May 2018 Drought Report

Overview

Like the last several months, the upper-level circulation pattern was quite active during May 2018 with ridge and trough patterns migrating through the jet stream flow over the CONUS. This activity occurred within a broadscale/long-wave ridge which dominated the overarching circulation over the CONUS. The ridge kept temperatures much warmer-than-normal across most of the country, with the excessive warmth increasing evapotranspiration in many places. Left to itself, the ridge would have given much of the CONUS a drier-than-normal month, but the frequent passage of the upper-level troughs and lows, in addition to Sub-Tropical Storm Alberto near the end of the month, gave several areas above-normal precipitation. The wet areas included parts of the interior West, central to northern High Plains, and southern Great Lakes, as well as a wet swath from the Southeast to Mid-Atlantic region. But the weather systems missed the coastal West, Southwest, parts of the Great Plains, much of the Mississippi and Ohio Valleys, and much of the Northeast. The dry conditions during May were a continuation of persistent dryness since October in the southern High Plains, and for much of the last 12 months in the Four Corners States (Standardized Precipitation Index maps for the last 1, 2, 3, 6, 9, 12 months). Drought and abnormal dryness expanded or intensified where the dry conditions continued, especially in the Southwest, Far West, parts of the Plains and Mississippi Valley, and parts of Hawaii. Drought and abnormal dryness contracted in other parts of the Plains and in the Southeast to Mid-Atlantic region. Contraction outweighed expansion again this month, so the USDM-based national moderate-to-exceptional drought footprint across the CONUS shrank from 28.6 percent of the CONUS at the end of April to 26.4 percent of the CONUS at the end of May (from 23.9 percent to 22.1 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 32.5 percent of the CONUS was in moderate to extreme drought at the end of May, increasing about 2.4 percent from the 30.1 percent at the end of April.

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 May 29^th, 2018 USDM map, included the following core drought and abnormally dry areas:

• A large area of abnormal dryness (D0) to moderate (D1) drought stretched from Oregon and California in the West, across the southern and central Great Plains, to the Mid-Mississippi Valley, with severe (D2) to exceptional (D4) drought across the Four Corners States to Texas, Oklahoma, and Kansas. —
  • The percent area of the West (from the Rockies to the West Coast) in moderate to extreme drought (based on the Palmer Drought Index) rapidly increased in December and continued to increase in January and February, growing from 21.5 percent at the end of October to 62.4 percent of the West at the end of February. Above-normal precipitation improved conditions in California and Nevada in March, so the Palmer percent area in drought shrank to 52.3 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, especially in Oregon and Washington, and the last two months were quite dry in the Four Corners States. The dry conditions were coupled with unusual May warmth which increased evapotranspiration, thus exacerbating drought conditions, especially in the Southwest. As a result, the Palmer Drought Index measure of drought area expanded to 58.3 percent of the West by the end of May.
  • The D3-D4 areas are regions which have experienced the most severe precipitation deficits over the last several months to two years based on the Standardized Precipitation Index (maps for the last 1, 2, 3, 6, 9, 12, and 24 months).
  • Drought conditions were reflected in several indices and indicators, including evapotranspiration (EDDI), soil moisture (SMOS satellite observations, and CPC, Leaky Bucket, VIC, NASA GRACE, NLDAS top and total layer models), Crop Moisture Index (CMI maps for weeks 1, 2, 3, 4, 5), streamflow, groundwater (NASA GRACE satellite-based estimates, USGS observations), the QuickDRI index, the satellite data-based VegDRI and VHI vegetation condition indices, some reservoirs, and USGS number of rain days and consecutive dry days. USDA observations of soil moisture and pasture and rangeland condition revealed significant impacts in the West and South. Topsoils were short or very short of moisture (dry or very dry) across 42 percent of Oregon on May 27, worsening to 56 percent by June 3; 61 percent of Texas, worsening to 77 percent a week later; and 55 percent of Louisiana, worsening to 73 percent. A third to half of Kansas (48 to 38 percent) and Oklahoma (46 to 49 percent) had dry topsoils, and three-fourths of New Mexico did (76 to 71 percent). Subsoil moisture conditions were equally bad (May 27 to June 3), with values as follows: New Mexico (78 to 75 percent), Texas (64 to 71 percent), Louisiana (46 to 61 percent), California (55 percent), Oklahoma (56 to 58 percent), Oregon (41 to 51 percent), and Kansas (55 to 48 percent). Pasture and rangeland (May 27 to June 3) were in poor to very poor condition across most of Arizona (85 to 94 percent) and New Mexico (66 to 68 percent), and a third of Colorado (37 to 40 percent) and Texas (31 to 37 percent).
• Abnormal dryness to severe drought continued in the northern Plains and the High Plains of Montana and expanded into the western Great Lakes. — This status mainly reflected a combination of short-term (last 1, 2, 3, to 6 months) and, especially for the Dakotas, long-term (last 9 to 12 to 24 months) precipitation deficits. Drought impacts were evident in several indices and indicators, including evapotranspiration (EDDI), soil moisture (SMOS satellite observations, and CPC, Leaky Bucket, VIC, NASA GRACE, NLDAS top and total layer models), the QuickDRI index, and the satellite data-based VegDRI and VHI vegetation condition indices. According to USDA observations, topsoils were short or very short of moisture (dry or very dry) across 37 percent of North Dakota on May 27, improving to 31 percent by June 3. The values for subsoil moisture were worse, from 49 percent to 43 percent of North Dakota.
• Heavy May rains eliminated drought from the Southeast, but a few areas of abnormal dryness lingered, and above-normal precipitation reduced the area of abnormal dryness in Alaska. But dry weather resulted in the expansion or introduction of abnormal dryness to parts of New England and Hawaii. Low streamflow was observed in parts of New England, and by June 3, according to USDA observations, topsoil moisture was dry to very dry across 70 percent of New Hampshire, 55 percent of Massachusetts, and 48 percent of Maine, and subsoil moisture was dry to very dry across 41 percent of New Hampshire. Puerto Rico continued free of drought and abnormal dryness on the USDM map.

