Weekly Weather Summary: The period began with a widespread area of severe weather (mainly strong winds and large hail) across the East Coast states, though with minimal tornadic activity, especially when compared to the previous week. Several other severe weather events followed in the days ahead, first over the middle Mississippi Valley and southern Great Lakes region, and then over the northern and central Great Plains. A widespread area of heavy precipitation (2 inches or greater) was observed from Kansas and Nebraska eastward across the middle Mississippi Valley and southern Great Lakes region. Unfortunately, this broad area doesn’t need the rain, whereas the drought areas to the south (southern Plains, Gulf Coast, and South Atlantic states) received little in the way of beneficial rain. Temperatures averaged anywhere from 4 to 10 degrees (F) above average across most of the East, Ohio and Tennessee Valleys, Gulf Coast states, lower half of the Mississippi Valley, the southern Plains, and the interior Southwest. Elsewhere, which includes much of the northern and western CONUS, temperatures generally ranged from 4 to 10 degrees (F) below average.

Important Note:  The drought depiction for south-central Louisiana was changed last week to show the areas of drought in the Atchafalaya Basin. As a result, this does not take into account the areas of flooding that were released into this basin from the Morganza Spillway courtesy of the lower Mississippi River flooding. The actual flooded areas have been less extensive and slower to spread than initially forecast; however, the amount of flooding is still significant.

For the latest text information about river levels in the region:

http://www.srh.noaa.gov/lmrfc/?n=lmrfc-mississippiandohioriverforecast

For satellite images (via ASTER on NASA’s Terra satellite) of the Morganza Spillway:

http://earthobservatory.nasa.gov/IOTD/view.php?id=50659&src=iotdrss

Upper Great Lakes Region: Light rains (0.5 to 1 inch) were reported in northeastern Minnesota and the UP of Michigan, with heavier totals well to the south of the D0(H) areas. NLDAS ensemble mean soil moisture anomalies (top 1-meter) reveal a slight degradation in the past month in northeastern Minnesota.  Stream flows have shown some improvement recently, but this area still needs a good widespread soaking to eliminate the lingering long-term (12- to 18-months) dryness.

South Atlantic Seaboard and Eastern Gulf Coast States:  Over the past 6 months, the southern Delmarva Peninsula and the adjacent tidewater area of southeastern Virginia have accumulated 5- to 10-inch precipitation deficits. Little if any rain has fallen during the past week.  Current stream flow values, as well as those for the past 7-, 14-, and 28-days, are located within the lowest ten percent of the historical record. On the Delmarva Peninsula, moderate drought (D1) conditions were expanded northward into central Sussex County, westward into eastern Dorchester County, and southward to include the southernmost counties. Given sandy soils, the recent heat, and the large precipitation deficits, this region will need to be monitored for rapid deterioration. The Washington, D.C. area has also been drying out, though a thin corridor of wetness remains along Chesapeake Bay’s western shoreline. If this drying trend continues, abnormally dry (D0) conditions may be warranted for next week’s depiction.

Light rain (0.5 inches or less) fell over eastern portions of the Carolinas during the past 7 days. Standardized precipitation indices (SPI) for the past 6-months support longer-term hydrological impacts, especially in eastern North Carolina. Stream flows for the past month in eastern North Carolina and southern South Carolina have remained within the lowest quartile of the historical distribution. Severe drought (D2(A,H)) conditions were introduced to the drought depiction over eastern North Carolina, emphasizing both short-term and long-term impacts. With much of upstate South Carolina receiving moderate to locally heavy rain (1-3 inches or greater), expanding dryness and drought has been temporarily halted; though with the recent high temperatures conditions are likely to deteriorate fairly quickly.

