December 2014 Drought Report

Overview

An active jet stream with a strong zonal flow dominated the weather pattern over North America during most of December 2014. By keeping cold arctic air masses bottled up in Canada and allowing milder Pacific air masses to move across the CONUS, this circulation pattern fed Pacific moisture into the West and Gulf of Mexico moisture into the Southeast, but blocked these moisture sources from most of the central part of the country for most of the month. Consequently, monthly precipitation was above normal across much of the West and Central Plains, and parts of the Northeast and Southeast, improving drought conditions in those regions; precipitation was below normal across much of the Southern and Northern Plains, Mississippi Valley, and southern Great Lakes, with drought conditions worsening in the Southern Plains. December was drier than normal across southeastern Alaska and much of the Hawaiian Islands. When integrated across the CONUS, December 2014 ranked as the 51^st wettest December in the 1895-2014 record. On balance, the national drought footprint contracted slightly when compared to last month, decreasing to about 24.0 percent of the U.S. as a whole, according to USDM statistics. According to the Palmer Drought Index, which goes back to the beginning of the 20^th century, about 10 percent of the CONUS was in moderate to extreme drought at the end of December, a decrease of about 3 percent compared to last month.

By the end of the month:

• moderate (D1) drought covered a large area of the U.S. from the West Coast to the Central and Southern Great Plains, connected across parts of New Mexico, with large areas of more intense drought within the moderate drought area;
• there were two epicenters of extreme (D3) to exceptional (D4) drought within this large moderate to severe (D2) drought area — one located in the California-Nevada-Oregon region and the other in the Southern Plains centered in northern Texas and adjacent Oklahoma; and
• areas of moderate drought persisted along the Gulf of Mexico coast from Louisiana to Alabama, in Hawaii, and in the Northern Plains.

Percent area of the CONUS in moderate to exceptional drought, January 4, 2000 to present, based on the U.S. Drought Monitor.

Palmer Drought Index

The Palmer drought indices measure the balance between moisture demand (evapotranspiration driven by temperature) and moisture supply (precipitation). The Palmer Z Index depicts moisture conditions for the current month, while the Palmer Hydrological Drought Index (PHDI) and Palmer Drought Severity Index (PDSI) depict the current month's cumulative moisture conditions integrated over the last several months. While both the PDSI and PHDI indices show long-term moisture conditions, the PDSI depicts meteorological drought while the PHDI depicts hydrological drought. The PDSI map shows less severe and extensive drought in the West and parts of the Plains than the PHDI map because the meteorological conditions that produce drought are not as long-lasting as the hydrological impacts.

Used together, the Palmer Z Index and PHDI maps show that short-term dry conditions occurred over parts of the Southern Plains, which had areas in long-term drought during November, maintaining or intensifying drought by the end of December. Short-term wet conditions occurred over parts of the West, reducing areas of long-term drought.

Standardized Precipitation Index

The Standardized Precipitation Index (SPI) measures moisture supply. The SPI maps here show the spatial extent of anomalously wet and dry areas at time scales ranging from 1 month to 24 months.

The SPI maps illustrate how moisture conditions have varied considerably through time and space over the last two years. December and the last 2 to 6 months have been dry across the Far Northern Plains, Lower Great Lakes, and Lower Mississippi Valley, but it was wet at longer time scales. The Southern Plains were dry during December and at longer (12-24 months) time scales, but neutral in the aggregate on the intermediate time scales. Precipitation during December and earlier months contributed to wetter-than-normal conditions across much of the West at the 1 to 2 month and 6 month time scales, but dryness is still evident in the Intermountain Basin and parts of the Southwest at 3 months and across most of the West at longer time scales (12 to 24 months).

Standardized Precipitation Evapotranspiration Index

The SPI measures water supply (precipitation), while the SPEI (Standardized Precipitation Evapotranspiration Index) measures the combination of water supply (precipitation) and water demand (evapotranspiration as computed from temperature). Warmer temperatures tend to increase evapotranspiration, which generally makes droughts more intense.