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 from the Southwest, across the southern and central Plains to the Ohio Valley, expanding and intensifying previous long-term drought. Short-term dry conditions occurred in parts of the northern Plains, expanding previous long-term drought. Short-term dry conditions occurred in parts of the Great Lakes to Northeast, and Lower Mississippi Valley, reducing previous long-term wet conditions. Short-term wet conditions occurred in the Southeast to Mid-Atlantic region, shrinking previous 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 across parts or most of the northern Plains and Southwest is evident at all time scales. The Pacific Northwest coast, and parts to most of California, are dry at the 1-month time scale, and parts of Oregon are dry at the 6- to 12-month time scales. Parts of the Great Plains to Mississippi Valley are dry for the last 1 to 12 months, with the Mid-Mississippi Valley also dry at 24 months. Much of the Northeast is dry at the 1-month time scale, and parts are dry at 3 months and 9 to 24 months. Small parts of the Southeast are dry at 9 to 12 months, with more dry at 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.

Temperatures during May 2018 were above normal across a large part of the country, with record warmth occurring from the southern Plains to Mid-Atlantic region. For areas that were drier than normal during May, this resulted in a more extreme SPEI compared to SPI.

Unusually warm temperatures for June 2017-May 2018 across the Southwest have resulted in a significantly more extreme 12-month SPEI compared to SPI. The temperatures have been record warm across Arizona for the last 12 months. When the record warmth is combined with the seventh driest June-May, the result is the most extreme SPEI in the 1895-2018 record. Unusual warmth and dryness have dominated Arizona for much of the last two decades.

Regional Discussion

Hawaii:

May 2018 was drier than normal at most of the Hawaiian stations, especially those in leeward locations. But a wet April resulted in a wetter-than-normal pattern for the last 2, 3, and 4 months. A few stations were drier than normal, but a wetter-than-normal pattern still predominated during the last 5, 6, 8, 12, 24, and 36 months. Streamflow was mostly near to above normal. Abnormal dryness returned to about a fifth (21 percent) of the state, as seen on the May 29^th, 2018 USDM map.

Alaska:

