April-June 2005

Latest Seasonal Assessment - Drought has rapidly worsened across the Northwest from Washington and Oregon eastward to Montana, as mountain snowpacks have dropped to record or near-record lows across the region. Although forecasts for the last half of March show a promising tendency for increased precipitation, especially in western parts of Washington and Oregon, it is very unlikely that the Northwest will see significant improvement in the hydrological drought picture this late in the wet season, given the difficulty of making up the deficits following four consecutive months of below-normal precipitation. Spring-summer water supply outlooks made in early March would place this year's flows among the bottom one or two of the last 70 years in the Pacific Northwest. Storms have continued to ease drought across the Southwest and Great Basin, and the latest seasonal drought outlook calls for additional improvement for lingering drought areas. Melt from the extraordinary mountain snowpack in this region will boost streamflows this spring, improving reservoir supplies. Although even the larger reservoirs will benefit from this past winter's prolific snows, one season will not be enough to bring full recovery to the largest reservoirs, such as Mead and Powell on the Colorado River. In the northern High Plains, including the upper Missouri River basin, below-normal mountain snowpacks will mean that hydrological drought will be ongoing, although late winter storms and spring rains should offer limited improvement by benefiting soil moisture. In the South, spotty dry areas resulting from below-normal rainfall since October are not expected to develop into large-scale drought. Dryness that has recently developed in Puerto Rico will likely grow somewhat worse in coming weeks. The onset of the normal seasonal rains that begin in April-May will have a significant impact on whether dry conditions intensify. Forecast models for April-June suggest the dryness will not persist.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for April-June, the drought termination and amelioration probabilities for June, various medium and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the soil moisture tools based on the GFS model and the Constructed Analogues for the season, and the latest western water supply forecasts.

The current drought outlook focuses on the prospects for the drought in the Northwest. There has been a rapid deterioration in the drought picture in recent weeks from Washington and Oregon eastward through Idaho and Montana, as the cumulative precipitation deficits increased and snow water content reached record or near-record lows. Thirty-day precipitation amounts through mid-March have been less than 25 percent of normal across Washington, Oregon, and northern Idaho. Dozens of SNOTEL sites show record low snow water content in Washington, Oregon, Idaho, western Montana, and northwestern Wyoming. Offical streamflow forecasts for this spring and summer prepared on March 1 project 25 to 49 percent of normal flows for large parts of the Pacific Northwest. Given the lateness in the snow season, it would be extremely difficult to make up for the snowpack deficits already incurred, so the water supply picture this spring and summer is looking dismal. The good news is that the weather outlook has improved over the short and medium range as well as the seasonal range. For the latter period, the various forecast tools no longer show a consensus for below normal precipitation for the next 3 months, resulting in the official CPC April-June outlook depicting equal chances for wet or dry. Over the next 2 weeks, the high pressure ridge responsible for the abnormal dryness is forecast to retrogress westward toward Alaska, and even continue westward near Siberia, while a jet stream tracks across the Pacific Ocean, bringing a series of storms toward the U.S. West Coast. As a result, the March 15 CPC forecast shows above-normal precipitation over the Northwest for both the 6-10 day and 8-14 day periods.

The new Drought Outlook only shows some improvement in the areas expected to see the highest precipitation totals in the next 2 weeks, although the entire region will see a pickup in rain or snow. Hydrologists believe that it is too late in the season for rain or snow to offset in any meaningful fashion the snowpack deficits already incurred, so even above-normal precipitation in the next 2 weeks followed by a more normal pattern during April-June will not make a material difference in the pessimistic water supply outlook for the Northwest. Indeed, drought will likely get worse from the water supply viewpoint, despite attempts by some reservoir managers to store more water than normal in anticipation of poor inflows later in the season. For farmers and ranchers not dependent on irrigation water, any increase in rainfall would be welcomed, so late-season precipitation is not without some benefit.

In the northern Rockies and northern High Plains, the drought outlook is somewhat more optimistic than last month despite recent deterioration in the Dakotas and Montana, with limited improvement shown for this region. Below-normal snowpack in the headwaters of the Missouri River Basin should result in ongoing hydrological drought this spring, but a return to a more normal precipitation pattern would benefit ranchers, farmers and firefighters. The seasonal forecast models show a tendency toward wetter and cooler prospects than last month, although it is believed that the CFS model has a wet and cool bias in the recent runs, possibly because of unrealistic initial soil moisture input. Even discounting this model, there are few if any forecast tools over the seasonal time frame that depict dry and warm conditions for the Plains, with a consensus for wetness for the southern Plains and east of the drought area in the Great Lakes region.

In the Southwest, the Drought Outlook map emphasizes the optimist streamflow forecasts and the impending improvement in the water supply picture. However, it should be noted that much of the desert Southwest is heading into the dry season, so actual soil moisture will drop as evaporation increases. The latest CPC precipitation outlook for April-June shows a tilt toward wetness in New Mexico, and that could result in further drought improvement in areas with lingering concerns.

Some of the areas of dryness in the South shown in the latest Drought Monitor will be seeing near-term relief as the storm system that brought up to 34 inches of snow to New Mexico on March 14-15 dumps heavy rain over Florida and the southeast Atlantic coast. Given the seasonal outlook for below-normal rainfall in Florida, there is the potential for drought development there, but the heavy rains forecast in the next several days have offset drought concerns for the time being.

Below-normal rainfall since November, and especially in recent weeks, has resulted in drought development for parts of southern Puerto Rico. This is still the dry season there, so dryness will likely worsen over the short term. Normal or above-normal rain during the upcoming rainy season, which starts in April-May, would bring relief. Both the IRI Multi-Model outlook for April-June and the Scripps GSM model show above-normal rainfall for the eastern Caribbean region, including Puerto Rico, so dryness is expected to ease by June. Because of the small size of the new drought area and the expectation that the drought will not expand once the rainy season gets underway, the area of D1 drought recently introduced in the Drought Monitor is not shown on the Seasonal Drought Outlook map.