National Weather Service

Monsoon 2009 Review
Updated: 4:00 AM October 1, 2009

With the close of September, Monsoon Season 2009 has come to an end. Across much of Arizona this will go down as one of the hottest and driest Monsoon Seasons on record. At Phoenix Sky Harbor International Airport the seasonal precipitation total was 0.87" (1971-2000 Normal: 2.77"), the 10th driest on record. The Phoenix Rainfall Index, representing the average rainfall across the Valley, was 0.62" which is well below the 1998-2008 median of 1.16". Precipitation at Yuma for this Monsoon Season was 1.76" (1971-200 Normal: 1.32"), which was the 25th wettest. The average temperature during this Monsoon Season in Phoenix was 94.1 °F, which was the second warmest on record. The average temperature in Yuma was 93.6 °F, tied for fourth warmest on record.

Top 10 Monsoon Season Temperature Rankings for Phoenix, AZ

Rank	Year	Average High Temperature (°F)
1	1989	106.4
2	2002	105.7
3	2009	105.6
4	2007	105.5
5	2003	105.3
6	2001	105.2
	1979	105.2
8	2005	105.1
9	1981	105.0
10	1988	104.8
Long-term Avg		102.2

Rank	Year	Average Temperature (°F)
1	2007	94.5
2	2009	94.1
3	2003	94.0
4	2002	93.9
	1989	93.9
6	2001	93.8
7	1981	93.7
8	2008	93.3
9	2005	93.2
	1988	93.2
Long-term Avg		89.2

Rank	Year	Average Low Temperature (°F)
1	2007	83.4
2	2003	82.8
3	2009	82.6
4	1981	82.4
5	2001	82.3
6	2002	82.2
7	2008	82.0
8	1988	81.6
9	2006	81.4
	1989	81.4
Long-term Avg		76.2

Top 10 Monsoon Season Temperature Rankings for Yuma, AZ

Rank	Year	Average High Temperature (°F)
1	1960	108.1
2	1994	107.8
3	1981	107.7
4	1956	107.1
	1879	107.1
6	1962	107.0
	1951	107.0
8	1995	106.9
	1989	106.9
	1957	106.9
Long-term Avg		103.9

Rank	Year	Average Temperature (°F)
1	1994	94.6
2	1995	93.7
	1981	93.7
4	1960	93.6
5	2009	93.5
6	2006	93.5
7	1998	93.2
8	2002	92.9
	1990	92.9
10	2003	92.8
Long-term Avg		89.6

Rank	Year	Average Low Temperature (°F)
1	2006	81.5
2	1994	81.4
3	1998	81.0
4	2003	80.8
	1999	80.8
6	2009	80.6
7	2000	80.5
	1996	80.5
	1995	80.5
10	1997	80.4
Long-term Avg		75.5

Why did this Monsoon Season turn out to be so dry? While the reasons are many and complicated, as well as not all fully known, a few simple explanations can be offered. Figure 1 below shows the average location and intensity of the jet stream during this past Monsoon Season. Figure 2 shows how this year differed from an average year - the jet stream was stronger across the southern U.S. and weaker across the north. This means the jet stream was displaced further south then what is typically seen. The reason for this southward displacement was likely the developing El Niño across the Pacific basin.

Fig. 1. Average jet stream winds (250 mb, ~25,000 feet) during Monsoon Season 2009. (Click for larger image.)

Fig. 2. Average jet stream winds (250 mb, ~25,000 feet) difference from climatology during Monsoon Season 2009. The pattern indicates that the jet stream was displaced to the south from its typical location (stronger winds, warm shading; weaker winds, blue shaded areas). (Click for larger image.)

With the jet stream displaced further south, this inhibited high pressure in the mid-levels of the atmosphere from building near the Four Corners area. Typically with the high near the Four Corners, clockwise flow around the high means east or southeast winds across southern portions of New Mexico and Arizona northern Mexico which then transports moisture northwest into Arizona. (More) This year, the more southern location of the jet stream meant the high was suppressed further south or was weaker. Figure 3 shows the average height of the 500 mb level during this monsoon season and figure 4 the difference from climatology. Clearly the weather pattern was dominated by a stronger low over the east coast (blue shaded areas) and stronger highs over Canada, the northwest U.S., and Mexico (warm shaded areas). This also influenced the wind patterns closer to the ground, as can be seen in Figure 5. Winds at 10,000 feet tended to have a stronger northerly component, meaning drier air would move south from the Great Basin region. The unusual cyclonic circulation can also be see over the eastern U.S. where low pressure often existed.

Fig. 3. Average 500 mb height during Monsoon Season 2009. (Click for larger image.)	Fig. 4. Average 500 mb height difference from climatology during Monsoon Season 2009. Lower heights were common over the east coast (blue shaded area) while higher heights were common across Canada, the western U.S., and Mexicon (warm shaded areas). (Click for larger image.)
Fig. 5. Wind anomaly at 10,000 feet during Monsoon Season 2009. Winds over Arizona tended to have a more northerly component than usual, which resulted in either drier air moving south from the Great Basin area and/or the southward suppression of moist air over Mexico. (Click for larger image.)