February - April 2007

Latest Seasonal Assessment - Storms bringing rain, ice, and snow created much havoc across the Plains states from the end of November into January, but they have also provided long-term benefits for areas experiencing dry soils and low water levels. Looking toward early spring, the Drought Outlook depicts continuing improvement for remaining drought in the central and southern Plains, but a tendency for drought to persist in the northern Plains, especially from Montana to Minnesota. Drought is also expected to largely persist over western Wyoming. In the Southwest, the storms have largely bypassed southern and central California and Arizona, leading to very dry conditions and the expansion of drought into southern California. With the spring and summer streamflow forecasts indicating below normal conditions in most of Arizona, Nevada, and California due to low mountain snow pack, the confidence for major relief from this season’s dryness has diminished. Nevertheless, El Niño-related precipitation may bring some improvement by the end of April. The Florida peninsula has also seen below-normal rainfall since autumn, but drought improvement is expected there. Most of Hawaii has been abnormally dry, and the odds continue to favor drought development across the island chain.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for February-April, 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.

The latest CPC February-April precipitation outlook continues to indicate 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 lingering droughts in the central and southern Plains and in Florida. In the Southwest, however, the season into mid-January has been unusually dry, and mountain snowpacks in Arizona, California, and Nevada were generally around one-half of normal. As a result, the spring-summer streamflow forecasts as of January 1 prepared by USDA indicated flows less than 70% of normal for some basins in the region, and a few less than 50%. With the El Niño expected to enhance the odds for wetness across the Southwest during February-April, some relief is expected. The forecast was changed to a less optimistic status of “some improvement” rather than “improvement” because of 1) the shortened window for improvement as the season marches into late January with little sign of needed precipitation and 2) a number of forecast models (UKMET, CFS, CAS) indicate below-normal precipitation for California and Arizona for February-April. With Sierra snow pack around one-half normal and the upper air ridge setting up in the medium range, late-season snows may become very important across the region.

Elsewhere, confidence for improvement is relatively high for the Texas area based on recent trends and forecasts for additional storms early in the period. As is usually the case, forecast confidence diminishes to the north, but the odds for drought-easing precipitation increase by the end of April due to the typical increase in precipitation at that time. Even a weakening El Niño should help to tilt the odds away from drought for most of the Plains. To the north, however, better odds for improvement should come later in the spring, and the drought is expected to largely persist from Montana into Minnesota and northwestern Wisconsin into April. The CAS soil moisture forecast for late April and some El Niño analogues suggest that drought relief may be difficult in this region. The Palmer probability data show that significant relief may be especially difficult in northern Minnesota.

As in the Southwest, rainfall in Florida has been surprisingly low given the ongoing El Niño. Some seasonal models continue to suggest that dryness could persist, especially across the southern peninsula, but medium-range forecasts show that the long-expected change to a wetter pattern could be developing during the last half of January. Improvement is indicated for Florida, but the confidence is not high.

The Outlook continues to call for drought development across Hawaii, due to El Niño-related forecasts of below-normal rainfall through April and beyond. Abnormally dry conditions had developed by mid-January across all of the islands.