The year 2009 in Phoenix ended as the (tie) 4th warmest year on record (since 1896) and the 7th driest.

Temperature got off to a warm start early in the year, with high temperatures often well above normal for extended periods with intermittent bouts of cooler weather, especially during April (figure 1). Much of June featured below normal temperatures and was in fact the coolest since 1998.

Once summer set in, temperatures - both highs and lows - generally remained a few degrees above normal. The biggest story during the summer was the overall lack of thunderstorm activity during the monsoon season (June 15 - September 30). The0.87" of rain that fell at Phoenix Sky Harbor Airport was the 10th lowest on record (figure 3). The Phoenix Rainfall Index total was 0.62". It was also one of the hottest monsoon seasons on record, with an average high temperature of 105.6 °F (3rd warmest) and an average low temperature of 82.6 °F (3rd warmest). By average temperature, July 2009 was the hottest month on record (98.3 °F).

Temperatures remained warm and above normal through much of the fall (November), finally cooling in December. The development of El Niño, which started in the summer and may have been partially responsible for the dry monsoon season, continued to strengthen through the fall and into the early winter. Several storm systems moved through the region in December, bringing the first widespread precipitation to the region since the previous spring.

Low temperatures were above normal for much of the entire year, with only brief periods of below normal lows (figure 2).

High and low temperatures have been trending warmer since temperature records began in 1896 (figures 4 and 5). This warming is attributable to a combination of local warming (urban heat island effect) and regional climate change. The average high temperature in 2009 tied for the 8th warmest on record while the low temperature was the 5th warmest. The decade that just ended (2000-2009) featured the warmest average high temperatures on record for any ten year period (previous was 1984-1993). Likewise, the average low temperature from 2000-2009 was the warmest on record. As for precipitation, 2000-2009 was the 6th driest ten-year period on record (driest was 1953-1962).

Figure 1. Observed daily high temperatures (°F) at Phoenix Sky Harbor Airport (top) and departure from normal (bottom) for 2009.

Figure 2. Observed daily low temperatures (°F) at Phoenix Sky Harbor Airport (top) and departure from normal (bottom) for 2009.

Figure 3. Observed daily precipitation (inches) at Phoenix Sky Harbor Airport (top). Bottom graph represents accumulated daily precipitation (green line) and normal (black line). Brown shaded areas represent below normal precipitation with blue representing above. Precipitation was below normal nearly the entire year, with exception of mid-February when ~40% of the year's total precipitation fell in a three day span.

Figure 4. Annual average high (top) and low (bottom) temperature in Phoenix from 1896 through 2009 (°F).

Figure 5. Annual precipitation in Phoenix from 1896 through 2009 (inches). The black line is a ten year average. The driest periods occurred in the 1950s/1960s and the 2000s.

Prolonged heat throughout much of the month of July contributed to 10 heat-caused deaths, according to the Maricopa County medical examiner. July 2009 was not only the hottest July on record but the hottest month of all-time in Phoenix. Records for Phoenix began in 1896. There were 15 days in July where highs reached 110 or hotter at Phoenix Sky Harbor airport. EPISODE NARRATIVE: Strong high pressure aloft dominated the southwestern states for most of the month. July 2009 was the hottest month on record in Phoenix...wth a mean monthly temperature of 98.3F, which was 3.5F hotter than normal. The period from July 10 to July 20 was the hottest in the Valley's climatological history, with an average maximum temperature of 112.7 degrees during those 10 days. Yuma was also very hot with a mean monthly temperature of 97.5F, or 3.8 degrees above normal.