Weekly Weather Summary: An unsettled weather pattern brought widespread light to moderate rains to most of the eastern half of the Nation, and a series of Pacific frontal systems dropped unseasonable precipitation on the Northwest and parts of California.  A slow-moving upper-air low meandered along a stalled front, traveling from the central High Plains to the Ohio Valley and then tracking southeastward into the Carolinas. The system triggered numerous showers and thunderstorms from northeastern Colorado eastward to the mid-Atlantic and Carolinas, while welcome rains fell on drought-stricken portions of central and eastern Texas and central Oklahoma. More rain fell on the saturated middle Mississippi, Tennessee, and Ohio Valleys, although totals were much less than previous weeks. The record crests along the lower Mississippi River flood waters were diverted into central Louisiana’s Atchafalaya River Basin (via the Morganza spillway) to ease the potential flooding of Baton Rouge and New Orleans, LA. Unfortunately, little or no rain fell on drought-hit areas of the Southwest, parts of the southern High Plains, and along sections of the central Gulf Coast. Temperatures were subnormal along the West Coast and in the Plains, and above-normal in the northern Rockies and eastern third of the U.S.

Upper Great Lakes Region: Light to moderate rains (1 to 2 inches) across most of northern Wisconsin, the Upper Peninsula of Michigan, and northern lower Michigan was enough to alleviate D0(H) in much of the region. Precipitation has been 1.5 to 2 times above-normal at the short and medium-term (30-, 60-, and 90-days), and even above normal at the long-term (180- and 365-days). USGS average stream flows (1-, 7-, 14-, and 28-days) were mostly in the upper 75th percentile and soil moistures indicated wet conditions. However, there were a few spots where the precipitation was less than the surrounding areas, and this included the Arrowhead of Minnesota and the Upper Peninsula of Michigan’s Keweenaw Peninsula and adjacent area. In Minnesota, stream discharge was in the lower end of normal, fire danger was moderate, and April 1-May 16 precipitation was near to somewhat below normal. In the Upper Peninsula of Michigan, long-term deficits remained (e.g. Houghton, MI was 85 percent of normal during the past 34 months, with a deficit of 13.87 inches). Accordingly, D0(H) remained in these two areas.

South Atlantic Seaboard and Eastern Gulf Coast States: Scattered showers and thunderstorms provided localized relief to some areas in Virginia and the Carolinas where the rains exceeded 2 inches (near Richmond, VA, Wilmington, NC, Myrtle Beach, SC, and north-central South Carolina and south-central North Carolina), but elsewhere overall precipitation (0.5 to 1 inch) kept conditions status-quo. An exception to this was in extreme eastern North Carolina where weekly rain totals were less than 0.3 inches and short-term precipitation remained well below-normal (30-days: 25-50 percent; 60-days: 30 to 70 percent; 90-days: 40 to 75 percent). The two separate D1 areas in southeast Virginia and southeast North Carolina were merged to include northeastern North Carolina, with D1(A) emphasizing the short-term dryness. Additionally, the D0(A) was pushed northward into the Delmarva Peninsula where precipitation totals for the past 30-, 60-, 90-, and 180-days were less than 75 percent of normal.

Farther south, another week of subnormal rainfall in southern Mississippi, southern Alabama, most of Georgia, and western South Carolina, combined with near to above-normal temperatures, produced deteriorating soil moisture conditions along the central and eastern Gulf Coast. Both short- and long-term deficits exist, and USGS average stream flows have dropped into the lower 10th percentile in much of this region. Along the Gulf Coast where La Nina-induced winter precipitation shortages (6-months: 50 to 70 percent of normal), the spring rains have also been disappointing. As a result, D3 was added to areas where less than 50 percent of normal precipitation was observed at 30-, 60-, and 90-days. D1 and D2 were also edged northward in eastern Alabama and central Georgia, and extended into southwest South Carolina. Fortunately some rain (0.75 to 1.5 inches) fell on southeastern Georgia, maintaining D3 conditions there. In Florida, a general increase in shower activity provided some stability to conditions, and where heavier rains (1 to 3 inches) fell, some slight improvements were made. This included parts of north-central, central, and southwestern Florida.

