PHOENIX (AND ARIZONA) WEATHER HISTORY

four tornadoes were confirmed in northern Arizona near Flagstaff, with two of the tornadoes hitting Bellemont, a suburb to the west. Arizona averages slightly less than four tornadoes in a year, and the most on a single day was six on June 21, 1972.

*** Update made on October 8 - NWS has now confirmed 5 tornadoes in Arizona on October 6 ***

The first of the tornadoes struck Bellemont prior to dawn at about 5:15 AM MST and was preliminarily rated an EF1, causing extensive roof damage to homes, overturning RVs at a dealership, and derailing more than 20 railroad cars. The second tornado hit Bellemont about an hour later.

While October is not typically that active tornado-wise for the United States as a whole, it is one of the active periods for Arizona. For Coconino County, home to Bellemont and Flagstaff, it has actually been the most active month of the year in data since 1950.

The tornadoes were triggered as a cold pocket aloft revolved into the area about the cutoff low over the Southwest, as shown in the figure below. It gave a few severe thunderstorms in southeast NV and west AZ on Monday October 4, hailstorms in the greater Phoenix area on Tuesday October 5, and then tornadoes near Flagstaff on the morning of October 6. That number -18 indicates that the temperatures was -18C at Flagstaff, AZ at 500 mb (about 19,000 feet).

*** Update made on Oct 8 - air in the upper low was up to 9 degrees Celsius below average for this location and time of year ***

The surface temperature at the time of the first tornado was only 46F, so the limited instability was driven primarily by the cold pocket aloft. But it was enough to tap strong low-level wind shear - associated with a low-level jet, having winds a few thousand feet above ground that exceeded 60 mph --and convert that into tornado rotation.

The tornadic thunderstorms Wednesday weren't your typical Tornado Alley monsters, size-wise. They were low-topped supercells with strong low-level rotation, cores that weren't very intense, and low tops. The next three figures show the storm on radar using data closest to the time that the first tornado (shown by triangle) was hitting Bellemont (shown by B). Its strongest returns are in the red range (no lavenders or whites that would depict huge hail, although the storm to its southeast has a lavender core).

Despite not being that intense, the thunderstorm developed a significant Tornado Vortex Signature (TVS), as shown in the storm-relative velocities below. Reds indicate winds toward the northwest and greens toward the southeast. The storm to the southeast has a weaker TVS, but no known affiliated tornado. This would be a sign of strong low-level cloud rotation.

If we take a slice through the storm from north (left on the figure below) to south (right side), we can see that the storm was not that tall. The storm top was about 27,000 feet, and the red core of heavy rain and small hail doesn't reach as high as 15,000 feet. The northward lean of the storm, though, reflects that strong wind shear mentioned above.

Different atmospheric ingredients give different outcomes! If we go back to Phoenix AZ a day earlier, there was a lot more instability, tapping surface temperatures that were about 86F (a far cry from Flagstaff's 46) and giving greater instability. But there was much less wind shear, so storms were tall and intense, and produced hail up to tennis ball size (2.5 inch diameter). The radar image flow shows the lavender and white cores of one of those just northeast of Phoenix -- much more intense than the Flagstaff storm. The stronger thunderstorm updrafts, tapping that greater instability, allowed for the development of the large hail. Incidentally, that little"unicorn horn" extending northwest from the white hail core is a false echo known as a "hail spike" or "flare echo." More on that later.

The slice through this storm shown below, reveals a storm top over 42,000 feet and a lavender hail core that extends upward to about 25,000 feet -- nearly as high as the entire storm top at Flagstaff!

What causes that hail spike to show up in a false location beyond where there is actually precipitation? The true edge of the storm is not very far beyond the edge of the green-colored area. The false return has to do with the path taken by the radar beam interacting with hailstones. Most of the radar beam energy bounced directly back from the hailstones to the radar, giving that white intense core. But part of the radar energy glanced off the sides of some of the hailstones at an angle down toward the ground, and then off the ground to the radar. Since this part of the energy follows a longer, indirect path, it takes it longer to get back to the radar. That "delayed arrival time" is interpreted by the radar as if the target had been farther away. Hence, the spike extending northwest beyond the edge of the true storm edge. The radar displays the position of the storm or portion of the storm based upon the time lag between when its signal was transmitted and when the energy returns. The energy travels of the speed of light, so the radar needs to have a very accurate timer! So the hail spike is "late return" from the hail.

