Minimal drought coverage continued across the contiguous United States throughout April. Remaining areas of drought across the Four Corner’s region, extreme southwestern Wyoming, and southwestern Texas are forecast to either improve or be removed by the end of May 2019, despite May typically being a climatologically dry month for many of these areas prior to the onset of the seasonal summer monsoon.
Other areas of lingering drought in north-central portions of Washington and Wyoming are forecast to persist, due to insufficient precipitation predicted during May. Climatology and precipitation outlooks at most time ranges out to a month in advance tilt the odds towards drought development in western and central Washington, though precipitation amounts that fell in April will have to be offset first.
The May Drought Outlook for the Southeast is perhaps the most challenging part of this Outlook. Although a majority of precipitation outlooks out to a month in advance favor slightly above normal rainfall across this region, it does not appear to be enough to outweigh longer-term deficits. Therefore drought persistence is depicted.
There is a slight tilt in the forecast precipitation probabilities (considering all relevant time-ranges) that drought could develop in eastern Georgia and southern portions of South Carolina by the end of May.
Persistence of drought is favored for the southern Alaska Panhandle, which is heading into a climatologically drier time of year. Low reservoirs are anticipated to continue to have an impact on hydropower generation in the region. Hawaii is also entering a drier time of year, and the seasonal increase and consistency of easterly trade winds favor heavy precipitation along the windward (east or northeast-facing) slopes, but accentuates the rain shadow effect to the lee of the mountains.
Accordingly, drought persistence is expected for the leeward slopes of Hawaii. Drought persistence is also favored in interior portions of central Puerto Rico. Whatever rainfall the Commonwealth receives in May is not expected to be enough to offset 60-day rainfall deficits (4-8 inches). Drought development was not introduced since it is early in the rainy season.
Forecast confidence is high for continued improvement/removal for the southern Rockies/Four Corners region, and moderate for Washington state.
- The Advanced Hydrologic Prediction Service (AHPS) Percent of Normal Precipitation (PNP) values for the past 30-days range from 200-400 percent of normal in Montana and New Mexico, down to 5-25 percent of normal in Arizona, southern Utah, and central and southern portions of California, including neighboring portions of Nevada.
- SWC is well above average in the California Sierras (ranging from 175% to 245%), above average in the Wasatch Mountains (115% to 144%), and below average in the Cascade Mountains (60% to 91%).
- Across the West, 28-day streamflows from the U.S. Geological Survey depict values in the lowest quartile of the historical distribution for this time of year in western Washington, far southern California, central Arizona, and in other very scattered locations.
- May precipitation climatology favors increasing dryness across the West Coast states and the Southwest (pre-monsoon), and relatively wet conditions in Montana. Precipitation outlooks for the next 30-days favor relatively wet conditions across the Four Corners drought area (including southeast New Mexico), and relatively dry conditions for north-central Washington.
- Accordingly, drought conditions are likely to be improved or removed across the southern Rockies/Four Corners region during May, though drought conditions are likely to persist and expand in Washington state.
Forecast confidence is high for most of the High Plains region, though low for northern North Dakota.
- The High Plains region is mostly drought-free, though there are two small areas of moderate drought (D1 on the U.S. Drought Monitor) currently indicated in Wyoming; one in north-central Wyoming, and the other in extreme southwestern Wyoming.
- For the first area (Big Horn Mountains), SWC is only about 70 percent of average, while for the second area (extreme southwestern Wyoming), SWC is near 110 percent of average.
- Precipitation for nearly all time-scales out to a month in advance is expected to be above normal. These precipitation outlooks and May climatology favor the removal of drought in southwestern Wyoming, but it will be harder to erase the drought in north-central Wyoming because of the lower SWC and related longer-term hydrological impacts.
- Elsewhere, above average precipitation this winter resulted in major flooding along the banks of the Middle and Lower Missouri River. Drought development is unlikely by the end of May since soil moisture ranks above the 90th percentile from South Dakota into Kansas.
- Although development is not anticipated across northern North Dakota by the end of May, this region will be closely monitored later this spring since 30-day precipitation has averaged between 50-75 percent of normal.
Forecast confidence is moderate to high for the South.
- Only three small areas of moderate drought remain in the South, all in southern and southwestern Texas.
- May climatology, a relatively wet April, soil moisture ranking between the 80th and 85th percentiles, and above normal precipitation forecast at most time ranges out to 30-days in advance, all point to drought removal in this region.
Forecast confidence is high for the Midwest Region.
- Widespread flooding continues across parts of the Midwest region, especially along the banks of the Mississippi River and the Red River of the North.
- During the past 60 days, precipitation surpluses range from 2 to 6 inches throughout much of the Midwest with soil moisture ranking in the 95th-99th percentiles for this time of year.
- Based on these wet initial conditions, and the expectation of near to above-normal precipitation at all timescales out to 30-days, drought development is unlikely during the outlook period.
Forecast confidence is moderate for much of the Southeast Region; but low along and near the South Atlantic Coast.
- Abnormal dryness (D0) and moderate drought (D1) affected the area from Alabama to South Carolina during April. Sixty-day precipitation deficits range from 2 to 4 inches in these D0 and D1 areas, and 28-day streamflows fall within the lowest quartile.
- May precipitation climatology favors a slight tilt toward dryness in this region. Climatologically, sea-breeze induced and/or pulse convective activity in this area ramps up significantly by about mid-June, which is beyond the range of this drought outlook.
- Most precipitation outlooks during the next few weeks to a month out favor a slight tilt toward above normal rainfall. Despite this, the predicted precipitation amounts are not expected to be enough to erase longer-term deficits.
- Therefore, drought persistence (and even some development) is considered more likely.
Forecast confidence is moderate to high for the Northeast Region.
- The Northeast region has remained drought-free since early November 2018. During the last 30-days, widespread precipitation surpluses have been observed across most of the Northeast, and soil moisture ranks above the 80th percentile for nearly all of this region.
- Based on these initial soil moisture conditions, a wet April, and the absence of a dry signal for all time ranges out to the end of May, development is unlikely to occur.
Forecast confidence is moderate to high for Alaska.
- Moderate to severe drought (D1 to D2) coverage remained nearly steady along the southern Alaska Panhandle this past month. Since May is a relatively dry month, and there is not a wet signal in the constituent precipitation outlooks that comprise the monthly accumulation (including almost all of the input models that go into the North American Multi-Model Ensemble), persistence is forecast for the southern Alaska Panhandle.
Forecast confidence is high for Hawaii.
- Abnormal dryness (D0) and moderate drought (D1) returned to Hawaii during the last 30 days. The North American Multi-Model Ensemble indicates below normal precipitation during May, and the archipelago is entering its dry season with a weak El Niño in progress.
- These factors support drought persistence for the leeward slopes of the Hawaiian Islands.
Forecast confidence is moderate for Puerto Rico.
- Drought coverage dropped from 30.38 percent in late March to 12.61 percent in late April across Puerto Rico. The large reduction in drought and abnormal dryness occurred over the western third of the Commonwealth in early to mid-April, as heavy rains affected that area.
- In contrast, conditions have been deteriorating farther east. Sixty-day precipitation deficits range from 4 to 8 inches in the interior portion of central Puerto Rico, and dryness/drought is expanding towards the southeast coast.
- Aquifers in eastern parts of the south coast of Puerto Rico (Guayama-Salinas and Santa Isabel areas) are reaching critical levels, according to the Department of Natural Resources. Although climatology becomes increasingly wet during late spring, it may not be enough to erase the longer-term deficits noted above. Therefore, drought persistence is expected in Puerto Rico.