August - October 2003

Latest Seasonal Assessment - Record-setting heat spread from the West into the Plains during the first half of July, resulting in rapidly-declining soil moisture levels. Based on the summer thunderstorm season forecast, there may be some localized relief following rain events for parched areas from the Southwest northward into the central Rockies, as well as parts of the northern Rockies, but significant easing of the water shortages will wait until next winter, at the earliest. Farther west, drought should persist across the Great Basin- Intermountain West, as the summer thunderstorm season is expected to bring less relief to this region. The official August-October forecast calls for below-normal rainfall from California through Nevada to western Utah and northward into Idaho and Oregon. To the east, the recent hot, dry weather has hit spring-planted crops in the Plains at their most critical growth stages, so drought is currently developing from Nebraska southward into Oklahoma, and this is indicated in the drought outlook. However, indications are that the extreme heat and dryness will not persist, as the long-range forecasts for August through October call for equal chances of above normal, normal, and below normal rainfall and temperatures, and shorter-range forecasts show improved chances for precipitation in the Plains during the last 10 days of July. In the Pacific Northwest, unusually warm, dry weather in June dried out topsoils from Washington and northern Idaho into Oregon, and July weather has not been much help. Although dry weather is typical from now into September for this region, additional dry and warm weather will result in problems for crops that cannot be irrigated. Forecasts call for little rain and above-normal temperatures across the Northwest during the last half of July. Above- normal temperatures are expected to persist into August, further worsening dryness. In Texas, Hurricane Claudette eliminated lingering dryness in eastern parts of the state, and drought relief was ongoing or imminent in southwestern Texas as the seasonal outlook was being issued. In Hawaii, drought has recently expanded northward to include Oahu, and significant relief is not expected by October.

Tools used in the drought outlook included: the official CPC long-lead precipitation outlook for August-October, the Palmer Drought Index probability projections for October and various medium and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts. The latest western water supply forecast is also considered.

The forecast continues to call for persisting drought in the Great Basin-Intermountain region where the hydrologic drought is well entrenched and summer rainfall is unlikely to offer much improvement. Indications that August-October rainfall may be even lower than normal across the region, with above-normal heat, result in the drought forecast confidence being quite high. Some worsening of the drought is even possible, but the most recent Drought Monitor already has much of the region at D3 levels, so it would be difficult to forecast worsening. In the Pacific Northwest, warm, dry weather in June and the first half of July resulted in very dry conditions across the region beyond those areas already experiencing hydrologic drought. With most of Washington and Oregon in near-drought conditions already from the standpoint of soil moisture and rain-fed agriculture, development of drought is likely. Little rain with above-normal temperatures is on tap during the next 2 weeks for the Northwest, according to the latest NCEP forecasts and the latest GFS model run. The location for drought expansion is based on the areas that are experiencing the greatest current dryness, according to the CPC soil moisture model, USGS streamflow data, and ground reports. Farther east, the summer monsoon started late in the Southwest, but moisture has been streaming northward from Arizona and New Mexico into the Rockies, and forecasts indicate a good chance for at least normal monsoon rains. This is based on the recent evolution of large-scale weather systems, and some indications from seasonal forecast tools, especially the constructed analogues from soil moisture. Confidence is quite high that localized relief can extend northward into the Rockies, although the rains cannot be expected to make a big difference in the water shortages affecting the region.

In the central states, the improvement forecast last month for the northern and central plains was ongoing until July, when record heat spread over the Plains. Temperatures in the 90s or even triple digits accompanied by little or no rain rapidly depleted soil moisture from South Dakota to northern Texas during early to mid-July. With spring-planted crops in their crucial growing stage, there is a significant potential for crop damage as each day goes by without substantial rainfall. The drought outlook calls for developing drought from Nebraska into Oklahoma due to the short term heat and dryness. But the extreme heat and dryness may not persist very much longer. Frontal passage around July 20-22 should trigger showers and lower temperatures in the Plains, and a number of long-range forecast tools indicate that the odds do not favor continuation of the high pressure ridge over this region in coming weeks and months. There are some indications for slight retrogression of the western ridge-eastern trough complex during the next 2 weeks, and this would increase the chance for rain in the central states. In addition, based on historical correlations, normal to below-normal 500-hpa heights in the central plains would be consistent with the current status of the SST gradient in the equatorial Pacific as well as the negative SOI and QBO currently being experienced. Regarding ENSO, the assumption is that neutral conditions--neither El Niño nor La Niña--will prevail during this forecast period.

In western Texas, the rains from Claudette were approaching the Big Bend drought area as this forecast was being composed, and the moisture should put a dent in the drought in southwest Texas in coming days. Some of this moisture may very well become wrapped into the monsoon circulation, and enhance the rainfall in the Southwest.

In Hawaii, drought is forecast to persist because this is a relatively dry time of the year, and significant relief for leeward areas on the islands would be difficult.