December 2018 Drought Report

Overview

During December 2018, the upper-level circulation pattern consisted of numerous shortwave troughs and closed lows which migrated through the jet stream flow. Their associated fronts and surface lows brought Pacific or Gulf of Mexico moisture, and warmer-than-normal southerly air or cold Canadian air, with them in alternating waves of weather. For the month as a whole, the weather systems brought above-normal precipitation to large parts of the Great Plains, South, and East — but mostly to areas that were drought-free. Above-normal precipitation fell in pockets across the drought-stricken West, but much of the West was drier than normal for the month. Drought and abnormal dryness contracted in parts of the West and Great Plains, but expanded in other parts of the West and in parts of the Plains and southern Florida. Drought and abnormal dryness expanded in Hawaii, where drier-than-normal weather continued, and in Puerto Rico, which was also drier than normal. Severe drought contracted slightly in Alaska. Drought contraction was a little more than expansion this month, so the USDM-based national moderate-to-exceptional drought footprint across the CONUS shrank slightly from 22.2 percent of the CONUS at the end of November to 21.9 percent of the CONUS at the end of December (from 18.9 percent to 18.7 percent for all of the U.S.). According to the Palmer Drought Index, which goes back to the beginning of the 20^th century, about 22.0 percent of the CONUS was in moderate to extreme drought at the end of December, increasing about 1.4 percent from the 20.6 percent at the end of November.

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

Drought conditions at the end of the month, as depicted on the January 1^st, 2019 USDM map, included the following core drought and abnormally dry areas:

• A large area of abnormal dryness (D0) to severe (D2) drought stretched across the West, with centers of extreme (D3) drought in the Pacific Northwest and southern California, and extreme to exceptional (D4) drought in the Southwest (Four Corners States). According to the USDM, more than half (53.3 percent) of the West was experiencing moderate to exceptional drought at the end of December. This is a decrease from last month's 54.5 percent. But the percent area of the West (from the Rockies to the West Coast) in moderate to extreme drought (based on the Palmer Drought Index) increased to 67.6 percent compared to 61.3 percent at the end of November.
• Abnormal dryness to severe drought continued in Alaska, mainly in the panhandle.
• There were a few areas of moderate (D1) drought in parts of the northern and southern Plains, and moderate drought expanded in southern Florida. Abnormal dryness expanded and moderate drought was introduced in Puerto Rico and Hawaii. Spots of abnormal dryness lingered in the Northeast.

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 (as well as wet spell conditions) in some parts of the country than the PHDI map because the meteorological conditions that produce drought and wet spell conditions are not as long-lasting as the hydrological impacts.

Used together, the Palmer Z Index and PHDI maps show that short-term dry conditions continued in December in parts of the West over areas that were in drought at the end of November, intensifying or expanding long-term drought. Short-term wet conditions occurred over central Florida, reducing or eliminating previous areas of long-term drought. Short-term near-normal conditions occurred over parts of the West, reducing previous areas of long-term drought in northern Utah and southern California. Short-term wet conditions occurred across much of the CONUS east of the Rockies, expanding or intensifying previous areas of long-term wet conditions.

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. Wet conditions dominate much of the CONUS east of the Rockies at all time scales. As far as dry conditions are concerned, parts of the West are dry at the 1- to 3-month time scales, with dryness more widespread at 6 to 12 months. Dryness dominates the Southwest at 2 and 9-12 months, with wet conditions interrupting at 3 months. Parts of Oregon are consistently dry at 3 to 24 months. Southern Florida is dry at 3 to 6 months. A large portion of the northern Plains shows dry conditions at the 24-month time scale.

Regional Discussion

Hawaii:

Following a dry November, December 2018 was drier than normal across most of the Hawaiian Islands. This makes the last 2 months drier than normal for most of the state, with the last 3 months having large areas of below-normal precipitation. Wetter-than-normal weather dominated at longer time scales (last 4, 6, 9, 12 months). Some stations were drier than normal at the longest time scales (last 24 and 36 months), but other stations were wetter than normal at these time scales. Streamflow was still above normal across the island chain. Moderate drought (D1) returned at the end of December, with abnormal dryness to moderate drought covering about 43 percent of the islands on the January 1^st USDM map.

Alaska:

December 2018 was drier than normal across much of the Alaskan panhandle, interior east central region, and west coastal area. This pattern continued at the 2- to 4-month time scales. At 6 months, dryness persisted along the west coast and panhandle, but was especially severe in the southern panhandle. The panhandle dryness continued quite severe at the 9-12 month time scales, with record dryness in some areas, and the dryness was even evident at the 24-36 month time scales (low elevation station precipitation maps for the last 1, 2, 3, 4, 6, 9, 12, 24, and 36 months) (gridded precipitation percentile maps for last 1, 3, 12 months) (climate division precipitation maps for last 1, 3, 6, 12 months). Temperatures during December were mostly near to warmer than normal, but warmer-than-normal temperatures dominated at 2- to 12-month time scales with some record warm temperatures at the longer time scales (low elevation station temperature maps for the last 1, 2, 3, 4, 12 months) (gridded temperature percentile maps for last 1, 3, 12 months) (climate division temperature maps for last 1, 3, 6, 12 months) Snow cover was below average in the panhandle and in parts of the south coastal and interior sections. Streamflow was mostly near to above average for those streams that haven't frozen over yet. Severe drought contracted slightly this month, but abnormally dry to severe drought conditions continued in the panhandle. About 5.8 percent of the state continued in abnormally dry to severe drought conditions on the January 1^st USDM map.

