October 2018 Drought Report

Overview

Like the last several months, the upper-level circulation pattern was quite active during October 2018 with ridge and trough patterns migrating through the jet stream flow over the CONUS. Frequent ridging or a northwesterly flow kept much of the West Coast drier and warmer than normal. But frequent troughs, closed lows, and moisture from remnants of tropical cyclones brought above-normal precipitation to the Southwest. The tropical moisture combined with Gulf of Mexico moisture to feed fronts and low pressure systems which dumped above-normal precipitation across the southern and central Plains to Great Lakes, while Hurricane Michael and frontal rainfall gave parts of the East Coast above-normal precipitation. Parts of the Southeast to Ohio Valley, and northern High Plains, missed out on the precipitation. Hawaii and parts of Alaska were wetter than normal, but the Alaska panhandle continued dry. Much of the widespread above-normal precipitation fell on drought or abnormally dry areas, bringing much-welcomed improvement. As a result, drought and abnormal dryness contracted across the West, Plains, Great Lakes, Northeast, and parts of the Southeast, and in Hawaii and Puerto Rico. Abnormal dryness expanded in other parts of the Southeast and a little in the southern Plains. Drought contraction far outweighed expansion this month, so the USDM-based national moderate-to-exceptional drought footprint across the CONUS contracted from 29.0 percent of the CONUS at the end of September to 22.0 percent of the CONUS at the end of October (from 24.6 percent to 18.8 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 21.4 percent of the CONUS was in moderate to extreme drought at the end of October, decreasing about 13.1 percent from the 34.5 percent at the end of September.

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 October 30^th, 2018 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 (52.3 percent) of the West was experiencing moderate to exceptional drought at the end of October. This is a decrease from of the last month's 59.3 percent. The percent area of the West (from the Rockies to the West Coast) in moderate to extreme drought (based on the Palmer Drought Index) grew slowly in the fall 2017 then rapidly increased in December and continued to increase in January and February, growing from 18.7 percent at the end of October 2017 to 62.4 percent of the West at the end of February 2018. Above-normal precipitation improved conditions in California and Nevada in March, so the Palmer percent area in drought shrank to 50.2 percent by the end of March. April was wet in northern California and northwest Nevada, and Washington to the northern Rockies, and May was wet in Nevada to southern California, but extremely dry conditions occurred along the coast in May, especially in Oregon and Washington; California to Colorado were dry in June; the Pacific Northwest to northern and central Rockies were dry in July; several parts of the West were dry in August; and most of the West was very dry in September. The dry conditions were coupled with unusual warmth in parts to much of the West during May, June, July, August, and September which increased evapotranspiration, thus exacerbating drought conditions (5-month SPEI vs. SPI). As a result, the Palmer Drought Index measure of drought area expanded to 84.5 percent of the West by the end of September, approaching values that have been exceeded only a few times in the past — 1934, 1977, and three times since 2000 (2002, 2003, and this year). But October was wetter and cooler than normal across much of the West, especially the Southwest, which shrank the Palmer measure of drought area to 63.1 percent of the West. In spite of the wet conditions, most of the West began this snow season with a below-average snow pack.
• Areas of moderate (D1) to severe drought covered parts of the northern Plains.
• A few areas of abnormal dryness to moderate drought lingered from the central Plains to Mid-Mississippi Valley, and in the Southeast.
• A few areas of abnormal dryness to severe drought persisted in the southern Plains and Northeast.
• Abnormal dryness to severe drought continued in Alaska.