May 2018 was wetter than normal across Alaska according to the climate division analysis, but pockets of drier-than-normal conditions could be found on the gridded percentile, low elevation station, and SNOTEL high elevation station maps. The pockets of dryness are a little larger at 2 months and the last 3 months (climate division, gridded percentile, low elevation station maps), where they become evident especially in the panhandle, and the last 4 to 5 (climate division, gridded percentile, low elevation station maps) months from the panhandle to the south central coast. For the last 6 months, dryness is still evident in the panhandle with a slightly mixed pattern elsewhere. For the water year to date (October 2017-May 2018), dryness along the south central coast to the panhandle is again apparent (low elevation station and SNOTEL high elevation station maps). Wet conditions dominate at the 11- and 12-month (climate division, low elevation station maps) time scales, except in the southern locations. Dry conditions are a little more widespread at the 24- and 36-month time scales, but wet conditions still dominate in the northern half of the state. Temperatures during May were near to warmer than normal (gridded temperature percentile, climate division, Leaky Bucket model temperature percentile maps), with some cooler-than-normal stations mixed in. Cooler-than-normal stations were a little more widespread for the last 2 and 4 months, but warmer-than-normal temperatures dominate for the last 3 (gridded temperature percentile, climate division, low elevation station maps), 5 (gridded temperature percentile, climate division, low elevation station maps), 6, 8 and 12 (climate division, low elevation station maps) months. Streamflow was mostly near to above normal. The Leaky Bucket model suggested a some lingering dryness in the soils in the south and eastern sections. Abnormally dry (D0) conditions shrank to about 2 percent of the state on the May 29^th USDM map.

Puerto Rico: Puerto Rico was drier than normal in the south central sections, and wetter than normal across the rest of the island, during May. This pattern was evident for the last 2, 3, 5, 6, and 8 months, although the south central dry area became less dry at 8 months. Soils were dry in the south central region to southwest coast, but streamflow was mostly near normal. It should be pointed out that the island still has not recovered from the damage to the weather observation network suffered during last year's hurricane season. As seen on the May 29^th USDM map, Puerto Rico was free of drought and abnormal dryness.

CONUS State Precipitation Ranks:

May 2018 was drier than normal across much of the West Coast, Southwest, Lower Mississippi Valley, and Northeast, and across parts of the Great Plains, and Midwest. On a statewide basis, 15 states ranked in the driest third of the 124-year historical record, with Louisiana ranking in the top ten driest category at tenth driest. New Hampshire and Washington state were close at eleventh and twelfth driest, respectively, although it was record dry along parts of Washington's coast.

March-May 2018 was drier than normal across the Southwest, much of the Great Plains and Mid- to Upper Mississippi Valley, and parts of the Northeast. On a statewide basis, eleven states ranked in the driest third of the 124-year historical record, with Arizona and New Mexico ranking in the top ten driest category at tenth driest each. It was record dry in parts of Arizona.

January-May 2018 was drier than normal across the Southwest to central and southern Plains, northern Plains to Upper Mississippi Valley, and parts of the West Coast. On a statewide basis, nine states ranked in the driest third of the 124-year historical record, with none in the top ten driest category.

December 2017-May 2018 had a similar pattern, except the dryness was more severe and was more widespread across the West. On a statewide basis, eleven states ranked in the driest third of the 124-year historical record, with top ten driest rankings for New Mexico (eighth driest), Kansas (ninth driest), and Colorado (tenth driest). Arizona was close at twelfth driest.

The last 12 months had a similar pattern to December-May. On a statewide basis, eleven states ranked in the driest third of the 124-year historical record, with top ten driest rankings for Arizona (seventh driest) and Utah (tenth driest). North Dakota was close at twelfth driest. North Dakota's dryness during June 2017-May 2018 is a notable departure from the unusual wetness which has dominated much of the last 30 years.

Agricultural Belts

For the Primary Corn and Soybean agricultural belt during May 2018, the precipitation pattern varied but temperatures were warmer than normal across the region. The month ranked as the warmest and 60^th driest May, regionwide, in the 1895-2018 record. March is the beginning of the growing season for the Primary Corn and Soybean agricultural belt. March-May 2018 also had a spatially variable precipitation anomaly pattern, while temperatures averaged near normal across the region. March-May 2018 ranked as the 49^th driest and 61^st coolest March-May, regionwide, in the 1895-2018 record.

The prolonged below-normal precipitation has dried soils and inhibited crop growth in many states in the Plains and Southwest. According to statistics from the U.S. Department of Agriculture (USDA), 28 percent of the topsoil and 29 percent of the subsoil was rated short or very short of moisture (dry or very dry) nationwide, 34 percent of the winter wheat was rated in poor to very poor condition, and 17 percent of the pasture and rangeland was rated in poor to very poor condition. Conditions were much worse on a state by state basis, especially in the Southwest and Great Plains:

Percentage of winter wheat and pasture and rangeland in poor to very poor condition, and percentage of topsoil and subsoil moisture short to very short (dry to very dry), based on May 29, 2018 USDA reports.

