Tools used in the U.S. Seasonal Drought Outlook (SDO) included the official Climate Prediction Center (CPC) temperature and precipitation outlooks for
December 2014 and
December 2014 - February 2015,
various short- and medium-range forecasts and models such as the forecast 5-day and 7-day precipitation totals
from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC forecasts, the
NAEFS precipitation outlooks, the soil moisture tools
based on the Constructed Analog on Soil Moisture (CAS), dynamical models (CFSv2, NMME, IRI, and IMME), the 384-hour total precipitation forecasts from several runs
of the GFS, the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions. An El Niño (ENSO) Watch is currently in
effect, with the November 6 ENSO Diagnostic Discussion indicating a 58% chance of El Niño conditions during the winter and early spring.
A protracted, intense drought is affecting California, with 99.5% of the state designated in some degree of drought by the Drought Monitor, and 55% of the state in
exceptional drought (D4, the most intense classification). Statewide precipitation data from the National Climatic Data Center (NCDC) indicates that
December - February, the period covered by this SDO, brings slightly over half of the precipitation California receives each year on average, and closer to 60% in
the central valleys and along the western tier of the state. Thus, climatological statistics alone favor at least some drought improvement. Even if precipitation
were simply near normal, it would be considerably wetter than the past 3 wet seasons have been, engendering at least some improvement. Short-term forecasts call for
fairly wet weather across northern parts of the state. From the northern Sierra Nevada and the southern Cascades westward to the coast, at least an inch of
precipitation is expected by Thanksgiving, with 2 to locally over 6 inches anticipated from approximately Santa Rosa northward through northwestern California. Odds
generally favor a drier period from late November through early December, but longer-term precipitation outlooks show enhanced chances for above-normal precipitation
statewide for the month of December, and across roughly the southern half of the state for the December - February period as a whole. Based on these factors, drought
improvement is expected across most of the state. However, it must be emphasized that improvement does not mean elimination or removal. In particular, most areas of
extreme to exceptional drought likely will still be in the grip of a serious drought as winter ends despite any improvement. One factor lowering the confidence of
the improvement forecast is the deep and protracted nature of the current drought situation, which may require fairly robust winter precipitation totals for a
significant reduction in severity. Similar reasoning implies improvement will also occur in southwestern Oregon.
Forecast confidence for California and southwestern Oregon is moderate to high in the southern half of the region, and low to moderate farther north, where a wet
winter is less likely.
Drought is forecast to persist in The Great Basin and northern Intermountain West, with some expansion expected in adjacent areas where there is currently abnormal
dryness (D0) indicated by the Drought Monitor and/or subnormal precipitation has been observed over the past few months. In the northern Intermountain West,
above-normal precipitation may occur in the short-term through the first few days of December, but odds favor below-normal precipitation for the 3 winter months as a
whole, implying drought persistence and expansion. Generally 30% to 40% of each year's precipitation falls during these 3 months, so any precipitation deficits that
accumulate during this period are somewhat difficult to overcome during the typically drier spring and summer months. Farther south, persistence is forecast through
central and northern sections of Nevada and Utah where the December - February outlook shows no tilt of the odds toward either wetter- nor drier-than-normal
conditions overall. The winter is slightly wetter than other times of the year, at least in Nevada, but the long-term nature of drought conditions will make
improvement difficult in this region, where normal precipitation in an absolute sense is scanty. Compared to California's statewide average precipitation of nearly
12 inches for the 3 winter months, Nevada and Utah both average under 3.5 inches during this period.
Forecast confidence is high in the northern Intermountain West, and moderate across central and northern sections of Nevada and Utah.
In the upper southern Plains, from the western half of Oklahoma and the northern Texas Panhandle northward through the drought areas in Colorado and Kansas, drought
persistence is forecast. Light precipitation at best is anticipated through the end of November, and while surplus precipitation is favored during
December - February, this is a relatively dry time of year, bringing only 5 to 15 percent of the annual total on average. In addition, soil moisture approximations
show decreased moisture values significantly more often than increased numbers. As a result, it seems unlikely that even a relatively wet December - February
will bring much drought relief given the climatological considerations. Still, the odds favoring wetness do bring some uncertainty in.
Forecast confidence for the upper southern Plains is moderate.
In the northern Plains, the small area of drought is expected to persist. Soils should freeze up early in the period, so any improvement in soil moisture content
will have to wait for spring thaw. In addition, only about 5 to 10 percent of the annual average precipitation falls during this 3-month period. Given the small
potential for substantial changes by the end of February, drought is forecast to persist where it exists, but the odds for development in any given region are
relatively low, though not out of the question.
Forecast confidence for the northern Plains is high.
A small area of drought is expected to develop in southeastern Michigan and perhaps some adjacent areas by the end of February. This area has been drier than normal
for the last 3 months or so, and while moderate precipitation on the order of an inch or so is forecast by Thanksgiving, odds favor dryness during
December - February 2014/15, allowing moisture deficits to increase. In conjunction with the antecedent dryness, it seems likely that at least a small area of
drought will develop.
Forecast confidence in the southeastern Great Lakes is moderate.
Improvement is forecast for the areas of drought that exist from the southern Rockies eastward through the southern Plains, and the southeastern states. In the short
term, the period up through Thanksgiving should be wet in most areas from central Texas and eastern Oklahoma eastward. Totals over an inch should be fairly
widespread, with 2 to 3 inch amounts anticipated in the central and eastern Red River Valley and along the central Gulf Coast region. In the longer term, surplus
winter precipitation is favored throughout the southern tier of states, although late November and early December look dry at this point.
Forecast confidence for the southern tier of states is moderate to high.
The small areas of drought in the lower Northeast are expected to be removed by the end of February. Near to above normal precipitation is forecast on all time
scales, particularly near the coast, and over an inch is anticipated by Thanksgiving. Still, this is a difficult region to forecast. Some upper-level air pressure
anomaly patterns, such as the Atlantic Oscillation, can be the primary factors determining precipitation over the winter, and these are always difficult to forecast
in the longer term.
Forecast confidence for the Northeast is moderate.
There is a strong likelihood of below normal precipitation through the end of the period across Hawaii, which is typically the wettest time of the year. To wit,
precipitation amounts during the cooler time of the year are the most critical in determining the evolution of any drought across the islands. As a result, the small
areas of existing drought are expected to persist, and drought is forecast to broadly expand, covering much if not all of the state by the end of February. There is
some inherent uncertainty in forecasting conditions far enough removed from normal as to induce drought where there is only spotty dryness to begin with, so our
confidence is reduced slightly.
Forecast confidence for Hawaii is moderate.
Forecaster: R. Tinker
Next Seasonal Drought Outlook issued: December 18, 2014 at 8:30 AM EDT