Lower Mississippi Valley: The opening of the Morganza Spillway has relieved pressure on Mississippi River levees downriver, producing lower crests earlier than what had been predicted at Baton Rouge and New Orleans, LA. While sparing Louisiana’s largest cities from widespread Mississippi River flooding, hundreds of thousands of acres of farmland and thousands of homes in south-central Louisiana will be flooded as water from the spillway pours out into the Atchafalaya River basin. Since the last time this area was intentionally flooded (1973), the heart of Cajun country is much drier and absorbing more water, thus slowing the flow. Accordingly, this area of south-central Louisiana is depicted with no drought as drawn last week. And fortunately, little or no rain fell on most of Louisiana, western Mississippi, and southern Arkansas, preventing additional runoff into the lower Mississippi River basin but providing no relief to drought-stricken and non-flooded locations. Additional moderate-heavy rains (1.5 to 3 inches) fell on northern Arkansas, western Tennessee, and northern Mississippi, with the latter region receiving some improvement in the D0 and D1 areas.

Central and Southern Plains: Persistent rains that had brought relief to parts of north-central and northeastern Texas since early April finally became more widespread and heavier, soaking parts of  central and eastern Texas, near Del Rio, TX, and portions of the TX Panhandle with 1.5 to 4 inches. Unfortunately, little or no rain fell on southwestern, extreme western, west-central, and extreme southern Texas, and along the western Gulf Coast. Accordingly, where the heaviest rains occurred, a 1-category improvement was made, while continued dryness in the west & southwest maintained or expanded D3 and D4. In north-central Texas where 2 to 4 inches of rain fell, conditions improved to D1 from Runnels and Coleman counties northeastward to Tarrant county, and to D0 from Tarrant and Dallas counties to the Oklahoma border (Grayson to Bowie counties). D2-D4 were also improved by one category in surrounding areas where the rainfall was greatest. Similarly, in south-central Texas, 1.5 to 4 inches of rain eased D4 conditions near Del Rio (Val Verde county), in La Salle, Dimmit, and Webb counties, and Bell, Burnet, & Llano counties. In eastern Texas, D4 was eased from Houston, northern Trinity, Angelina, southern Nacogdoches, and southern Cherokee counties. Although this week's rains fell on the watershed for Sam Rayburn Reservoir, the lake continued to drop, reaching 1.99 million acre-feet (normally at 2.9 million acre-feet), and much of what remained was inaccessible.  In the extreme northern Panhandle, 1 to 2 inches of rain diminished the D3 in Hansford & Sherman counties. Farther north, moderate to heavy rains (1 to 3 inches) fell on central and northeast Colorado, most of Nebraska, and parts of eastern Kansas and central and eastern Oklahoma. A 1-category improvement was made where the largest totals occurred. In addition, the eastern edge of the drought (D0-D2) in the central Great Plains was trimmed as decent rains fell there. In Nebraska, releases on the Platte River from Lake McConaughy increased on May 17. Flows doubled to 4000 cfs, with 7000 cfs entering the lake. Low land flooding will increase in intensity with North Platte River stage expected to reach the 1971 record. A more dire situation is possible as 2 million acre feet of water is estimated in the central Rockies snow pack, but only a million acre feet of storage is available in McConaughy now. Any additional heavy rains in this region will aggravate conditions.

In contrast, exceptional short-term dryness continued in western sections of the state, expanding D4 into the Big Bend region and D3 into the northern Texas Panhandle. Elsewhere, another dry week in northeastern New Mexico pushed D3 northward and into southeastern Colorado. According to AHPS precipitation, this region has observed less than 10 percent of normal precipitation the past 90-days, and less than 25 percent the past 6 months. Similar conditions existed in western Oklahoma northward into south-central Kansas, and D3 was added here. According to the USDA/NASS, May 15 state winter wheat conditions rated very poor or poor included: CO 41 percent; KS 55 percent; OK 80 percent, and TX 75 percent. Similarly, May 15 pasture and range conditions considered very poor or poor included: AZ 55 percent; CO 41 percent; KS 35 percent; NM 80 percent; OK 50 percent, and TX 71percent.

