Weekly Summary: A fairly typical La Niña weather pattern occurred this week, with heavy precipitation in the Northwest, very dry conditions across most of the southern tier of States (from southern California to Florida), and above-normal precipitation in the Tennessee and Ohio Valleys into the Northeast. An intensifying storm dumped heavy snows on parts of the upper Midwest (southern Minnesota and Wisconsin) and brought rain to the East, but after the storm left, strong gusty winds and frigid air produced heavy lake-effect snows in the Great Lakes snow belt regions. Well above-normal temperatures enveloped the Rockies westward, while subnormal temperatures chilled the eastern half of the U.S., including sub-freezing readings deep into southern Florida. Heavy rains also soaked Hawaii, especially the eastern-most islands (Kauai and Oahu), although all islands (leeward and windward sides) received beneficial and surplus rains. Alaska was fairly tranquil, with light precipitation limited to the southeastern Panhandle.

Southeast and mid-Atlantic: Weekly precipitation totals generally increased from south to north along the East Coast, with light rainfall (0.1 to 0.3 inches) in much of Florida and eastern Georgia and heavy precipitation (2 to 4 inches) in eastern New England. In-between, 1 to 1.5 inches of rain in eastern New Jersey was enough to alleviate the D0, while the 1 to 1.5 inches of precipitation in southeastern Virginia (west of Norfolk) and eastern North Carolina (north of Wilmington) allowed for trimming of the D0 edge. Similar to the Ohio and Tennessee Valleys, the Northeast has also seen increased precipitation since October which has led to drought improvement or removal, as has parts of the Southeast, namely northern sections of Mississippi, Alabama, and Georgia. The trend continued this week in the northern parts of the Southeast as another 1 to 1.5 inches of rain maintained surpluses out to 90-days, resulting in additional trimming of D0-D1 in northern Mississippi, central and northern Alabama, and northern Georgia. In contrast, subnormal precipitation (weekly totals less than 0.5 inches) in the eastern Gulf and southern Atlantic Coasts further degraded conditions. Less than 25 percent of normal precipitation has fallen in portions of eastern Georgia and northern Florida the past 60-days, and since mid-September (90-days), under half of normal rainfall was observed in eastern Georgia and most of north-central Florida. Most USGS average stream flow sites at all time periods (1-, 7-, 14-, and 28-days) were at near or record lows in northern and central Florida, eastern Georgia, and the central Carolinas, while soil moisture indices, blends, and models were depicting percentiles in the lower 5th percentile in eastern Georgia and north-central Florida. Accordingly, D1-D3 crept northward into eastern Georgia and southeastern South Carolina; the two separate D3 areas in eastern Florida were joined, encompassing the Florida Space Coast; D1 and D2 expanded westward in north-central Florida; D1 extended into Collier County and surrounding areas in southern Florida; D1 slightly expanded eastward in central South Carolina; and the last remaining no drought area was removed in extreme southern Florida.

Ohio, Tennessee, and lower Mississippi Valleys: Light to moderate precipitation (0.5 to 1.5 inches), including snow, fell on the Ohio and Tennessee Valleys, continuing a pattern of wet weather that started about 2-3 months ago that is consistent with expected La Nina precipitation signals (surplus precipitation during the winter months). In the lower Mississippi Valley, however, precipitation has been more scattered and generally lighter (this week, less than 0.5 inches). Accordingly, short-term conditions (60-days and less) in the Ohio and Tennessee Valleys had above-normal precipitation and accumulated surpluses, especially in the eastern sections, resulting in some improvements along the D0-D3 edges where an inch or more fell this week. Elsewhere, near- to slightly subnormal precipitation in the western Ohio and Tennessee Valleys and lower Mississippi Valley in the short-term (less than 60-days) mandated little or no improvement. From 60- to 90-days, under half of normal precipitation has fallen on southern Missouri, western Arkansas, and western Louisiana, and conditions remained status-quo or slightly degraded. USGS average stream flows at 14- and 28-days depicted the short-term improvement or lack of it, with most sites in the normal percentiles (25th to 75th) in the Ohio and Tennessee Valleys into northern Mississippi and northern Alabama, but less than 25th percentile in lower Mississippi Valley. The Impact designator was also changed to H only (from AH) as most short-term impacts (e.g. agriculture) do not exist during the winter, especially in the north, but was kept AH in the lower Mississippi Valley as short-term impacts can still occur.

Central and Southern Plains: Similar to the Southwest, little or no precipitation fell on the southern half of the Plains, although temperatures were close to normal for the week. Since receiving moderate to heavy precipitation back in early November (mainly Texas Panhandle, northwest Oklahoma, central Kansas), much of Oklahoma, Kansas, and Texas have recorded little or no precipitation, and moisture conditions have deteriorated. More rain had fallen to the east (Arkansas and Louisiana) during November, but precipitation also decreased during December here. As a result, based upon the greatest deficits at 90-days and less, the Texas SPI blends, USGS stream flows below the 10th percentiles, and local input, D3 increased in east-central Texas (90-day percent of normal precipitation [PNP] less than 25 percent and deficits between 7 and 10 inches) and in south-central Texas; D2 expanded in north-central and eastern Texas (90-day PNPs less than 50 percent and deficits between 4 and 7 inches); D1 spread into west-central Texas near Midland, and into north and northeast Texas (30-day PNPs less than 25 percent and deficits between 2 and 4 inches), and northward into central Oklahoma (30- and 90-day PNPs are less than 25 and 50 percent, and deficits between 2 to 4 and 4 to 7 inches, respectively). In Oklahoma’s Osage and Creek counties, farm ponds are quite low, even lower than the drought of 2005 and 2006, and Skiatook Lake (Osage County) is 4 feet below normal, significant for this small lake. D2 stretched eastward into western Kansas from eastern Colorado as conditions have been as dry or even drier at 90-days and longer, while dry conditions in southeastern Kansas warranted D0. D1 was also extended into southeastern Oklahoma from western Arkansas where they have also missed out on decent rains the past 3 months. The D0 hole in northeastern Texas also shrank with another dry week, although the Texas Panhandle (from Lubbock to Amarillo), northwest Oklahoma, and south-central Kansas continued to benefit from heavy rains in mid-October and early November.

