December 2013

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Latest Monthly Assessment - The December 2013 Drought Outlook is based on initial conditions, short and medium range forecasts, the CPC updated monthly outlooks for temperature and precipitation, and climatology. During the first half of November, relatively tranquil conditions prevailed across the lower 48 States, with light to moderate precipitation limited to the Pacific Northwest Coast, and from extreme southern Texas northeastward into the upper Great Lakes region. Very dry weather continued in the Southeast and New England as 90-day precipitation was less than half of normal, accumulating deficits between 4 and 8 inches. In addition, the wet season has been slow to commence in northern California and southern Oregon, with even lower precipitation percentages (<25%) and larger deficits (up to 12 inches) since October 1. With the first half of December expected to be drier than usual and the central West Coast station normals rather high, some development was added where D0 existed.

After mid-month, weather conditions finally changed as a vigorous upper-level storm dropped plentiful moisture on the Southwest and slowly tracked eastward, tapping Gulf moisture. With the precipitation from the late November storm already improving drought conditions in Arizona plus favorable odds of above-median precipitation in the Four Corners region in early December – versus a contradictory tilt of below-median precipitation for DJF, no changes were made for December in the Southwest. Farther north, however, with the recent precipitation, 1- and 3-month probabilities of above-median precipitation, and short-term odds for wetness, improvement and removal was made in Idaho, Utah, and Wyoming, and extreme northern Arizona and New Mexico. Farther east, widespread and ample precipitation fell on the southern Plains, lower Mississippi Valley, most of the Southeast, and the Atlantic Coast States during the last week of November, putting a significant dent in the month-to-date (MTD) deficits, or in some areas, creating MTD surpluses. But with short-term, 1-, and 3-month forecasts pointing toward sub-median precipitation in the southern High Plains, some development where D0 existed was added to western Texas and southern New Mexico. In eastern Texas and central Arkansas, recent wetness, surpluses out to 90-days, and short-term forecasts for above-median rain should be enough to remove lingering D1 areas.

Some drought development is possible across parts of the Southeast, especially in Florida where D0 already exists, short-term and monthly forecasts edge toward dryness, and 28-day USGS stream flows are low. Development is a bit more uncertain in Alabama and Georgia as short-term forecasts hint at above-median rainfall, the monthly precipitation is EC (equal-chances), and the seasonal precipitation outlook tilts toward dryness. Therefore, development was added in the areas with the greatest 90-day deficits, where recent rainfall was less, and stream flows were below normal. Short-term drought also expanded across coastal New England in mid-November, but with recent heavy rains reducing 90-day deficits and early December forecasts pointing toward wetness, removal of D1 is anticipated before the ground freezes. December is a climatologically dry time of year across the upper Midwest and Plains. With no clear signal during early December and slight odds for below-median December precipitation from the southern Plains northeastward into Missouri, drought persistence is most probable in these regions.

In Alaska, wet and mild weather has continued in east-central Alaska, edging year-to-date percentages closer to normal and improving the chances for drought reduction. In Hawaii, a wet first half of November was replaced by much drier weather during the second half, but both the monthly and seasonal precipitation outlooks call for above-median rainfall, hence the improvement.

Tools used in the monthly U.S. Drought Outlook (MDO) included the official CPC temperature and precipitation outlooks for December 2013, various short- and medium-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the NAEFS and ESRL precipitation outlooks, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, dynamical models (CFSv2, NMME, and IMME), the 384-hour total precipitation forecasts from several runs of the GFS, the 7-day quantitative precipitation forecast from the NCEP Weather Prediction Center (WPC), climatology, and initial conditions. ENSO-Neutral conditions are expected to continue through the winter season, the Atlantic and Caribbean hurricane season are over, and little mid-latitude impacts from the current MJO pattern are expected.

During the first 3 weeks of November, well under half of normal precipitation was observed across most of the southeastern and eastern sections of the U.S., with many areas reporting less than 25% of normal. This continued a dry pattern across both regions since September as large deficits accumulated. Fortunately, a wet spring and summer tempered the effects of the short-term dryness, although there were a few isolated impacts from the dryness. During the last week in November, however, a vigorous upper-air storm dropped moderate to heavy precipitation on both regions, reducing deficits and bringing some areas surplus November precipitation. With some of the storm’s precipitation occurring after the Nov. 26 Drought Monitor cut off period, abnormal dryness (D0) still covered parts of the Southeast and most of the Atlantic Seaboard, with moderate drought (D1) in portions of southern and coastal New England. During the first half of December, a slow-moving cold front should stall in the Southeast, bringing ample precipitation from the central Gulf northeastward into New England. Most of Florida, however, is not expected to benefit from this moisture. In addition, the updated 1-month precipitation outlook (and the 3-month precipitation outlook) slightly favors below-median totals in the eastern Gulf and southern Atlantic. Based on recent heavy precipitation not yet incorporated into the Drought Monitor and an expected wet first half of December, removal is forecast for the D1 areas of the Northeast. Farther south, a small area of drought development is indicated where the 90-day deficits are the greatest, the recent rains were lower, USGS stream flows are in the lower 25th percentile, and D0 currently exists (e.g. southern Alabama, central Georgia, eastern Florida, eastern Carolinas).
Forecast confidence for the Gulf and Eastern Seaboard is low (New England, AL-GA-Carolinas) to moderate (FL).

