September 2018 Drought Report

Overview

Like the last several months, the upper-level circulation pattern was quite active during September 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 drier and warmer than normal. The Southwest was especially warm, with the excessive heat increasing evapotranspiration (water demand). Subtropical high pressure (Bermuda High), coupled with upper-level ridging, kept the eastern half of the CONUS warmer than normal. The high pressure pattern kept fronts and low pressure systems out of the eastern CONUS at first, resulting in drier-than-normal conditions. But the fronts eventually penetrated the South and East, and their precipitation was supplemented by tropical systems which came in from the Gulf of Mexico (Tropical Storm Gordon) and Atlantic (Hurricane Florence). Another tropical system (Olivia) soaked Hawaii. As a result, drought and abnormal dryness expanded, intensified, or developed in parts of the West, northern Plains, Southeast, and Northeast, as well as southeast Alaska, but contracted across much of Texas, parts of the central and northern Plains to Midwest, and other parts of the West, Southeast, and Northeast, as well as Hawaii and Puerto Rico. The dry conditions during September were a continuation of persistent dryness that has lasted 24 months, especially for the Southwest (Standardized Precipitation Index maps for the last 1, 2, 3, 6, 9, 12, 24 months). Drought contraction outweighed expansion this month, so the USDM-based national moderate-to-exceptional drought footprint across the CONUS contracted from 34.4 percent of the CONUS at the end of August to 29.0 percent of the CONUS at the end of September (from 29.2 percent to 24.6 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 34.7 percent of the CONUS was in moderate to extreme drought at the end of September, decreasing about 4.2 percent from the 38.9 percent at the end of August.

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 2^nd, 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 extreme to exceptional (D4) drought in the Southwest. —
  • According to the USDM, more than half (59.3 percent) of the West was experiencing moderate to exceptional drought at the end of September. 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 82.9 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).
• Areas of moderate (D1) to extreme drought covered parts of the southern and northern Plains to Upper Mississippi Valley, and moderate to exceptional drought covered the central Plains to Mid-Mississippi Valley.
• Moderate to severe drought persisted in the Northeast.
• Pockets of moderate drought developed in the Southeast and lingered in the Great Lakes region.
• Abnormal dryness and moderate drought persisted, and severe drought developed, in Alaska.
• Severe drought ended with abnormal dryness to moderate drought lingering in Hawaii, and abnormal dryness shrank but lingered in Puerto Rico.

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 September across much of the West over areas that were in drought at the end of August, expanding and intensifying long-term drought. Short-term dry conditions occurred in the northern portions of the Northeast and parts of the Great Lakes, resulting in new areas of long-term drought. Short-term dry conditions occurred in parts of the Southeast, eliminating previous areas of long-term wet conditions. Near-normal to wet short-term conditions occurred across much of the Plains and in the Mid-Atlantic region, shrinking previous 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. Dryness dominates the West at the 1- to 3-month time scales, is present in parts of the West for the last 6 to 12 months, and afflicts the Southwest at all time scales. Parts of the Southeast are dry at the 1- to 2-month time scales. Parts of the northern Plains are dry at the 2- to 24-month time scales. Dryness is evident from the central Plains to Mid-Mississippi Valley at the 6- to 24-month time scales.

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.

Temperatures during September 2018 were well above normal across much of the West, especially the Southwest, with record warmth occurring in some areas. For areas that were drier than normal during the month, this resulted in a more extreme SPEI compared to SPI.

Regional Discussion

Hawaii:

September 2018 was wetter than normal across the Hawaiian islands, except for a few stations on the Big Island, due to heavy rains from Tropical Storm Olivia. The wetness of September (and August) tipped the multi-month precipitation totals for almost all of the stations into the wet category for the last 2, 3, 4, 6, 9, and 12 months. Dryness at some stations became evident at the longer time periods (last 24 and 36 months). Olivia (and Hurricane Lane last month) left well-above-normal streamflow across the island chain. Moderate to severe drought covered about a third (35.8 percent) of the state at the end of August, but by the end of September the severe drought had disappeared and abnormally dry to moderate drought conditions had shrunk to about four percent of the state.