Southwest: Another week of little or no precipitation and above-normal temperatures (4 to 8 deg F, as expected during a strong La Nina) continued to accumulate precipitation deficits and delay the start to the 2010-11 water year. Except for some unseasonably heavy rainfall back in early and mid-October, most of southeastern California, Arizona, New Mexico, southwest Texas, and southern sections of Utah and Colorado have seen little precipitation. Fortunately, temperatures have generally been at or below normal since late October, however, this week's warmth exacerbated the lack of rain. Accordingly, D0 was expanded across nearly all of New Mexico, into southeastern Utah and southwestern Colorado, and D1 was introduced into southeastern Arizona and southwestern New Mexico and southern Colorado. In the latter area, precipitation deficits have accumulated over the past few months, SNOTEL precipitation percentiles are fairly low, and STD basin precipitation and SWC on December 13 ranged between 50 to 75 percent. In contrast, after a reassessment of several indices, D0 was removed from northwestern Arizona where the heavy rains (2 to 6 inches) in early and mid-October still affected the short-term blends and indices (no drought), and nearby southwestern Utah SNOTEL Basin Water Year to Date (WYTD) precipitation was at 199 percent.

Intermountain West and north-central Rockies: The wettest storm in the past two years hit western Washington Saturday, with weekly totals of 6 to 12 inches common, with locally up to 18 inches in western Washington. The moisture fetch also generated ample precipitation to much of the Northwest, in line with expected La Nina conditions. Accordingly, the continued wet winter and this week's precipitation (0.5 to 2 inches) was enough to remove lingering D0 in southwest and southeast Idaho, northwestern Utah, and the western edges in southwestern Oregon and northwestern Wyoming. SNOTEL basin averaged WYTD precipitation and Snow Water Content (SWC) on Dec. 13 was 125 to 150 percent of normal, an excellent start to this year's water year. In western Wyoming, higher-elevation sites are doing well, but lower-elevation stations are much drier, especially in Sublette and Sweetwater counties. As a result, the D1 remained there. Additional precipitation (1 to 1.5 inches) fell on the remainder of southern Oregon and northern California, requiring further trimming of D0 and D1 where the largest totals occurred. Farther south, even though no precipitation fell and temperatures were well above-normal, no degradation was made (D0 and D1H remained) in central California and southern Nevada thanks to a wet November.

Hawaii and Alaska: Widespread, moderate to heavy rains fell across the entire state including both windward and leeward sides, although Kauai and Oahu received the bulk of the rainfall. The rains started late Wednesday and tapered off by early Saturday. Totals of 4 to 10 inches were common on Kauai, 3 to 6 inches on Oahu, 2 to 4 inches on Molokai and Lanai, 2 to 5 inches on Maui, and 1 to 3 inches on Big Island. The two western most islands (Kauai and Oahu) have been in recovery mode for the past month, so a 1-category improvement was made there. The D4 in southwestern Maui was removed after Kula Branch, Ulupalakua, and Kihei measured 2.35, 1.84, and 2.04 inches, respectively. The rest of the state, however, was just recently introduced to heavy rainfall, so conditions were left status quo pending additional assessments next week. But as an example, the Kualapuu Reservoir in western Molokai did not rise at all after last week's rains. And on the Big Island, 1 to 2 inches of rain did little to ease brush fire threats (high to extreme), and ranchers in the south are still hauling water.

In Alaska, mostly cold and tranquil conditions covered the state, with little or no precipitation and subnormal temperatures common. Fort Yukon plunged to -53 deg F on Dec. 12, with Bettles not far behind at -47 deg F. Significant precipitation was limited to extreme southeastern Panhandle where Ketchikan and Annette Island measured a weekly total of 4.93 and 2.13 inches. With most of the state in deep freeze, no changes were made in Alaska.

Looking Ahead: December 16-20, 2010 weather should be somewhat similar to this week’s weather with the exception of a storm developing in the eastern Gulf and tracking northeastward. This should provide beneficial precipitation to the eastern Gulf and southern Atlantic Coast States. The West should see a continuation of stormy weather later in the week, even extending south into southern California. A weak system currently in the Southwest may bring light precipitation to parts of the Four Corners region, but dry conditions should prevail in the Nation’s mid-section (Plains). Readings will remain chilly in the northern Plains, upper Midwest, and across the eastern third of the U.S. while the Southwest and southern Plains observe above-normal temperatures. The Northwest and New England should be close to normal.

For the ensuing 5 days (December 21-25), above-normal precipitation is expected to continue in the West, particularly in California and the Great Basin, while subnormal precipitation is forecast across the southern tier of States and along the East Coast, especially in the Southwest, southern Plains, and lower Mississippi Valley. Alaska should see subnormal precipitation across the south and east, and surplus precipitation in the southwest. Readings are predicted to be unseasonably mild in the western half of the Nation, while subnormal temperatures persist in the Southeast and mid-Atlantic. The southern half of Alaska should experience below normal readings.

Author: David Miskus, CPC/NCEP/NWS/NOAA

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 Drought
D2 ... Severe Drought
D3 ... Extreme Drought
D4 ... Exceptional Drought

Drought or Dryness Types
A ... Agricultural
H ... Hydrological