1-month SPEI for current month.

1-month SPI for current month.

Temperatures were unusually warm during December. Normally that would increase evapotranspiration and make drought conditions worse. But since this is the winter and temperatures are normally low, the effect of the warm temperature anomalies was muted and made a noticeable difference in the SPEI and SPI only in the Southern Plains. But when the entire year is considered, the record warm temperatures in the West significantly intensified drought conditions when the 12-month SPEI is compared to the SPI.

12-month SPEI for current month.

12-month SPI for current month.

Agricultural, Hydrological, and Meteorological Indices and Impacts

USDA western states reservoir status

USGS monthly streamflow percentiles

Drought conditions were reflected in numerous agricultural, hydrological, and other meteorological indicators, both observed and modeled.

Well-above-normal precipitation fell across parts of the West in December, especially in California, restoring soil moisture and increasing streamflow. The precipitation was beneficial but was not enough to erase three years of deficits. While some smaller reservoirs in northern California were replenished by the rain, most California reservoirs were still well below average. Statewide, western reservoirs were below average from New Mexico to Oregon. Based on preliminary information, New Mexico's storage on December 31 was 53 percent of average, compared to 51 percent a year ago. In Arizona, statewide storage stood at 73 percent of average at the end of the year, down from 85 percent on December 31, 2013. The rains increased flow into western streams, at least temporarily, but many streams in the West, as well as in the Great Plains, were still well below average. Rocky Mountain snowpack was near to above average. But the fact that the western storms were "warm" storms, dropping their precipitation as rain instead of snow, left many mountain ranges further west with well below-normal snowpack and snow water content. Winter snowpack is important to western hydrology since it provides an important water source during the spring and summer melt season.

The reduced number of days with precipitation and prolonged dryness (last 3 months) in the Northern Plains, as well as the Southern Plains, continued to dry soils. Texas reservoirs have not recovered from the long-term dryness (36 months) in the Southern Plains, especially in the western half of the state. At the end of 2014, cumulative reservoir storage in Texas was 62.6 percent of capacity, compared to 64.3 percent at the end of 2013. Historically, reservoirs in Texas are collectively about 80 percent full in late December.

Regional Discussion

Hawaii: December 2014 was drier than normal at most of the reporting stations in Hawaii and marks the second consecutive dry month. Rainfall during October and earlier in the year gave many stations near- to above-normal precipitation at these time scales (last 3, 6, 9, and 12 months), but dryness is evident at the longer time scales (last 24 and especially 36 months). On the USDM map, moderate drought covered less than three percent of the state, reflecting the low reservoir situation in central Molokai and the Upcountry Maui area.

Alaska: Alaska had a mixed precipitation anomaly pattern during December, with drier-than-normal conditions in the south and near the Seward Peninsula in the west. The below-normal precipitation pattern was widespread at 2 to 3 months, both at the lower elevation stations as well as the higher elevation SNOTEL stations. Dryness shifted to the southwestern stations at 6 to 9 and 12 months, but was less widespread at the longer time scales (last 24 to 36 months). The dryness during the current water year (October 1-September 30) has reduced the water content of the mountain snowpack, as well as dried soils (according to model calculations). December was also predominantly warmer than normal. In fact, the state has been warmer than normal for much of the last year. The December 30^th USDM map was free of drought and abnormal dryness.

Puerto Rico: December was drier than normal for most of Puerto Rico except the southwest. The rain that has fallen throughout the course of the year has resulted in a pattern of dryness in the central to northwest areas at the 2-month time scale, widespread dryness at 3 months, dryness in the southeast at 6 months, and dryness across much of the eastern half of the island at 12 months. On the December 30^th USDM map, abnormal dryness continued for about 3 percent of Puerto Rico.