Alaska:

September 2018 was drier than normal across western and southern Alaska, and wetter than normal in the northeast. Record dryness occurred across parts of the northern panhandle to south central coast. At longer time scales, dryness continued widespread across the panhandle and south central regions, and lingered in some parts of the northwest. The southern portions of the panhandle were record dry for the last 12 months, with dryness still widespread across the southern portions of the state at 24 to 36 months. (Low elevation station precipitation anomaly maps for the last 1, 2, 3, 4, 6, 9, 12, 24, 36 months.) (Divisional precipitation rank maps for the last 1, 3, 6, 9, 12 months.) (Gridded precipitation percentile maps for the last 1, 3, 9 months.) Temperatures during September were warmer than normal across most of the state, but cooler than average in the northeastern portions. This pattern held for the last 2 to 6 months. Warmer-than-normal temperatures began to dominate across the state at the 9-month time scale. Record warmth was evident at some time scales. (Low elevation station temperature anomaly maps for the last 1, 2, 3, 4, 9, 12 months.) (Divisional temperature rank maps for the last 1, 3, 6, 9, 12 months.) (Gridded temperature percentile maps for the last 1, 3, 9 months.) Streamflow was below normal in the panhandle, south central, and northwest regions. According to reports from the National Weather Service, the Metlakatla area had low water at their hydroelectric facility, and the low stream flows posed a threat to fisheries. Abnormally dry to moderate drought conditions continued in the panhandle, with the southern portions of the panhandle going to severe drought. About 5.8 percent of the state was in abnormally dry to severe drought conditions on the October 2^nd USDM map.

Puerto Rico: Southern and eastern portions of Puerto Rico were drier than normal during September, with wetter-than-normal conditions in the northwest and north coastal regions. This anomaly pattern persisted at the 2- and 3-month time scales. Drier-than-normal conditions dominated the south-central to southwestern areas, with wetter-than-normal conditions to the north and east, at longer time scales (last 6, 9, 12 months). Soils were dry in the south central region to southwest coast, but streamflow was mostly near normal except for some low streams in the southwest. As seen on the October 2^nd USDM map, abnormal dryness contracted to the south central areas, covering about 14 percent of the island.

CONUS State Precipitation Ranks:

Four states in the West had a top ten dry September, including Utah (second driest), Idaho (fourth driest), California (sixth driest), and Nevada (seventh driest). Wyoming ranked twelfth driest in the 1895-2018 record. For the last three months (July-September 2018), Idaho and Oregon clocked in at second driest, while California ranked eleventh driest.

Utah had the ninth driest April-September and Colorado the tenth driest. Colorado also had the eighth driest January-September. The Southwest states stood out for the last twelve months (October 2017-September 2018), with Utah ranking driest on record, Colorado second driest, Arizona eighth driest, and New Mexico tenth driest.

Agricultural Belts

For the Primary Corn and Soybean agricultural belt during September 2018, temperatures were warmer than normal and precipitation was mostly wetter than normal. The month ranked as the sixth wettest and 13^th warmest September, regionwide, in the 1895-2018 record.

March is the beginning of the growing season for the Primary Corn and Soybean agricultural belt. For March-September 2018, temperatures were mostly near to warmer than normal and precipitation mostly wetter than normal. This 7-month period ranked as the tenth wettest and 26^th warmest March-September, regionwide, in the 1895-2018 record.

