U.S. Drought Outlook: Improvements Expected Across the Plains04/22/2016
As the wet season nears its end in the West, coincident with the waning El Niño, a return to climatologically dry conditions is favored across the region. Though the final drought-related statistics for California are still being determined, it appears that northern portions of the state fared well this past winter season in such areas as precipitation, snowpack, and reservoir levels.
Southern California did not fare as well, despite the presence of one of the strongest El Niño’s on record. Late season precipitation was indeed welcome in southern California, but unfortunately, significant moisture deficits remain.
Much of the Desert Southwest also missed out on anticipated El-Niño related precipitation this past winter. During the next two weeks, above-median precipitation is favored across portions of California, the Great Basin, and the Southwest. However, the heaviest precipitation is expected to occur outside the ongoing drought areas.
For the upcoming May-June-July (MJJ) season, drought persistence is forecast for most areas west of the Continental Divide. For the Great Plains and Middle Mississippi Valley, however, MJJ is climatologically a wet time of year, due to the passage of frontal systems and nocturnal thunderstorm clusters (MCS’s).
About one-half of the annual precipitation that falls over the lower Plains typically occurs during MJJ, and approximately one-third of the annual precipitation that falls over the High Plains usually occurs in MJJ. Precipitation outlooks out to 90-days in the future also favor a relatively wet pattern. Based on these factors, a one-category improvement in drought conditions is expected across the Great Plains, the eastern foothills of the northern Rockies, and the Middle Mississippi Valley.
In the subtropical North Pacific, the climatological trade wind regime has returned to the Hawaiian Islands, after some disruption associated with the El Niño. This pattern change favors heavy precipitation for east- and northeast-facing (windward) slopes, and drought removal. Leeward slopes, situated in the rain shadow areas of high terrain, are favored to experience drought persistence during the MJJ season.
At this time, there is no drought in Alaska. Across the Caribbean, the annual return of the Inter-Tropical Convergence Zone (ITCZ) and the Atlantic hurricane season signals a return of the wet season. A one-category improvement in drought conditions is therefore predicted in Puerto Rico.
Forecast confidence for The West is moderate to high.
- Across the West, the wet season and the El Niño are beginning to wind down. During the past 30-days, AHPS Percent of Normal Precipitation (PNP) values range from 25-75 percent in Utah, 10-75 percent in Arizona, 150-400 percent in southwestern New Mexico, 25-75 percent in southern and western Nevada, 10-75 percent in California, and 25-75 percent in southern interior Oregon.
- According to the Western Regional Climate Center in Reno, Nevada, SNOTEL basin-wide Snow Water Content (SWC) values range from about 50-75 Percent of Average (POA) in Utah, less than 50 POA in Arizona and southern New Mexico, 75-90 POA in northwestern Nevada and the California Sierras, and 50-75 percent in southern interior Oregon.
- For the upcoming 7-day period, moderate to heavy precipitation (0.5-3.0 inches) is expected across northern portions of California (including the Sierras), 0.25-0.50 inch for northwestern Nevada and the southern interior of Oregon, with little to no rainfall forecast across remaining portions of the Western drought area.
- The Week-2 precipitation outlook from CPC favors a tilt in the odds towards above-median precipitation, while the Weeks 3,4 outlook favors above-median precipitation for the Four Corner states, and EC elsewhere. The May 2016 and May-Jul 2016 precipitation outlooks favor above-median precipitation across much of this region.
- Though above-median precipitation is favored for most time periods out to 90-days, it is not expected to be enough to warrant at least a one-category improvement in the U.S. Drought Monitor depiction. One possible exception is the southern portion of the Persistence area in Utah, which may get enough rainfall to justify a one-category improvement.
- The northern portion of this Persistence area is also less likely to receive significant monsoonal moisture in July, increasing support for drought persistence.
Forecast confidence for the south-central Plains is moderate to high.
- MJJ climatology favors wetness across the south-central Plains, extending into Missouri. Approximately 30-45 percent of the annual precipitation received in this region usually falls during the MJJ season. This is also attributed to, in large part, to passing low pressure systems and MCS’s.
- Up to an inch of rain is forecast during the next 7-days across this area, with Week-2 and Weeks 3,4 precipitation outlooks tilting toward the wetter-than-median tercile.
- Probabilities for above-median precipitation are higher across the south-central Plains than they are for the northern Plains. CPC’s monthly and seasonal outlooks also favor above-median precipitation.
- Based on the above factors, a one-category improvement is anticipated, warranting drought removal for nearly all areas.
Forecast confidence for the Northern Plains is moderate.
- The May-July season is climatologically a wet time of year for the northern Great Plains states.
- The Dakotas typically receive about 40-50 percent of their annual precipitation during the late spring and early summer. This is due, in large part, to passing low pressure systems and overnight thunderstorm clusters (MCS’s).
- For the northern High Plains and adjacent eastern foothills of the Rockies, the climatological percentages for MJJ are less, ranging between 20-40 percent.
- The North American Land Data Assimilation System (NLDAS) ensemble mean soil moisture anomalies (current, top 1-meter of soil) in North Dakota and northern South Dakota range from 1-3 inches below normal.
- Up to 1-2 inches of precipitation is forecast across this region during the next 7-days. The Week-2 and experimental Weeks 3 and 4 precipitation outlooks indicate the best chances for above-median precipitation is for southern portions of this region.
- CPC’s 30-day outlook for May, and 90-day outlook for May-July, do not favor any particular category, so Equal Chances (EC) is indicated.
- Considering these factors, a one-category improvement is forecast in this region, resulting mostly in drought removal.
Forecast confidence for Hawaii is moderate.
- The return of the trade winds and the resulting shower activity to the east- and northeast-facing slopes of the Hawaiian Islands signals a more typical circulation pattern, now that the El Niño is fading away.
- Any residual areas of moderate drought (D1 on the U.S. Drought Monitor depiction) are ripe for removal along the eastern slopes.
- Drought persistence is predicted along the lee (western) slopes of the Hawaiian Islands.