July - September 2008

Latest Seasonal Assessment - Improvement is on tap for southern and eastern Texas with more limited relief for central Texas. West-central and far eastern Texas experienced beneficial rains in the latter half of June, though east and south Texas recorded little rain. Showers increased over southern and eastern Texas near the start of July, and extended-range forecasts suggest improvement during the first half of July due to a more favorable upper air pattern. Later in the season there is an overall climatological increase in the odds for rain from tropical activity. For central Texas, some improvement is forecast due to the odds favoring improved soil moisture by late in the season. However, the water supply situation, including groundwater levels, should see little improvement unless a tropical weather system strikes the area. To the west, with the typical onset of the summer monsoon during the first week in July, the Southwest can expect to see a gradual increase in rainfall amounts from southeastern Arizona and the Mogollon Rim area into New Mexico and far western Texas. Farther west, the official extended-range forecasts show little rainfall across California, and it is unlikely that the state will experience significant improvement during the ongoing dry season. North Dakota, after receiving some much-needed rainfall in June, now appears poised to return to a drier pattern. In the Southeast, an expanded area of improvement is indicated across coastal areas and into the Piedmont region of the Carolinas, associated with the anticipation of a more favorable upper air pattern, and the increased possibility of tropical systems later in the summer. The Hawaiian Islands are likely to experience expanding drought, especially along the leeward slopes.

Tools used in the Drought Outlook included the official CPC precipitation outlook for July 2008 the long lead forecast for July - September 2008, the four-month 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 CFS seasonal precipitation forecasts, and climatology.

Improvement for drought across the interior Southeast now appears more promising than it did two weeks ago, as one or two troughs set up near this region. The Outlook expands improvement into the coastal Plain and southern Appalachians of the Southeast from Alabama into the Carolinas and Virginia. This is consistent with the latest CPC rainfall outlook for July-September, which shows a slight tilt of the odds toward wetness along the Gulf Coast. The improvement is based on extended-range prediction models and, closer to the coast, it is also based on historical outcomes as shown by the NCDC Palmer probability maps for September. Eastern areas of North Carolina, for example, show at least a 60 percent chance for drought amelioration by September. For the short term, improvement in the core drought areas of the southern Appalachians is also consistent with the July 2-7 QPF, which depicts around 1 inch of rain over this region. This overall improvement in the core drought area is predicted despite the fact that many of the wells in this region are below the 10th percentile. In contrast, the CAS soil moisture tool indicates drier than normal soil moisture conditions extending from the Southeast back into east Texas. This scenario is thought to be less likely, given the predicted upper air height patterns during the 6-10 day and Week 2 time periods, as well as the climatological increase in tropical activity later in the July-September season. The 6-10 day and Week 2 forecasts anticipate wetter than normal conditions across most of the Southeast during these ranges. However, hydrological drought indicators such as reservoirs and groundwater are unlikely to see significant improvement during the summer in the absence of a tropical system. The area of persisting drought shown over the western North Carolina area in the previous Outlook has transitioned to a more favorable designation due to more confidence for relief from the weeks 1 and 2 rainfall forecasts. Medium and long-range rainfall forecasts and seasonal outlooks for soil moisture, runoff and the Palmer Drought Indices continue to show lower odds for meaningful improvement around the western North Carolina area than farther south and west; hence, the more limited improvement depicted there.
Confidence of the interior Southeast: Moderate

In Texas, significant rains (over several inches) fell in the last half of June from the Texas Panhandle southward to the Big Bend area, and also over far eastern portions of the state. Much of east Texas, however, had not received much in the way of beneficial rains. As noted earlier, the CAS soil moisture tool paints a more pessimistic picture for drought alleviation in east Texas during the summer, though extended-range models, as well as the updated 30-day rainfall forecast, support above-average rainfall amounts for the area. The July 2-7 QPF forecast from HPC shows between 2 and 3 inches of rain across southern Texas in the short-term, while medium range forecasts out to 2 weeks also point to a wet pattern. East Texas will be more vulnerable to tropical cyclone activity, and west Texas should be increasingly affected by monsoonal showers and thunderstorms as the summer progresses. For the Drought Outlook, key long-range indicators included the Palmer drought alleviation probabilities going through September and consideration of monthly normal rainfall totals (which includes the increased likelihood of tropical activity). Improvement that is forecast near the Gulf Coast extends deeper into the Texas hill country compared to the previous forecast due to wetter forecasts for the first half of July. The forecast for central Texas has been upgraded to indicate some improvement. Experimental 3-month soil moisture and runoff forecasts from the University of Washington show a strong tendency for improvement across nearly all of Texas by the end of September based on historical rainfall and temperature data. This long-range improvement extends into central Texas, although the bulk of the relief occurs between late August and late September. Given the time of the year, with the typical tendency for lakes and wells to drop, it is unlikely that central Texas will see significant drought relief for water supplies, especially groundwater, during the summer absent a tropical storm or other tropical weather system.
Confidence for Texas and eastern Colorado: Moderate
Confidence for the Southwest: High
Confidence for Nebraska: Moderate (nighttime thunderstorm clusters (MCS's) are common in western Nebraska during the summer).

In west-central North Dakota, a recent increase in beneficial rains has provided some help in alleviating drought conditions there. However, 6-10 day and 8-14 day extended-range model guidance is not very optimistic in drought amelioration for this region. In addition, Palmer probabilities for drought amelioration are only between 10% and 30%.
Confidence for North Dakota: Moderate

For the West, the ongoing dry season results in expected persisting drought for the California area, while the CPC July-September Outlook indicating a tilt of the odds toward dryness over the Northwest into the northern Rockies contributes to persisting drought from southeast Oregon into southwest Wyoming. Given short and long-range forecasts for above-normal temperatures, the western drought could expand or intensify to some extent.

In Hawaii, the drought forecast is unchanged from the preceding forecast, with development expected in many leeward areas, consistent with the seasonal forecast for below-normal rainfall.

Forecaster: A. Artusa and D. Le Comte

Next Outlook issued: July 17, 2008 at 8:30 AM EDT