In Georgia, Florida, Alabama, and Mississippi, significant rainfall (1-2 inches, locally greater) once again missed the drought areas which needed it most. Extreme (D3) drought was expanded in southern Georgia to include the entire southern half of the state, while areas of lesser intensity drought (D2, D1, D0) over central and north-central Georgia were expanded northward. In the western panhandle of Florida, a general 1-category degradation was made to the drought depiction to be consistent with the short- and long-term objective blends, in addition to the 30-, 60-, 90-, 120-, and 180-day SPI indications. Stream flows in the western Florida panhandle, southern Alabama, and southwestern Georgia have been below the tenth percentile for at least the past month. In southern Florida, the combination of short-term dryness (90-days: 25 to 50 percent of normal rainfall) and long-term deficits (since October 1: West Palm Beach and Fort Lauderdale deficits exceed 20 inches) have greatly lowered river and lake levels and some wells. West Palm Beach International Airport (PBI) reported its driest Water-Year-To-Date (October 1, 2010 to May 30, 2011) since 1850, with only 10.39 inches of rain. Lake Okeechobee stood at 10.09 feet (normal 13.13 feet) on June 1st, below the critical line for water shortages. An area of exceptional (D4) drought was introduced to southeastern Palm Beach County and eastern Broward County to reflect the very low well conditions and soil moisture, the very high fire danger, and the large rainfall deficits noted earlier.

Lower Mississippi Valley: As mentioned in the above “Important Note”, the opening of the Morganza Spillway on May 15 (last opened in 1973) had projected to flood much of the Atchafalaya River basin in south-central Louisiana by now. But due to the basin’s hydrological complexity and uncertainty, especially in the southern end, the rate and extent of flooding was less than initially forecast. Where it has flooded, however, the flooding is still significant. In contrast, where the flood waters had not yet reached, U.S. Forestry officials reported that many marshes were dry with fish kills from lack of oxygen in the remaining pools of water in coastal parishes of southwest and south Louisiana. Therefore, due to the severity of the ongoing drought and the lack of a recent image of actual flooded land, the area of drought depiction was returned to all of Louisiana. And of course, the flooded regions of the Atchafalaya basin (not depicted) would be drought free.

Elsewhere across southern Louisiana, little or no rain fell once again, except for a few stations close to the Mississippi border (1-2.5 inches). Light rains (0.5 inches) occurred in northeastern Louisiana and southeastern Arkansas. Unfortunately, additional heavy rains (2 inches or greater) fell across the middle Mississippi Valley, adding more water downstream into the swollen main stems of the Mississippi River. Large long-term (12-months) deficits (15 to 20 inches) remained in portions of the lower Mississippi River Valley, and were between 25 to 35 inches at 18-months. In southern Mississippi, large rainfall deficits (12 inches or more) since October 1st suggest a slight northward displacement of the various drought categories (Dx).

Central and Southern Plains: Widespread showers and thunderstorms dropped heavy rain (2 to 3 inches, locally greater) across portions of southern Nebraska, northern and central Kansas, and northeastern Colorado. Moderate rain (0.5 to 1.5 inches) fell over central and eastern portions of Oklahoma, and across the Red River in nearby northern Texas. Standardized Precipitation Index (SPI) maps going back at least 6 months (as well as recent 2 to 3 inch rains) provides support for a one-category upgrade in northeastern Colorado and south-central Nebraska. In southeastern Nebraska, however, conditions did not appear to warrant the removal of abnormal dryness (D0), especially when considering the longer-term (90- to 180-days) precipitation indicators.

In Texas, USDA Topsoil Moisture (May 29th) was rated short to very short over 86 percent of the state, and in general, the state is experiencing harsh drought impacts in both the short-term (up to 90-days) and the long-term  (beyond 90-days). For the March to May period, San Antonio reported only 0.88 inches of rain, the second driest such period since 1885, with the driest being in 1961, when only 0.52 inches of rain fell. Del Rio experienced its third driest March to May period (1.12 inches) since 1906. A number of adjustments (mostly minor) were made to the drought depiction in Texas, with the most significant being a broad expansion of exceptional (D4) drought across the Panhandle region. USGS stream flows continued to be in the lower tenth percentile in southern and south-central Texas, but improving to near-normal in the northeast. According to USDA/NASS, Texas winter wheat was rated 76 percent poor or very poor, while similarly-rated pastures and ranges stood at 73 percent. Temperatures ranged from 6 to 10 degrees above average across the state, causing even further dessication of soils.

