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Climate Prediction Center


December 2012 - February 2013


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Latest Seasonal Assessment - During the past two weeks, heavy precipitation (low elevation rain and mountain snow) has fallen across northern and central California, the coastal ranges and Cascades of the Pacific Northwest, and the northern Rockies. Light to moderate precipitation was observed over portions of the northern and eastern U.S., with little to no precipitation for the Southwest. The heaviest precipitation during the next two weeks is favored across the northwestern and eastern CONUS, while northern and central California get a respite from the recent series of storm systems. The updated monthly forecast (valid for the remainder of December) indicates enhanced odds of above-median precipitation across the northern half of California, the Pacific Northwest, the northern Rockies, and the central Great Lakes region. For the December-February 2012/13 seasonal drought outlook, some improvement in drought conditions is expected over California, and portions of Montana and Wyoming. Drought persistence is favored across the rest of the West and most of the Great Plains region, as well as over interior portions of the east Gulf Coast region, and drought development is possible over the eastern panhandle of Florida and adjacent portions of southeastern Georgia. Remaining areas of drought east of the Mississippi River are expected to see at least some improvement in conditions due to the occasional passage of storm systems. In Hawaii, a slackening of the trade winds has contributed to recent rainfall deficits over the Islands, most notably the Big Island, where Hilo has reported a 9.5 inch deficit during the past 30 days. Drought persistence is thought to be the best bet for the Big Island, while some improvement seems prudent for the more northern Islands, given the occasional approach of upper-air disturbances.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the updated official CPC temperature dnd precipitation outlooks for December 2012, the long lead forecast for December 2012 through February 2013, various short- and medium-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the NAEFS precipitation outlooks, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, the Climate Forecast System (CFS version 2), 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. To some degree, negative PDO (Pacific Decadal Oscillation) composites were also considered in this Drought Outlook.

During the past two weeks, a series of Pacific storm systems have brought abundant amounts of precipitation (rain at lower elevations, snow at higher elevations) to northern and central California (especially the Sierras), the coastal ranges and Cascades of the Pacific Northwest, and the northern Rockies. According to AHPS, anywhere from 10 to 20+ inches of precipitation (liquid equivalent) has fallen across California and the Pacific Northwest, with about half those amounts observed in the northern Rockies, encompassing much of Idaho and western Montana. In the next 5 days, another 0.5 -1.5 inches of precipitation is predicted over north-central portions of California, and 1.5 3.0 inches over western portions of both Washington and Oregon. Generally 1 2 inches of precipitation is anticipated over Idaho. In the next few days, the upper-air pattern is forecast to transition from fast zonal flow to a ridge over the eastern Pacific and a downstream trough over the interior West. This is likely to result in a drier pattern across California and the Pacific Northwest, with enhanced chances for near- to below-median precipitation anticipated for both the 6-10 day and Week 2 periods. However, the updated CPC monthly outlook for the remainder of December favors a tilt in the odds towards above-median precipitation for the northern half of California, the Pacific Northwest and the northern Rockies. The winter outlook for December February favors a tilt in the odds towards below-median precipitation across northern and central California, and Equal Chances (EC) over the Pacific Northwest and northern Rockies.
Forecast confidence for the Northwest is moderate.

Little to no precipitation has fallen across much of the Southwest during the past two weeks. A few exceptions include the Wasatch Range of northern Utah, and far northern and western Nevada which received moderate to heavy precipitation (0.5-inch to as much as 3 inches locally). In the next 5-days, light to moderate amounts of precipitation (less than 2 inches) are anticipated over northern Nevada, and much of Utah and Colorado, with little to no precipitation elsewhere. A drier pattern is expected during the extended-range period (days 6-14), with near to below- median amounts of precipitation across the Southwest. Beyond two weeks, EC is considered the best bet. No changes were made to the Drought Outlook in this region this time around.
Forecast confidence for the Southwest is moderate.

Between 0.5 and 1.0 inch of precipitation was observed over parts of the northern High Plains and far southern Texas during the past 2 weeks. Little to no precipitation was reported elsewhere across the Great Plains. For eastern Oklahoma and eastern Texas, this equates to a deficit of one to as much as 3 inches over this period. Up to half an inch of precipitation is predicted over eastern portions of both Oklahoma and Texas within the ensuing 5-day period, while a corridor of light precipitation (0.5-inch or less) is expected over South Dakota and Nebraska. CPC's 6-10 day and 8-14 day precipitation outlooks show enhanced chances for above-median precipitation across the northern Plains, and enhanced chances for near to below-median precipitation elsewhere. The updated December 2012 outlook favors below-median precipitation over most of Texas, above-median precipitation over Montana and portions of Wyoming, and EC elsewhere across the Plains. With no clear signal available for the December-February 2012-13 period, the forecast was left as EC. With a drier winter climatology across the northern Plains, it was decided to alter the outlook depiction in this area from Some Improvement (from two weeks ago) to Persistence.
Forecast confidence for the Plains is moderate.

