March - May 2007

Latest Seasonal Assessment - Short-term improvement is expected in much of California and Nevada, but cumulative rain and snow remains far behind normal for the water year starting last October, stacking the odds against receiving enough precipitation in coming months to offset the large deficits. In California, statewide water-year precipitation as of mid-February was running 55% of normal and Sierra snowpack was 48% of normal. In contrast, groundwater levels were near normal due to recent wet years and reservoir storage was at 110% of normal. The pattern was also looking somewhat wetter for the short term in Arizona, but the odds favor no significant change in drought status there. Texas should see some improvement going into May, but probably not enough to end the long-running drought there. Elsewhere, improving conditions are anticipated in the central Plains for some lingering drought in Oklahoma and Kansas, as well as Nebraska and parts of South Dakota, Colorado, and Wyoming. Improvement is on tap from northwest Wyoming into Montana, but more limited improvement is forecast for the rest of the northern Plains, with persisting drought in northern Minnesota. Florida has seen some recent heavy rains, and additional improvement is forecast there, as well as for an area of recently-developed drought in eastern Tennessee. Puerto Rico should see improvement for an area of drought in the east.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for March-May, the four-month drought termination and amelioration probabilities, various medium and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, and the soil moisture tools based on the GFS model and the Constructed Analogue on Soil moisture.

With the rapid fading of the El Niño, the odds for above-normal rainfall have likely diminished in the Southwest, making it less likely that enough rain and snow will hit the region before the dry season sets in during the spring to offset the large deficits that have accrued since the water year began in October. Cumulative precipitation and current Sierra snowpack and streamflows remain around 50% of normal despite some recent improvement. In contrast, the most recent medium-range forecast models show a series of Pacific storms affecting California during the last week of February, likely resulting in significant short-term improvement for the state, as well as adjacent areas of Nevada. Late-season storms in 1991 and 2000 did bring enough precipitation to turn drought situations around, so it can happen, but the odds are against it. In addition, a number of dynamic seasonal forecast models, such as the CFS (U.S.), ECMWF (European), and the UKMET (British), are tilting toward below-normal precipitation for March-May in Arizona and parts of California, and the official CPC long-lead outlooks for March and March-May reflect the less optimistic scenario for this region.

The forecast this month edged toward a less optimistic depiction for the remaining area of drought in Texas. Climatology through May, as indicated in the NCDC drought amelioration probability maps, favors at least some improvement for drought in Texas, and this is the depiction shown in the new Drought Outlook. Few seasonal statistical or dynamic forecasts favor wet for this part of Texas, and the official CPC March-May precipitation outlook has a wet area to the west and a dry area to the east of the drought region, resulting in equal chances wet or dry. As a result, the forecast for straight improvement indicated last month for conditions through April would appear to be somewhat optimistic going into May given the forecast tool guidance and the dissipating El Niño. Indeed, with some forecast models indicating a shift toward La Niña conditions by summer, the southern Plains region needs to be monitored closely, as this could tilt the odds toward more dryness in future months.

To the north, odds continue to favor improvement in the central Plains as seen in the drought probability charts for May and in CPC’s March and March-May precipitation forecasts, although the latter’s wet areas only include the western portions of the drought in Oklahoma and Kansas. Confidence for any forecast for this area is low, given conflicting signals for the medium range and long term. Climatology and analogues for rapidly-fading El Niño’s have the odds tilting toward drought improvement in South Dakota, Nebraska, and the lingering drought in northeastern Colorado and adjacent Wyoming. Similar reasoning applies to northwestern Wyoming and southwestern Montana, with 2-week soil moisture forecasts showing considerable improvement in both states. The consensus of the analogues and the statistical and dynamic forecasts is for persisting drought in Minnesota, although a short-term increase in soil moisture is anticipated.

Confidence is high for drought improvement in eastern Tennessee due to the wet short-term outlook, and not as high for improvement in Florida, with uncertainty surrounding the March-May outlook there. Recent trends favor continued improvement on the Florida Peninsula, and the fading El Niño analogues still favor a tilt toward wetness for southern Florida. The coming of the spring wet season supports drought improvement in Puerto Rico. In Hawaii, the demise of the El Niño lowers the odds for drought development, but the risk for drought remains elevated due to the ongoing dry conditions and forecasts from statistical and dynamic seasonal precipitation models. Looking much farther northward, there is some ongoing dryness in parts of Alaska, and forecasts are indicating below-normal precipitation in the medium range, but the March-May forecast shows equal chances wet or dry, so drought development is not expected at this time.