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Climate Prediction Center


March - May 2009


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Latest Seasonal Assessment - Heavy rain and snow from mid-February to early March raised river levels, boosted snow pack, and increased reservoir storage in drought-affected areas of California, but major reservoirs remained below normal. The seasonal drought outlook indicates continued improving conditions for northern and central parts of the state, but with the pace of improvement slowing due to forecasts of less rain and snow in coming weeks. Statewide reservoir storage increased from 58 percent of normal on February 17 to 71 percent of normal on March 3. Despite the improvement in water supplies, it is unlikely that shortages will be erased before the dry season sets in. With less precipitation in the forecast, little change in the drought situation is expected for those areas of southern California where drought exists, as well as in northwestern Nevada. Some improvement is forecast for other areas in the Great Basin. Elsewhere, drought has further worsened in the southern Plains, and short and long-range forecasts of below-normal precipitation and above-normal temperatures mean that drought could expand in Kansas, eastern Colorado, western Texas, and southern New Mexico. In contrast, heavy showers forecast during the first 2 weeks of the forecast period in March are likely to bring at least short-term relief to eastern drought areas of Texas and the northern Gulf Coast, with even some improvement possible in the hard-hit areas of south-central Texas. The drought has aggravated wildfire danger and damaged winter crops across the southwestern Plains. In early March, 63 percent of the Texas winter wheat crop rated poor to very poor. To the east, heavy rain and snow over the South at the end of February benefited remaining drought areas. More relief is anticipated from central Georgia northward, while drought should persist over southern Georgia and the Florida Peninsula. Development is forecast in northern Florida while lingering drought in parts of Hawaii should ease.

Discussion for the Seasonal Drought Outlook

Tools used in the Drought Outlook included the official CPC precipitation outlook for March 2009 and the long lead forecast for March May 2009, the four-month Palmer drought termination and amelioration probabilities, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil (CAS) moisture, the Climate Forecast System (CFS) seasonal precipitation forecasts, and climatology.

Heavy rain and snow significantly boosted moisture conditions for California from around February 13 through early March, with the early-March storm bringing up to several feet of snow to the Sierra. As a result, total reservoir storage to 71 percent of average on March 3, compared with 58 percent on February 17. Statewide snowpack water content on March 4 was about 90 percent of average, up from around 60 percent at the end of January. However, many of the larger reservoirs remain well below average, with Shasta Lake at 65 percent of average storage on March 3 and Lake Oroville at 60 percent. It is unlikely the reservoirs will climb to near normal levels before the dry season sets in. The latest drought outlook scales back the improvement shown in the prior forecast since some improvement has already occurred, and precipitation totals are forecast to be lower for the first 2 weeks of the period. The CPC 6-10 day and 8-14 day forecasts issued on March 4 show below-normal precipitation for California, while the monthly forecast shows above normal and the March-May forecast shows equal chances wet or dry. The below-normal temperatures forecast in days 6-10 will support continued beneficial snowpack numbers. Even with normal- to below-normal precipitation for the first 2 weeks of the forecast period, the recent rain and snow will likely lead to continued improvement in some drought indicators, such as groundwater and, perhaps, lake levels, so continued improvement is indicated. Also, the CFS soil moisture forecast indicates improving soil moisture into May for California, as does the Constructed Analogue soil forecast (CAS) from March to May.
Forecast confidence for central and northern California is moderate.

The persistence shown for southern California is based on the climatologically earlier end of the wet season in the spring for this area as well as dry precipitation forecasts for the first 2 weeks. Climatology as shown on the University of Washington Ensemble Streamflow Prediction models supports persisting drought in western Nevada, a more pessimistic forecast than shown last month. Medium-range forecasts and climatology are more supportive of at least limited improvement elsewhere in the Great Basin, with high snow pack leading to a more optimistic forecast for southeastern Nevada, as well as normal to above-normal precipitation in days 6 to 10. The CFS model also supports improving soil moisture into May for this area.
Forecast confidence for southern California and the Great Basin is moderate.

Improvement is continued for the small drought areas in Montana based mostly on climatology but also consistent with the CAS forecasts. The dry medium-range forecasts lower forecast confidence to some degree.
Forecast confidence for Montana is moderate

Drought expansion is again forecast for western Texas into New Mexico based on precipitation and temperature forecasts for most time periods. Recent trends toward worsening soil moisture as well as short- and long-range precipitation and temperature forecasts, and supported by La Niña analogues, led to forecast drought development in parts of Kansas and Colorado. This is supported by the CFS soil moisture forecasts but not by the CAS forecasts. May is typically one of the wettest months for Kansas, so the chances for beneficial precipitation by the end of May is high, but drought could develop quite quickly with rising temperatures given current low soil moisture, and this could cause a marked change in winter crop conditions.
Forecast confidence for west Texas and the Southwest and High Plains is moderate.

This forecast has added an area of improvement to eastern Texas and the Gulf Coast in areas that showed persistence in the prior Drought Outlook. This is largely supported by wet medium-range forecasts. A number of recent model runs show several inches of rain during March 10-14 from central Texas northeastward. This is in contrast to dry monthly and seasonal forecasts for the region, but the models have been quite consistent in recent days in showing this wetness. The forecast area of improvement is relatively conservative given the potential 2-week precipitation totals because of the contrasting longer-range forecasts.

Heavy precipitation at the end of February eased dryness in the Southeast outside of Florida. Medium-range (2 weeks) and longer range (1 month to 3 months) forecasts are consistent with a La Niña-type of pattern in showing dryness near the South Atlantic coast transitioning to near or above-normal rainfall in the Tennessee and Ohio Valley. All forecasts are dry for the Florida Peninsula, where drought has already been deepening and expanding.
Confidence for the Southeast is high for Florida and moderate elsewhere

In Wisconsin and adjacent Michigan, the 6-10 day forecast, climatology, the CFS soil forecasts, CAS, and La Niña composites all support improvement.
Confidence for the Upper Midwest is high.

In Hawaii, the Outlook continues to indicate improvement due to seasonal forecasts and climatology. A few seasonal dynamic models show below-normal rainfall over the southern islands, and this decreases confidence levels to some extent.
Confidence for Hawaii: Moderate.

In other areas, streamflows and groundwater are unseasonably low in parts of the mid-Atlantic region in response to below-normal rain and snow since the start of the year. This area, which includes the Potomac Basin, could easily slip into drought during the spring as temperatures rise. However, with none of the medium or long-range forecasts showing below-normal precipitation, drought development was not shown on this forecast, but the area will be monitored.
Short-term dryness could also transition to drought in parts of the Northwest, including Washington and western and northern Oregon, and across parts of Puerto Rico. These areas will also be monitored. Most seasonal forecasts do not indicate drought development in either region at this time.

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Climate Prediction Center
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Page Author: Climate Prediction Center Internet Team
Page last modified: February 5, 2009
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