Most areas of US receive some precipitation

Precipitation fell across much of the United States this week, with widespread moderate amounts (1-2 inches) falling on the eastern third of the contiguous U.S., as a strong storm system moved eastward and exited the Northeast early in the week (Dec. 24-25). Ahead of the frontal boundary associated with this system, strong southerly flow resulted in a rapid warm up and heavy rainfall (2-3 inches, with locally heavier totals), leading to increased snowmelt throughout the Northeast.

Behind this system, and in the following days due to lake effect, new snow blanketed many of the same locations that experienced the rapid snowmelt. Toward the end of the week, moderating temperatures led to additional snowmelt across the region. As a result, much of the Northeast saw 1-category improvements (D1 to D0, and D0 to removal). The Southeast also saw improvements in D0 coverage as the frontal boundary from this system extended all the way to the Gulf Coast as it tracked eastward.

In the western CONUS, only minor improvements were made in Oregon as long-term indicators (going back to last year’s below normal rainy season) have improved enough to warrant some reduction in extreme drought (D3) coverage. Late in the week (Dec. 28-29), a storm system finally tracked southward this season, bringing above-normal weekly precipitation to coastal southern California. Unfortunately, long-term deficits (beyond 60 days) remained, foregoing D1 improvement.

Status-quo was warranted from the Great Basin eastward to the Great Plains as the energy from this system propagated eastward, building upon snow water equivalent values and preventing further deterioration; however, the precipitation that did fall was not enough to improve conditions either. In the Northern Plains, snowpack remains well below-normal, leading to some minor degradation in areas where temperatures averaged above freezing and winds were gusty. Elsewhere, the time of year has minimized degradation of drought in many locations, with low temperatures, little or no evapotranspiration, and frozen ground (upper Midwest and Great Plains).

High Plains

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.

South

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 ground water data and the National Land Data Assimilation System (NLDAS) also support this deterioration.

Midwest

The Midwest region saw modest precipitation this week region-wide, except for northern Missouri, southern Iowa, and west-central Illinois, which missed out entirely. Slight surpluses in precipitation were observed mainly from southern Minnesota northeastward to the Arrowhead, northwestern Wisconsin, and the Upper Peninsula of Michigan, but given low normals (liquid water equivalent amounts of 0.1 to 0.5 inches) across the upper Midwest, these small gains resulted in status quo this week.

Further south and east through the Corn Belt where precipitation was lacking, some D0 expansion was warranted from central Missouri to northwest Ohio, as well as targeted D0 introduction in southern Indiana as SPIs have trended toward further degradation (widespread D0 and D1 depictions in the last 60 and 30 days, respectively). USGS 7-day average stream flows have also fallen below-normal (10th to 24th percentile). D1 was also expanded southwestward to Pike County in western Illinois and southward to northern Boone and Audrain Counties in northern Missouri for similar reasoning. Additionally, these areas experienced temperatures 6°F to 12°F above-normal this week (and above freezing), along with high winds early in the period.

West

Despite near to below-normal precipitation across the Pacific Northwest this week, precipitation for the season as a whole has finally improved long-term SPIs and 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.