Here’s to a great 2010, and an even better 2011. May the weather be interesting wherever you live!

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!

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.

In an online article at The Guardian, locals are concerned about the lack of lightning :

Fishermen in the village of Congo Mirador, a collection of wooden huts on stilts at the phenomenon’s epicentre, are puzzled and anxious by its absence. “It has always been with us,” said Edin Hernandez, 62. “It guides us at night, like a lighthouse. We miss it.”

There has been lightning in the skies around here for most of recorded history :

Electrical storms, product of a unique meteorological phenomenon, have lit up nights in this corner of Venezuela for thousands of years. Francis Drake abandoned a sneak attack on the city of Maracaibo in 1595 when lightning betrayed his ships to the Spanish garrison.

“This is unprecedented. In recorded history we have not had such a long stretch without lightning,” said Erik Quiroga, an environmentalist and leading authority on the Relampago de Catatumbo, or Catatumbo Lightning.

It appears to scientists that El Niño is to blame for the disruption of this phenomenon. The theory is that El Niño has caused a drought in the area, drying up the lakes and rivers that normally exist, and which contribute to the creation of the lightning. Another theory links it to decomposing organic matter which release methane. Yet another theory links it to Andean winds blowing across marshes, generating low pressure and building up an electrical charge in the atmosphere.

The last time the lightning disappeared? In 1906 when a catastrophic 8.8 magnitude earthquake off the coast of Ecuador and Colombia unleashed a tsunami. The lightning returned three weeks later. But the long delay in the lightning’s return this time have locals worried.

Losing the lightning is a symbolic blow. In addition to warding off Drake’s naval assault – an event celebrated in Lope de Vega’s 1598 epic poem – it is credited with helping independence fighters defeat a Spanish fleet in 1823. The state of Zulia, which encompasses Lake Maracaibo, has a lightning bolt across its centre and refers to the phenomenon in its anthem.

Quiroga worries that when rains return the lightning may not recover its former glory. It was dwindling in frequency and force even before the drought, probably because deforestation and agriculture had clogged the Catatumbo river and several lagoons with silt.

“This is a unique gift and we are at risk of losing it,” said Quiroga, who has led scientific teams to its epicentre. He has lobbied Venezuelan authorities to protect the area and the United Nations to recognise it as a world heritage site. A Unesco spokeswoman said there were no plans to do so because electrical storms did not have a “site”.

For more information about the Catatumbo Lightning, check out this brief Wikipedia article.

Current observations are an important part of determining what will happen with the weather. If you know how conditions are now, and how they have changed from the past, you can get a clue as to how they might be in the future. One tool that the NWS provides is a Surface Plot graph.

Surface Plot Chart

The official term for this is Mesonet Observations. According to wikipedia, “a mesonet is a network of automated weather stations designed to observe mesoscale meteorological phenomena. Dry lines, squall lines, and sea breezes are examples of phenomena that can be observed by mesonets. Due to the space and time scales associated with mesoscale phenomena, weather stations comprising a mesonet will be spaced closer together and report more frequently than synoptic scale observing networks, such as ASOS. The term mesonet refers to the collective group of these weather stations, and are typically owned and operated by a common entity.” So, basically, this is a series of stations that report current observations on wind speed and direction, temperature, relative humidity, and other items. If you know how to read these reports, and can spot trends, you can tell when a frontal system moves through, or when you might expect a thunderstorm. It’s good on the ground information.

One area at the NWS site that I spend a lot of time checking during stormy periods is the local weather radar image. This image gives me a great idea of where precipitation is occurring at the moment, and in what amounts and intensity. This is particularly valuable if you are going storm chasing, as you can see where the local thunderstorm cells are dumping their rain. It also provides a time-lapse ability, so you can see what direction the storm is moving. I enjoy watching the radar, especially when storms are headed my way.

Radar Image

Now, let’s say you’re interested in what the current forecast is for your region. The NWS is particularly good at providing that information, and does so in a wonderful graphical format. You can find out great things like temperature, wind speed and direction, dew point (handy in the summer for monsoon prediction), sky cover, precipitation, and more for up to the next week or so. This is very handy for planning activities, and the graphical format of the data is exceptional.

Graphical Forecast

Finally, though by no means even close to the end of the great things you can get from the NWS, is the Weather Story that they produce from time to time. In Tucson, I tend to see these when a weather event is on the way, and right after an event. Any significant change in the expected weather pattern also seems to generate a Weather Story. Essentially, these are a couple slides that show the basics of what’s going on in the area, and what to expect in the near future. I find them interesting because it’s a good synopsis of what’s going on. I don’t know if all major locations produce weather stories, but I suspect they do.

Weather Story

So, there you have it…some of the free products that you can get from the NWS. These items only touch on the rich repository of information that is the National Weather Service. If you are interested in weather, I encourage you to take some time and look around. You’ll find data on things that you didn’t even know they collected data on!

I hope you like the new direction of this blog, and I look forward to and welcome your comments. Thanks for visiting “Ive Been Thunderstruck!”

An awesome looking supercell!