From the NWS

Tornado Outbreak Strikes Northern Arizona

October 6, 2010

Updated October 12, 2010 at 500 PM MST.

Lastest update includes: Total number of confirmed tornadoes is now 8 (Garjon Tank tornado and Cordes Junction tornado have been added). Map has been updated to include additional tornado tracks. A synopotic overview of the event is now included. See update log at bottom of the page for a complete history of updates.

If you are a resident or visitor of northern Arizona, and if you experienced any wind or tornado damage related to the storms on October 6, 2010, please send your stories and pictures to the National Weather Service Flagstaff office by e-mail to W-fgz.Webmaster@noaa.gov. In addition, please e-mail us if you observed any large hail. Pictures to go along with your reports are always greatly appreciated. With any reports you send us, it is important to include the exact location of the wind or tornado damage or hail, along with details of the time that it occurred. Please let us know if we can include your pictures in our storm reports. Thank you!

A major severe weather event struck northern Arizona early on Wednesday, October 6, 2010. With at least 8 confirmed tornadoes, this event will go down in history as the most tornadoes to strike the state in a single day. Not only was the number of tornadoes impressive, but several of the tornadoes were damaging and long tracked events. One of the tornadoes had a nearly continuous path exceeding 30 miles, while another tornado southeast of Tuba City was violent enough for a preliminary rating of EF-3 on the Enhanced Fujita scale. Highlighted below is a meteorological summary of the outbreak.

On October 5th (the day prior to the tornado outbreak), a strong low pressure system in southern California forced strong southerly winds across the state, allowing rich moisture to move northward into the state from Mexico. Skies across much of northern Arizona were mostly cloudy, however, skies were clear across the southern half of the state. As the deserts warmed during afternoon, the atmosphere became very unstable and storms erupted across the state, most notably with several severe storms striking the Phoenix metro area. These storms continued to develop after nightfall and race northwards across northern Arizona.

Visible Satellite Loop from 4:00-4:30 PM MST Tuesday October 5, 2010

Shortly after midnight, new storms began to develop along the I-17 corridor. At this time, the low in southern California inched closer, and a strong short wave evident in the water vapor imagery approached from the west. The wind shear (change in wind direction and speed with height) become very strong across northern Arizona, veering from southwesterly in the mid levels, to southeasterly in the low levels. It was at this time that the environment became increasingly favorable for tornadic thunderstorms, with the first tornado touchdown near Blue Ridge at approximately 1:58 am MST.

Water Vapor Loop from 3:15-5:30 AM MST Wednesday October 6, 2010

NWS Flagstaff Radar Loop 1:00-5:00 AM MST (8:00-12:00Z) Wednesday October 6, 2010

As the early morning progressed, more storms developed and moved quickly northward, becoming supercells with strong rotational signatures on radar. The low pressure system driving the thunderstorm activity was not moving significantly, and remained west of the Arizona border. The strong southerly flow, and atmospheric forcing in place caused severe and tornadic storms to continually redevelop over the same areas (a highly unusual event!). The 12z (5 AM MST) weather balloon launch from Bellemont indicated that despite being fairly cool with surface temperatures near 50F, the atmosphere was quite unstable due to the very cold temperatures aloft. Strong wind shear was present in the profile, causing the storms which formed to rotate, and engender the tornadic production. The very cold temperatures in the profile (-14C, or 7F at around 19,000 ft) aided in hail production within the storms.

