The West
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Despite near to below-normal precipitation across the Pacific Northwest this week, precipitation for the season as a whole has finally improved long-term Standardized Precipitation Index (SPIs) and Standardized Precipitation Evapotranspiration Index (SPEIs) below D3 status, warranting some minor reduction in D3 (extreme drought) coverage.
Ground water has been slower to recover, but given the continued region-wide improvements to SWE, the various indicators supported the improvement this week. Further south, coastal areas of southern California saw above-normal weekly rains late in the period, from San Luis Obispo County southward to the Mexico border. The heaviest rainfall was observed in Santa Barbara County, with some southeastern locations receiving more than 4 inches. However, this was not enough rainfall to overcome long-term deficits (2 to 4 inch deficits going back 60 days). Elsewhere in the Western Region, much-needed precipitation fell across the Great Basin and the Four Corners Region, adding some additional snowpack and preventing further degradation this week. Basin SWE values remain below 75 percent of normal for the southern Great Basin and the Four Corners, but some gains were made in northeastern New Mexico and southern Colorado.
The High Plains
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A low-pressure system developed over the Central Plains on Monday, associated with energy exiting the eastern Rockies Monday into Tuesday (Dec. 28-29).
This provided above-normal precipitation, mostly in the form of snow, across the central and eastern High Plains Region (liquid water equivalent precipitation greater than 200 percent of normal). Despite below-normal precipitation southward, it was enough to stave off degradation across Kansas and Colorado. Further north, however, in northwestern South Dakota and southwestern North Dakota, average maximum temperatures remained above freezing, aided by reduced seasonal snowpack.
Additionally, high winds, mainly early in the period, and 1 to 2-month SPEIs ranging from D1 to D4, led to degradation to D1, extending westward into southeastern Montana. SWE percent of normal values have continued to improve; however, most basins in the High Plains Region remained below-normal for the season. The exception was in southern Colorado (near to above-normal SWE) where near to above-normal weekly precipitation added to the snowpack.
The South
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Most of the precipitation fell on the eastern third of the Southern Region associated with a frontal boundary during Wednesday and Thursday (Dec. 23-24). This led to some minor trimming of D0 in south-central Tennessee and extreme northwestern Alabama which received 1.5 to 2 inches of rain.
However, locations along and west of the Mississippi River observed below-normal weekly totals, warranting further expansion of D0 in eastern and northern Arkansas. Some slight improvement (D1 to D0) and removal (D0) north of Houston was also justified in localized areas that received more than 1.5 inches of rainfall. Light precipitation fell over parts of Oklahoma and Texas Monday into Tuesday (Dec. 28-29) as energy exiting the southern Rockies and intensified across the Central and Southern Plains.
Despite the late arrival of the system in the Plains this week, the previous week’s release of the Drought Monitor remained representative of conditions across Oklahoma and most of Texas. However, southern Texas saw some slight degradation along the Lower Rio Grande Valley, with D3 expanded northward from Starr County to Webb County and D2 expansion southward into southeastern Hidalgo and western Cameron Counties. This region has experienced low relative humidity and topsoil moisture.
NASA SPoRT soil moisture data depicts the top 10 cm of soil below the 2nd percentile in areas where D3 was expanded and between the 5th and 10th percentiles where D2 was expanded. NASA GRACE groundwater data and the National Land Data Assimilation System (NLDAS) also support this deterioration. —UNL Drought Monitor
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