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 October across the West Coast over areas that were in drought at the end of September, intensifying long-term drought. Short-term dry conditions occurred over Florida which had neutral long-term conditions in September, and the long-term conditions continued neutral in October. Short-term wet conditions occurred across much of the Southwest, Great Plains, and Northeast, reducing or eliminating previous areas of long-term drought. Short-term conditions occurred across the Great Lakes and Ohio Valley to Mid-Atlantic region, 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. Dryness dominates much of California for most of the time scales (all but 24 months) and much of the Pacific Northwest (especially Oregon) for all but the 1- and 24-month time scales). The wet conditions in the Southwest during October eliminated dryness at the 1- and 2-month time scales, with the southern portions wet at the 3- to 9-month time scales. But dryness was still evident in the northern portions at 6 to 9 months and 24 months, and across most of the Southwest at 12 months. Dry conditions were evident in the central to northern Rockies at 2 to 6 months and in the northern Plains at 24 months. Florida was dry at the 1- to 3-month time scales, and other parts of the Southeast had a few areas of slightly dry conditions at 6 to 24 months.

Regional Discussion

Hawaii:

Heavy rains this month and from tropical systems in previous months gave the Hawaiian Islands widespread above-normal precipitation for October 2018 and for the last year (last 2, 3, 4, 6, 7, 10, 12 months). Some stations were drier than normal at longer time scales (last 24 and 36 months), but other stations were wetter than normal at these time scales. Streamflow was still well above-normal across the island chain. Drought and abnormal dryness were eliminated on the October 30^th USDM map.

Alaska:

October was drier than normal across parts of the panhandle, northwestern Alaska, and central to interior southeastern Alaska (low elevation station, high elevation SNOTEL station, gridded percentile, climate division, Leaky Bucket model percentile maps). The last 2 months were drier than normal across much of the state. Dryness occurred at a few stations along the northwest coast, and was widespread and severe in the southern panhandle, for much of the last year (low elevation station maps for the last 3, 4, 6, 7, 10 months) (high elevation SNOTEL station maps for the last 3, 6, 10 months) (gridded percentile maps for the last 3 and 10 months) (climate division maps for the last 3, 6, 10 months). At longer time scales, the dryness was concentrated along the panhandle to southern coastal areas (climate division map for the last 12 months) (SNOTEL map for the last 12 months) (low elevation station maps for the last 12, 24, 36 months). Record dry conditions have occurred in southern parts of the panhandle during the last 3, 6, and 12 months. The dryness there is especially obvious on the SPI maps (last 1, 2, 3, 6, 9, 12, 24, 36, 48 months). Very dry to exceptionally dry conditions extend out over the last 3 years, with abnormally dry conditions even at the 4-year time scale. Temperatures during October were unusually warm across the state, with record warmth in the south and west (low elevation station, gridded percentile, climate division, Leaky Bucket model percentile maps). Statewide, Alaska had the warmest October in the 1925-2018 record. The record warmth in western Alaska extended back 3 months (August-October temperature maps based on low elevation stations, gridded analysis, climate divisions), and the last year as been warmer than normal for most to all of the state (low elevation station maps for the last 10 and 12 months) (gridded percentile map for the last 10 months) (climate division maps for the last 6, 10, 12 months). Snow pack was below average due to the warmer-than-normal conditions. Streamflow was below normal in the southern portions of the panhandle but near to above normal elsewhere. Abnormally dry to severe drought conditions continued in the panhandle. About 5.8 percent of the state was in abnormally dry to severe drought conditions on the October 30^th USDM map.

Puerto Rico:

The northern coast and part of the southwest were wetter than normal, but drier-than-normal conditions dominated Puerto Rico during October 2018. This anomaly pattern was evident at the 2-month time scale, but conditions were wetter in the northwest at 3 months with the dry areas in the south shrinking further at the 6- to 10-month time scales. Soils were dry along the southern coast and a little toward the interior in the southeast, but streamflow was mostly near normal. As seen on the October 30^th USDM map, only a small amount of abnormal dryness remained, covering about 4 percent of the island.

CONUS State Precipitation Ranks:

With October 2018 being so wet across much of the nation, only California and Florida were dry, ranking 40^th and 24^th driest, respectively, in the 1895-2018 record. For the last three months (August-October 2018), Florida and five states in the West ranked in the dry third of the historical record.