The prolonged below-normal precipitation has dried soils and inhibited rangeland, pasture land, and crop growth in many states in the West as well as Missouri and, in the Southeast, Georgia, South Carolina, and Florida. According to September 30^th statistics from the U.S. Department of Agriculture, 23 percent of the topsoil and 28 percent of the subsoil was rated short or very short of moisture (dry or very dry) nationwide, 23 percent of the pasture and rangeland was rated in poor to very poor condition, and (only) 12 percent of the corn crop and 10 percent of the soybean crop was rated in poor to very poor condition, but 25 percent of the cotton crop was rated poor to very poor. As of October 2^nd, drought was affecting five percent of corn production, seven percent of soybean production, 20 percent of hay acreage, 19 percent of cattle inventory, 16 percent of winter wheat production, and 42 percent of spring wheat production.

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, it was a tale of extremes regarding precipitation in September, as some areas were excessively wet while others were extremely dry. Some areas in drought across the region saw the continuation of dry conditions in September. For instance, western Colorado, southern Wyoming, portions of the Dakotas, and northeastern Kansas did not receive any drought relief. Although the primary growing season is coming to an end, winter wheat planting is underway. However, winter wheat producers in these areas were concerned about continued dryness, as moisture is needed going into winter for adequate growth of the crop. In western Colorado, streamflows continued to be well below normal, as precipitation was less than 25 percent of normal in many locations. September was also dry for much of Wyoming, which caused streamflows to fall below normal throughout portions of the state.

Drought conditions improved in some areas but worsened in others in September, according to the USDM. Overall, the area in drought (D1-D4) in the High Plains increased by approximately three percent. For much of the region it was a dry month, causing drought to intensify. Western and northern Colorado experienced warm and dry conditions during September, which exacerbated impacts. For instance, streamflows were low, with many streams experiencing flows that were less than the 10th percentile for this time of year. The drought has impacted water supplies, as reservoir levels continued to drop. Reservoir levels at Lake Powell and Lake Mead, which provide water to multiple states in the West, were less than the 10th percentile in September. Drought also intensified in the Dakotas where it has been persistently dry. In North Dakota, producers reported stressed pastures, a shortage of hay, and disappointing soybean yields in dry areas. Producers in the Dakotas have voiced concerns about the dryness coinciding with winter wheat planting. Areas that received drought relief in September included northeastern and southwestern portions of Kansas, as well as southeastern Colorado. Heavy rains early in the month brought about a one to two-category improvement in these areas, which helped improve soil moisture conditions.

As explained by the Southern Regional Climate Center, precipitation values for the month of September were above normal across much of the Southern region, while temperatures varied spatially. The few dry areas included parts of northern and extreme western Texas as well as northeastern Oklahoma, which received 25 percent or less of normal precipitation, and parts of northern, western, and extreme western Texas, northeastern and northwestern Oklahoma, northwestern and southeastern Arkansas, and part of western Mississippi, which received 50 percent or less of normal precipitation.

At the end of September, exceptional drought conditions were no longer present across the Southern region. Extreme drought classifications were present in northern and northwestern Texas. Severe drought classifications were present throughout northern, northwestern, extreme western, and east-central Texas as well as southwestern and northeastern Oklahoma. Moderate drought classification was present throughout extreme western, northern, central, southeastern, and eastern Texas, southwestern and northeastern Oklahoma, northwestern, southwestern, and southern Arkansas, northwestern and northern Louisiana, and northeastern Mississippi. There were no drought conditions in Tennessee. Across the Southern region, the total area not experiencing at least abnormally dry conditions increased from just over 40 percent at the end of August to just under 71 percent at the end of September. Over 90 percent of both Arkansas and Tennessee were not experiencing at least abnormally dry conditions, while over 80 percent of Louisiana and Mississippi were not experiencing at least abnormally dry conditions. Texas, at just under 57.5 percent, contained the smallest area lacking at least abnormally dry conditions.

As described by the Midwest Regional Climate Center, regionwide, September 2018 precipitation across the Midwest ranked as the 11th wettest September in history (beginning 1895) with 149 percent of normal, and September temperatures ranked as the 11th warmest in history (since 1895) averaging 3.7 degrees F (2.1 C) above normal. Drought eased slightly in September. As of the August 28th USDM, just over 16 percent of the Midwest was in drought, 7 percent in severe drought, 2 percent in extreme drought, and a half percent in exceptional drought. Those coverages dropped, particularly in the first half of the month, to 8.5 percent, 1.7 percent, 0.4 percent, and 0.01 percent respectively as of the September 25th USDM.

