Click on a thumbnail graphic below to view a gallery of drought data from the US Drought Monitor.
Click here to go to the US Drought Monitor website. Here’s an excerpt:
This Week’s Drought Summary
Precipitation fell across much of the United States this week, with widespread moderate amounts (1-2 inches) falling on the eastern third of the CONUS, 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 (SWE) 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).
Much of Alaska has received near to above-normal precipitation during the first half of Fall, with some sporadic stations depicting some minor dryness in the last month. However, snowpack is above-normal everywhere south of the Brooks Range for the season as a whole. Hawaii has experienced a dry December and this has led to some D0 degradation to D1 on the Big Island, as pasture conditions have begun to worsen. Puerto Rico observed below-normal precipitation in areas already experiencing abnormal dryness and moderate drought (D0 and D1, respectively), but USGS 7-day average stream flows remain steady from last week…
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…
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…
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…
Looking Ahead
The 5-day Quantitative Precipitation Forecast (QPF) from the NWS Weather Prediction Center (December 31, 2020 – January 4, 2021) depicted heavy precipitation along the West Coast, from northern California northward to Vancouver Island, associated with a series of potent storm systems. Across the eastern CONUS, an active storm track favors widespread precipitation totals exceeding an inch from the Southern Great Plains eastward and northeastward to the Atlantic Coast, excluding the Florida Peninsula. This is in addition to precipitation falling in the first 2 days of the period across many of these same areas. Heavier precipitation (more than 3 inches) is likely from eastern Texas to the Tennessee Valley over the next 5 days, and west of the Big Bend area in Florida. Little to no precipitation is expected from the Four Corners Region to the Northern Plains and Upper Midwest. A southern shift in the western storm track is expected to occur toward day 5, leading to below-normal temperatures across the southwestern CONUS. The lack of seasonal snowpack is expected to remain intact across the Northern Plains, contributing to large positive temperature anomalies. In the eastern CONUS, mean southerly flow is expected to dominate, keeping temperatures anywhere from 5°F to 10°F above-normal.
The 6-10 day CPC extended range outlook (January 5-9, 2021) favored amplified mean troughing from Alaska southeastward to the southwestern CONUS, indicating a southward shift in the storm track along the West Coast. Ridging is favored to dominate much of the eastern CONUS, with the highest mean mid-level positive geopotential height anomalies over the Great Lakes and Northeast. Below-normal temperatures and precipitation are favored over western and northern Mainland Alaska, respectively, while odds tilt toward above-normal precipitation along the southern Alaska coast. Increased odds of below-normal temperatures in the southwest CONUS are associated with below-normal mid-level heights. Ahead of the mean trough over the West, above-normal precipitation is favored over the Central Plains and western Corn Belt. Additionally, southerly to southwesterly mean flow enhances odds of above-normal temperatures across much of the eastern two-thirds of the CONUS. With the amplified troughing across the western CONUS, enhanced odds of above-normal precipitation extend along the entire West Coast and into the Great Basin and Four Corners region. Probabilities of below-normal precipitation are increased for the Northeast, underneath positive mean mid-level height anomalies, and in Florida and southeastern Georgia, as any passing frontal boundaries are expected to remain farther north.
Here’s the calendar year US Drought Monitor animation.