In Oklahoma, USDA Topsoil Moisture (May 29th) was rated short to very short over 53 percent of the state, substantially higher than the 5-year average of 34 percent. The objective short- and long-term blends, in addition to the Worst Drought Indicator Blend, continue to favor mostly agricultural, shorter-term impacts across the western part of the state.

The West: In the Southwest, nearly all areas experienced no rain during the past week, as would be expected at this time of year, in the weeks prior to the seasonal monsoon. The few, spotty exceptions included several tenths of an inch of precipitation in northern New Mexico, southwestern Colorado, and southeastern Utah. Agricultural impacts are notable in New Mexico, where 95 percent of the topsoil moisture (USDA) was rated as short- to very short, compared with 60 percent of the state for the 5-year average. Farther north and west, continued widespread precipitation across northern California, northern Nevada, Washington, Oregon, Idaho, northern Utah, Montana and Wyoming warranted no new introduction of abnormally dry (D0) conditions this week. Heavy precipitation amounts (in excess of 2 inches) were reported over a significant portion of the Northwest, primarily in orographically favored areas such as the Cascades and coastal ranges, the northern Rockies, and the Wasatch Mountains. Unseasonably cool temperatures have also reduced natural water demand by vegetation and decreased evaporation. Snow fell in many high elevation areas this week in the Sierra Nevada, and with the cool temperatures the snowpack is melting at a slower rate. Many areas of northern California and northern Nevada reported May average temperatures anywhere from 2 to 6 degrees below the long-term average.

Hawaii and Alaska: Very little rain fell on Maui, Lanai, and Molokai this past week, though scattered light to moderate showers (up to 1 inch) were reported on Oahu and Kauai. On the Big Island, moderate showers (1 to 2  inches) were observed over the windward (eastern) slopes, though for most other areas, less than 0.5 inch of rain fell. No changes were deemed necessary in the drought depiction for Hawaii this week. In Alaska, most of the southern coast received little if any precipitation, with the exceptions being Kodiak Island and portions of the Panhandle where light precipitation (generally 0.5 inches) was noted. However, amounts were not large enough to eliminate long-term deficits. In the short-term (30- to 90-days), precipitation in southwestern and southeastern Alaska is close to or above-normal, however, in the long-term (since September 1, 2010, the start of the cold season), much larger deficits have accumulated. This included departures of -10 to -30 inches since September 1 along the normally wetter coastal locations. Last week, the D0(H) area was slightly modified to depict the larger long-term deficits. This week, no changes were made to the Alaska drought depiction.

Looking Ahead: During the next 5 days (June 2-6), a frontal system moving across the northern Rockies and northern Great Plains will bring as much as an inch of precipitation to that region. Another storm system moving into northern California will bring 1 to 2 inches of rain. Between 0.5 and 1.0 inches of  rain is expected to fall over New Mexico, east Texas, and southern Louisiana during the period, which would be beneficial to those areas. However, much more rain would be needed to make a significant impact on the drought in those areas.

The 6-10 day CPC outlook (June 7-11) calls for above-normal temperatures over most of the eastern lower 48 states, and most of central and eastern Alaska. Below normal temperatures are predicted over most of the western lower 48 states, and southwestern Alaska. Above median precipitation is forecast from the northern and central Great Basin northeastward across the northern Plains to the central Great Lakes region.   Unfortunately, the drought areas of the southern CONUS, southern Atlantic, and middle Atlantic states are expected to receive below median rainfall during the period.

Author: Anthony Artusa, 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: Heavy rainfall hits the Midwest, temperatures warming up