December 24, 2104 - The West
Another in a series of Pacific systems affected the West, with the latest one dropping the greatest precipitation totals on western Oregon and Washington (4-12 inches), although the northern half of California received 2-6 inches, with lesser amounts to the south (generally an inch or less). Although the recent precipitation was not as great as the past 2 weeks, the total capacity of water at the major reservoirs in northern and central California (e.g. Trinity, Shasta, Oroville, Folsom, and San Luis Lakes) did slightly increase on Dec. 16 from 29, 32, 33, 38, and 33% to 32, 38, 36, 41, and 36% as of Dec. 22, respectively. Unfortunately, these values are still well below the historic average capacity for Dec. 22 (47, 62, 59, 85, and 54%, respectively), with the Sierra Nevada basin average SWEs also well below normal (45-72%). The Northern Sierra 8-station precipitation index on Dec. 22 was at 22.4 inches, or 146% of normal, while the San Joaquin 5-station index was at 8.8 inches, or 78% of normal. And similar to last week, major reservoirs to the south showed little or no increase this week with the lower totals. Therefore, even with the additional precipitation, no modifications were made to California this week.In the Pacific Northwest, with most basins reporting near to above normal WYTD precipitation, generally normal to surplus precipitation at short, medium, and long-term periods (except in southern Oregon, closest to California), the heavy precipitation was enough to slightly trim (improve) the edges of the D0-D2 in northwestern and northeastern Oregon, western Washington, and western Idaho, especially where the long-term SPIs (12- and 24-months) were wet. Since improvements were made in north-central Washington last week, no modifications were made this week. A major concern this Water Year has been the warm component of the storms. Even though WYTD average basin precipitation for the Cascades ranged between 103-134% of normal as of Dec. 22, SWEs remained quite low, ranging between 10-65% of normal. Fortunately, SWEs were close to normal in the northern and central Rockies. Across the West, not only will continued ample moisture be crucial, but also occur in combination with enough cold air to produce adequate winter mountain snow pack for spring snow melt, runoff, and reservoir recharge.
Looking Ahead
For the upcoming 5-day period (December 24-28), light to moderate precipitation (0.5-2 inches, locally to 3 inches along the Pacific Northwest Coast) is forecast for the Northwest, and with temperatures expected to be below normal, the precipitation should mainly fall as snow on the Cascades, northern Sierra Nevada, and the northern and central Rockies. Farther east, moderate (more than 0.75 inches) to heavy precipitation (over 2 inches) is forecast for the eastern third of the U.S., with most of the Southeast expecting over 2 inches of rain, and locally 4-6 inches along the east-central Gulf Coast. Readings should average above normal across the eastern half of the Nation.For the ensuing 5-day period (December 29-January 2), the CPC 6-10 day precipitation outlook paints a dry outlook for the West (good odds of below median precipitation), but favorable probabilities of above median precipitation for Alaska, the southern Plains, Southeast, and mid-Atlantic, with the best chances along the Gulf Coast. Temperatures are expected to average below normal in the western two-thirds of the Nation, with above median temperature probabilities limited to southern Florida and Alaska.
Author(s):
David Miskus, NOAA/NWS/NCEP/CPC
Dryness Categories
D0 ... Abnormally Dry ... used for areas showing dryness but not yet in drought, or for areas recovering from drought.Drought Intensity Categories
D1 ... Moderate DroughtD2 ... Severe Drought
D3 ... Extreme Drought
D4 ... Exceptional Drought
Drought or Dryness Types
S ... Short-Term, typically <6 months (e.g. agricultural, grasslands)L ... Long-Term, typically >6 months (e.g. hydrology, ecology)