The Southwest: Little or no rain fell on much of Arizona, New Mexico, southern Utah, and southeastern Colorado. Normal precipitation totals in the Southwest are generally low during the spring months, although rainfall will usually increase during the late summer southwestern monsoon. An exception to this is in the southern High Plains (west Texas, eastern New Mexico, and eastern Colorado) where precipitation typically increases during mid- to late spring. Unfortunately, the spring rains have failed to materialize in the southern High Plains, and with the subnormal winter precipitation courtesy of the strong La Nina, deficits have gradually accumulated. During the past 90 days, less than 5 percent of normal precipitation has fallen on southwestern Texas and the southern two-thirds of New Mexico, and less than 25 percent was measured in west Texas, southeastern Colorado, the Oklahoma Panhandle, and eastern Arizona. Since mid-November (6-months), less than 5 percent of normal precipitation fell on southwestern Texas and southeastern New Mexico, and less than 25 percent in southeastern Arizona, southern half of New Mexico, and west Texas. For example, Midland-Odessa, TX, has received only 0.11 inches since October 1, 2010 (normally about 6 inches, or 2 percent of normal). With deteriorating pasture and range conditions and poor to very poorly rated non-irrigated crops, D3 was expanded northward into northeastern New Mexico & southeastern Colorado, and D4 increased into nearly all of southwest Texas, eastern New Mexico, and extreme southeastern Colorado.

Hawaii and Alaska: Scattered light to moderate (1 to 3 inches) showers fell on the eastern (Kauai and Oahu) and western (Big Island) Hawaiian Islands, while little or no rains fell on the middle (Maui, Molokai, Lanai) islands. Recent past rains, however, have been adequate enough to maintain current conditions. According to the May 5 Hawaii drought information statement from Honolulu, agricultural conditions have recovered enough from rains earlier in the year in parts of Maui to justify the improvement of D2 to D1 in southwestern sections. On the Big Island, rain totals increased in April over the leeward slopes, but it was too soon to determine how much improvement would occur over the existing drought areas. On Molokai, water levels in the Waimanelo Reservoir remained steady during the past month, leaving the mandatory 30 percent cutbacks in irrigation water consumption in place.

In Alaska, mostly dry and seasonable temperatures were observed across the state. Weekly temperatures generally averaged within 2 to 4 degrees F of normal, and the largest precipitation totals (0.5 to 1.5 inches) were found in the southwest and southeast sections of Alaska. No changes were made to this week's depiction as adequate precipitation fell on most of the D0 area last week. However, as temperatures normally begin to increase during the late spring and summer months, subnormal rainfall will need to be closely monitored.

Looking Ahead: During the next 5 days (May 19-23), heavy precipitation (more than 2 inches) is forecast for the north-central Rockies and Plains (Wyoming and South Dakota), and for the central and south-central Great Plains and lower Missouri Valley. Additional light to moderate rains may fall on northeastern and central Texas. Unsettled weather is also expected across the Midwest and Northeast, and in the Great Basin. Unfortunately, little or no rain is forecast for most of the Southwest, southern High Plains, and the central and eastern Gulf and southern Atlantic Coast States. Temperatures are expected to average below normal in the western half of the Nation, and above-normal in the Southeast.

The 6-10 day CPC outlook (May 24-28) calls for a continued wet pattern across the northern portion of the Nation, from the Pacific Northwest to the north-central Plains to the Ohio Valley and northern Appalachians. Odds favor subnormal precipitation along the southern tier of the U.S., from southern California to the southern Great Plains and along the Gulf Coast into Florida. Above-normal temperatures are expected along the Gulf and Atlantic Coast States, with subnormal readings forecast for the North-Central States. Alaska should have mostly dry and mild conditions, especially in the northern half of the state.

Author: David Miskus, NOAA/NWS/NCEP/CPC

Dryness Categories
D0 ... Abnormally Dry ... used for areas showing dryness but not yet in drought, or for areas recovering from drought.

Drought Intensity Categories
D1 ... Moderate Drought
D2 ... Severe Drought
D3 ... Extreme Drought
D4 ... Exceptional Drought

Drought or Dryness Types
A ... Agricultural
H ... Hydrological

Drought Monitor: Rain covers east half of the U.S.