CONUS State Ranks:

The December precipitation anomaly pattern of dryness in the Southern and Northern Plains was reflected in the state ranks, with four states ranking in the driest third of the historical record, including North Dakota which had the ninth driest December. Portions of some states in the southern Great Lakes region were dry, but other parts of those states were not as dry and this tempered the statewide rank.

A variable pattern of precipitation characterized the rest of 2014. Five states (in the Northern Plains, Lower Mississippi Valley, and Intermountain Basin of the West) ranked in the driest third of the historical record for October-December, four states east of the Rockies were that dry for July-December, and three were that dry (in the Southern Plains and Central Appalachians) for January-December. The most notable statewide climate extreme was the record heat in the West, where four states (California, Nevada, Oregon, and Washington) had the warmest July-December and three states (California, Nevada, and Arizona) had the warmest year in the 1895-2014 record.

Western U.S.

Percent area of the Western U.S. in moderate to extreme drought, January 1900 to present, based on the Palmer Drought Index. As noted above, dry weather has dominated the West for much of the last several months, resulting in significant hydrological (low lake, reservoir, and stream levels) and agricultural impacts. According to the USDM, 54.5 percent of the West was experiencing moderate to exceptional drought at the end of December, which is slightly less than the previous month. The Palmer Drought Index percent area statistic for the West was 29.5 percent, a drop of about 8 percent compared to the previous month.

Agricultural Belts

Primary Hard Red Winter Wheat Belt precipitation, December, 1895-2014.

Primary Hard Red Winter Wheat Belt precipitation, October-December, 1895-2014.

October marks the start of the growing season for the Primary Hard Red Winter Wheat agricultural belt. While portions of the region were drier than normal this month, December 2014 ranked as the 39^th wettest and 23^rd warmest December in the 1895-2014 record regionwide. The growing season to date ranked as the 57^th driest and 39^th warmest October-December. Regionwide, 2014 ranked as the 40^th driest and 59^th warmest year on record, or in the middle of the historical distribution. According to the January 6^th, 2015 USDM, as of December 30^th, 37 percent of the winter wheat regions were experiencing drought, while 29 percent of cattle, 18 percent of hay, five percent of corn, and three percent of soybeans were in drought areas.

NOAA Regional Climate Centers:

A more detailed drought discussion, provided by the NOAA Regional Climate Centers and others, can be found below.

Southeast — South — Midwest — Northeast — High Plains
West

As described by the High Plains Regional Climate Center, December was generally a wet month for the High Plains region with the main exception being North Dakota, and the month was generally mild regionwide. Although there was a slow start to the snow season, end of the month snowpack was near normal in the Rockies in Colorado and Wyoming, with statewide snowpacks of 103 percent and 107 percent, respectively. Additionally, a large area of the region had precipitation totals which were greater than 200 percent of normal including central Wyoming, eastern Colorado, western Kansas, western and central Nebraska, and southern South Dakota. A few localized areas had greater than 400 percent of normal precipitation. Meanwhile in North Dakota, precipitation was lacking with widespread precipitation totals of less than 25 percent of normal. For this part of the country, liquid equivalent precipitation is typically light at this time of the year so although there were extremes, the actual amounts were quite low. These precipitation extremes led to some top 10 rankings on both ends of the spectrum. Some areas of North Dakota had a lack of snowfall this December. For example, Fargo, North Dakota only had a Trace of snow on the ground on Christmas and has yet to receive its first 1-inch (3 cm) snowfall of the season (although smaller snowfall events under one inch have occurred). Only two other times has the first 1-inch snowfall happened so late in Fargo - January 22, 1914 and January 27, 1944. With only 0.9 inches (2 cm) of snow, Fargo tied for its 3rd least snowiest December on record (period of record 1885-2014). This was 10.3 inches (26 cm) below normal.