Oregon had the seventh driest May-October and California ninth driest. Three other states in the West ranked in the driest third of the historical record.

Four states, all in the West, ranked in the driest third of the historical record for the year so far (January-October). For the last 12 months, Colorado had the ninth driest November-October and Oregon the eleventh driest. Five other states in the West ranked in the driest third of the historical record.

Agricultural Belts

October 2018 was wetter than normal across the Primary Hard Red Winter Wheat agricultural belt, with cooler-than-normal temperatures. The month ranked as the third wettest and 21^st coolest October, regionwide, in the 1895-2018 record.

The above-normal precipitation this month improved drought conditions across the primary agricultural belts. As of October 30^th, drought was affecting only one percent of corn production and one percent of soybean production, 11 percent of hay acreage, nine percent of cattle inventory, seven percent of winter wheat production, and 25 percent of spring wheat production. These values are all less than they were a month ago. Nationwide, 13 percent of the topsoil moisture and 19 percent of the subsoil moisture was dry or very dry, and 20 percent of pasture and rangeland and 14 percent of the winter wheat were rated in poor to very poor condition; these values are less than a month ago. But conditions have not improved much in the West and, for some states, have gotten worse (U.S. Department of Agriculture [USDA] statistics for October 29 compared to October 1). Half or more of the pasture and rangeland is rated in poor to very poor condition in Oregon (71 percent), Utah (68 percent), California (65 percent), Colorado (58 percent), and Washington (50 percent), as well as Maine (68 percent) and New Hampshire (54 percent) in New England. Soil moisture continues to dry out where it hasn't rained, with (according to October 29^th USDA reports) 80 percent of topsoil and 80 percent of subsoil short or very short of moisture (dry or very dry) in California, 77 percent of topsoil and 89 percent of subsoil dry or very dry in Oregon, and 95 percent of topsoil and 50 percent of subsoil dry or very dry in Nevada.

NOAA Regional Climate Centers:

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

As summarized by the High Plains Regional Climate Center, October was cool and wet for the majority of the High Plains. The cool and wet conditions relieved drought and improved soil moisture conditions throughout the Dakotas, Kansas, and Colorado. While it did not help crops that were already being harvested, the rains were beneficial for the emerging winter wheat crop in Colorado, and it helped improve streamflow conditions across the state. However, October conditions caused a variety of issues. For instance, harvest was delayed due to excessively wet conditions, and some crop disease issues were reported. Additionally, the first fall freeze came early in parts of Kansas, causing damage to sorghum.

With the growing season ending, excess moisture could soak into the ground and recharge soil moisture, and the abundant rainfall significantly improved drought conditions across the region. The area experiencing drought or abnormal dryness (D0-D4) decreased from 48 percent to 37 percent over the course of the month, according to the USDM. The greatest improvements occurred across eastern Kansas where some areas saw a 3-category improvement — primarily from extreme drought (D3) to abnormally dry conditions (D0). This area received approximately 200-300 percent of normal precipitation during October, which was very beneficial for recharging soil moisture before the winter season. The small area of exceptional drought (D4) was eliminated, and by the end of the month, only small pockets of moderate drought (D1) remained. Colorado also received much-needed relief from drought conditions. Several snows occurred in the mountains to start off the water year with ample precipitation, resulting in the reduction of area in D3 or D4. Meanwhile, heavy rains fell across the Eastern Plains of Colorado, improving conditions in a large area experiencing severe drought (D2). The rainfall greatly benefited recently-planted winter wheat. Conditions improved throughout portions of the Dakotas as well, as D2 and D3 were eliminated in South Dakota and North Dakota, respectively. Improvements in soil moisture and ground and surface water conditions were reported.

