I've Been Thunderstruck

I stumbled across this today, and thought it was worth sharing. There are lots of ways to forecast the weather without relying on gadgets. Knowing how to do it old school is a great skill to have, and helps you understand the fundamental properties of weather. Check out this graphic, and have fun learning!

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

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. 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.

When 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!

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.

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!

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.

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

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!

When people hear the term “Tornado Alley,” they tend to think of the area from mid-Texas up through the heartland that spawns a greater number of tornadoes annually than any other area of the country. However, recent research by Michael Frates of the University of Akron, reported on MSNBC, suggests that there are actually four regions of active tornado development in the US, and the original Tornado Alley is not the most active!

Michael Frates, a graduate assistant at the University of Akron in Ohio, devised the new boundaries and a more nuanced set of “Tornado Alleys” by analyzing the spatial distribution of F3 to F5 tornadoes with tracks greater than 20 miles in the Central and Eastern U.S. from 1950 to 2006. The output of that work is spread across a grid of more than 3,000 cells across the region.

Each cell was then given a different “frequency value” depending on the frequency of tornadoes with intersected the unit, and out of this process came “major spatial patterns, which served as the basis for delineating new tornado alleys,” as shown on his map, above.

“Results from this analysis indicate that Dixie Alley has the highest frequency of long-track F3 to F5 tornadoes, making it the most active region in the United States,” Frates concluded. Dixie Alley had a frequency value of 2.92, followed by Tornado Alley (2.59), Hoosier Alley (2.37) and Carolina Alley (2.00).

When Frates’ data is presented on a map, it gives the regions indicated below as the four Tornado Alley regions:

Four Tornado Alley Regions

This new data should help the National Weather Service understand better where to focus tornado predicting technologies, and where to concentrate research efforts. This years spring tornado season, while delayed likely due to the El Nino effect, has been particularly active.

Given that tornado season appears to be in full swing now (a recent tornado killed 11 in Mississippi), I thought it would be a good time to describe how tornadoes are rated. From time to time you might hear the weather reporter talk about an “F0″ or “F3″ tornado, and you might not know what they mean. I’m hear to set you straight!

Tornadoes are measured using the Fujita Scale, which was developed by Ted Fujita of the University of Chicago back in 1971. The scale Fujita developed is used to determine the intensity of a tornado after it has passed through an area and done its damage on human-based structures and vegetation. After a tornado has gone through an area, meteorologists and engineers will do a ground and/or aerial damage survey. If possible, they will also take into account things like eyewitness accounts, ground-swirl patterns, radar tracking and media reports and imagery. After everything has been considered, a rating of F0 to F5 is assigned to the tornado. Here is the criteria that they use to determine the rating:

Keep in mind that the rating cannot be applied until after the tornado has gone through and done its damage…the rating depends entirely on a guess of wind speed, and a measure of the damage done to structures and vegetation.

All of the world uses the Fujita Scale except the United States, which moved to the Enhanced Fujita Scale in 2007. It was revised to reflect better examinations of tornado damage surveys, so as to align wind speeds more closely with associated storm damage. Better standardizing and elucidating what was previously subjective and ambiguous, it also adds more types of structures, vegetation, expands degrees of damage, and better accounts for variables such as differences in construction quality.

The Fujita/Enhanced Fujita Scales are good ways to gauge just how bad a tornado was…not that you have to tell the people who were in them!

As the temperatures in Southern Arizona heat up, and the moisture in the air decreases, we start getting a lot of Red Flag Warnings for the various parts of the state. I’ve known intuitively what these warnings are about, especially since I was a wildland firefighter for a season, but wanted to find out some more details.

Red Flag Warnings are issues by the National Weather Service to inform firefighting authorities and land management offices that the conditions are right for wildland fire ignition and propagation. For these offices, the issuance of a Red Flag Warning helps them prepare for the potential fires brought on by drought, low humidity and high winds with potential lightning. For the general public, a Red Flag Warning means there is a high fire danger with increased probability of a quickly spreading vegetation fire in the area in the next 24 hours.

The weather criteria for fire weather watches and red flag warnings varies with each Weather Service Office’s warning area based on the local vegetation type, topography, and distance from major water sources but usually includes the daily vegetation moisture content calculations, expected afternoon high temperature, afternoon minimum relative humidity and daytime wind speed.

Related to, but of less severity than, a Red Flag Warning is a Fire Weather Watch. which alerts the public and fire fighting agencies that conditions may exist for a Red Flag Warning after the initial forecast period (12 hours). It is generally issued 12 to 48 hours in advance of the conditions, but can also be issued up to 72 hours in advance. That watch then remains in effect until it expires, is canceled, or upgraded to a Red Flag Warning.

Here’s an example of a Red Flag Warning for Flagstaff, AZ.

WWUS85 KFGZ 120342
RFWFGZ

RED FLAG WARNING
NATIONAL WEATHER SERVICE FLAGSTAFF AZ
842 PM MST WED JUN 11 2008

AZZ111>117-140-120445-
/O.EXP.KFGZ.FW.W.0023.000000T0000Z-080612T0400Z/
CHUSKA MOUNTAINS AND DEFIANCE PLATEAU (FIRE WEATHER ZONE 111)-
LITTLE COLORADO RIVER VALLEY IN COCONINO COUNTY (FIRE WEATHER
ZONE 112)-
LITTLE COLORADO RIVER VALLEY IN NAVAJO COUNTY (FIRE WEATHER ZONE
113)-
LITTLE COLORADO RIVER VALLEY IN APACHE COUNTY (FIRE WEATHER ZONE
114)-WESTERN MOGOLLON RIM (FIRE WEATHER ZONE 115)-
EASTERN MOGOLLON RIM (FIRE WEATHER ZONE 116)-
WHITE MOUNTAINS (FIRE WEATHER ZONE 117)-
NORTHEAST PLATEAUS AND MESAS SOUTH OF HWY 264 (FIRE WEATHER ZONE
140)-
842 PM MST WED JUN 11 2008

...RED FLAG WARNING WILL EXPIRE AT 9 PM MST THIS EVENING...

THE RED FLAG WARNING WILL EXPIRE AT 9 PM MST THIS EVENING. 

WINDS WILL CONTINUE TO DIMINISH AND RELATIVE HUMIDITIES WILL
INCREASE THIS EVENING...THUS THE RED FLAG WARNING WILL BE ALLOWED TO
EXPIRE.

$$

So, not the best picture in the world, but I was able to catch some growing mamatus clouds this morning on my way to work. I had to use my iPhone camera to take the pic. I love how these clouds look!

Mamatus Clouds in Tucson, AZ