State Climate Update for Arizona

Conditions through May 2006

 

The primary climate variables of temperature and precipitation are monitored for the state of Arizona by climatically homogeneous areas termed ‘climate divisions’. There are 7 climate divisions for Arizona (see map). From the stations within each climate division, mean values of temperature and precipitation for the division as a whole are computed. Calculated from the divisional temperature and precipitation data is the Palmer Drought Severity Index (PDSI), a type of wetness/dryness index based on budgeting input (precipitation) against demand (evapotranspiration) in simulating soil moisture through time.

 

Here, values for the most recent month along with 3-, 6-, 12-, 24-, 36-, and 48-month total precipitation values are ranked within a historical context (1895-present) and the rankings are divided by the number of years of data and multiplied by 100% to create percentiles. Low percentile values indicate cool or dry conditions while high values indicate warm or wet conditions. For example, a precipitation total that falls at the 100th percentile means that it is the highest precipitation total for the period of record. As the PDSI is a cumulative index that describes conditions as they have evolved up to the present, only the value of the current month is translated to a percentile.

 

Based upon analysis by the Monitoring Group of the Governor’s Drought Task Force, precipitation data are stratified into the following drought categories:

           

Numerical Category

Description

Precipitation Percentile

0

no drought

40.01 – 100

1

abnormally dry

25.01 – 40.00

2

moderate drought

15.01 – 25.00

3

severe drought

5.01 – 15.00

4

extreme drought

0.00 – 5.00

 

 

SHORT TERM CONDITIONS: 1-6 MONTHS

The month of April was characterized by precipitation at or below average across the entire state, with very dry conditions across the south-central (“severe drought”) and southeastern (“moderate drought”) portions of Arizona. Precipitation totals for the past 3-month period are characteristic of “abnormally dry” and “moderate drought” conditions across all but the northwestern portion of the state. However, the past 6-month period has been dramatically dry across the entire state with precipitation totals characteristic of either “severe drought” or “extreme drought” conditions. The Palmer Drought Severity Index (PDSI) has fallen dramatically since the wet winter of 2004-05. The PDSI indicates moist conditions only in the southwestern corner of the state (CD 5), but the recent dryness in this area is likely to significantly drop the PDSI value in the coming months. Elsewhere, the PDSI signals very dry conditions.

 

 

MAY 2006: percentiles of precipitation (with drought category) and Palmer Drought Severity Index for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(X)

Bill Williams

(1)

Lower Colorado

(0)

Upper Colorado

(X)

Little Colorado

(2)

Agua Fria

(1)

Verde

(1)

Salt

(2)

Upper Gila

(2)

Lower Gila

(0)

San Pedro

(0)

Willcox Playa

(1)

WhiteWater Draw

(0)

Santa Cruz

(X)

San Simon

(3)

 

 

 

MARCH 2006 - MAY 2006: percentiles of precipitation (with drought category) for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(X)

Bill Williams

32.6(1)

Lower Colorado

36.7(0)

Upper Colorado

42.8(0)

Little Colorado

50.4(0)

Agua Fria

31.9(1)

Verde

49.6(0)

Salt

60.6(0)

Upper Gila

54.4(0)

Lower Gila

61.4(0)

San Pedro

30.8(2)

Willcox Playa

20.0(2)

WhiteWater Draw

37.4(1)

Santa Cruz

36.7(1)

San Simon

40.1(1)


 

 

DECEMBER 2005 – MAY 2006: percentiles of precipitation (with drought category) for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(X)

Bill Williams

5.1(3)

Lower Colorado

6.9(3)

Upper Colorado

9.3(3)

Little Colorado

5.6(3)

Agua Fria

4.7(4)

Verde

9.2(3)

Salt

9.3(3)

Upper Gila

7.5(3)

Lower Gila

10.2(3)

San Pedro

4.1(4)

Willcox Playa

4.4(4)

WhiteWater Draw

6.6(3)

Santa Cruz

3.1(4)

San Simon

3.4(4)


 

INTERMEDIATE TERM CONDITIONS: 12-24 MONTHS

The intermediate time periods of 1 and 2 years define the break point in the more recent drought conditions. The past 12 months have been marked by dryness characteristic of “extreme drought” across 4 of the 7 Arizona climate divisions. This 1-year period represents the conditions generally since the end of the wet winter of 2004-05. The area of exception is again in the southwestern corner of the state, where the 12-month precipitation value is of an amount indicative of “abnormally dry” conditions.

 

There is very little evidence of drought within the state when examining precipitation totals for the past 2 years. Only one region registered precipitation totals suggestive of drought; precipitation for southeastern Arizona (CD 7) ranked only in the 8th percentile, which is indicative of “severe drought”.