As explained by the Southern Regional Climate Center, Precipitation values for the month of October varied spatially across the Southern region, and temperatures varied spatially as well, but generally exhibited a pattern with warmer temperatures in the eastern half of the region and cooler temperatures in the western half of the region. At the end of October, drought conditions continued to improve. Extreme drought classifications were no longer present in the region, a change from the beginning of the month. Severe drought classifications were present across extreme western Texas, while moderate drought classifications were present in parts of extreme western and northern Texas as well as northeastern Oklahoma. There were no drought conditions in Tennessee, Louisiana, Mississippi, and Arkansas; however, there were some small patches of abnormally dry conditions.

As described by the Midwest Regional Climate Center, October temperatures and precipitation varied across the region, with the month ranking as the twelfth wettest October regionwide. Drought areas continued to show improvement due to the October rains with the Midwest becoming less and less affected over the course of the month.

As noted by the Southeast Regional Climate Center, temperatures across the Southeast were warmer than normal in October, while precipitation was highly variable. In Puerto Rico, rainfall amounts were generally much lower than normal at most locations, such as Juncos (1931-2018; 22nd driest) with 5.05 inches (128 mm), which was 3.2 inches (81 mm) drier than normal. In spite of the dryness, the USDM showed a reduction in abnormally dry conditions by the end of the month. A few scattered locations in the Florida peninsula received less than 0.50 inch of rain during the month as high pressure dominated the region. Scattered areas in eastern NC and western AL also received less than 25% of normal rainfall. By comparison, a number of stations in the mountainous areas of western North Carolina and in the Florida Panhandle received over ten inches, mainly from precipitation associated with Hurricane Michael.

Dry conditions were found in many areas not affected by Hurricane Michael during the month of October. An area of abnormally dry conditions expanded by the end of the month in Georgia and Alabama, as the absence of rain and the warm temperatures increased evaporation rates in the first two weeks. Rainfall from Hurricane Michael reduced dry conditions along the path of the storm but led to tremendous damage to agriculture, including livestock, timber, crops and the infrastructure needed to support agriculture, including storage facilities and barns.

As explained by the Northeast Regional Climate Center, the Northeast wrapped up October with an average temperature of 49.9 degrees F (9.9 degrees C), 0.5 degrees F (0.3 degrees C) above normal. It was another wetter-than-normal month in the Northeast as the region saw 4.36 inches (110.74 mm) of precipitation in October, 113 percent of normal, with ten states wetter than normal. The USDM released on October 4 showed 5 percent of the Northeast in a moderate or severe drought and 17 percent of the region as abnormally dry. While rainfall during the month allowed dry conditions to slowly ease, streamflow and groundwater levels remained below normal in parts of northern New York and northern New England. The USDM released on October 25 showed 2 percent of the Northeast in a moderate or severe drought and 14 percent of the region as abnormally dry.

As summarized by the Western Regional Climate Center, much of the Southwest observed well above normal precipitation this month in association with remnant moisture from decaying tropical systems. October temperatures averaged to within a few degrees of normal across the West.

At the beginning of the month, remnant moisture from former Hurricane Rosa streamed into the Four Corners states, setting precipitation records in some cases. This moisture interacted with an upper-level low pressure system to produce abundant precipitation. Mid-month, similar conditions occurred with remnant moisture from former Hurricane Sergio. Following a fairly dry first half of the month, an atmospheric river impacted the Pacific Northwest during the last week of the month, helping to bring precipitation totals near to slightly above normal in parts of Washington, Idaho, and northern Oregon. This month's precipitation brought widespread improvement of drought conditions to Arizona, New Mexico, and Utah on the USDM. Improvements were also seen in small areas of western Washington, central Idaho, and western Montana.

During the first month of what is typically considered the wet season, precipitation was below 75% of normal across southern Oregon, western Nevada, and much of California. Drought conditions persisted, but did not worsen, in these areas on the USDM.

