Drought affected large portions of the United States during 1999. Few areas remained consistently entrenched in drought throughout the year, but Palmer Drought Index (PDI) data from the National Climatic Data Center (NCDC) indicated that 46 of the contiguous 48 states were affected by at least mild drought at some point during the year, and 29 of those states experienced severe to extreme drought. Furthermore, the Secretary of the U. S. Department of Agriculture (USDA) had approved Drought Disaster declarations for all or part of 35 states by year’s end, with 3 additional states pending.

The areal coverage and severity of drought changed as the year progressed, but the most significant drought conditions occurred (a) across the Northeast, mid-Atlantic, central Appalachians, and middle Ohio Valley during April-August, with long-term dryness dating back to July 1998 in some areas; (b) through the Ohio Valley, Tennessee Valley, interior Southeast, lower Mississippi Valley, and Texas from February through the end of the calendar year, with long-term precipitation shortages dating back to May 1998 along the southern tier of this region; (c) the interior Pacific Northwest during Spring and Summer; and (d) across the Hawaiian Islands throughout the year, where dryness began affecting some areas as far back as October 1997. Significant dryness also affected the Southwest early and late in the year, and the central and northern Plains from late summer through year’s end. However, very wet weather dominated the June–August period in both of these regions, keeping impacts to a minimum.

Very dry weather began affecting the Northeast, mid-Atlantic, central Appalachians, and middle Ohio Valley in April and continued through at least July. Accumulated precipitation deficits through this four-month period ranged from 200 to 250 mm in northern New Jersey and southeast Pennsylvania, and were at least 100 mm in many areas from central Kentucky and southeastern Indiana eastward through the mid-Atlantic states and northeastward across central and eastern sections of the Northeast. In many areas from central Maryland northeastward through southeastern Maine, this represented less than half of normal precipitation. According to NCDC, statewide-average precipitation for April-July was the lowest in 105 years of record for Delaware and Rhode Island; the second-lowest on record for Connecticut, Maryland, New Jersey, and West Virginia; and one of the seven lowest on record for six other states from the Virginias to Maine.

August brought limited relief to the mid-Atlantic region in the form of near-normal precipitation, but dryness continued in the rest of the Northeast, central Appalachians, and middle Ohio Valley. By the end of August, precipitation deficits of 150 to locally 300 mm since April were common along the Eastern Seaboard and in the middle Ohio Valley, and 13 states in the region recorded one of the seven driest such periods on record, according to NCDC.

Two factors accentuated the effects of this dryness. One was timing, as the dryness coincided with the primary agricultural growing season. For the lower Northeast, middle Ohio Valley, and mid-Atlantic regions, the second was that the dryness came shortly after extremely dry conditions during July-December 1998. This was particularly true for the mid-Atlantic region. Despite near- to above-normal January-March 1999 precipitation, the 13-month period from July 1998 through July 1999 brought precipitation anomalies below –200 mm to most areas from central Kentucky and southeast Indiana eastward across the central Appalachians and mid-Atlantic and northeastward through central and eastern Pennsylvania, central and southeastern New York, and New Jersey. From central and western Virginia northeastward to the New York City area, 13-month totals were 300 to 450 mm below normal, which represented only 60% to 65% of (and more than 2.5 standard deviations below) normal in northern Virginia and central Maryland. NCDC reported that this July-July period was the driest in 104 years of record for Virginia, the second-driest in Maryland, the third-driest in both New Jersey and West Virginia, and the fourth-driest in Delaware. When PDI values reached their lowest (driest) during July 1999, the historic proportion of the long-term drought became evident. These PDI values were approximately equal to those observed during the mid- to late-1960’s and significantly less severe than those of the early 1930’s, making this drought the second or third most severe since records began in 1895.

The spring and summer dryness across the east-central and northeastern United States was directly caused by an anomalous jet stream position. For much of this period, the jet stream was oriented in a southwest-to-northeast position from the interior Southwest to the western Great Lakes region and into southeastern Canada, resulting in a position considerably north of normal across eastern North America. As a result, storminess was enhanced through the southwestern, central, and north-central United States, where near-record precipitation was observed, and suppressed near the longitude of the ridge axis in the eastern states.

