January - March 2007

Latest Seasonal Assessment - Above-normal El Niño-related precipitation is forecast to bring drought improvement to the Southwest, southern and central Plains, Florida and southern Georgia by the end of March. Farther north, the major winter storm crossing the Plains on December 20-21 was bringing significant relief from Colorado into Nebraska, with over a foot of snow in some locations. More limited improvement is expected farther north for the forecast period and, for most of the northern Plains, including the Dakotas, drought should largely persist through March, a period that typically features low precipitation relative to other times of the year. The December Plains storm, however, was expected to bring some improvement to northern parts of Wisconsin and Michigan, as well as parts of Minnesota. Montana and northern and western Wyoming experienced some drought reduction during November-December, but the latest January-March outlook indicates a tendency for below-normal precipitation for much of this area, resulting in an outlook for persisting drought. In Hawaii, an enhanced risk for drought development continues across the islands due to the continuing El Niño. Abnormally dry conditions had already developed on the Big Island and Kauai by mid-December.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for January-March, the four-month drought termination and amelioration probabilities, various 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.

The latest CPC January-March precipitation outlook indicates an El Niño-related tendency for above-normal precipitation from southern California across the southern Plains into Florida and up the southern Atlantic coast. As a result, the Drought Outlook continues to indicate drought improvement for the ongoing droughts in the Southwest, southern Plains, and the Southeast. This month’s outlook is more optimistic in the central Plains, now showing improvement in Kansas, northern Oklahoma, and the lingering drought area in western Missouri. Precipitation tends to increase from February into March in the central and southern Plain overall and, in addition, the January-March El Niño signal for wetness is quite strong for this region. El Niño soil moisture composites show increased moisture from February to March across the Southwest into the Plains states.

The subtropical jet has been slow to bring rain or snow to southern California and the Southwest during the current rainy season well into December, with the December 19 Drought Monitor showing expansion of drought into interior southern California. It is still believed that, based on historical El Niño’s, the jet will bring moisture to the region between now and the end of March, resulting in varying amounts of relief. However, it should be noted that a number of seasonal forecast models are showing below-normal precipitation for January-March over Arizona and southern California, including the CAS (CPC Constructed Analogue from Soil moisture), the UKMET (British model), and the ECMWF (European model). As a result, confidence for major drought relief is not as high as it could be given the ongoing moderate El Niño.

The major Plains storm bringing heavy snow to Colorado and western Nebraska and freezing rain farther east during December 20-21 is a major contributor to the indicated improvement depicted over northeast Colorado and western Nebraska. Melting of the foot or more of snow should significantly boost soil moisture, ground water, and stream flows. The long-term nature of the drought and the attendant impacts on water supplies mean that one storm will not be enough to end the drought, especially in western Nebraska. Looking beyond the storm, El Niño soil moisture composites for March, Palmer Drought amelioration probabilities, and the CAS also argue for at least some improvement by the end of March for the central Plains. To the northeast, the December Plains storm should offer some relief to the drought in northern parts of Wisconsin and Michigan, but climatology suggests little change in drought status for the Dakotas by the end of March. El Niño soil moisture composites show that drought should tend to linger into March for areas still experiencing drought in Montana and northwestern Wyoming, so persisting conditions are indicated. The January-March long-lead CPC outlook indicates the odds favor below-normal precipitation for Montana and northwest Wyoming. For the rest of Wyoming, the outlook is more uncertain, but persistence is shown for this outlook outside of southern areas. Precipitation tends to increase after March in many parts of Wyoming, upping the odds for drought relief after the end of the current drought outlook period.

An area worth watching for possible drought development extends from eastern Tennessee northeastward through West Virginia into adjacent southwest Pennsylvania. This area experienced short-term dryness into mid-December, resulting in low stream flows and dropping well levels. In addition, this area is situated along the eastern edge of the area of forecast below-normal precipitation in the long-lead monthly and seasonal precipitation outlooks. Forecasts for abundant rainfall in the first 2 weeks of the outlook period precluded indicating drought development at this time, but the situation bears watching. The dry signal shown on the monthly and seasonal precipitation outlooks for the Midwest did not result in a forecast of drought development because of above-normal moisture conditions at the start of the Outlook period.

The Outlook continues to call for drought development across Hawaii, due to El Niño-related forecasts of below-normal rainfall through March. Abnormally dry conditions had developed by mid-December across much of the Big Island and Kauai.