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Tornado Alley Actually Four Regions?

April 28th, 2010 No comments

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

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.

How Are Tornadoes Rated?

April 27th, 2010 No comments

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:

Scale

Wind Estimate (MPH) Typical Damage

F0

<73

Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.

F1

73-112

Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.

F2

113-157

Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

F3

158-206

Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.

F4

207-260

Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.

F5

261-318

Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur.

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!

Red Flag Warning

April 21st, 2010 No comments

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.

$$


Picture Of The Week

April 19th, 2010 No comments

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

Mamatus Clouds in Tucson, AZ

Picture Of The Week

April 11th, 2010 No comments

Got this with my iPhone while at the airport in Dallas. It’s that time of year!

Categories: General, Tornadoes, Weather Tags: ,