Drought news: Still plenty of time to build snowpack in the west

Click here to go to the US Drought Monitor website. Here’s an excerpt:


Strong storm systems brought heavy, widespread precipitation to both the Northeast and West Coasts, providing beneficial moisture and drought relief to both regions. In the West, a broad trough of low pressure over the eastern North Pacific Ocean funneled several storm systems into the Pacific Coast that tapped subtropical moisture. The week’s greatest precipitation amounts fell on central and northern California where many locations totaled 4 to 12 inches. This precipitation came after the previous week’s moderate to heavy precipitation in the same area, continuing a wet pattern from northern California northward into the Pacific Northwest since mid-October. The wet weather finally allowed ample runoff (while producing stream and river flooding) that raised major reservoir levels (as of Dec. 16) in most of northern and central California by 6 to 10 percentage points from normal capacity (compared to Nov. 28 values). However, major California reservoir capacities still remained below normal, and due to above-normal temperatures accompanying these Pacific storms, more rain than snow has fallen on the lower elevations of the Sierra Nevada and Cascades, producing well below normal snow pack and water content this Water Year (since Oct. 1). Only the highest elevations have seen abundant snows. Fortunately, the West still has plenty of time this winter to build its snow pack for spring and summer stream runoff. Early in the week, a developing storm off the mid-Atlantic Coast looped back eastward, then tracked slowly northward, and eventually stalled over New England, dropping more than 1.5 inches of precipitation from eastern New York state eastward, with locally more than 4 inches in Maine. Late in the period, a storm system in the Nation’s mid-section brought widespread light to moderate precipitation to most of the Plains and Midwest, finally bringing welcome moisture to the central Plains and western Corn Belt after a very dry November and early December. Weekly temperatures averaged well above normal in the western two-thirds of the Nation and in New England, while the Southeast experienced subnormal temperatures…

Central and Southern Plains

The storm system previously mentioned in the Northern Plains and upper Midwest brought beneficial and welcome precipitation to the southern Great Plains (northeastern Texas, eastern Oklahoma) and central Plains (eastern and northern Kansas and most of Nebraska). The greatest totals (between 1.5 and 2 inches, locally to 2.7 inches) fell along the eastern Red River Valley, with 1 to 1.5 inches occurring in eastern Oklahoma, eastern and northwestern Kansas, and central and eastern Nebraska, with most locations (even Nebraska) seeing this precipitation fall as rain on unfrozen ground. With winter precipitation totals typically low, this was a significant moisture event. But due to the very dry autumn weather, this event was not quite enough to erase D0 over most of the central Plains; however, where November was wetter (Kansas southward) and this event had higher totals (at least an inch), a slight 1-category improvement was made. This included northwestern and eastern Kansas, southeastern Oklahoma, and northeastern Texas. In contrast, minimal or no precipitation fell on western and southern sections of Texas, and some slight deterioration was warranted in Texas in the northwestern Panhandle, along the southeastern coast near Matagorda Bay, and northeast of Houston…

Northern Plains and upper Midwest

After a rather dry November and early December, a storm system developed over the Southwest and intensified over the south-central Plains while tracking northeastward, bringing welcome moisture not only to the southern and central Plains, but also to parts of eastern South Dakota, Iowa, and much of Minnesota. The precipitation (0.25 to 0.75 inches, locally to 1.2 inches) was enough to stave off any deterioration in the region, and was a nice moisture bonus where the soils were not frozen (and since December normals are quite low). In the western Dakotas where little or no precipitation fell, short and medium-term surpluses existed, so dryness was not a factor. Accordingly, status-quo was applied here…


With most of this week’s significant precipitation concentrated in the Far West (e.g. California), only light amounts (0.1 to 0.5 inches, locally to 1.2 inches) fell on most of the Four-Corners States. Toward the east, little or no precipitation was reported in eastern New Mexico and western Texas. After the previous week’s light to moderate totals in Arizona and northwestern New Mexico, and a rather robust and long-lasting Southwest monsoon, surplus precipitation still lingered at 90- and 180-days across much of the region, thus no deterioration was needed. This is fortunate as the WYTD (since Oct. 1) basin average precipitation and SWE are both below normal, ranging from 50-70 percent of normal (precipitation) and 10-80 percent of normal (Dec. 15 SWE), with conditions closer to normal in central Colorado and northeastern New Mexico…