Upper Air Sounding from Bellemont, AZ at 5 AM MST Wednesday October 6, 2010

NWS Flagstaff Radar Loop 5:00-9:00 AM MST (12:00-16:00Z) Wednesday October 6, 2010

By mid-morning, strong storms continued across the Interstate-17 corridor northward towards Utah, with new development across the little Colorado River Valley east of Flagstaff, and south of Tuba City. The lead shortwave forcing the thunderstorm activity was now moving through the state, pushing the strongest activity north and eastward, giving the Prescott and Bellemont areas a much needed break, while putting Flagstaff and areas eastward under a greater threat.

Water Vapor Loop from 10:15-11:30 AM MST Wednesday October 6, 2010

NWS Flagstaff Radar Loop 9:00 AM -1:00 PM MST (16:00-20:00Z) Wednesday October 6, 2010

By 3 PM MST, the lead shortwave was now moving across the northern part of the state, pushing the strongest activity northeastwards towards Southern Utah and the four corners, with scattered activity remaining across Northern Arizona. As the shortwave passed, the strong vertical forcing causing the thunderstorm activity diminished, and the winds shifted to more southwesterly direction, lessening the wind shear across the region. This marked the end of the severe weather threat for the region, as the best combination of instability and wind shear had now been displaced further north into Utah.

Water Vapor Loop from 1:30-3:00 PM MST Wednesday October 6, 2010

The following is a preliminary summary of the hails events, and will be updated as additional information becomes available.

From the NWS

http://www.wrh.noaa.gov/psr/general/history/

Many people perceive Phoenix, as well as Arizona, to be a place of unrelenting sunshine - nearly devoid of stormy, active or hazardous weather. With the exception of the intense summer heat, Arizona is a great place to live, or retire to. However, those who have lived here, and experienced the fury of Monsoon thunderstorms know that Arizona can hold its own with any state in the country when it comes to severe weather - especially during the summer thunderstorm season. The following cases represent some of the more impressive weather events that have occurred across Arizona, with special emphasis on the Phoenix Metropolitan area.

I. ARIZONA SUPERCELL - 1999

Arizona experiences few, if any, tornadoes each year, quite unlike the "tornado alley" states of the Great Plains. However, it is possible for a tornado to occur in Arizona, or in the Phoenix Metro area, when the atmospheric conditions become "just right". Tornadoes, for the most part, do not spring from garden-variety thunderstorms - they form in association with thunderstorms known as SUPERCELLS. Supercells are thunderstorms that are unique in that they contain strong rotation, or spin, within the core of the storm. A tornado is a violently rotating column of air, on a rather small scale, and this rotation is derived from the larger-scale rotation present within the supercell.

In order for a thunderstorm to develop this strong rotation, and thus become a supercell, the atmosphere must possess substantial amounts of WIND SHEAR. Wind shear is a change in the wind's direction, or speed, or both, with height. For example, if the winds at the surface were from the southeast at 10 mph, and at 10000 feet aloft they were from the southwest at 50 mph, the atmosphere would possess strong wind shear. A supercell storm is considered a severe thunderstorm, in that it can produce tornadoes, as well as damaging winds and large hail. Severe, supercellular storms need more than just wind shear to develop, they need an atmosphere that is very unstable.

Herein lies the problem: during the Monsoon, the atmosphere is very unstable virtually every day. However, on most occasions, the winds aloft are rather weak, and the wind shear in the atmosphere is not sufficient to promote the development of supercells. Thus, supercells and their associated tornadoes are very rare during the Arizona summer. It is a somewhat different story during the spring and fall months, however. It is possible to have both strong wind shear, and strong atmospheric instability during these months, and this increases the chances for a supercellular thunderstorm to form.

One such supercell did indeed develop on September 14, 1999. The storm was located near Crown King, north of the Phoenix metro area and southeast of Prescott. Although no tornadoes were reported with this storm (largely due to the fact that the storm moved over an area of low population density), the potential for tornadoes with this storm was very high! The following graphics, taken from the Phoenix WSR-88D Doppler Radar, show both the reflectivity, and velocity, structures of this supercell. A very prominent "hook echo" can be seen in the first image - this feature is associated with strong rotation within the storm and in some cases a tornado will develop in the vicinity of this echo! In fact, when a genuine hook echo is seen on radar, the NWS will issue a tornado warning! (Click on the image to display larger, hi-res, version)

The following 2 images compare the Arizona supercell with an Oklahoma supercell that spawned tornadoes on the ground, including an F5 tornado in the Oklahoma City area. Note that the reflectivity structures of the two storms are nearly identical...including the presence of and position of the hook echo. Oklahoma is noted for its supercellular, tornadic thunderstorms - yet it is possible for such storms to occur in Arizona, even though they do so infrequently.