Puerto Rico:

Most of Puerto Rico was drier than normal during December 2018. Most of the island was drier than normal for the last 2 to 3 months, but wet conditions became evident in the western and northern areas at 6 to 12 months. Soils continued dry along the southern coast and a little toward the interior in the southeast, and were drying in parts of the northwest. Streamflow averaged for the month was was a mixture of near normal to below-normal flows. As seen on the January 1^st USDM map, abnormal dryness expanded and moderate drought was introduced to the state, with abnormal dryness to moderate drought covering about 60 percent of the island.

CONUS State Precipitation Ranks:

With December and October 2018 being so wet across much of the nation, only a few states ranked in the dry third of the historical record for this period — four states in December and one state for October-December, all in the West. Six western states ranked in the dry third of the historical record for July-December, and six for the year (January-December). The driest rank of all of these states for all of these periods was 13^th driest for Oregon for the year, followed by Colorado at 14^th driest.

Agricultural Belts

December 2018 was wetter than normal across the Primary Hard Red Winter Wheat agricultural belt, with temperatures near to warmer than normal. The month ranked as the tenth wettest and 34^th warmest December, regionwide, in the 1895-2018 record.

October marks the beginning of the growing season for the Primary Hard Red Winter belt. October-December 2018 was wetter and cooler than normal across the region. The 3-month period ranked as the third wettest and 32^nd coolest October-December, regionwide.

Pacific Islands: The NOAA National Weather Service (NWS) offices, the Pacific ENSO Applications Climate Center (PEAC), and partners provided reports on conditions across the Pacific Islands.

In the U.S. Affiliated Pacific Islands (USAPI) (maps — Federated States of Micronesia [FSM], Northern Mariana Islands, Marshall Islands [RMI], Republic of Palau, American Samoa, basinwide), December 2018 was drier than normal at Chuuk, Kapingamarangi, Lukonor, and Kosrae; near normal at Kwajalein and Pohnpei; and wetter than normal at Koror, Yap, Guam, Saipan, Majuro, and Pago Pago.

It was a dry month in terms of drought at Kosrae, Lukonor, Fananu, Nukuoro (FSM); and Kwajalein, Ailinglapalap, Jaluit, Utirik, and Wotje (RMI), with rainfall amounts below the minimum thresholds (4 or 8 inches) required to meet most monthly water needs. It was a wet month (above the minimum thresholds) at the rest of the regular reporting stations in the USAPI. The 4- and 8-inch thresholds are important because, if monthly precipitation falls below the threshold, then water shortages or drought become a concern.

Kosrae had the second driest December in a data record spanning 51 years. Lukonor ranked fifth driest (out of 35 years of data) for December and Nukuoro seventh driest (out of 36 years). Jaluit had the driest December in a 35-year record, although instrument exposure may have contributed some to the low rainfall totals at Jaluit. For the year, Lukonor ranked fourth driest (out of 22 years) and Nukuoro ninth driest (34 years). At the other end of the spectrum, 2018 was the wettest year on record at Pohnpei (based on 67 years of data) and Kwajalein (66 years), and second wettest at Saipan (30 years).

As measured by percent of normal precipitation, Kosrae and Lukonor have been drier than normal in the short term (December and the last 3 months [October-December 2018]) and drier than normal in the long term (last 12 months [January-December 2018]). Chuuk was drier than normal in the short term and wetter than normal in the long term. Kapingamarangi was drier than normal for December but wetter than normal for the other two time periods. Guam, Kwajalein, Majuro, Pohnpei, Saipan, and Yap were drier than normal for the last 3 months, but near to wetter than normal for the other two time periods. Koror and Pago Pago were wetter than normal for all three time periods (except the last 12 months are missing for Koror).

The following analysis of historical data for the USAPI stations in the Global Historical Climatology Network-Daily (GHCN-D) dataset, augmented with fill-in data from the 1981-2010 Normals, helps put the current data into historical perspective by computing ranks based on the period of record. The table below lists the precipitation ranks for December 2018, July-December 2018 (the last 6 months), and January-December 2018 (the last 12 months). Some stations have a long period of record and their dataset is fairly complete, while other stations have a shorter period of record and the dataset has some missing data.

Station	Dec 2018 Rank	Dec No. of Years	Jul- Dec 2018 Rank	Jul- Dec No. of Years	Jan- Dec 2018 Rank	Jan- Dec No. of Years	Period of Record
Jaluit	1	35	1	35	MSG	32	1981-2018
Koror	37	68	MSG	67	MSG	66	1951-2018
Woleai	23	38	7	26	12	23	1968-2018
Yap	42	68	28	68	44	67	1951-2018
Majuro	42	65	21	65	62	64	1954-2018
Mili	22	35	MSG	33	MSG	33	1981-2018
Ailinglapalap	15	35	8	34	11	34	1981-2018
Kosrae	2	51	5	38	11	32	1954-2018
Lukonor	5	35	6	22	4	22	1981-2018
Saipan	25	31	30	31	29	30	1981-2018
Pohnpei	36	68	38	68	67	67	1951-2018
Kwajalein	36	67	34	67	66	66	1952-2018
Kapingamarangi	9	28	15	18	14	16	1962-2018
Chuuk	22	68	38	68	42	67	1951-2018
Guam	51	62	52	62	40	61	1957-2018
Nukuoro	7	36	17	35	9	34	1981-2018
Pago Pago	46	53	47	53	49	52	1966-2018
Wotje	28	34	MSG	33	MSG	33	1981-2018
Utirik	13	19	5	8	4	4	1985-2018

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

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