According to the USDM, drought conditions remained largely unchanged over the past month. Winter is typically the driest time of year, so drought improvements or developments are not usually expected and impacts of drought, if any, are minor. The total area in drought (D1-D4) remained at about 11 percent with only a very slight expansion of moderate drought (D1) in South Dakota and a slight contraction in severe drought (D2) in northwestern Kansas. Drought conditions in the southern part of the region, across Colorado and Kansas, persisted. Abnormally dry conditions (D0) have expanded westward in the Dakotas and northward in Kansas where dryness has been occurring over the past three months.

As explained by the Southern Regional Climate Center, with the exception of a few spots, December was a drier than normal month across the Southern region and temperatures were warmer than normal for all six states in the region. Conditions were quite dry in northern Texas, central Texas and south western Oklahoma. A bulk of the stations in these areas averaged only between 5 to 50 percent of normal. It was also a very dry month for Arkansas and western Tennessee, where most stations only received between 50 and 75 percent of their normal monthly precipitation. This was also the case though most of Louisiana and north western Mississippi. Due to dry conditions in the northwestern corner of the Southern region, drought conditions over northern Texas and Oklahoma remained relatively unchanged. Conversely, wetter than normal conditions in east central Mississippi has helped replenish soils and subsequently alleviate drought conditions there.

In Texas, the drought has caused many rice farmers to lose their jobs. In order to fix this problem, an additional reservoir is going to be made in Lake City. The new reservoir will be the size of the Marble Falls and Lady Bird area lakes combined. Some ranchers still have to look to other sources of income, due to their cattle struggling in the prolonged drought. Some have even had to move their cattle northward, starting back in 2011. There are currently only 3.91 million cows in Texas as of 2014 compared to the 5.35 million cows in 2005. Drought conditions tended to worsen over the course of the month, though changes from last month were not large. Dallas area reservoirs were at 62.3% of capacity, compared to 61.6% at the beginning of the month. The Mineral Wells area is making plans to install a reverse osmosis well in order to have enough water, costing $6 million. If they did not do this, the county might run out of water by spring. Population growth is still a concern for reservoir use moving forward, and there are concerns about the $2 billion SWIFT initiative's ability to combat this (Information provided by the Texas Office of State Climatology).

As summarized by the Midwest Regional Climate Center, December's precipitation primarily fell as rain across much of the region resulting in below normal accumulated snowfall values. This resulted in more snowfall being recorded during November rather than in December across much of the region. Total monthly accumulated precipitation was concentrated along the Ohio River Valley with observed values of 3.00 to 4.00 inches (76 to 102 mm) across southern Illinois, Indiana, Ohio, and most of Kentucky primarily from events on December 1st, 2nd, and 6th. Additional areas of higher precipitation were located across northern Wisconsin, the Upper Peninsula of Michigan, and far northern Minnesota, south of Lake of the Woods. These areas saw precipitation at 100 percent to 200 percent of normal. The remaining central, far southern, and far northwest portions of the region recorded precipitation at 25 percent to 75 percent of normal for the month. December regional precipitation was 0.40 inch (11 mm) below normal, ranking December 2014 as the 36th driest in 120 years of records.

Abnormally dry conditions persisted in much of Minnesota (expect for far southeast counties and east-central counties) through December despite having the second smallest departure from normal precipitation in December. The dry conditions stem from below normal precipitation beginning in September 2014.

As noted by the Southeast Regional Climate Center, precipitation was highly variable across the Southeast during December and mean temperatures were slightly above average across much of the region. The wettest locations were found across portions of central Alabama, southwestern Georgia, and north-central Florida. In contrast, the driest locations were found across central and southern Florida, where monthly precipitation ranged between 5 and 50 percent of normal. Fort Myers, FL (1902-2014) tied its fourth driest December on record with only 0.05 inch (1.3 mm) of precipitation. Precipitation was near average for much of Puerto Rico and the U.S. Virgin Islands, though Aibonito, PR (1906-2014) observed its third driest December on record with only 1.3 inches (33 mm) of precipitation. Very little snowfall was recorded across the region throughout December, with only 3.1 inches (78.7 mm) observed at Mount Mitchell, NC during the month.