II - PHOENIX METRO MACROBURST - 1996

Thunderstorms are a common occurrence during the Arizona Monsoon; on any given day scattered storms are possible across the southern deserts and many of them can produce strong, gusty winds, along with heavy rain and small hail. In some instances, the downdrafts associated with the thunderstorms are very strong, but very localized, with damaging winds reaching from 60 to over 80 mph. Winds such as these are known as "microbursts", as they only last for a short time, and affect a small area.

On a much larger scale, in both time and space, there is the phenomenon known as the "MACROBURST". This is sort of like the "big brother" to the microburst. The strong, rain-cooled downdrafts from the monsoon thunderstorms become well organized and persistant, and can last for a much longer time, and cover a much greater area. One such notable macroburst affected the Phoenix metropolitan area on August 14 of 1996. In this case, strong thunderstorms between Paradise Valley and Crown King organized into a massive cluster of storms in the vicinity of Carefree; this cluster of storms marched rapidly southwestward across the west valley, producing widespread damaging winds and very heavy rainfall. Peak wind gusts of up to 115 mph were measured at the Deer Valley Airport, and the storm caused over 160 million dollars of damage over several west valley cities, including Buckeye. The measured speed of 115 mph set the all time peak gust record record for Phoenix, as well as for the entire state of Arizona!

It should be noted that macroburst winds, unlike tornadic winds, are STRAIGHT-LINE winds - they do not contain strong rotation such as would be observed with the passing of a tornado. These strong winds descend from the lower levels of a thunderstorm, then hit the ground and spread outwards, moving in a straight line.

III - 100 DEGREE DAY STATISTICS FOR PHOENIX

It seems, at times, that 100 degree weather and Phoenix go together like 'mom' and 'apple pie'. However, this is a bit of a misconception, as the mercury stays in double-digits for the bulk of the year. In fact, in 1913, Phoenix only registered 48 days where the mercury hit 100 degrees or higher. Nevertheless, there is a lot of interest by the public and media in 100 degree statistics, especially with regards to the average first 100 degree reading at Sky Harbor. Because of this interest, this section was created. Click on the following link to see a wealth of statistical information concerning 100 degree and hotter temperatures in Phoenix:

100 Degree Day Statistics for Phoenix

IV - NIGHTIME INFRARED (IR) IMAGE OF PHOENIX

Over the two decades, as the Phoenix Metropolitan area has grown dramatically in size, the "urban heat island" effect has developed, which has caused temperatures in the center of the city to become much warmer than those on the outskirts of the valley. The concrete and asphalt of the city retains the heat of the day, and releases it slowly as compared to the surrounding desert terrain, which cools much quicker at night. The ASOS weather sensor has always been located near the Sky Harbor runway complex, and as the heat island effect intensifies, the nighttime lows at Phoenix keep rising every year. The summer of 2003 saw the all time record high minimum temperature at Phoenix (93 degrees) shattered as a new mark of 96 degrees was established! Several times during the summer the old mark of 93 was tied or broken, as well.

As you can see from the thermal imagery map of Phoenix, the hottest temperatures in the valley occur at the Sky Harbor Airport runway complex, clearly shown by the bright yellow colors at the top center of map. The most significant concentration of asphalt in the Phoenix Metro occurs at the runway complex, and you can clearly see the yellow stripes in the IR imagery below, which correspond to the east-west runways at Sky Harbor. With the weather sensor located very close to this location, no wonder Phoenix has been seeing increasingly warm mornings over the past decade. As time goes by, it is possible that Phoenix will see a morning where the temperature never drops below 100 degrees!!