California and National Drought Summary for May 15, 2018
May 17, 2018 - During the 7-day period ending Tuesday morning, areas of locally heavy rain provided drought relief from the Plains to the East Coast, though much of the Southeast was dry. Toward the end of the time frame, an influx of tropical moisture associated with a slow-moving disturbance generated heavy to excessive rainfall in Florida, with rain associated with this broad area of unsettled weather overspreading the Southeastern and Mid-Atlantic States after the data cutoff for this week’s analysis; any rain that falls after 12z Tuesday (8 a.m., EDT) will be incorporated into the following week’s drought assessment. In contrast, dry, hot weather maintained or exacerbated drought from the southern High Plains into the Southwest. Likewise, despite the generally unsettled weather pattern, pockets of dryness and drought lingered or intensified in the Upper Midwest and northern Plains.
Generally dry conditions in northern portions of the region contrasted with locally heavy rain farther south. Short-term dryness has begun to develop over southern portions of Vermont and New Hampshire, with some locales reporting locally less than 50 percent-of-normal rainfall over the past 60 days. If rain does not materialize soon, these areas will be primed to slip into D0 (Abnormal Dryness). Across the southern third of the region, an increasingly active weather pattern brought a large area of moderate to heavy rainfall (1-3 inches, locally more), easing lingering D0 across central and southern Maryland.
A pronounced weather pattern change began to unfold during the period, with the leading edge of soaking rainfall arriving in Florida while the rest of the region remained mostly dry. A sub-tropical low developed in the eastern Gulf of Mexico and drifted north, pulling copious tropical moisture and associated heavy downpours northward into the region. As of Tuesday morning (12z, or 8 a.m., EDT), 7-day rainfall totals topped 2 inches (locally more than 4 inches) across much of southern Florida, resulting in widespread reductions of drought intensity and coverage. Conversely, rain had yet to reach the remainder of the Southeast as of Tuesday morning, though heavy showers from this system accelerated rapidly over the Carolinas and Virginia as of Wednesday. As a result, the remainder of the East Coast States were kept status quo, pending the placement and intensity of rainfall from the system advancing currently over the region. Farther west, Moderate Drought (D1) was expanded across central and northeastern Alabama to reflect the increasingly dry conditions noted over the past 60 days (locally less than 50 percent of normal) as well as declining streamflow levels.
Spotty heavy downpours brought localized drought relief to portions of Oklahoma and Texas, while dry, warmer-than-normal weather prevailed across the rest of the region. Locally heavy showers (2-4 inches) dotted drought areas from central Texas northward into western Oklahoma. Where rain was heaviest, reductions in drought intensity and coverage were made; however, considerable longer-term deficits remained, with the remaining Exceptional Drought (D4) areas reporting less than 25 percent of normal precipitation (locally less than 10 percent) over the past six months. Along the Gulf Coast, Abnormal Dryness (D0) was expanded eastward to reflect increasingly dry conditions over the past 30 to 60 days, while D1 and D2 were added in coastal locales where 90-day rainfall was less than half of normal. Farther inland, Moderate to Severe Drought (D1 and D2) were expanded in western Texas to reflect increasingly dry conditions at both the shorter term (60-day rainfall averaging 10 to 25 percent of normal) and longer time frame (6-month precipitation averaging less than 50 percent of normal, locally less than 25 percent).
Warm, wet weather in southern portions of the region compared with dry albeit cooler conditions farther north. A wide area of moderate to heavy rain (an inch or more) was observed from Missouri and Iowa eastward into the lower Great Lakes Region. Rainfall locally topped 2 inches, resulting in modest reductions in Abnormal Dryness (D0) and Moderate Drought (D1) across Missouri, Iowa, as well as northwestern Minnesota. However, the heaviest rain largely missed the lingering core drought areas, so overall the central Corn Belt’s drought depiction remain largely unchanged. Farther north, increasingly dry conditions over the past 60 to 90 days (25-60 percent of normal) led to an expansion of D0 and D1 in northern portions of Minnesota and Wisconsin as well as the upper peninsula of Michigan, particularly in areas which reported less than 0.5 inch of rain during the 7-day period.
