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      Drought Monitor Weekly: Heavy Rains Along West Coast; Severe Storms Hit Central U.S.

      Summary

      This U.S. Drought Monitor (USDM) week was marked by a much more active storm pattern across the West Coast and Northern Rockies with areas of heavy rain along the coast and valley locations. Further inland, the higher elevations of the Cascade Range, Klamath Mountains, Sierra Nevada Range, northern Great Basin, and the Northern Rockies have all received significant snowfall accumulations this week.

      The storm event, fueled by an atmospheric river, provided a much-needed boost to snow water equivalent (SWE) levels in the Far West.

      On December 14, the NRCS SNOTEL network was reporting normal-to-above-normal SWE in Oregon’s Willamette (100% of median) and Southern Oregon Coastal (116%) basins as well as in the Lower Sacramento (113%), San Joaquin (106%), Tulare-Buena Vista Lakes (119%), Truckee (137%), Carson (120%), Walker (122%) and Mono-Owens Lakes (122%) basins of the Sierra Nevada Range.

      In the Lower Mississippi, Tennessee, and Ohio Valley, an outbreak of violent thunderstorms including dozens of tornadoes, erupted along a path spanning from northeastern Arkansas to northeastern Kentucky. The tornados moved very rapidly through the region on Friday night with devastating effect, especially in the southeastern Kentucky town of Mayfield.

      In Kentucky, latest reports announced at least 74 people lost their lives, and the death toll is expected to increase.

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      In the Mid-Atlantic, unseasonably warm temperatures were observed over the weekend with daily high-temperature records broken in Pennsylvania, West Virginia, Virginia, Maryland, Delaware, New Jersey, and North Carolina where high temperatures ranged in the mid-60’s to mid-70’s.

      On the map, short-term precipitation shortfalls and anomalously warm temperatures led to the degradation of drought-related conditions in portions of the Mid-Atlantic, Southeast, South, and in the Midwest, while some improvements were made in the High Plains and isolated areas out West.

      According to NOAA’s National Center’s for Environmental Information (NCEI), November 2021 was the 7th warmest on record for the contiguous U.S. as well as the 7th warmest January-November period on record. In terms of precipitation, November marked the 8th driest for the contiguous U.S., while year-to-date precipitation ranked in the middle third (61st wettest).

      At a regional level, the Rockies and Westward Region saw its warmest November (+5.8 deg F anomaly) on record as well as its warmest March-November period on record.

      Southeast

      During the past week, light to moderate rainfall accumulations (1 to 3 inches) observed across isolated areas of Alabama, Georgia, and the Carolinas led to minor reductions in areas of Moderate Drought (D1) and Severe Drought (D2). Conversely, short-term dryness and declining soil moisture levels expanded areas of Abnormally Dry (D0) in northern Florida.

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      In North Carolina, USGS stream gauges registered below-normal flows across the entire state. Additionally, several soil moisture analysis products showed negative soil moisture anomalies in the Carolinas.

      Average temperatures for the week were near normal across most of the region with average maximum temperatures ranging from 55 to 65 deg F in the northern extent of the region while temperatures in the southern extent ranged from 70 to 80 deg F in Florida and in southern portions of Alabama and Georgia. According to NOAA NCEI, the Southeast Climate Region experienced its 15th driest November on record.

      South

      In the South, conditions on the map continued to degrade across areas of Texas and Oklahoma where unseasonably warm and dry conditions prevailed this week. The dry conditions showed up on a variety of drought indicators including satellite-based soil moisture and evaporative demand tools.

      Additionally, numerous rivers and streams in the western half of the state showed flows dipping below the 20th percentile during the past 7-day period. Rainfall deficits (ranging from 3 to 6+ inches) during the past 60-day period were greatest in southeastern Oklahoma, eastern Texas, Louisiana, and southwestern Arkansas.

      For the week, average temperatures were above normal (2 to 10+ deg F) across Texas, Oklahoma, Louisiana, Arkansas, and Mississippi.

      Midwest

      On this week’s map, minor changes were made on the map including expansion of areas of Moderate Drought (D1) and Severe Drought (D2) in southern Wisconsin where precipitation has been below normal during the past 90-day period with deficits ranging from 2 to 4+ inches. Similarly, short-term dryness led to expansion of areas of Abnormally Dry (D0) and Moderate Drought (D1) in Missouri.

      Conversely, improvements were made on the map in northern Michigan where recent snowfall has helped to improve drought-related conditions. According to NOAA NOHRSC, 50.7% of the Northern Great Lakes Region was covered in snow with an average depth of 3.8 inches and a maximum depth of 21.6 inches.

      New snowfall observed in the Upper Peninsula of Michigan this week ranged from 2 to 20 inches while areas in southeastern Minnesota and northern Wisconsin received amounts ranging from 4 to 14 inches.

