U.S. Drought Monitor Report for the Week Ending Sept. 17 (Text)Stephen Rose
Following is the text of the weekly U.S. Drought Monitor as released by the National Drought Mitigation Center in Lincoln, Nebraska:
Weather Summary: The combination of ample Gulf and Pacific tropical moisture (in part from Tropical Storms Manuel (Pacific) and Ingrid (Gulf) which inundated Mexico), stalled frontal systems, and upsloping conditions produced widespread heavy to copious rainfall (widespread 2 to 6 inches, locally 12 to 18 inches especially near Boulder, CO) and severe flash flooding in parts of New Mexico and Colorado. Moderate to heavy rains (1.5 to 4 inches) also drenched portions of Arizona, eastern Nevada, Utah, Wyoming, south-central Montana, western sections of Nebraska, Kansas, Oklahoma, and northern and southern Texas. September monsoonal rains have generated welcome relief from the drought in the Southwest, central Rockies, and High Plains, but unfortunately have been accompanied by flash flooding. Elsewhere, a pair of cold fronts during the week brought relief from last week’s unseasonable heat in the Midwest and Northeast, along with light to moderate rain that generally prevented further deterioration of conditions. Hit and miss (mostly miss) showers occurred in the Southeast, with the most significant rains (more than 2 inches) limited to southern Florida. Warm and mostly dry weather returned to the Northwest after a wet first week of September. Wet weather continued across most of Alaska, while decent windward showers returned to the Hawaiian Islands. The Northeast: The passage of two cold fronts brought both rain and heat relief to New England and the mid-Atlantic. The rains were more widespread and heavier in New England (2 to 5 inches), but unfortunately less so to the south. Although 0.5 to 1.5 inches fell across the D0 areas in Pennsylvania, Long Island, and the mid-Atlantic, short-term deficits (at 60- and 90-days) continued to accumulate. As a result, a slight expansion of the abnormal dryness was made by linking the two areas in central Pennsylvania and Maryland together where 50-75% of normal precipitation has fallen the past 60 and 90 days, creating shortages of 2 to 4 inches. Fortunately, a wet spring and early summer plus cooler weather has tempered the potential negative impacts of the recent dryness. Most USGS 14- and 28-days average stream flow values in the region are near normal, although some sites in the D0 areas are below the 25th percentile, and a few in Maryland are below the 10th percentile. The Midwest: After several weeks of rapid deterioration due to “flash drought” conditions in the Midwest (lack of rain + heat), this week finally brought some relief with lower temperatures and light to moderate rain via two separate cold fronts. Locally heavy rains (1.5 to 3 inches) that fell across west-central Minnesota was enough to remove the D0 there, and 2-weeks of moderate rains in west-central Illinois reduced shortages enough to improve to D0. The widespread 0.5 to 1 inches of rain that occurred elsewhere was enough to prevent further deterioration, but not enough to significantly dent the 90-day accumulated deficits. In contrast, rainfall amounts diminished across southern sections of the Midwest, with weekly totals under 0.5 inches. With increasing deficits at 60- and 90-days, abnormal dryness was expanded across most of Indiana, southwestern Ohio, and southern Illinois. D1 similarly increased near St. Louis, MO area, central Illinois, and central Indiana. 60- and 90-day deficiencies have reached 3-6 and 4-8 inches, respectively, in the aforementioned D1 areas. Historically, after a relatively wet June (and spring), the summer months (Jun-Aug) were ranked as the 13th, 32nd, and 36th driest since 1895 (119 years) for Iowa, Minnesota, and Illinois, respectively, according to NCDC, depicting how dry July and August were. Fortunately, summer temperatures averaged below normal. Agriculturally, the recent heat (and Jul-Aug dryness) created declines in potential crop yields while it accelerated the denting and maturity stages of corn to values well above the 5-year averages. Corn and soybean crop conditions rated good to excellent fell from early July highs of 68 and 67% to 56 and 54% by September 1, respectively, according to NASS/USDA. The Lower Mississippi Valley: Since late July, much of the lower Delta region (Louisiana, southern Arkansas, western Mississippi) has missed out on significant rainfall that has fallen on nearby areas. This week was no exception as Louisiana averaged only 0.24 inches this past week with a departure from normal of -0.85 inches. Many locations received no rainfall, and temperatures averaged 2 to 4 degrees F above normal. Precipitation has been under 25% of normal the past 30 days, and between 50-75% of normal during the past 60- and 90-days, with shortages of 3-6 and 4-8 inches, respectively. The drier weather has started to creep eastward, with