On Dec, 14, a strong mid-level low pressure system progressed inland into southern California where heavy precipitation (0.5 to 3 inches, liquid equivalent) occurred. As this area of mid-level low pressure continued to track eastward, a powerful surface low developed across the high Plains with a subsequent track northeastward to the upper Mississippi Valley. This intense low surface low resulted in widespread damaging winds and a dust storm across the central and southern Plains.
The high winds and antecedent dryness contributed to a number of wildfires across Kansas, Oklahoma, and the Texas Panhandle. According to the Storm Prediction Center preliminary data, nearly two dozen tornadoes were reported across eastern Nebraska and Iowa on Dec. 15. As this storm tracked northeast to the upper Mississippi Valley, widespread precipitation (0.5 to 2 inches, liquid equivalent) fell over this region with heavy snow blanketing northern Minnesota.
A slow-moving cold front resulted in a widespread area of rainfall (0.5 to 3 inches) from the Ozarks Region south to the western Gulf Coast. On Dec. 18, a wave of low pressure developed along the front which led to rainfall (0.5 to 3 inches, locally more) overspreading the Southeast. However, this rainfall remained south of the increasingly dry Mid-Atlantic. A series of low pressure systems and frontal passages resulted in 7-day amounts of 0.5 to 1.5 inches, liquid equivalent, throughout the Northeast. Periods of enhanced onshore flow continued to bring rain and high-elevation snow to the Pacific Northwest and northern Rockies through mid-December. The heaviest 7-day precipitation (2 to 5 inches, or more, liquid equivalent) was observed along and west of the Cascades in Oregon. Month-to-date temperatures (Dec 1-20) have averaged more than 6 degrees F above normal across the central and southern Great Plains. In contrast to the above-normal temperatures throughout nearly all of the central and eastern continental U.S. so far this month, temperatures have averaged below normal across most of California. Following the heavy precipitation associated with a Kona low earlier this month, enhanced trade winds continued to maintain a fairly wet pattern for Hawaii through mid-December. More than 1 inch of rainfall occurred across northeast Puerto Rico from December 14 to 20.
South
Based on worsening soil moisture conditions and recent warmth and enhanced winds, severe drought (D2) was expanded to include all of the Oklahoma Panhandle and this D2 expansion included the northeast Texas Panhandle. Extreme drought (D3) increased across southwestern Oklahoma where a few locations have received less than 0.10 inch of precipitation during the past 70 days. As of December 21, no measurable precipitation has been recorded at Amarillo, Texas for 70 consecutive days which is the 4th longest streak on record. Abnormal dryness (D0) and D1 to D2 drought continue to expand across west-central Texas. Month-to-date temperatures (Dec. 1-20) have averaged 6 to 10 degrees F above normal throughout Texas. Impacts, related to the expanding and worsening drought conditions across the southern Great Plains, include poor grazing for livestock. Farther to the east, moderate to heavy rainfall (1 to 3 inches, locally more) resulted in a 1-category improvement to parts of Arkansas, Louisiana, eastern Oklahoma, and eastern Texas. Despite the recent rainfall, 60 to 90-day SPI values continue to support D0 and D1 across eastern Texas and northern Louisiana. Moderate to heavy rainfall (1 to 3 inches) prompted a decrease in the abnormal dryness (D0) for parts of western Tennessee and northern Mississippi.
Midwest
Heavy precipitation (0.5 to 2 inches, liquid equivalent) for this time of year and longer term SPI values supported a 1-category improvement to parts of central and northeast Minnesota. Widespread precipitation of more than 1.5 inches prompted a decrease in abnormal dryness (D0) and moderate drought (D1) across southern Missouri along with a small reduction in D0 for southwestern Illinois, near the Mississippi River. However, a slight expansion of D1 was made to east-central Missouri which remained north of the swath of moderate to heavy rainfall this month.
High Plains
Based on 60- to 90-day SPI values and soil moisture, abnormal dryness (D0) and moderate drought (D1) were expanded across parts of southern Nebraska and western Kansas. Severe drought (D2) increased across southeastern Colorado and southwestern Kansas due to rapidly worsening soil moisture and the EDDI. Based on below-average SWE and SPI, the Sangre de Cristo Mountain Range was degraded to extreme drought (D3). Minor improvements were made to parts of North Dakota due to recent snowfall and above-normal precipitation dating back through the fall.
West
During early to mid-December, precipitation averaged more than 150 percent of normal across much of California along with western areas of Oregon and Washington. According to the California Department of Water Resources as of Dec 21, snow water equivalent statewide is 93 percent of normal. During the past two weeks, temperatures have averaged 2 to 4 degrees F below normal which has contributed to the favorable snow water content numbers through mid-December. Above-normal precipitation during the past two weeks, current SWE numbers, and 12- to 24-month SPI considerations supported a reduction of D4 (exceptional drought) across much of the Sierra Nevada Mountains. Also, D4 was decreased to the north of Los Angeles due to recent heavy precipitation. As a result of December precipitation, a slight decrease in extreme drought (D3) was made to the north of San Francisco. The exceptional drought (D4) was eliminated from southern Nevada and southeastern California due to the lack of support from long-term SPI values. A persistent wet pattern resulted in a 1-category improvement to western Oregon as longer term indicators slowly improve. As of Dec 20, Oregon statewide SWE is at 95 percent of normal with generally above average SWE along and west of the Cascades but continued below average SWE throughout northeastern Oregon. WYTD (Oct. 1-Dec. 20) precipitation along with assessment of long-term SPIs resulted in a 1-category improvement to western Idaho and adjacent Washington. Recent heavier precipitation, especially at the higher elevations, resulted in improving drought conditions throughout many locations of Montana. The areas of exceptional drought (D4) were decreased due to recent wetness and lack of support from longer term SPIs. Due to current low SWE, there is growing concern for worsening conditions across the Bear and Snake River basins of southern Idaho. These ongoing low SWE numbers supported a 1-category degradation for parts of southern Idaho and adjacent areas of western Wyoming. Due to recent dryness and rapidly worsening soil moisture, D3 was expanded to include more of northeast New Mexico.