A nearly complete elimination of drought conditions was noted for the Southeast region during December. The percentage of the region under drought-free conditions (less than D1) increased from 96 percent on the 2nd to 99 percent on the 30th. The small areas of moderate (D1) drought in extreme southwestern and east-central portions of Georgia dissipated. Moderate-to-severe (D1-D2) drought conditions were alleviated in east-central Alabama, and only a small portion of coastal Alabama centered on Mobile Bay remained in moderate drought by the end of the month. Agricultural impacts were relatively minimal across the region. In Florida, the initial harvesting of sugarcane, strawberries, citrus, and vegetables was aided by modest rainfall and relatively warm temperatures. Winter pasture conditions for grazing livestock also continued to improve across portions of the state.

As explained by the Northeast Regional Climate Center, the last month of the year proved to be a warm one in the Northeast, and December brought wetter-than-normal conditions back to the region. The region picked up 3.98 inches (101.09 mm) of precipitation, 114 percent of normal. Nine of the twelve Northeast states were wetter than normal, with three ranking this December among their top 20 wettest. Departures for the dry states ranged from 88 percent of normal in Pennsylvania to 94 percent of normal in Maryland. At the start of December, 19 percent of the Northeast was abnormally dry and 2 percent of the region was experiencing moderate drought, according to the USDM. Dryness eased in New England and West Virginia by the end of the month. However, dry conditions were introduced in western and northern New York and lingered in other parts of the Northeast. Overall, 13 percent of the region was abnormally dry as of December 31.

As summarized by the Western Regional Climate Center, a series of storms brought beneficial precipitation to drought-stricken areas such as California, eastern Oregon, southern Idaho, and the Great Basin this month. Throughout the West, temperatures averaged above normal despite an outbreak of very cold air over the last few days of the month.

Several areas of the West received above normal precipitation this month. A series of storm systems during the first half of the month brought significant precipitation to California and the Pacific Northwest and helped to alleviate drought conditions in localized areas. Unfortunately, snow levels were generally high during these storms and thus did not contribute significantly to the development of Sierra Nevada and Cascade snowpack. At month's end, Sierra Nevada snow water equivalent (SWE) stood at 40-60% of normal. The southern Cascades were generally in the 30-40% of normal SWE range. The northern Cascades saw SWE values in roughly the 40-70% of normal range. Further east, mountain ranges of the northeastern Great Basin and the Rocky Mountains ended the month with near to above normal SWE. Many locations in the Rocky Mountain states also recorded above normal precipitation this month. The Southwest saw areas of above normal precipitation as well. Despite above normal precipitation in many areas this month, 54% of the West is experiencing moderate to exceptional drought. A large area of drought improvement was seen in northern California this month as well as small areas of improvement elsewhere in California, Oregon and Washington. Extreme to exceptional drought persists in a large area of California, western Nevada, and southeast Oregon.

Precipitation across Hawaii was generally below normal. In southwestern Oahu, Kalaeloa received 0.76 in (19 mm) precipitation, 22% of normal. On the Big Island, Hilo received 6.10 in (155 mm), 53% of normal. At month's end, 29% of the state was abnormally dry and 3% was in moderate drought. Further north, temperatures throughout Alaska were warmer than normal. The greatest departures from normal were observed in the Southcentral and Interior regions. Fairbanks (8.0 F/-13.3 C) and McGrath (10.7 F/-11.8 C) both observed their warmest average December temperatures on record. Records in Fairbanks and McGrath began in 1929 and 1941, respectively. Anchorage reported its second warmest December in a 101-year record at an average 25.3 F (-3.7 C). Precipitation was variable across the state, though much of the Southcentral and Southeast regions observed below normal precipitation.

For additional, or more localized, drought information, please visit:

• The U.S. Drought Portal
• National, Regional, and State Drought Resources