Valuable Weather Information!

May 6th, 2010 No comments

There is a site on the Internet that is an absolute GOLDMINE of weather data – TwisterData.com. This is a one-stop-shop for all the awesome information you need to know when the next storm will here, where it will hit, and how much rain/hail/wind and other stuff to expect. Here’s a list of some stuff you can find there:

  • Air and Theta-E Temperature
  • Dewpoint, Mixing Ratio, Relative Humidity
  • Relative Vorticity, Vertical Vorticity
  • Precipitation
  • Cape, CIN, EHI
  • Storm Motion, Storm Relative Flow
  • Shear

And here’s a shot of what some of their beautiful data looks like:

TwisterData.com - Storm Motion

TwisterData.com - Storm Motion

The quality of the data here is simply amazing, and it’s free! If you’re going to be a storm chaser, you should probably become familiar with this site. Here’s what the site owners have to say about what they’re doing:

As avid storm chasers and weather watchers, we make extensive use of internet weather data websites. Unfortunately, most websites use outdated technology to produce small graphics, resulting in images that are often difficult to read and interpret, and at worst virtually useless. In addition, these resources are often poorly-organized, so that locating even basic products can be a time-consuming task. As a consequence, we placed a great deal of emphasis on intuitiveness and usability when designing the TwisterData.com website. Our goal was to create a website that gets out of the way of the user, allowing them to focus on the data. We hope we have succeeded and plan on continually improving and enhancing the website’s user experience.

Take some time, check out the data, learn how to interpret it, and become a very educated, and safe, storm chaser!