A series of slow moving cold fronts during the past 90-days have produced widespread rainfall along a swath from eastern Texas northeastward into the Great Lakes region. The late-November storm also dropped plentiful moisture on most of Texas, further increasing the short-term surpluses. Additional precipitation is expected across the lower and middle Mississippi Valley during the next 6-10 days as another cold front slowly progresses eastward, however, the 8-14 day forecast hints at drier weather for the middle Mississippi Valley. This additional moisture falling on short-term surpluses should provide further relief from drought in eastern Texas and central Arkansas. Farther north in Missouri and Illinois, however, the short-term forecasts are less certain as to whether decent precipitation will fall over the middle Mississippi Valley. The updated 1-month precipitation outlook also has a slight tilt toward below-median totals from northern Texas into Missouri. And as the ground begins to freeze in the Midwest, water infiltration into the soils will basically cease. Therefore, based on the mixed 6-10 and 8-14 day signals, drought persistence is anticipated in the middle Mississippi Valley.
Forecast confidence for eastern Texas, lower and middle Mississippi Valley is moderate.

Although surplus precipitation fell on parts of eastern Iowa, northern Illinois, and southern Wisconsin during the past 30 days, 90- and 180-day accumulations were still below normal across most of the upper Midwest. Both November and December are a climatologically dry time of the year in the upper Midwest, and the soils generally begin to freeze or are frozen during December, limiting moisture infiltration. With the CPC 6-10 and 8-14 day outlooks indicating near to slightly above normal precipitation, the updated December outlook maintaining equal chances of below, near, and above-median precipitation, and a dry climatology with increasingly impermeable frozen soils, drought persistence is forecast for the upper Mississippi Valley.
Forecast confidence for the upper Mississippi Valley is moderate to high.

After a very wet October, dry weather returned to the Dakotas, with many locations measuring less than 25% of normal precipitation. Subnormal November precipitation also occurred across most of central Nebraska, eastern Kansas, central Oklahoma, and northern Texas, although parts of the central and southern Plains received surplus November precipitation. However, November (and December) climatology in the Plains is dry, so it does not take much precipitation to create a surplus. With little or no precipitation expected during the next 7 days, the extended range forecasts showing generally near to below normal median precipitation, the updated December outlook favoring dryness in the south-central Plains, and dry climatology, drought persistence is anticipated across the Plains, with some slight development possible in western Texas where D0 already exists.
Forecast confidence for the Plains is moderate to high.

Climatologically in the Far West, precipitation increases during the late fall and peaks during the winter months. As a result, a slow start to the wet season can accumulate large deficits rather quickly. In contrast, winter normals generally decrease the farther one heads eastward. Unfortunately, this season has started out slowly, with 60-day shortages between 6 and 16 inches in parts of the Cascades and Sierra Nevada. In contrast, a late-November storm dropped ample precipitation on southeastern California, Arizona, southern sections of Nevada, Utah, and Colorado, and western New Mexico, improving drought conditions somewhat. Looking ahead into early December, mostly dry and cold weather is on tap for much of the Far West, with Arctic air enveloping much of the region. Farther east, however, the odds favor above-median precipitation in the central and southern Rockies, lingering into days 8-14 in the central Rockies. Late in the period, high pressure should retreat, allowing for milder and wetter weather to enter the Northwest. The December updated outlook does not indicate a wet or dry signal across the Far West, although it tilts the odds towards above-median precipitation in the northern Rockies and High Plains, and below-median precipitation in eastern New Mexico. Based on short-term and monthly wet signals, development is forecast for northern California and southern Oregon (where large deficits already exist), improvement for parts of the central and southern Rockies, and status-quo for the recently-improved Southwest.
Forecast confidence for the West is moderate.

A very wet and mild October weather pattern continued into November, most likely providing additional drought relief to east-central Alaska as year-to-date totals have recently risen close to normal in the lingering D1 area. With this latest information from the state, it is expected that the remaining moderate drought area will be removed from the Drought Monitor in early December. Therefore, drought removal is anticipated.
Forecast confidence for Alaska is moderate to high.

The first half of November saw an increase in the frequency and amount of showers across most of Hawaii, and some drought relief was made, especially on the windward sides. As the month progressed, however, dry weather returned to the islands, halting the improvement. The winter season is a wet time of year across Hawaii as mid-latitude storms generate increased precipitation across the islands. Climatologically, most of this activity starts in December and continues into the winter months. The 1- and 3-month Hawaiian precipitation outlooks also tilt the odds toward above-median rainfall. Therefore, drought improvement is likely by the end of December, with possibly more relief during early 2014.
Forecast confidence for Hawaii is moderate to high.