western portions of Alabama now below normal at 30- and 60-days. The USGS 1-, 7-, 14-, and 28-day stream flows have dropped below the tenth percentile in central parts of Louisiana and Mississippi. The summer months were also quite dry in Louisiana, with a ranking of 22nd driest summer (June-August) since 1895, according to NCDC. Accordingly, D0 through D3 was expanded eastward to cover the driest areas during the past 2 to 3 months. Northern and Central Great Plains: The Dakotas observed opposite conditions as decent rains (2-4 inches) the past two weeks fell on most of North Dakota, easing drought conditions along the western edge and southeastern section as 60-day deficits were reduced while 90-day surpluses were found. In contrast, most of South Dakota was dry, with only half an inch of rain measured in the extreme northeastern part of the state. Although near to surplus rains have fallen across western and southern South Dakota the past 60-days, the northeastern corner has measured less than 25% of normal precipitation. For example, Aberdeen (1.02”), Webster (1.20”), Clear Lake (1.12”), Watertown (0.96”), and Bryant (1.62”) totals since August 1 are at near-record dry levels. This dryness, plus the recent heat that caused loss of potential crop yields (especially soybeans) justified a return of D2 in this area. In Nebraska, rains were limited to the southern half and far western portions of the state. Weekly totals generally ranged around an inch, keeping conditions status-quo. An exception was along the Kansas border and in the extreme southwestern and far western areas where amounts exceeded 2 inches, allowing for some slight drought reductions to be made. In Kansas, heavy rains (more than 2 inches) fell across the northern half of the state, with up to 7 inches falling in the northwestern corner, while decent rains also occurred in western and southern sections. As a result, D4 was eliminated from Kansas (to D3) while a reduction in the eastern D0-D3 edges were made. The D0 edge in eastern and southern Kansas was also pared back. Some small 2-category improvements were done in northwestern Kansas in association with the heaviest rains. Southern Great Plains: In Oklahoma and Texas, general improvements were made in western sections while eastern portions deteriorated. In the Oklahoma Panhandle, copious monsoonal rains that inundated parts of the Southwest and central Rockies and caused flash flooding also soaked the extreme western Panhandle (and southeastern Colorado) with over 5 inches of rain, enough for a 2-category improvement to D1. With lesser totals (1.5 to 3 inches) just to the east, a 1- category improvement was made to the rest of the Oklahoma Panhandle and in northwestern Oklahoma. Similarly, 2 to 4 inches of rain along the KS-OK border was enough to erase D0 in Kay and Osage counties. However, little or no rain along the Red River Valley continued the dry trend in southern sections of the state as D2 and D3 expanded in extreme southern Oklahoma and across much of eastern Texas (and Louisiana). 30-day rainfall was under 25%, while 60- and 90-day precipitation hovered around 50%, creating 3-6 and 4-8 inch deficits, respectively. In contrast, tropical moisture from Tropical Storm Ingrid in the western Gulf pushed enough moisture northward to dump 2 to 7 inches of rain on southern Texas. Frequent tropical showers have brought Brownsville, TX, 11.29 inches of rain so far this month, with Harlingen at 8.14 inches and McAllen at 5.99 inches. Accordingly, drought was reduced a category where the heaviest rains fell. The Southwest: The robust southwestern summer monsoon exploded with copious rainfall (6 to 12 inches, locally over 18 inches near Boulder, CO) across portions of New Mexico and Colorado, producing severe flash flooding, loss of lives, and the destruction of property and infrastructure. The combination of ample Gulf and Pacific tropical moisture (in part from Tropical Storms Manuel (Pacific) and Ingrid (Gulf) which inundated Mexico), stalled frontal systems, and upsloping conditions produced the widespread rainfall. Other states in the surrounding region (Arizona, Nevada, Utah, southern Idaho, Wyoming, south-central Montana) and the High Plains also received beneficial moisture from the monsoon, not only this week but in weeks past. In Colorado, widespread flooding was realized from these rains on the Cache la Poudre, South Platte, Big Thompson, and St. Vrain Rivers where communities were stranded as roads collapsed. This was a historic flood (estimates are currently a 100 year flood) for the Front Range, and as such, many improvements are warranted. In some cases, 2-3 category improvements were recommended as 3 inches of rain is approximately 20% of the normal ANNUAL total at many locations. This event was not convective activity, but more tropical in nature, falling for several days in succession. This time of the year is also a huge consideration for improvement