      High Plains

      On this week’s map, eastern Wyoming, eastern Montana, central North Dakota, southern South Dakota, and northwestern Nebraska saw improvements based on short-term precipitation (30 to 90-day period). This included beneficial snowfall in southern portions of South Dakota where observed totals ranged from 6 to 18 inches with the highest totals in southwestern South Dakota.

      In these areas, soils have yet to freeze throughout the soil column and melting snows are infiltrating and helping to improve soil moisture levels. According to NOAA NOHRSC, snow coverage in the Upper Midwest Region (includes portions of Wisconsin, Minnesota, the Dakotas, eastern Montana, and northwestern Wyoming) is currently at 52.5% with an average depth of 1.5 inches and a maximum depth of 28.8 inches.

      Despite some improvements on the map, it is noteworthy that average temperatures across the High Plains region have been well above normal since September. This includes numerous high-temperature records that were recently broken across the region during the first week of December when high temperatures soared into the 70’s.

      West

      Out West, a weak-to-moderate atmospheric river made landfall in the Pacific Northwest on Saturday and moved southeastward bringing coastal and valley rains as well as heavy mountain snowfall accumulations across California and the Pacific Northwest.

      For the week, rainfall accumulations along the coastal areas from Washington to California ranged from 3 to 13+ inches with the highest accumulations observed in the coastal ranges of northwestern Oregon and along the Central Coast of California. In terms of snowfall during the multi-day storm event, total accumulations exceeded 6 feet in areas of the Central Sierra while areas in the southern Cascades received up to 3 feet.

      Further inland, areas of the Northern Rockies in Idaho and northwestern Wyoming, observed snowfall totals ranging from 12 to 20 inches.

      Despite the beneficial nature of this week’s storm event, significant precipitation deficits (ranging from 4 to 20+ inches) still exist across California and the state’s largest reservoirs are still at critically low levels, with Lake Shasta currently at 46% of the historical average (25% of capacity) and Lake Oroville at 62% of average (31% of capacity).

      In other areas of the West, basin-level SWE is well below normal, especially in New Mexico where median SWE ranged from 12% to 77% of normal as of Dec 14.

      On the map, some improvements were made in areas of Extreme Drought (D3) and Exceptional Drought (D4) in Montana, Oregon, and Utah as well as improvements in areas of Severe Drought (D2) and Moderate Drought (D1) in Idaho and Wyoming. According to NOAA NCEI, November 2021 was the 2nd warmest on record for the West and Southwest climate regions.

      Moreover, California and Wyoming both recorded their warmest average minimum temperatures on record for November while Nevada, Utah, and Colorado observed their 2nd warmest on record. In terms of precipitation, the Southwest Climate Region was notably dry having its 5th driest November on record.

      Northeast

      In the region, short-term dryness (2-to-4-inch deficits during the past 30-day period) and declining streamflow levels led to minor expansion of areas of Abnormally Dry (D0) in southwestern Pennsylvania, West Virginia, eastern Maryland, Delaware, and southern New Jersey.

      For the week, precipitation was light across the region with light-to-moderate snowfall accumulations (1 to 6 inches) in areas of Upstate New York as well as northern portions of Vermont, New Hampshire, and Maine. Average temperatures for the week were warmer than normal across much of the region with departures ranging from 1 to 8 deg F above normal.

      According to NOAA’s National Operational Hydrologic Remote Sensing Center (NOHRSC), 17.4% of the Northeast region was covered by snow with an average depth of 0.7 inches and a maximum depth of 16.9 inches on December 14. In terms of climatological rankings, the August-November and June-November periods were the warmest on record according to NOAA NCEI.

      Looking Ahead

      The NWS WPC 7-Day Quantitative Precipitation Forecast (QPF) calls for moderate-to-heavy liquid (liquid = rain + SWE) precipitation accumulations ranging from 2 to 5 inches in an area extending from Northern California to Washington state.

      Similar accumulations are forecasted in the Sierra Nevada Range, Cascade Range, and Olympic Mountains. In the Intermountain West, 1-to-2-inch (liquid) accumulations are expected across the Central and Northern Rockies.

      Elsewhere, 1-to-5-inch (liquid) accumulations are expected in far southeastern portions of the Southern Plains as well as in the Lower Mississippi, Tennessee, and Ohio River valleys. Along the Eastern Seaboard, light accumulations of generally <1 inch are expected in New England while most of the Mid-Atlantic and Southeast will continue to be generally dry.

      The CPC 6-10-day Outlooks calls for a moderate-to-high probability of above-normal temperatures across the southern half of the conterminous United States excluding California and Nevada. Below-normal temperatures are expected across the remainder of the West including the Pacific Northwest and the Northern Rockies.

      In terms of precipitation, the wetter-than-normal pattern is expected to persist across the western U.S., while there is a low-to-moderate probability of below-normal precipitation across the Central and Northern Plains as well as across most of the South. In the eastern third, near-normal precipitation is expected.




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