Precipitation over most of the Mississippi-, Tennessee-, and Ohio Valleys was mostly light (less than 0.5-inch) over the past 2 weeks. A few areas received moderate precipitation (0.5 - 2.0 inches) which was observed in east-west oriented bands. These areas included the Arrowhead region of Minnesota, near and along the Ohio River, southwestern Tennessee, southwestern Missouri, and much of both Mississippi and Louisiana. During the upcoming 5-days, a broad band of moderate to heavy precipitation (0.5 3.0 inches) is expected from the lower Mississippi Valley northeastward across the lower Tennessee Valley and most of the Ohio Valley, attributed to the passage of two separate cold fronts and associated centers of low pressure. This wet pattern is predicted to continue through the Week 2 period, as evidenced by CPC's extended-range outlooks which favor a tilt in the odds towards above-median precipitation. Equal chances is the predominant forecast across these regions during the remainder of December, with only a marginal tilt in the odds for heavier-than-median precipitation across Indiana and nearby sections of Ohio and Illinois. The winter outlook for December February 2012-13 indicates a tilt in the odds towards above-median precipitation for the south-central Mississippi Valley and adjacent Tennessee and southern Ohio Valleys, with EC indicated elsewhere. The only alteration made to the drought outlook depiction was over western Minnesota, where some-improvement was changed to Persistence. This area shows up in seasonal DJF precipitation composites as being similar to that of the northern Great Plains.
Forecast confidence for the Mississippi-, Tennessee-, and Ohio Valleys is moderate to high.

During the past 2 weeks, 0.5 1.0 inch of rain fell across western parts of Alabama, and in a band from Mobile Bay and the western Florida panhandle northeastward across southern sections of Georgia and coastal South Carolina. Little if any precipitation was observed elsewhere. During the next 5-days, up to 0.5-inch of rain is anticipated across the region. Odds favor above-median precipitation during the extended-range period across all but the Florida peninsula, where there are enhanced odds of near-median rainfall. CPC's updated December outlook favors below-median precipitation across Florida and southern Georgia, with EC predicted elsewhere. For the December February 2012-13 time frame, EC is generally forecast across the Southeast, except for a tilt in the odds towards wetter-than-median precipitation over the southern Appalachians. Seasonal climatology favors drought development over the Florida Panhandle, while composites which assumed negative PDO and ENSO-neutral conditions favored the dryness over the eastern portion of the Panhandle, and parts of eastern Georgia. This is also consistent with extended-range predictions which show a storm system affecting mostly the western and central Gulf region, with the area of heaviest rainfall (several inches) extending northeastward toward the Carolinas.
Forecast confidence for the Southeast is moderate.

Light to moderate precipitation (1.5 inches or less) was reported over the middle and northern Atlantic coast states and the Great Lakes region during the past 2 weeks. Locally higher amounts (1.5 3.0 inches) were observed over the Tug Hill Plateau in upstate New York, and parts of Michigan. In the ensuing 5-days, light to moderate precipitation (1.5 inches or less) is forecast across most of the region, while the odds for above-median precipitation are elevated in the extended-range period. For the remainder of December, odds slightly favor above-median precipitation over the Great Lakes region, with EC indicated elsewhere. There are no clear climate signals for the upcoming winter forecast across this area, so EC is deemed the best bet.
Forecast confidence for the middle and northern Atlantic region, and the Great Lakes region is moderate.

During the past 30-days, significant rainfall deficits have accumulated over the Hawaiian Islands. Deficits for a few selected locations include 3.7 inches at Lihue, 1.9 inches at both Honolulu and Kahalui (Maui), and 9.5 inches at Hilo. This overall drop in precipitation is related to the slackening of the trade winds in this area. If the past month is any indication, no change to the drought outlook depiction appears warranted. Any approaching short-wave troughs are more likely to bring at least limited precipitation to northern portions of the Archipelago.
Forecast confidence for Hawaii is moderate.

NOAA/ National Weather Service
NOAA Center for Weather and Climate Prediction
Climate Prediction Center
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Page Author: Climate Prediction Center Internet Team
Page last modified: December 6, 2012
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