Nationwide
[inline_image file=”cdcc5f71d6c8d7d6cc817e9383f1c3ff.jpg” caption=”20201222_usdm.jpg”]
Most of this week’s precipitation across the contiguous U.S. (CONUS) fell on portions of the Pacific Northwest, Southeast, and Northeast.
Typical of La Nina conditions, the northward displacement of the storm track across the West so far this season has resulted in near to above-normal snowpack across the Pacific Northwest and northern Rockies and below-normal southward. This week mainly saw a continuation of that seasonal signal, with above-normal precipitation falling again along with coastal ranges and the Cascades from central Oregon northward to Canada, leading to some minor improvements, mainly in northwestern Oregon. However, this week did see a slight southward shift in the storm track, providing central and northern California some much needed, albeit below near-normal, precipitation. Over the eastern United States, a storm system developed over the Southeast early in the period and transitioned into a strong Nor’easter that impacted much of the Mid-Atlantic and Northeast, with a swath of 1 to 2 feet of snowfall extending northeastward from southern Pennsylvania into southern Maine. Several short-wave troughs provided some additional precipitation across the Great Lakes and Northeast in the wake of that system.
Elsewhere in the CONUS, several areas of the High Plains, Southwest, and western sections of the Midwest saw little to no precipitation. However, the time of year has minimized drought degradation in many of these areas with low temperatures, frozen ground, and little or no evapotranspiration. Additionally, temperatures averaged near to below-normal across the southern tier of States, further minimizing any deteriorations.
The West
[inline_image file=”c6e42e1b78dfd7d3659e15181c368997.jpg” caption=”20201222_West_trd.jpg”]
Many improvements in the Western Region this season have been designated to the Pacific Northwest due to typical La Nina conditions aiding in a northward-displaced mean storm track.
As a result, the Cascades and northern Rockies have accumulated above-normal snowpack, with snow water equivalent (SWE) of 100-125 percent. Meanwhile, areas southward have experienced below-normal precipitation and reduced snowpack, ranging from 75 percent of normal in the central Rockies to less than 25 percent of normal in the Southwest.
This week saw a continuation of the La Nina projection on seasonal precipitation, with the heaviest precipitation (150-300 percent of average) in the coastal ranges and Cascades, resulting in some localized D1 and D2 improvement in western and northeastern Oregon. Additionally, USGS 7-day average stream flows are above-normal, and these same areas are showing surpluses in water year-to-date (WYTD) precipitation estimates.
Some precipitation also fell across northern and central California, the Great Basin, and central Rockies. However, amounts were modest, resulting in no major changes this week. Despite the Southwest missing out on precipitation this week, precipitation from the prior week and below-normal average temperatures this week warranted no further degradation.
The High Plains
[inline_image file=”8bdc26f1b36501678dd053387662bf42.jpg” caption=”20201222_High_Plains_trd.jpg”]
Colorado and Wyoming’s mountainous areas saw some additions to their snowpack this week, but amounts were near-normal at best.
Above-normal temperatures were widespread across the region, with the highest positive average temperature anomalies (greater than 8 F) across the Dakotas. These positive anomalies can mainly be attributed to below-normal snowpack across much of the Northern Plains.
Liquid-equivalent precipitation amounts averaged between 0.01 and 0.25 inches across several locations in the High Plains Region, with several spotty near-normal amounts reported in the Central Plains (co-located with areas seeing the highest average high temperatures) and portions of the Dakotas, reducing further impacts and degradation. Several USGS stations across the region are reporting near and above-normal 7-average stream flows. As such, no changes to drought coverage are warranted this week across the High Plains Region.
The South
[inline_image file=”f2035019a4f1eb897eb3025cc7af772f.jpg” caption=”20201222_South_trd.jpg”]
Much of the region saw below-normal temperatures, and the largest precipitation amounts fell across southeastern Texas, extending eastward to Louisiana and northeastward to the Tennessee Valley.
Locations east of Austin, Texas, and along the western Gulf Coast, saw above-normal precipitation (1 to 1.5-inch totals, with some localities receiving more than 2 inches), leading to some D1 and D2 improvement near Austin and a reduction in D0 coverage in southern Louisiana. Western and central Texas saw some degradation in D1-D3 areas, with a lack of precipitation (less than 10 percent of average rainfall in the last 90 days) and low relative humidity.
Some slight trimming of the abnormally dry (D0) area in north-central Oklahoma, with 0.25 to 0.5 inches of precipitation falling last week, near-normal average temperatures, and 30-60 day precipitation totals ranging between 150 and 175 percent of normal. Some D0 reduction was also warranted in northern Louisiana and central Mississippi, as Standardized Precipitation Index (SPIs) across several periods show mixed weak above and below-normal signals, indicating near-normal conditions.
Some expansion of D0 conditions occurred in north-central and central Arkansas in favor of D0-D1 SPIs across various periods, coupled with 90-day deficits of 4 to 6 inches (localized 6 to 8 inches). Some southward D1 expansion in southern Tennessee was also warranted in areas missing out on relatively higher rainfall this week, continuing to add to deficits there (6 to 8-inch deficits going back 90 days). —UNL Drought Monitor