Unseasonably mild weather was observed across Alaska this month, with several locations reporting their warmest October on record. Precipitation was variable across the state, with locations in the Southcentral region experiencing above-normal precipitation while many Interior locations reported below-normal precipitation. Dry conditions continued in Southeastern Alaska; Ketchikan recorded 11.58 in (294 mm), 60% of normal. Below-normal precipitation continued to impact hydropower generation in this area. Further south, precipitation was above normal across Hawaii due to several storms impacting the state during the course of the month. Landslides on Maui and Oahu occurred in the later part of the month, causing damage to roads and travel impacts. Recent wet weather has made it difficult for farmers in Hawaii to produce healthy crops.

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), October 2018 was drier than normal at Yap, Guam, Saipan, Chuuk, Lukonor, Kosrae, Kwajalein, and Majuro, and wetter than normal at Koror, Pohnpei, Kapingamarangi, and Pago Pago.

It was a dry month in terms of drought at Kosrae, Woleai, and Yap (FSM), and Jaluit, Kwajalein, 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.

Jaluit had the second driest October and July-October, and driest August-October, in a data record spanning 35 years. Woleai had the second driest October in 34 years of data and seventh driest August-October (in 30 years of data). Kosrae had the second driest October (out of 50 years of data), second driest September-October (44 years), and seventh driest June-October (40 years). October was only 16^th driest out of 34 years at Lukonor, but March-October ranked fourth driest out of 22 years. Similarly at Nukuoro, October was 16^th driest (out of 36 years), but August-October ranked third driest (35 years) and February-October fourth driest (34 years). Yap had the sixth driest October and tenth driest September-October (out of 68 years). It was dry in the short term but wet in the long term at Kwajalein and Majuro. Kwajalein had the fourth driest October and sixth driest September-October (out of 67 years), but wettest November-October (66 years). Majuro had the eighth driest October and seventh driest September-October (out of 65 years), but second wettest November-October (64 years).

As measured by percent of normal precipitation, Lukonor been drier than normal in the short term (October and the last 3 months [August-October 2018]) and drier than normal in the long term (year to date [January-October] and last 12 months [November 2017-October 2018]). Kosrae, Majuro, and Yap were drier than normal in the short term and wetter than normal in the long term. Chuuk, Guam, Kwajalein, and Saipan were drier than normal for October but wetter than normal for the other three time scales. Kapingamarangi, Pago Pago, and Pohnpei were near to wetter than normal at all four time scales. Koror was wetter than normal for October (the other time scales are missing).

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 October 2018, May-October 2018 (the last 6 months), and November 2017-October 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	Oct 2018 Rank	Oct No. of Years	May- Oct 2018 Rank	May- Oct No. of Years	Nov 2017- Oct 2018 Rank	Nov- Oct No. of Years	Period of Record
Jaluit	2	35	MSG	34	MSG	32	1981-2018
Koror	43	68	MSG	66	MSG	66	1951-2018
Woleai	2	34	13	25	11	24	1968-2018
Yap	6	68	21	67	40	67	1951-2018
Majuro	8	65	55	64	63	64	1954-2018
Mili	26	35	MSG	33	MSG	31	1981-2018
Ailinglapalap	31	35	8	34	16	32	1981-2018
Kosrae	2	50	12	39	21	32	1954-2018
Lukonor	16	34	5	22	5	22	1981-2018
Saipan	14	38	37	38	28	29	1981-2018
Pohnpei	33	68	41	67	67	67	1951-2018
Kwajalein	4	67	63	67	66	66	1952-2018
Kapingamarangi	25	29	12	18	17	17	1962-2018
Chuuk	23	68	48	67	42	67	1951-2018
Guam	11	62	58	62	39	61	1957-2018
Nukuoro	16	36	4	34	13	33	1981-2018
Pago Pago	39	53	36	53	47	52	1966-2018
Wotje	7	35	MSG	34	MSG	31	1981-2018
Utirik	16	17	5	5	1	1	1985-2018

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

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