November 2004 - January 2005

Latest Seasonal Assessment - Powerful Pacific storms striking the West during the early part of the forecast period, especially during the last 10 days of October, will bring substantial rain and snow to parts of the region. The greatest drought benefits are expected over the western portions of the western drought region, with the October 19-20 storm dropping 1 to 3 feet of snow over the high Sierra just before the start of this outlook period, and another major storm forecast during the following week. Unfortunately, the water supply situation in drought-affected areas for most of the interior West is not expected to change significantly through early winter, and the overall drought outlook calls for only limited improvement in these areas. The ongoing multi-year drought has left major reservoirs near their lowest levels in decades, and it will take more than a few storms to recover. Also, the recent stormy circulation pattern will likely not persist through the forecast period, with a drier and milder regime expected for much of the November-January period. Looking beyond this outlook period, various seasonal forecast models, as well as historical analogues with weak to moderate El Niño events, suggest improving odds for significant rain and snow toward late winter and spring across the Southwest, including the Colorado River Basin, and that could bode well for mountain snowpack this spring.

The outlook also calls for limited improvement in the High Plains drought region, with the odds for improvement less in northeastern Wyoming and southeastern Montana, an area that is experiencing extreme to exceptional drought. Farther east, storms over the short term should ease lingering drought in the Wisconsin-Michigan area. The odds favor a return to drier weather for the Great Lakes region, as well as southward into the Ohio Valley, later in the outlook period, so the entire region will be monitored closely.

El Niño tends to favor dryness in Hawaii, and the seasonal precipitation outlook for November-January indicates a tilt of the odds toward dryness in this region. Although some rain is expected for the short-term, the longer-term outlook implies a risk for expanding drought by the end of January. The U.S. Drought Monitor already depicts drought over eastern and northern parts of the Big Island, with some abnormally dry conditions in Maui.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for November-January, the drought termination and amelioration probabilities for January, 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 Analogues for the season. For this outlook, a number of analogues based on historical instances of weak to moderate El Niño events were consulted, as well as updated runs of the Coupled Forecast System (CFS).

Once again, the circulation pattern at the start of the forecast period features a deep upper level trough near the West Coast. The mean trough position is expected to persist near the West Coast at least into late October. This should lead to more storms dumping rain and snow on the western drought area. The storm lashing California on October 19-20, just before the start of the outlook period, was expected to bring 1-3 feet of snow to the high Sierra and heavy rains to lower elevations. Models indicated another storm should strike the Pacific states around October 26-28, with a deep upper level trough or cutoff low positioned near California. Snow and rain would again spread eastward well beyond the Pacific states. Although a highly amplified pattern is likely to continue into November, there are some indications that the mean trough will migrate eastward, with ridging developing over the West, resulting in a transition to a positive PNA pattern during early November. This would lead to a milder and drier pattern for the West. Seasonal models, such as the CFS, as well as weak El Niño analogues, suggest a tendency toward positive PNA (ridge west, trough east) patterns during the winter, and experimental RSM and GSM weekly forecasts depict this pattern by January, as well as periodically during November-December. Drought improvement is shown for the western fringes of the drought region largely because of the short-term storm pattern, and because the drought is less firmly entrenched in California, where low reservoirs are less of an issue than farther east. Only limited improvement is indicated for most of the western and High Plains drought region because of the difficulty in relieving water shortages this early in the snow season. Also, the official November-January precipitation outlook calls for equal chances wet or dry. The latest CFS run, updated on October 20, shows an area of below-normal precipitation during November-January for the Upper Colorado River Basin extending northwestward to Washington and Oregon. The expected benefits from this El Niño, should they occur, would tend to take place more toward late winter and spring, based on historical analogues and the CFS model, and this would mainly affect the Southwest and Colorado River Basin. The area of persisting drought in Montana and Wyoming is based on climatology and the expectation of relatively limited benefit from the near-term storms.

The near-term outlook for the lingering drought areas in the Great Lakes region is promising, with a number of low pressure systems expected to bring rain during the first two weeks of the outlook period. The longer-term outlook is problematical, with both the CFS model runs and El Niño analogues showing a tendency for dryness during November-January from the Great Lakes southward into the lower Mississippi Valley. The drought outlook is focusing more on the shorter-term outlook in this area, where the confidence is higher, and the fact that recent rains have already made the drought area in Michigan very marginal. If drought does persist or return, indications are that this is more likely in Wisconsin, where the drought is more firmly established and analogues and models both tilt toward November-January dryness.

In Hawaii, dry weather since September has already resulted in drought development in parts of the Big Island and dryness in parts of Maui. Further drought development is indicated because of a dry November-January precipitation outlook due to the El Niño. Confidence is low for the drought development outlook given a near-term forecast for rain and historical analogues showing dryness tending to kick in late in the outlook period rather than early. In addition, the complex climates on the islands make long-range forecasts especially difficult, so a broad-brush approach was used for this drought outlook.