Tools used in the U.S. Monthly Drought Outlook (MDO) included the official Climate Prediction Center (CPC) updated (0.0 month lead) temperature and precipitation outlooks for May 2016, various short- and medium-range forecasts and models such as the 7-day precipitation totals from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC Extended-Range Forecasts (ERFs), Weeks 3 and 4 experimental outlooks, the NAEFS precipitation outlooks, dynamical models including the CFSv2 and GFS, climatology, and initial conditions.

According to the National Weather Service's Advanced Hydrologic Prediction Service (AHPS), notable precipitation deficits (2-6 inches, locally greater) have mounted across much of the southern and central Appalachians, and mid-Atlantic during the past 90-days. These same regions experienced deficits of 1-3 inches during the past month. As of April 24, the North American Land Data Assimilation System (NLDAS) depicts top 1-meter soil moisture percentiles in the lowest third of the historical distribution during the past month. In parts of West Virginia, and southwestern and central Pennsylvania, the soil moisture percentiles range within the lowest 10 percent of the historical distribution. Though increasingly dry conditions were beginning to produce some impacts (such as the Shenandoah National Forest wildfire which burned in excess of 10,000 acres), impending drought conditions over a large area appear to have been offset, or at least delayed by the arrival of a cold front that stalled across the mid-Atlantic on April 28-29. This front, in addition to a surface low pressure trough that extended across the central and northern Appalachians, brought substantial rainfall (0.5-2.5 inches) to the region. It is difficult to get a persistent, firmly established drought across the mid-Atlantic during the springtime, because of the frequent passage of low pressure systems that typically occur during this season. The southern Appalachians, however, missed out on this rainfall, and it is easier for this more southern location to experience longer-lasting drought conditions. During the first few days of May, moderate rain (0.5-2.0 inches) is predicted for the central Appalachians/mid-Atlantic region, and about 1.25 inches is expected for the southern Appalachians. For Week-2, and Weeks 3 and 4, odds favor below-median rainfall across these areas. The monthly update for May favors Equal Chances (EC), which acknowledges the wetness early in the month, followed by a drier pattern later in the month. Drought removal is favored across all areas, especially the central Appalachians and mid-Atlantic.

Forecast confidence for the mid-Atlantic and central Appalachians is moderate. Confidence for northeast Georgia is considered low to moderate.

During the past 30- and 90-days, precipitation across most of the Great Plains has been above-median, and the upcoming May-July season is climatologically the wettest time of the year for the Plains. In nearby Missouri, however, abnormally dry conditions have prevailed during the past 90-days, especially across central and southern portions of the state. In the first week of May, the more substantial precipitation amounts are predicted across the southern Great Plains (1.5-2.5 inches), and the central High Plains (1-2 inches). For the remainder of May, odds favor above-median precipitation, and therefore drought removal for most areas. Persistence of drought is favored in northwestern Montana and in Missouri.

Forecast confidence for the Great Plains (and Missouri) is considered moderate.

Though observed precipitation during the last 90-days across the northern and central portions of both the Rockies and Intermountain West was highly variable in coverage, southern portions of those areas were decidedly below-median. Practically all of Arizona and far western New Mexico did not receive the quantity of beneficial precipitation so typical of strong wintertime El Nino events. During the past month, below-median precipitation was noted across the Northern Intermountain Region, and much of the Colorado Plateau. For the first week in May, moderate precipitation (0.5-2.5 inches) is expected across much of the Intermountain Region and the Rockies. There are elevated odds of upper-tercile precipitation for most of the region during the second week of May, Weeks 3 and 4, and for the updated 30-day outlook. Improvement/Removal of drought is predicted for most areas, but drought is likely to persist in Arizona.

Forecast confidence in the Rockies and Intermountain West is moderate.

Observed precipitation in Washington state during the past three months was generally above-median, though for most of Oregon and California, below-median precipitation amounts were more commonplace. This relatively wet season for Washington state is also very unusual for a strong wintertime El Nino, during which the subtropical jet stream and associated cyclonic activity is typically confined to the southern tier of the U.S. However, during the past month, Washington received only 25-50 percent of normal precipitation, as did Oregon and parts of northern California. Interior portions of far southern California ended up with well above-normal precipitation (150-400 percent of normal), thanks to late-in-the-season storm activity associated with the fading El Nino. Up to a half-inch of precipitation is expected across portions of the Pacific Northwest and the California Sierras. Though wetter-than-normal conditions are favored for Week 2, Weeks 3 and 4, and the updated monthly precipitation outlook, the monthly drought outlook for May is tempered by May climatology, which depicts dryness for California and much of Oregon. It is thought that even if above-median precipitation verifies across these states, it will not be enough to warrant drought removal or improvement.Therefore, drought persistence is forecast.

Forecast confidence for the West Coast states is moderate.

There is no drought at this time in Alaska.

With the seasonal fading of one of the strongest El Nino's ever recorded,  comes a return of the more persistent trade wind regime to the Hawaiian Islands. Windward (east and northeast-facing) slopes are expected to receive ample precipitation during the month of May, leading to either drought improvement or removal across southeastern parts of the Big Island. Drought conditions are favored to continue across the leeward slopes.

Forecast confidence for Hawaii is considered moderate to high.

As Puerto Rico emerges from its traditional dry season, rainfall is expected to increase. Therefore, drought improvement/removal is predicted across Puerto Rico in May.

Forecast confidence for Puerto Rico is moderate.


Forecaster: Anthony Artusa

Next Monthly Drought Outlook issued: May 31, 2016 at 3:00 PM EDT