It appears as though the ongoing La Nina contributed to this setup. Historically, La Nina tends to favor wetness in the Ohio Valley and dryness along the south Atlantic coast for much of spring and early summer. During 1999, the observed pattern was similar to the historic La Nina response, but shifted a few hundred miles to the west, thus bringing dryness into parts of the eastern Ohio Valley, mid-Atlantic, and Northeast that is more typical of historic La Nina conditions along the south Atlantic coast.

In mid-August, precipitation began increasing across the eastern Carolinas, mid-Atlantic, and lower elevations of the Northeast. Over the next two months, a series of tropical cyclones (especially Hurricane Floyd in mid-September) and some non-tropical systems dropped exceptional rainfall totals on these areas, eliminating the drought that had plagued these regions for at least several months. From February through mid-August, 300 to 450 mm of rain fell on most sites from southern and western New England southward through the Virginias and parts of interior North Carolina while 450 to locally over 600 mm were measured through the rest of the Carolinas. These amounts were 100 to 300 mm below normal across broad sections of southern New England, the lower Northeast, the mid-Atlantic, and the interior Carolinas, despite near-normal February and March totals in all but the latter region. But from mid-August through mid-October, a period only one-third as long as the earlier period analyzed, rainfall totals exceeded 300 mm (at least 100 mm above normal) over most areas from the central and eastern Carolinas northward up the Eastern Seaboard to the east of the Appalachians, with amounts of 600 to 900 mm (over 400 mm above normal) inundating extreme northeastern South Carolina, much of eastern North Carolina, and southeastern Virginia.

In contrast to the situation in the mid-Atlantic and Northeast, dryness began earlier in the year and persisted through year’s end across much of the Ohio Valley, Southeast, Tennessee and lower Mississippi Valleys, and central and north Texas; however, the dryness in these regions, which primarily occurred during the February-December period, was not as climatologically extreme as the drought that affected areas farther to the northeast. In fact, near- or only slightly below normal precipitation covered areas south of Virginia and the Ohio Valley during April-July as the drought peaked in other parts of the East. Still, February-December precipitation was at least 200 mm below normal from the central and western Carolinas, the central Appalachians, and the lower Ohio Valley westward across the lower Mississippi Valley to central Texas, with anomalies of –300 to –475 mm recorded in southeast Texas, much of the lower Mississippi and Tennessee Valleys, the lower Ohio Valley, part of the southern Appalachians, and southwestern Georgia. In addition, only 60% to 70% of normal precipitation was observed across central and north-central Texas, where normals are somewhat lower. NCDC indicated that Kentucky endured its third-driest February-December period in 105 years of record while Georgia, Indiana, Louisiana, Mississippi, Ohio, Tennessee, and West Virginia all experienced their sixth- to eight-driest such period.

For the southern tier of this region, and to a lesser extent the middle Ohio Valley and Appalachians, the dryness that extended through the end of 1999 could be traced back to May 1998. Most areas from central South Carolina southward to central Florida and westward along the Gulf Coast states to central Arkansas and southwestern Louisiana received at least 250 mm less than normal rainfall, as did north-central Texas and scattered sections of the Appalachians and middle Ohio Valley. In parts of northern and western Georgia, lower northern Florida, southern Mississippi, and southeastern Louisiana, precipitation totals for the 20-month period were more than 500 mm below normal. As a result, this was the fourth-driest May-December 20-month period in 104 years of record for the state of Georgia. It is significant that the dryness in the south-central and southeastern states began roughly at the same time in 1998 that the strong El Nino began its rapid transition toward moderate La Nina conditions, because La Nina favors subnormal precipitation through most of these regions.