 

 

 

JUNE 2005 – MAY 2006: percentiles of precipitation (with drought category) for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(0)

Bill Williams

6.7(3)

Lower Colorado

31.9(1)

Upper Colorado

11.3(3)

Little Colorado

3.7(4)

Agua Fria

4.3(4)

Verde

6.9(3)

Salt

4.4(4)

Upper Gila

3.4(4)

Lower Gila

11.5(3)

San Pedro

4.8(4)

Willcox Playa

3.4(4)

WhiteWater Draw

9.0(3)

Santa Cruz

2.7(4)

San Simon

3.3(4)


 

JUNE 2004 – MAY 2006: percentiles of precipitation (with drought category) for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(X)

Bill Williams

75.2(0)

Lower Colorado

89.0(0)

Upper Colorado

81.6(0)

Little Colorado

21.8(2)

Agua Fria

54.3(0)

Verde

70.9(0)

Salt

38.2(1)

Upper Gila

22.7(2)

Lower Gila

61.4(0)

San Pedro

7.1(3)

Willcox Playa

10.6(3)

WhiteWater Draw

20.0(2)

Santa Cruz

4.3(4)

San Simon

16.1(2)


 

LONG TERM CONDITIONS: 36-48 MONTHS

The pattern of precipitation rankings for the longer-term 3-year period across the state closely matches that for the past 2-year period. The 36-month precipitation totals were at or above average for 5 of the 7 climate divisions in Arizona. Only across the southeastern (CD 7) and east-central (CD 4) portions of the state did the precipitation fall far below average. Across southeastern Arizona the 3-year precipitation total is suggestive of “severe drought”, while that across east-central Arizona is indicative of “moderate drought” conditions.

 

Dryness is more evident for the longer 4-year period. Three regions of the state (northwestern (CD 1), southwestern (CD 5), and west-central (CD 3)) are characterized by 4-year precipitation totals that are at or above average, while two regions of the state (northeastern (CD 2), south-central (CD 6)) can be characterized as “abnormally dry” over the past 4 years. The long-term problematic areas within the state are the east-central (CD 4) and southeastern (CD 7) regions, where 4-year precipitation totals are suggestive of “severe drought” conditions.

 

 

 

JUNE 2003 – MAY 2006: percentiles of temperature, and precipitation (with drought category) for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(X)

Bill Williams

59.9(0)

Lower Colorado

79.1(0)

Upper Colorado

67.4(0)

Little Colorado

11.1(3)

Agua Fria

34.1(1)

Verde

49.6(0)

Salt

24.2(1)

Upper Gila

12.9(3)

Lower Gila

53.2(0)

San Pedro

(3)

Willcox Playa

6.3(3)

WhiteWater Draw

12.7(3)

Santa Cruz

8.2(3)

San Simon

23.0(2)


 

JUNE 2002 – MAY 2006: percentiles of temperature, and precipitation (with drought category) for the 15 Arizona watersheds

Watershed

Precipitation

Virgin River

(X)

Bill Williams

38.6(0)

Lower Colorado

70.2(0)

Upper Colorado

61.4(0)

Little Colorado

8.2(2)

Agua Fria

26.4(1)

Verde

37.8(1)

Salt

15.4(2)

Upper Gila

7.9(3)

Lower Gila

43.6(0)

San Pedro

(3)

Willcox Playa

3.8(4)

WhiteWater Draw

6.2(3)

Santa Cruz

5.5(3)

San Simon

12.1(3)


 

SUMMARY

● The recent extremely dry conditions that briefly abated in March returned in April across many areas of the state of Arizona, most greatly evidenced across south-central Arizona, which received no precipitation for the month.

 

● Precipitation totals for the past 3- and 6-month periods are very low, with much of the state characterized by amounts indicative of “severe” to “extreme” short-term drought.

 

● The Palmer Drought Severity Index (PDSI) has fallen dramatically since the wet winter of 2004-05. The PDSI indicates moist conditions only across the southwestern corner of the state (CD 5); elsewhere, the PDSI signals very dry conditions.

 

● The past 12 months have been marked by dryness characteristic of “extreme” or “severe” drought across 5 of the 7 Arizona climate divisions, and dryness or drought elsewhere.

 

● There is very little evidence of drought within the state when examining precipitation totals for the past 2 years; drought is evident only across southeastern Arizona (“moderate drought”) during the period.

 

● The 36-month precipitation totals are at or above average for 5 of the 7 climate divisions in Arizona; however, across southeastern Arizona the 3-year precipitation total is suggestive of “severe drought”, while that across east-central Arizona is indicative of “moderate drought” conditions.

 

● The long-term problematic areas within the state are the east-central and southeastern regions, where 4-year precipitation totals are suggestive of “severe drought” conditions.