This week’s summary: A low pressure system propagated eastward across much of the southern tier states early in the period (March 4-6) and merged with a short-wave trough dropping southeastward from the Midwest before moving off the Mid-Atlantic coast on March 7. This system dropped more than two inches of rainfall over large areas from extreme southeastern New Mexico eastward to South Carolina. Some areas of central Alabama and Georgia saw more than five inches of rainfall, which fell over saturated soil. However, much of the heavier rainfall remained north of the I-10 corridor from southern Texas to northern Florida, while areas south of I-10 received only modest amounts, which were not nearly enough to reduce deficits. The Pacific Northwest and California also saw some precipitation over the past week, but amounts were not enough to reduce any deficits. Some recent dryness over southern Iowa and northern Missouri was mitigated a bit with near- to above-normal precipitation falling last week as well. Deficits increased in the Mid-Atlantic and New England over the past 30 days, but were kept at bay, as these areas saw 0.1 to 1 inch and 0.1 to 0.5 inches of rainfall, respectively. The active storm track continued last week for Alaska, with the southeastern Panhandle receiving two to six inches of precipitation over many areas. This precipitation, along with near- to below-normal temperatures, has finally produced above-normal snowpack in the Alaska Panhandle for the first time in seven to eight years, warranting D0 removal. Hawaii remained dry on the leeward slopes last week due to persistent trade winds, leading to some D0 expansion and development on the Big Island and Oahu, respectively. Puerto Rico saw D0 removal, as northern portions of the island saw much above-normal precipitation, eliminating short-term deficits.
South: D0 was expanded northward from the Gulf Coast of Louisiana, Mississippi, and Alabama. Many of these areas have 6-month deficits of more than eight inches; 6-month deficits over 12 inches in southeastern LA (i.e. 50 to 75 percent of normal). Rainfall over the past 30 days has been particularly low, with precipitation falling to 25 to 50 percent of normal, with some locations in the new D1 area seeing 10 to 25 percent of normal. Some expansion of D1 was made northeast of Corpus Christi, and D2 toward the coast, as USGS stream flows were well below normal at many locations and soil moisture has diminished further in recent weeks. Meanwhile, many areas in West and North Texas (north of the I-10 corridor) saw anywhere from 0.5 to 3 inches (in isolated locations). The heavier rainfall extended southward into the upper Rio Grande Valley where many areas saw 0.25 to 1 inch of rainfall, warranted some reduction in D0, D1, and D2 areas along the river. It is status quo elsewhere for the Southern Region, including southwestern Oklahoma, whose 7-day totals (0.25 to 1 inch) were not enough to cut into rainfall deficits.
Looking ahead: During the next five days (March 12-16), low pressure will be moving into southern California and the Southwest. This will help to deepen troughing over the western CONUS allowing for a southern stream of moisture to develop, enhancing chances for precipitation over California, the Southwest, southern Great Plains, and the Ohio River Valley. Below-normal temperatures (5°F to 10°F) are also expected for much of the West Coast and northern Rockies. Meanwhile, the Gulf Coast is likely to see temperatures 5°F to 10°F above normal, south of a lingering frontal boundary.
The 6-10 day (March 17-21) extended range forecast favors an amplified 500-hPa height pattern with Pacific ridging building northward into Alaska, leading to above-normal temperatures and precipitation over much of Mainland Alaska, with near- and below- normal precipitation along the southern coast and southeastern Panhandle. Troughing is favored over much of the western CONUS, enhancing probabilities for below-normal temperatures and above-normal precipitation in Central and Southern California and the Southern Rockies, which would be welcome for areas with below-normal snowpack. Above-normal precipitation is favored for the eastern two-thirds of the CONUS, as the pattern is favorable for lee-side cyclogenesis east of the Rockies. Weakly above-normal chances for precipitation are favored along the Gulf Coast east of Texas, with below-normal probabilities favored over the drier areas of the Florida Peninsula. The odds favor above-normal temperatures in the eastern half of the lower 48 states.