PHOENIX (AND ARIZONA) WEATHER 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 supercell, located northwest of Carefree, and southeast of Prescott, is labeled as "A" in this reflectivity image. The orange and red colors refer to the high reflectivities at the core of the storm...and are associated with intense rainfall. A very prominent appendage, the "hook echo", is labeled as "B" here; it is the bright red pendant on the southwest flank of the storm. The strong rotation in the cell has actually wrapped some of the heavy rainfall around the main storm updraft. The presence of such an appendage, or hook, is sometimes associated with a tornado on the ground!

The same supercell is shown here in base velocity image, which gives information on speed and movement of air parcels within a storm. Green shades depict air parcels moving towards the radar (inbound), where red shades show parcels directed away from the radar (outbound). Points A and B define a rotational couplet - maximums of inbound and outbound velocities within a very small distance. Note the proximity of this couplet to point "B" in the reflectivity image. This reflects the strong rotation present in the updraft of this supercell...which created the hook echo, and possibly generated a tornado!

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.

	This image shows the severe storm in its incipient stages. At 00:38 GMT (5:38 pm MDT), a cluster of strong thunderstorms was beginning to organize, with the main cells located between Crown King and Fountain Hills. This cluster will continue to coalesce into a Mesoscale Convective System (MCS).
	At 01:02 GMT, the MCS has developed in the vicinity of New River, Carefree and Cavecreek.
	At 01:20 GMT the system has started to move southwestward into the northern and northwestern portions of the Phoenix Metropolitan area. As it moves to the SW, it will generate very strong, straightline winds which push out ahead of the system. These strong "macroburst" winds, will reach speeds of up to 115 mph...as recorded at the Deer Valley airport.
	At 01:37 GMT, the MCS continues to propogate quickly to the southwest, moving through the western portions of the metro area. It can be seen moving through Peoria and Sun City, among other places.
	This last image, taken at 02:01 GMT (7 pm), shows a well defined, leading edge to the system, moving quickly southwestward ahead of the main storm cluster and through the West Valley communities, such as Buckeye. The radar reflectivities seen with this MCS are intense, and associated with copious rainfall, as well as small to moderate hail...which accompanied the severe and damaging winds that lashed the west valley communities.

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!!

Infrared image taken on April 17, 2001, depicting the Phoenix Metropolitan area at night. The different colors depict the temperature variations across the valley, from the cool blue/green shades in the valley outskirts, to the hot red/yellow hues in the central part of the city. Note the city hot spot - shown in yellow - at the top center, which is the runway complex at Phoenix Sky Harbor Airport.