As noted by the Southeast Regional Climate Center, precipitation across the Southeast in September was highly variable. This was due to the presence of a dominating high pressure system that suppressed convective showers in the central part of the region and two tropical systems that brought record-breaking rainfall along the paths of the storms. Drier than normal conditions were observed in most of Georgia and the Florida Panhandle, with the driest areas found along the East Coast, especially in northeast Florida and the South Carolina coast south of Charleston. Monthly precipitation totals in the areas not affected by the tropical storms ranged from 70 to less than 25 percent of normal. The Melbourne, FL airport (1942-2018; 1st driest) received only 1.40 inches (36 mm) for the month, 6.24 inches (159 mm) drier than normal. Daytona Beach, FL (1923-2018; 2nd driest) received 1.44 inches (37 mm) of rain, 5.54 inches (140 mm) drier than normal. Albany, GA (1947-2018; 1st driest) received only 0.50 inch (13 mm) during the month. Drier than normal conditions were also observed across all of Puerto Rico and the US Virgin Islands, with amounts ranging from 1.7 to 6.1 inches (43 mm to 155 mm) drier than normal across the islands. Those dry conditions led to the expansion of drought across that area.

Dry conditions in areas of the Southeast that were not affected by tropical storms led to the development of moderate drought in northern Alabama, central Georgia and southern South Carolina during the month. Many agricultural producers noted that crop development came to a virtual standstill due to the dry conditions, with soybeans, cotton, and pecans all suffering from losses in yield due to lack of moisture, stressing the plants. Peanut farmers noted that it was so dry they could not even dig up peanuts to do maturity checks and observed that the ground was so hard that it would be difficult to dig the peanuts when they were ready. Livestock producers were forced to start feeding hay early when pastures dried up and hay production came to a halt due to the lack of forage growth. In Puerto Rico, the area of abnormally dry conditions contracted even though precipitation was less than normal across the island. In areas affected by Tropical Storm Gordon and Hurricane Florence, dry conditions were wiped out by the heavy rain, but damage to agriculture was much more severe than in dry areas.

As explained by the Northeast Regional Climate Center, the USDM released on September 6 showed 8% of the Northeast in a severe or moderate drought and 13% of the region as abnormally dry. These areas included northern and western New York, much of Vermont, parts of New Hampshire, portions of Maine, southeastern Massachusetts, and southern Rhode Island. Much-needed rain during the month improved conditions slightly in many of these areas, with the exceptions being eastern Maine and east of Lake Ontario in New York as abnormal dryness expanded. The USDM released on September 27 showed 5% of the Northeast in a severe or moderate drought and 16% of the region as abnormally dry. The dry conditions led to small fruit and a large loss of blueberries in parts of Vermont. In addition, farmers in parts of that state were weeks behind in planting fall cover crops. Farmers in northern New York reported decreased hay and pasture production. In Aroostook County, Maine, dry conditions contributed to a hay shortage, with some farmers likely having to use supplemental feed for their animals this winter. The dry conditions also allowed bedstraw, an invasive plant species, to flourish, reducing the number of acres of hay for some farmers. Growers in Maine also reported smaller sized apples. Springs and private wells continued to run dry in parts of northern Vermont. The town of Franklin trucked in water to replenish its water system, which is spring-fed. Residents were asked to conserve and boil water. The village of Waterville also asked residents to conserve water. Stowe's water system was okay, but one of the two primary wells was around ten feet lower this year (mid-September) compared to last year. Water levels at two dams on the Lamoille River were too low to make electricity. Companies that drill, deepen, or replenish wells have seen increased business.