State	Topsoil Moisture	Subsoil Moisture	Winter Wheat	Pasture/Rangeland
California	75%	55%	0%	30%
Colorado	36%	34%	19%	37%
Kansas	48%	55%	48%	29%
New Mexico	76%	78%	—	66%
North Dakota	37%	49%	—	27%
Oklahoma	46%	56%	63%	13%
Oregon	42%	41%	11%	11%
South Dakota	18%	26%	10%	10%
Texas	61%	64%	54%	31%
Utah	38%	39%	—	21%

NOAA Regional Climate Centers:

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

As described by the High Plains Regional Climate Center, above-normal temperatures prevailed across the region in May while precipitation conditions varied. The rapid change in temperatures from unusually cold in April to unusually warm in May impacted agriculture and livestock. For instance, warmer temperatures aided with crop progress after slow growth in April, but increased evaporative demand worsened winter wheat conditions in Colorado and Kansas where it has been dry since the fall. Additionally, oppressive heat and humidity in late May caused cattle deaths in South Dakota because the cattle were not yet acclimated to these conditions. Heavy rains improved drought conditions throughout a large part of Kansas and northeastern Colorado, while continued dryness caused drought to spread and intensify across portions of the Dakotas.

Soil moisture conditions are very important during this time of year, as most crops have been planted and the moisture is needed for them to grow. According to the USDA Weekly Weather and Crop Bulletin, beneficial rains improved topsoil and subsoil moisture in May in portions of Colorado, Kansas, North Dakota, and Nebraska. However, topsoil moisture declined in South Dakota, where most areas missed out on heavy rainfall and experienced much-above-normal temperatures. The decline of topsoil moisture is an indicator of worsening drought conditions, and it is something to watch closely as we move into the season when crop water demand is highest.

As noted earlier, both improvements and degradations in drought conditions occurred throughout the High Plains in May. According to the USDM, little change occurred to the region-wide percent area experiencing drought over the past month. However, changes in conditions were evident on a more local basis. For instance, heavy rains improved conditions across portions of Kansas, northern Colorado, and south-central Nebraska, while drought expanded and intensified in the Dakotas, southern Colorado, and southeastern Nebraska. Kansas experienced the greatest improvements in drought conditions during the month of May. Portions of central Kansas received up to 300 percent of normal precipitation, which was quite welcome after paltry amounts in April. Unfortunately, it was too late to improve winter wheat conditions in Kansas, as nearly half of the crop was still in poor to very poor condition by the end of the month. Meanwhile, continued dryness and high temperatures caused drought to expand and intensify in the Dakotas and parts of Colorado and Nebraska. In North Dakota, pastures were dry and low streamflows impacted flow releases from dams. Twenty-one counties were declared natural disaster areas due to lingering drought conditions that caused significant production losses last year. Impacts continued to mount in Colorado, as livestock producers reduced herds and hauled water, and visitation declined in Great Sand Dunes National Park due to low flows in Medano Creek.

As explained by the Southern Regional Climate Center, May temperatures were above normal throughout most of the Southern region, resulting in the warmest May on record for the region as a whole, while precipitation values for the month varied spatially throughout the region. Dry areas included parts of northwestern and western Texas which received 5 percent or less of normal precipitation. Areas of central, western, eastern, and southern Texas, southwestern Oklahoma, southern and eastern Arkansas, east central Mississippi, and most of Louisiana received 50 percent or less of normal precipitation.

At the end of May, exceptional and extreme drought classifications were still present throughout parts of western Oklahoma and northern Texas. Severe drought classifications were present throughout parts of southwestern, southeastern, and northern Texas and western and northern Oklahoma. The moderate drought classification remained throughout parts of western, central, northern, and southeast Texas. Moderate drought classification appeared in northeastern and southwestern Texas and in extreme southeastern Louisiana. There were no drought conditions in Arkansas, Mississippi, and Tennessee.

As summarized by the Midwest Regional Climate Center, May precipitation varied across the Midwest with the majority of the region below normal. Totals ranged from less than 50 percent of normal in parts of Illinois, Indiana, and Minnesota to more than twice normal in southern Wisconsin and southwestern Michigan. Spring precipitation totals were above normal in the easternmost states, along the southern extent of the region, and from the Iowa-Minnesota border eastward across southern Wisconsin into lower Michigan. Drier than normal swaths stretched across the northern extent of the region and through central areas including the southern two-thirds of Iowa, northwestern two-thirds of Missouri, much of Illinois, and nearly all of Indiana. Drought areas in the Midwest were less than 5 percent of the region throughout May. The area in northern Minnesota that started the month in moderate drought saw conditions improve and drought end. In northern Missouri and southern Indiana, a larger area of moderate drought also had an area of severe drought develop in northwest Missouri where spring rainfall was less than half of normal.

