Stormy and chilly weather, with temperatures averaging more than 4°F below normal, enveloped much of the nation, particularly east of the Rockies. Weekly departures exceeded -8°F in the south-central Plains, Midwest, and Northeast. A series of storm systems traversed the country, including a strong Nor’easter that dumped excessive rains on coastal New Jersey and New York, and blanketed interior New England with heavy snow. Very strong and persistent winds accompanied the system, affecting much of the East Coast. The storms, however, did bring beneficial rains to the Delta and Southeast. Light precipitation also fell on most of the West and Rockies, but after a very dry and warm March, short-term conditions continued to decline. Dry weather prevailed in the northern Plains, upper Midwest, Rio GrandeValley, and most of Puerto Rico, Alaska, and the leeward side of Hawaii.
The West: After hoping for a wet and cool March, the opposite occurred instead – unseasonably dry with near-record warmth – and nearly every Western basin registered a decline in snow packs with significant meltouts. According to NRCS’s April 1 Western Snowpack Conditions and Water Supply Forecasts, the losses were greatest in the Southwest and central Oregon, where snow packs declined more than 30%. A similar March decline was also observed in 2004. As a result, snow packs are extremely low in Arizona, Utah, Nevada, California, and eastern Oregon. As of April 16, many long-term NRCS SNOTEL sites in central Arizona, Utah, northern Nevada, and eastern Oregon had already melted out, with several nearby stations reporting only 1-39% of its snow water equivalent (SWE). This was also true in California’s Sierra Nevada, and should result in a very poor spring and summer stream flow and runoff. Fortunately, April 1 reservoir storages are above seasonal averages in California, Colorado, Idaho, Nevada, and Washington, and only slightly below-normal in Oregon and Utah, providing some cushion from the expected poor 2007 stream flow and runoff forecast. In contrast, Arizona, Montana, New Mexico, and Wyoming reservoirs are at below-normal capacities.
After March’s disappointing precipitation (except for above-normal precipitation in northwestern Washington and the eastern halves of Montana, Wyoming, and New Mexico), season-to-date totals (since October 1) were less than 50% in southern California, southern Nevada, and western Arizona; between 50-69% in northern California and western Nevada; and between 70-89% in eastern Nevada, southeastern Oregon, south-central Idaho, western Utah, and eastern Arizona. This week, light precipitation and slightly below-normal temperatures briefly halted the recent parade of dry and warm days, but did little to improve conditions except to slow the snow melt. During the past 6 months, deficits of 4 to 8 inches have accumulated throughout much of California and central Arizona, with shortages exceeding a foot around the Los Angeles area, in parts of the Sierra Nevada, and north-central California. At 4 southern California stations, downtown Los Angeles (2.67 in), Los Angeles Airport (2.27 in), Long Beach (1.62 in), and Burbank (2.39 in), this rainy season (since July 1, 2006) is currently the driest to date since records began at each location, or about 20% of normal, justifying a D3(A) expansion in the area. Similarly, moderate and severe non-hydrologic (A) drought increased and edged northward, with many California communities already requesting that residents voluntarily reduce water usage. According to USDA/NASS, some northern California foothill pastures benefited from the rain, but others had dried beyond the point of being helped by moisture. In central California, pastures were dry and feeder cattle were being shipped elsewhere. Farther north, the recent dryness, warmth, and rapidly declining basin SWE (between 50-75%) warranted an extension of D0(A) into the rest of eastern Oregon, central Idaho, and southwestern Montana. In southwestern Wyoming, little or no precipitation plus basin SWE between 54-63% deteriorated conditions by one category. Additionally, the Impact lines were redrawn to emphasize short to medium-term drought (A) in westernmost areas since hydrologic concerns (e.g. reservoirs) are currently adequate; H was placed in easternmost areas where long-term hydrologic concerns lingered but short-term moisture was adequate or excessive; and AH was labeled in transitional areas where both short- and long-term drought impacts were occurring.
The Plains and upper Midwest: Across the upper Midwest, all of the snow had finally melted this week, even though temperatures averaged below normal. Although little or no precipitation fell, conditions remained static as the subsoil remained frozen and the topsoil was very moist from the melted snow.
In the southern Plains, little or no rain fell on south-central Texas where long-term drought (at 12 to 24 months) still lingered. Farther north, moderate to heavy precipitation was measured across much of Oklahoma and Kansas, alleviating abnormal dryness in eastern Oklahoma except in the far southeast where less than 0.5 inches fell, and D0(A) remained. In the Dakotas and Nebraska, light precipitation (0.1 to 0.5 inches) and subnormal temperatures did little to dent long-term (H) drought; however, in North Dakota, moist topsoil, frozen subsoil (5-27 inches), and cold weather has delayed the start of fieldwork. In addition, a reassessment of products and indices indicated adequate to surplus moisture in central South Dakota and south-central North Dakota, and the D0(H) area was shifted westward.