as it allows for excellent soil moisture storage going into the fall when evapotranspiration rates are much less as compared to the height of the growing season. In New Mexico, similar 4-10 inch totals (minus the excessive 18 inches) fell, also leading to a widespread 1-category improvement statewide, but due to the prolonged 3-year drought, 2-category improvements were very limited. It will be interesting to see how quickly and how much the major reservoirs in New Mexico react to these rains. Similar 1-category improvements were made in Arizona, Utah, Wyoming, and south-central Montana where 2-4 inches of rain diminished long-term deficits. Numerous flood warnings were issued by the NWS in these states, and with no surprise, most USGS stream flow levels were currently at near or record high flows. Although Arizona saw less rain this week, last week’s downpours were enough to increase flows on the Gila River that raised water levels at the Coolidge Dam by 7 feet, with a few more feet still expected. The West: The heavy monsoonal rains that inundated the Southwest and central Rockies bypassed the West, leaving warm and dry weather instead. As this is the normally dry summer and early fall season, no changes were made to much of the region. An exception was made in extreme southern California where isolated heavy showers (1 to 2.5 inches) fell west of the Salton Sea last week, and that was enough to reduce Water Year-To-Date (WYTD) deficits and change D2 to D1. Similarly in eastern Nevada, additional showers (1 to 2.5 inches) continued to diminish the long-term deficits in the region, allowing for some minor improvements to D2 and D3 areas. In contrast, along the coast near San Diego, CA, D2 was expanded as the WYTD percentages were similar to areas just to the north (between 25-50%). In California, the 154 reservoirs are at 79% of average; last year at this time it was 90%. The reservoirs are not at critical levels yet as they need to be in the 30-40% range to be critical. However, the Department of Water Resources were informing water agencies to prepare for a dry 2014 as water deliveries will be less than normal so the reservoir storage can recover. In Nevada, drought declarations continued for a majority of the state, with agriculture the hardest hit from irrigation restrictions and cattle ranchers selling off more of their herd due to lack of grazing land. Hawaii and Alaska: In Hawaii, a couple of remnant frontal rain bands produced scattered light to moderate showers on the western half of Hawaii (Kauai, Oahu), helping to mitigate the worsening of drought conditions there. In the eastern portion of Hawaii, drier conditions (fewer and lighter showers) were enough to bridge the two separate D2 areas on the western Big Island as lack of rain has forced users on catchment systems to haul water and severely ration usage. In Alaska, another wet week, especially along the southern (1.5 to 4 inches) and northwestern (0.5 to 1.2 inches) coasts and in central Alaska (1 to 2.5 inches), allowed for another round of improvements to the state. Wet weather started in early August along the south-central coast, and has expanded northward each week. The recent rains and lower temperatures followed a warm and dry summer that produced significant deficits for the warm season. The main impact in the Alaskan interior was the deep drying of soils and another summer of significant stress on the primary tree species of boreal forest. Fortunately, there has been enough rain and a return to seasonable temperatures that the wildfire season is over; however, long-term deficits accumulated during the summer still linger in the interior. Looking Ahead: During September 19-23, a slow-moving frontal system will trigger showers and thunderstorms from the Great Plains eastward to the Atlantic Coast, with the largest totals (more than 1.5 inches) expected along the Rio Grande Valley eastward along the Gulf Coast and into the Southeast. An inch of rain is also forecasted from the central Great Plains northeastward into the upper Great Lakes region, with unsettled weather returning to the Pacific Northwest. Much drier weather should return to the Southwest, central Rockies, and High Plains, allowing for recovery from flooding. Temperatures should average above normal across the contiguous U.S. except for subnormal readings in the Far West. For the ensuing 5 days (September 24-28), the odds favor enhanced chances for precipitation from the Pacific Northwest eastward to the upper Midwest, in southern Florida, and along the southern coast of Alaska. Subnormal precipitation is likely in northern Alaska and the Tennessee and Ohio Valleys and eastern Great Lakes region. Above-normal temperatures are probable east of the Rockies and west of the Appalachians and in southeastern Alaska while a tilt toward subnormal readings are forecast for the West and southwestern Alaska.
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