May temperatures in the Midwest set a new record (1895-2018 period of record) averaging 66.3 degrees F (19.1 C) which was 6.9 degrees F (3.8 C) above normal. Five Midwest states (Illinois, Indiana, Kentucky, Missouri, and Ohio) set statewide records for May while the remaining four states ranked among the five warmest Mays in their respective histories. Spring temperatures averaged to near normal across the region with the very cold April (2nd coldest) and the record warm May largely offsetting each other.

Spring planting of corn and soybeans caught up to the 5-year average in most of the region in May. Cool and wet weather in April put most of the Midwest 1 to 3 weeks behind but as warmer and drier condition allowed farmers into the field they quickly made up for lost time. Soybeans were caught up across the Midwest and corn was as well except in Michigan where it remained about a week behind the 5-year average.

As noted by the Southeast Regional Climate Center, precipitation was well above normal across much of the Southeast region during May. Temperatures were well above average across much of the Southeast region during May, but near average to slightly below average across the Florida Peninsula, Puerto Rico, and the U.S. Virgin Islands.

During the first half of May, moderate-to-severe (D1-D2) drought covered approximately 15 percent of the Southeast, as above-average temperatures and below-normal precipitation were observed across much of the region. On May 8th, over 25 percent of Florida, Georgia, and South Carolina were classified in moderate-to-severe drought, with localized portions of Alabama, North Carolina, and Virginia in moderate drought. However, well-above-normal precipitation during the second half of the month rapidly eliminated drought conditions from the region. A prevalence of warm, dry weather during the first half of May allowed farmers to resume crop planting and other field activities, which were significantly delayed by unusually cool temperatures in March and April. However, a prolonged lack of rainfall and insufficient soil moisture in some portions of the region prevented crop planting and slowed the growth of livestock pastures and hayfields. The persistence of low humidity from March through mid-May reduced disease pressure on fruit and vegetable crops across North Carolina, with a good yield of blueberries expected for the sixth largest producer in the nation. While below-average temperatures in March and April reduced the quantity of the largest Vidalia onion sizes, favorable weather conditions during late April and May resulted in a near-perfect harvest of the onion crop in southeastern Georgia. During the second half of the month, persistent rainfall replenished soil moisture levels in crop fields and livestock pastures across much of the region, but heavy precipitation and flooding in some areas prevented fieldwork, increased disease pressure on crops, and required fields to be replanted. The quality of small grains (e.g., wheat, rye, and oats) and hay deteriorated due to lodging (i.e., when high winds or heavy rain cause crops to fall over) and excessive dampness, with many farmers unable to harvest these crops from wet or flooded fields.

As explained by the Northeast Regional Climate Center, the region had the fifth warmest May on record while precipitation regionwide was 97 percent of normal. At the beginning of May, 40 percent of Maryland was abnormally dry, and it was the only state in the region that was experiencing abnormal dryness. Throughout the month, these dry conditions improved as a result of the abundant rain that impacted much of the southern states within the region. The USDM released May 29 showed New Hampshire as the only state to have dry conditions, with 18 percent of the southern part of the state experiencing abnormal dryness. Drought conditions improved in the Northeast during spring, with two percent of the Northeast being abnormally dry at the beginning of March and less than one percent of the region experiencing abnormal dryness by the end of May.

As summarized by the Western Regional Climate Center, an active weather pattern this month brought above-normal precipitation to the Great Basin and Intermountain West with temperatures near normal across much of California, the Great Basin, and Arizona. In contrast, much warmer than normal and in many cases, record breaking, temperatures were observed across the Northwest as well as in New Mexico and Colorado.

Several closed low-pressure systems impacted the West during May. These systems contributed to above normal precipitation, much of it in the form of thunderstorms and intense rainfall, over the Sierra Nevada, Great Basin, and portions of the Inland Northwest. There were only small areas of drought amelioration in the West this month in central Idaho, northeastern Montana, northern Colorado, and northeastern New Mexico. The areas that received above normal precipitation were generally not experiencing significant drought conditions and rely primarily on the snowpack for water resources. In many areas, May precipitation acted to increase surface soil moisture but did little to offset deficits in snowpack accrued during the winter and early spring season. At many mid-to-low elevation locations in the Sierra Nevada, Oregon Cascades, Wasatch, and Great Basin ranges, snowpack completely melted out in May, which is 2-4 weeks earlier than normal.