In the western United States, drought was not as much of a concern as in areas farther east, primarily because (a) wet winter and spring conditions in many areas during 1997-1998, (b) excessive precipitation in the Pacific Northwest during the 1998-1999 cold season, and (c) surplus spring and summer 1999 precipitation in the Southwest and southern half of the Rockies combined to fill reservoirs above typical historical levels, where they remained through the end of 1999. However, subnormal April-July 1999 precipitation in the northern Great Basin and interior Pacific Northwest caused significant wildfire and agricultural impacts. Less than 65% of normal precipitation was observed throughout these regions, with under 45% of normal observed in northeast Oregon and central Washington. According to the National Interagency Coordination Center (NICC), more than 1.7 million acres were charred by wildfires in the western Great Basin during 1999, which was 65% more than the combined totals of the previous five years. During one 10-day period in August, more acreage was consumed by wildfires in this region than during any previous full year on record. In addition, the dryness impacted the planting and development of winter wheat.

Most locations across Hawaii experienced a second consecutive year with subnormal rainfall, although totals were generally greater than those measured in 1998. Rainfall in 1999 was near- or above-normal across much of Kauai, west-central Oahu, the higher elevations of eastern Oahu, northwesternmost Maui, and eastern Hawaii Island. However, most areas were drier than normal, and totals in extreme southeastern Oahu, central and southern Maui, most of Molokai and Lanai, and western sections of Hawaii Island were only 25% to 50% of normal. Rainfall for 1998-1999 combined was exceptionally low at a number of locations, including:

These deficits resulted in a variety of agricultural impacts, and an active year for wildfires; however, heavier precipitation in 1999 than in 1998, particularly late in the year, were slowly mitigating the effects of the drought as the year ended.

USDA’s national annual agricultural assessment for 1999 depicted significant production and quality reductions for a number of crops, and the various drought conditions that affected the country contributed to these occurrences. Of the major U. S. crops, total national corn production was down 3.2% from 1998, soybeans were down 4%, and the combined spring and winter wheat crops were down 9.6% to the lowest level since 1996. As November ended and winter wheat progressed toward dormancy in typical fashion, about 43% of the crop was reported in good or very good condition, compared to an average of 75% in late November during 1996-1998. In addition, tobacco production dropped 14% from last year to its lowest level since 1995 while the barley and oat crops were 29% and 33% below the 1990-1998 average, respectively. On statewide bases, some crop production drops were even more dramatic. Missouri and Ohio, major producers of both corn and soybeans, each reported 13% to 16% reductions in the output of both crops from last year. The corn crops in Pennsylvania and Maryland (both lesser but significant producers) declined 47% and 23%, respectively. Combined spring and winter wheat output in Kansas and Oklahoma declined 13% and 24%, respectively, while reductions of 21% and 40% were reported in Washington and Oregon. Georgia peanuts were down 6% from last year, and tobacco output dropped sharply in Kentucky (9%), Virginia (9%), South Carolina (15%), and Georgia (29%). Furthermore, hay production declined 16% to 31% from 1998 levels in several states from the middle Mississippi Valley eastward to West Virginia and Pennsylvania.

The various drought conditions that affected the country in 1999 also contributed to above-average wildfire activity. According to NICC, almost 5.7 million acres were consumed by wildfires in 1999, which was 65% above the average of the previous nine years and the highest total since 1996. In addition to the previously-discussed fires in the western Great Basin, 1999 was also an active fire year in Alaska, where over a million acres were scorched (nearly twice the 1994-1998 average), and across the southeastern quarter of the country, where the 962,000 total acres consumed was 26% above the 1994-1998 average.

Drought in the U. S. during 1999 also engendered a variety of additional impacts. Streamflows were often below the 10%ile levels during summer in the mid-Atlantic and Northeastern states, and throughout the last half of the year in most other areas east of the Mississippi River and south of the Great Lakes, according to the U. S. Geological Survey (USGS). Water supplies dropped to alarmingly low levels at times along the Eastern Seaboard and in parts of the Ohio Valley, requiring some local and state governments to implement mandatory water use restrictions, and both groundwater near the Chesapeake Bay and total inflow to the Bay dropped to near-record low levels in late summer before conditions improved. Furthermore, drought may have contributed to the first outbreak on record of West Nile Fever in the United States, which occurred in the Northeast during the summer.