The Delta and Southeast: A vigorous storm system that tracked across the Southeast and eventually morphed into a Nor’easter generated widespread, beneficial showers and thunderstorms, although numerous reports of severe weather accompanied the moderate to heavy rainfall. The rains tended to fall in stripes, with many areas receiving over 2 inches while others measured under an inch. Heavy rains (2 to 6 inches) and a general one-category improvement occurred from northern Louisiana/southern Arkansas northeastward into southern West Virginia, from southern Louisiana northeastward into North Carolina, and in the southern half of Florida. In the southern Appalachians, however, rainfall tended to be somewhat lighter (1 to 2.5 inches), and USGS stream flows, after an initial rapid rise, had started to drop back down into the lower 25th percentile by early Wednesday. In addition, 30-, 60-, and 90-day deficits still remained, and with 75 small wildfires still active in western North Carolina on Tuesday, D0 and D1 were retained in the western Carolinas.
In contrast, an inch or less was measured in central Louisiana, central Mississippi, north-central Alabama, northwestern and southeastern Georgia, northern Florida, and extreme southern South Carolina, including some locations in the aforementioned areas where less than 0.5 inches fell. Accordingly, drought conditions were maintained or even expanded where rainfall was meager. The latter included D2 across north-central Florida, southeastern and northwestern Georgia, and southwestern Mississippi where USGS stream flow values (1-, 7-, 14-, and 28-days) remained in the lower 10th percentile. Even with the recent rains, 60-day deficits were still 6 to 10 inches, and some locales had still measured less than 25% of normal precipitation. The D3 was shifted south and east in alignment with the drought blends, CPC soil moisture, 30-, 60-, 90-, and 6-month precipitation anomalies, and near-record low USGS stream flows. In southern Florida, the rains were welcome and provided some slight relief, especially in far southern sections, but more was needed. Lake Okeechobee’s level had dropped to 10.02 feet on April 17 (4 feet below average), and the wells around the lake and west coast areas were in the lowest 10th percentile. The South Florida Water District implemented Phase 3 for the Lake Okeechobee area (only 1 day watering/week), and Phase 2 for the east and west coast metro areas of south Florida (only 2 days watering/week). The Governor of Florida asked the people of South Florida to obey the water restrictions and to conserve water.
Hawaii, Puerto Rico, and Alaska: With typical trade wind conditions, the windward locations experienced light showers (0.1 to 0.5 inches, isolated 1-2 inches) while mostly dry conditions were observed on leeward sides (isolated 0.1-0.5 inches), maintaining abnormally dry conditions across Hawaii.
In Puerto Rico, after unseasonably heavy late March rains (2-4 inches) fell across the island and provided some relief, April has been relatively dry across the southern and eastern sections. As a result, short-term deficits have accumulated, with year-to-date rainfall less than 75% and about half of normal in the southeast. Accordingly, the D1 was redrawn to reflect where shortages exceeded 6 inches since January 1.
Above-normal temperatures and dry weather prevailed across much of Alaska, except along the southeastern Panhandle where light to moderate precipitation and subnormal readings occurred. According to the April 1 NRCS Water Supply Forecast Summary and Alaskan SWE sites, mountain snow pack was 50-69% of normal across central and southwestern Alaska after a very cold March that was dry (<50%) in the west but had ample snows in the northeast (snow pack increased by more than 20% between March 1 and April 1, easing D0 there). In addition, with season-to-date (since Oct. 1, 2006) precipitation less than 70% across the southwestern Panhandle, D0 was extended into this area.
Looking Ahead: During the next 5 days (April 19-23), an upper-air trough of low pressure will settle over the West and bring in a series of Pacific storms to the region. Precipitation will fall on most of the West (including snow in the northern Rockies, Utah’s Wasatch Range, and Sierra Nevadas) and eventually the northern half of the Plains, but will miss the Southwest. Temperatures should remain below-normal from the Rockies westward. In the East, leftover showers from the departing Nor’easter will fall over coastal New England while two weak low pressure centers will produce light rain in parts of the Corn Belt and Southeast. As the last system finally departs by late Friday, subnormal readings will slowly moderate to above-normal levels by Sunday. Only the north-central states will observe above-normal temperatures during most of this period.
The NWS 6-10 day outlook (April 24-28) calls for subnormal precipitation in the West and across most of Florida, with wetter-than-normal conditions expected from southern Plains and western half of the Gulf Coast northeastward into the Great Lakes region and Northeast, and the southern third of Alaska. Temperatures will average above-normal in the Far West and eastern third of the nation, with cooler-than-normal weather in the southern halves of the Rockies and Plains.
Source: David Miskus, Joint Agricultural Weather Facility