Dry conditions dominated along the West Coast and across the southern border of the region. Drought conditions worsened across much of the Southwest this month in the USDM, especially in Arizona and New Mexico. Much of the Four Corners region is currently experiencing extreme to exceptional drought. Severe drought was also introduced this month in eastern Oregon.

May temperatures were near normal across Alaska, while several locations received well above normal precipitation. The mean extent of May sea ice in both the Bering and Chukchi Seas was the lowest in 40 years of data. Further south, May precipitation was variable across Hawaii, though favored windward areas. For example, Hana, on the eastern side of Maui, reported 7.33 in (186 mm) of rainfall, 131% of normal. The leeward areas of Maui, Big Island, Oahu, and Molokai experienced a dry spring, prompting the introduction of abnormally dry conditions this month in the USDM.

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

Rainfall amounts were below the minimum thresholds (4 or 8 inches) required to meet most monthly water needs at Lukonor, Ulithi, and Yap (in the FSM) and Ailinglapalap (in the RMI). May rainfall was above the monthly minimum thresholds at the rest of the regular reporting stations in Micronesia. The 4- and 8-inch thresholds are important because, if monthly precipitation falls below the threshold, then water shortages or drought become a concern.

The beneficial rains of recent months have relieved stress from earlier dry conditions in the Marianas and northern Marshall Islands. But dry conditions developed or continued in parts of the FSM. Lukonor had the fourth driest May (out of 34 years of data) and fourth driest June-May (out of 21 years of complete June-May data). Even though Nukuoro had 8.69 inches in May 2018, which is above the 8-inch monthly minimum needed to meet most water needs, this May ranked as the second driest May in 35 years of data. Ulithi had the sixth driest May out of 36 years of data. In the Republic of Palau, Koror had the eleventh driest March-May out of 67 years of data. In the RMI, Alinglapalap had the fourth driest May in 35 years of data and ninth driest June-May in 32 years of data. Jaluit had the fifth driest June-May in 33 years of data.

As measured by percent of normal precipitation, Lukonor has been drier than normal in the short term (May, the last 3 months [March-May 2018], and the year-to-date [January-May]) as well as the long term (last 12 months [June 2017-May 2018]). Koror was drier than normal in the near term and near normal in the long-term. Kosrae, Kwajalein, and Majuro were near to wetter than normal in the short term and long term. The rest of the stations were drier than normal during one of these time periods and near to wetter than normal during the other three time periods. Kapingamarangi, Pago Pago, Pohnpei, and Yap were drier than normal for May. Chuuk was drier than normal for the last 3 months. Guam and Saipan were drier than normal for the last 12 months.

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 May 2018, March-May 2018 (last 3 months), and June 2017-May 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	May 2018 Rank	May No. of Years	Mar- May 2018 Rank	Mar- May No. of Years	Jun 2017- May 2018 Rank	Jun- May No. of Years	Period of Record
Jaluit	27	35	18	35	5	33	1981-2018
Koror	28	67	11	67	35	66	1951-2018
Woleai	20	36	18	32	9	21	1968-2018
Yap	28	67	44	67	41	66	1951-2018
Majuro	62	64	63	64	64	64	1954-2018
Mili	21	34	30	34	MSG	31	1981-2018
Ulithi	6	36	17	35	19	33	1981-2018
Ailinglapalap	4	35	20	35	9	32	1981-2018
Kosrae	41	49	38	44	24	31	1954-2018
Lukonor	4	34	5	34	4	21	1981-2018
Saipan	37	38	37	37	9	29	1981-2018
Pohnpei	25	67	67	67	66	66	1951-2018
Kwajalein	65	67	65	66	66	66	1952-2018
Kapingamarangi	10	24	20	24	14	14	1962-2018
Chuuk	41	67	18	67	26	66	1951-2018
Guam	53	62	52	62	19	61	1957-2018
Nukuoro	2	35	4	35	24	32	1981-2018
Pago Pago	25	53	28	52	47	52	1966-2018
Wotje	MSG	34	MSG	34	MSG	31	1981-2018
Utirik	18	19	10	11	2	3	1985-2018

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

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