The overall trend toward improving conditions in the south contrasting with increasingly dry weather in the north continued. From northeastern Colorado into central and southern Kansas, areas of moderate to heavy rain (1-3 inches) netted reductions in drought intensity and coverage. The most significant improvements were made in south-central Kansas, where a large area of 2 to 4 inches of rain (locally more) fell on areas of Severe (D2) to Extreme (D3) Drought. Moderate to heavy rain (1-3 inches) was similarly beneficial in northeastern Colorado, trimming the aerial extent of Abnormal Dryness (D0) and Moderate Drought (D1). Farther north, outside of locally beneficial downpours (1-3 inches) in northwestern South Dakota, acute short-term dryness over the past 30 days resulted in expanding D0 across southwestern South Dakota, southeastern North Dakota, and northeastern Montana, while a more prevalent dry signal over the past 90 days (50 percent of normal or less) led to expanding D1 and D2 in northeastern North Dakota.
utside of beneficial rain and high-elevation snow in the north, pronounced short- and long-term dryness led to drought intensification and expansion across the Southwest. In southern Idaho, Abnormal Dryness (D0) was removed from most locales north of the Snake River, as recent rain and snow have pushed Water Year (to date) precipitation at or above the 35th percentile. However, sub-par seasonal precipitation coupled with increasingly dry conditions over the past 30 days led to the expansion of D0 in northeastern Montana and Moderate Drought (D1) in southwestern Idaho. Abnormal Dryness was expanded westward across central and northern portions of California’s Coastal Range due to a sub-par water year coupled with increasingly dry conditions over the past 30 days (deficits of 2 inches or more, locally less than 10 percent of normal). Farther south, Severe, Extreme, and Exceptional Drought (D2-D4) were increased from southern California into much of New Mexico. Changes were most pronounced in eastern portions of the region, where Water Year precipitation was in the 2nd percentile or lower, particularly across the northern third of New Mexico. Satellite-derived vegetation health data indicated conditions have deteriorated rapidly across the region, with the worst vegetation indices with respect to normal noted from Arizona southeastward into southern New Mexico. Drought Monitor authors are in close contact with local and regional experts from the Southwest, and further detailed analysis will likely result in additional intensification and expansion of drought as the situation is assessed over the upcoming weeks.
Alaska, Hawaii, and Puerto Rico
There were no changes made to areas outside of the contiguous U.S. over the past 7 days. On Hawaii, northeast- and east-facing (windward) slopes were seasonably wet, while leeward areas have been generally dry. However, the leeward dryness has not yet reached a point to warrant D0. There were no changes made this week to the Abnormal Dryness depiction in Alaska. Puerto Rico remained free of Abnormal Dryness or drought.
An active pattern will continue, with three significant areas of wet weather over the next 5 days. Moderate to heavy rain will continue to soak locales from the Appalachians to the East Coast, with totals approaching or topping 5 inches in parts of the Mid-Atlantic. Meanwhile, an area of low pressure will develop over the middle part of the nation, triggering moderate to heavy rain (locally more than 2 inches) from the central Plains into the Dakotas, while a trailing cold front producers similar rainfall amounts over north-central Texas and environs. Finally, a pair of upper-air disturbances will generate periods of rain across the northern two-thirds of the western U.S. Despite the unusually active pattern, dry weather will prevail from central and southern California into the lower Four Corners. The NWS 6- to 10-day outlook for May 22 – 26 calls for below-normal rainfall across central and southern Texas and from the Great Lakes into New England. Conversely, wetter-than-normal conditions are expected from northern portions of the Rockies and Great Basin to the central and southern Atlantic Coast, with the greatest likelihood for abnormally wet weather in the Southeast. Abnormal warmth is expected over most of the nation save for near-normal temperatures in California and New England.
Eric Luebehusen, U.S. Department of Agriculture