The West

Two consecutive weeks of widespread heavy (7-day totals of 4 to 12 inches) precipitation, augmented by above-normal autumn precipitation, produced major stream and river flooding in north-central California. The flooding on the Sacramento River was the highest since Dec. 31, 2005. The runoff led to good capacity increases (6 to 10 percentage points) in major reservoirs across northern and central California; however, they were still below the historical averages for Dec. 16. For example, the Nov. 28 Trinity, Shasta, Oroville, Folsom, and San Luis Lakes percent of capacity was 23, 23, 26, 28, and 23; on Dec. 16, they increased to 29, 32, 33, 38, and 33 percent, respectively, but the Dec. 16 historical averages were 44, 53, 54, 79, and 52 percent. Based upon historical storage data, almost every major reservoir saw a 1-category drought improvement in their levels during the first half of December, and combined with wet start to the Water Year (and December), a broad 1-category improvement was made in north-central and along the central coast of California (D4 to D3, or 0-2 to 2-5 percentile). In addition, the mountains east of San Diego and south of San Bernardino in southern California received between 6-12 inches of December precipitation, improving overall moisture conditions there from D3 to D2 levels. Lastly, along the border of California and Arizona, a reassessment of conditions noted surpluses at 30- and 180-days, hence D0 was extended northward to the tip of southern Nevada.

Unfortunately, remaining areas to the south and east of the 1-category improvements were unchanged as the storms precipitation amounts were lower (but welcome), the autumn months were quite dry, runoff was minor to non-existent (albeit a few mudslides in fire-scarred slopes), and not surprisingly, the major reservoir levels in the south remained static. Furthermore, the rain shadow effect was notable on the Nevada side where much less moisture made it over the Sierra Nevada Mountains. Also, temperatures accompanying the storms have been above-normal, leading to more rain than snow at the higher elevations (e.g. Sierra Nevada). As a result, the snow water equivalents (SWE) for the northern, central, and southern Sierras were only at 43, 43, and 63 percent of normal for Dec. 16, which was actually higher than SWE values farther north in the Cascades of Oregon and Washington (10-30 percent). Fortunately, there are still many months left in their normal wet season for the Far West to build-up their snow pack, even though the Water Year-to-Date (WYTD) precipitation was at or slightly above normal in the Pacific Northwest. In north-central Washington, the past few months have been wet enough to justify a 1-category improvement to D1 as WYTD basin average precipitation was 112 to 120 percent. Similarly, surplus precipitation has fallen on extreme northern Idaho during the past 6-months to justify removal of D0. Elsewhere across the West and Rockies, enough precipitation (0.2 to 1 inch) fell from these Pacific storms to prevent any further deterioration, but not enough to warrant improvement.

In summary, a wet December (to date) has provided California a foothold for drought recovery, but 3 straight winters of subnormal precipitation will take time (possibly several consecutive wet winters) to fully recharge the reservoir levels and subsoil moisture back to normal. With several more months still left in the wet season, it is possible that additional storms similar to the ones that just occurred will continue to chip away at the long-term hydrological drought, and the addition of lower temperatures would help build the snow pack. “Cautious optimism, but still a long way to go” would be the very short summary for this week’s California drought picture…

Looking Ahead

For the upcoming 5-day period (December 18-22), heavy precipitation (up to 10 inches in northwest Oregon) is forecast for the Pacific Northwest (and southward to northwestern California) and northern Rockies, with lighter amounts in the central Rockies. A southern storm system should bring widespread moderate to heavy precipitation (1 to 4 inches) from central Texas eastward to Georgia and the Carolinas (including the expanding drought area in the Southeast), and lighter amounts along the Northeast Coast and in the central Great Plains. Mostly dry weather is expected in the Southwest (including southern California), High Plains, upper Midwest, Ohio Valley, and southern Florida. Near to above normal temperatures should envelop most of the lower 48 States, with the greatest positive departures in the northern Rockies and Plains and upper Midwest.

For the ensuing 5-day period (December 23-27), the CPC 6-10 day outlooks tilt the odds toward subnormal precipitation for eastern and southeastern Alaska, and from southern Oregon and California southeastward into the lower Mississippi Valley (lower Delta). Favorable chances of above median precipitation are expected in western Alaska, across the northern tier of States, and in the eastern quarter of the Nation, with the highest odds in the Great Lakes region and Northeast. Above median temperatures are expected in Alaska, the Far West, Southwest, and New England, with subnormal readings favored in the northern Plains and Florida.

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