NCDC added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.
- Based on the Palmer Drought Index,
severe to extreme drought affected about 17 percent of
the contiguous United States as of the end of March 2007, an
increase of about 8 percent
compared to last month. By contrast, about 8 percent of the
contiguous U.S. fell in the severely to extremely wet
- About 29 percent of the
contiguous U.S. fell in the
moderate to extreme drought categories (based on the
Palmer Drought Index) at the end of March .
- On a broad scale, the previous two decades (1980s and 1990s)
were characterized by unusual wetness with short periods of
extensive droughts, whereas the 1930s and 1950s were characterized
by prolonged periods of extensive droughts with little wetness
to extreme drought,
severe to extreme drought).
- A file containing the national monthly percent area severely
dry and wet from 1900 to present is available for the severe to extreme and moderate to extreme categories.
- Historical temperature, precipitation, and Palmer drought data
from 1895 to present for climate divisions, states, and regions in
the contiguous U.S. are available at the Climate Division:
Temperature-Precipitation-Drought Data page in files having
names that start with "drd964x" and ending with "txt" (without the
Detailed Drought Discussion
|At the end of March
extreme drought was concentrated in the Southwest, southern Texas,
the western High Plains, northern Minnesota, and the Tennessee
Valley. Drought and abnormal dryness covered a broad swath from the
Southwest through the northwestern Plains into the upper Midwest,
as well as another swath from eastern Texas eastward to the
Atlantic Ocean (March 27 Drought
Monitor). Conditions in southern California and southwestern
Arizona continued to deteriorate. Los Angeles is on track to record
its driest rainy season in 120 years.
A storm system at the end of the month brought much needed
precipitation to the Plains and upper Midwest; persistent drought
conditions were alleviated throughout the central part of the
country. Drought and abnormal dryness continued in Florida and
increased in severity in the central and southern Appalachians. In
the Southwest, Southeast and upper Midwest, soil
moisture was low. Vegetative
health was stressed from central California southeastward
through southern Arizona. Streamflow
was low in a wide area of the Southeast.
||Florida has had over 600
wildfires that consumed over 58,000 acres since the beginning of
the year. More acres have burned so far this year than in all of
2005. Burning bans are in effect in many areas of the State, water
restrictions are prevalent, and recreational uses of water bodies
have been reduced.
In Texas drought has led to increased expansion and contraction of
clay soils resulting in damages to house foundations.
California farmers were affected by poor pastures and water
Tennessee is under a statewide burning ban, and the Tennessee
Valley Authority produced about 25 percent less electricity than
usual because of low water levels and minimal rainfall.
Farmers in Alabama reported that current drought conditions aided
operations because the dry, broken soil made for excellent planting
conditions. However, once the planting is completed, rainfall will
be needed for crop growth.
Impacts in drought-stricken areas have been collected and
summarized by county at the National Drought Mitigation Center's
Drought Impact Reporter.
State/Regional/National Moisture Status
March 2007 Paleoclimatic Analysis for Southern
California, most of the annual precipitation falls in the 6-month
cool season from October to March. After a string of dry winters in
1999-2004 was broken by very wet conditions in 2004-2005, severe
drought conditions have returned to this region. In California
Climate Division 6 (South Coast), which includes Los Angeles and
San Diego, precipitation for the last six months was only 4.05
inches (28% of normal), the driest October-March period in the
entire 112-year record. The U.S. Drought Monitor for late March and
showed southern California classified from category D1 (moderate
drought) to D3 (extreme drought), with rapidly worsening conditions
The graph below (annual values in light blue, 5-year weighted
average in dark blue) shows the cool season (October-March)
precipitation, 1896-2007, for California Division 6. The
instrumental record is notable for extreme variability, with a
six-fold difference between the driest and wettest winters. Long
strings of dry winters (four or more consecutive years below
average) are common, having occurred in 1896-1900, 1917-1921,
1923-26, 1928-1931, 1946-1951, 1974-77, 1987-1990, and 1999-2004.
The period from 1917-1931 had below-average winter precipitation in
13 of 15 years (yellow arrow).
large image (50 KB)
larger image (175
|The graph to the left
also shows a 595-year tree-ring record (1409-2003; annual values in
light red; 5-year smoothed values in dark red) that corresponds
very well to the variability in October-March precipitation. This
record is the average of four blue oak tree-ring chronologies from
central and south-central California. The correlation between the
annual values of the tree-ring record and October-March
precipitation is 0.858, which indicates a very high degree of
shared variance. The tree-ring record captures the multi-year
variability of the observed precipitation record extremely
|The tree-ring record,
as a proxy for winter precipitation, can put the droughts of the
last century in southern California into a much longer perspective.
Standing out is that the years with very low tree-ring values
(index below 0.4; dashed orange line)--suggesting very low winter
precipitation in those years--are much more numerous prior to 1900
(an average of five per century) than since 1900 (none). The winter
of 2006-2007 may not be very unusual, relative to the nearly
600-year tree-ring record.
|Also, the tree-ring
record shows some persistent multi-year low-growth anomalies,
suggesting that droughts in the past have been more sustained than
those in the instrumental record. In the early 1500s, the tree-ring
index was below the long-term mean for eight straight years
(1515-1522; orange arrow). More strikingly, in the mid-1700s, two
7-year periods of below-mean tree growth (1751-1757, 1759-1765) are
separated by a single year. The average tree-ring index for the
entire 15-year period (1751-1765; red arrow) is very similar to
that for 1917-1931, suggesting these events were of similar
multi-century tree-ring record strongly confirms the impression
from the shorter instrumental record that drought is a recurrent
feature of the southern California climate, whether severely dry
single winters like this year's, or more persistent dry
- Divisional climate data, including precipitation for California
Division 6 as shown above, can be obtained from NCDC.
- The four California blue oak tree-ring chronologies were
obtained from R. Daniel Griffin and David Stahle of the University
of Arkansas. These four chronologies have also been used to
reconstruct annual streamflow for the Salinas River in California,
as described at the World
Data Center for Paleoclimatology.