As summarized by the Western Regional Climate Center, September remained dry in Utah punctuating one of the driest water years on record. At Salt Lake City a trace of precipitation was recorded for only the third time since records began in 1928, tying 1943 and 1951 for the driest September on record. Oregon, eastern Washington, and Idaho also saw a well below normal month for precipitation. Moscow, Idaho recorded 0.05 in (1.27 mm), 4% of normal, which ties 1990 and 2012 for the third driest September since records began in 1894. But southeast Arizona and southern New Mexico benefitted from an active September Monsoon. According to the USDM, conditions degraded to extreme drought in eastern Oregon and severe drought in southern Idaho with small areas of improvement to moderate drought in northeast New Mexico. A number of large wildfires continue to burn across the west and several new fires ignited in September.

September was another dry month in one of the driest water years on record for much of Utah and the agricultural communities have seen major impacts. For the first time in ten years an emergency meeting of the Utah Drought Review and Reporting Committee was called to investigate the potential need for an emergency declaration across the state. Many farmers in southern Utah have run out of surface water. Pasture and range conditions are some of the worst in the country with 71% of the grazable land in poor or very poor condition according to the Climate Prediction Center.

Hawaii was affected by another tropical system this month but impacts were minor compared to the devastating rain from Hurricane Lane last month. To the north in Alaska, a strong upper-level ridge was parked over the region for much of the month centered over the western part of the state. This brought widespread above-normal temperatures and below-normal precipitation. Many locations in south central and western Alaska saw record or near record warmth for the month. In the southeast part of the state Juneau, 2.3 in (57.9 mm) and 26% of normal, Cordova, 3.6 in (92.5 mm) and 27% of normal, and Sitka, 2.4 in (70.0 mm) and 20% of normal, all recorded the driest September on record. Widespread D2 (severe drought) and D1 (moderate) is now present over much of southeast Alaska.

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

It was a dry month in terms of drought at Koror (in the Republic of Palau), Nukuoro (FSM), and Jaluit 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 driest August-September in a data record spanning 35 years, Nukuoro had the second driest August-September in 35 years of data, Ailinglapalap ranked sixth driest for August-September and fifth driest for May-September in 34 years, and Wotje had the fourth driest September in 35 years of data. For the year to date, it was the fifth driest January-September in 22 years of record at Lukonor and fourth driest in 34 years at Nukuoro. On the other hand, long-term wet conditions resulted in the wettest year to date for Pohnpei (out of 67 years of data) and Kwajalein (66 years).

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

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 September 2018, April-August 2018 (the last 6 months), and October 2017-September 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	Sep 2018 Rank	Sep No. of Years	Apr- Sep 2018 Rank	Apr- Sep No. of Years	Oct 2017- Sep 2018 Rank	Oct- Sep No. of Years	Period of Record
Jaluit	4	35	MSG	34	MSG	32	1981-2018
Koror	9	68	MSG	66	MSG	66	1951-2018
Woleai	21	37	23	29	18	25	1968-2018
Yap	42	68	27	67	55	67	1951-2018
Majuro	25	65	60	64	64	64	1954-2018
Mili	20	34	MSG	33	MSG	31	1981-2018
Ulithi	MSG	36	MSG	35	MSG	32	1981-2018
Ailinglapalap	14	35	6	34	13	32	1981-2018
Kosrae	7	47	19	39	20	30	1954-2018
Lukonor	17	35	5	22	7	21	1981-2018
Saipan	36	38	37	38	28	29	1981-2018
Pohnpei	16	68	39	67	67	67	1951-2018
Kwajalein	20	67	64	66	66	66	1952-2018
Kapingamarangi	23	27	13	19	17	17	1962-2018
Chuuk	60	68	35	67	43	67	1951-2018
Guam	59	62	59	62	45	61	1957-2018
Nukuoro	5	36	3	34	18	33	1981-2018
Pago Pago	39	53	40	53	51	52	1966-2018
Wotje	4	35	MSG	34	MSG	31	1981-2018
Utirik	9	15	7	7	2	2	1985-2018

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

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