July - September 2010

Outlook Graphic: GIF   PDF 

Latest Seasonal Assessment - Continued off-season rainfall helped alleviate drought across northern Idaho and western Montana during the previous two weeks. Farther south, drought areas expanded in the central Rockies and persisted in the Great Basin region. Drought conditions in these regions are projected to persist throughout the summer dry season, though monsoon-related thunderstorm activity may effect some drought improvement in northeastern Arizona and north-central New Mexico. Recent episodes of heavy rainfall eroded drought areas previously entrenched in northern Wisconsin and northeastern Minnesota. Further improvement is expected with additional rainfall across portions of western Wisconsin, but widespread drought reductions are less certain across the remainder of the Great Lakes region due to the highly variable distribution of thunderstorm and MCS-based rainfall as well as the long-term nature of the drought. Drought conditions expanded along the lower Mississippi Valley and across north-central Texas, with extreme drought conditions observed in northern Louisiana. Forecasts of very heavy rainfall along the entire Gulf Coast during the upcoming two weeks, however, coupled with enhanced chances of above-median precipitation during the three-month outlook period support the potential for significant drought improvement, particularly throughout much of Texas and Louisiana. In the East, increased dryness has been noted from the Carolinas through southern New England, and drought has developed across eastern Maryland and the southern Delmarva Peninsula. Continued dryness is forecast in the short term, which may lead to an expansion of short-term agricultural drought across the mid-Atlantic region, but a longer-term establishment of drought conditions in the East during the summer months is much less certain, particularly due to a climatological likelihood of summer air-mass thunderstorms and the potential for tropical cyclone activity during the projected very active season. In Hawaii, drought expanded eastward across Hilo and is expected to persist across the leeward side of the island chain. Drought conditions are also expected to persist and expand across interior Alaska.

Tools used in the U.S. Drought Outlook (USDO) included the official CPC precipitation outlook for July 2010 and the long lead forecast for July - September 2010, 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, climatology, and initial conditions.

The Upper Midwest continued to receive beneficial rainfall during the past several weeks, and stream flows have accordingly been given a much-needed boost. The occasional passage of frontal systems and thunderstorm complexes has resulted in a chipping away of the drought area, especially westernmost portions. Additional heavier rain events will be needed however, to overcome what has been a long-standing, nearly two year drought in some areas such as northern Wisconsin. Prospects for substantial mitigation of drought are anticipated to improve during the first half of July, based on extended-range predictions of a weakness in the summertime ridge across this region. At least some improvement is forecast for western sections of the drought area, with the Outlook not quite as optimistic for eastern Upper Michigan and northern Lower Michigan, where the drought appears to be more entrenched. In the longer ranges, such as the 30- and 90-day periods, the odds favor neither abnormally wet nor dry conditions.
Forecast confidence for the Upper Midwest and northern Michigan is moderate during the first few weeks of the Outlook period, but low thereafter.

Moderate to extreme drought currently over portions of the lower Mississippi Valley and northern Texas has been compounded by above-average temperatures during the past several weeks. Current total column Soil Moisture Anomalies (CSMA) indicated by various NLDAS-related models show negative departures on the order of 50 to 150 mm (2 to 6 inches) below normal across Louisiana and small portions of adjacent states. For reference, these anomalies are based on a 28 year climatology (1980 - 2007). A significant number of stream flow values are within the lowest quartile of their historical distributions. At the start of the Outlook period, heavy rains from Hurricane Alex are expected to result in substantial improvement over southern and central sections of the drought areas. The odds for at least some improvement are enhanced over northern sections. The official, updated CPC monthly precipitation outlook for July indicates above-median rainfall across portions of this drought region. CPC's July-September 2010 precipitation outlook also favors above-median rainfall for much of this area. Scattered airmass thunderstorms are a common occurrence across the lower Mississippi Valley in the summertime, and across the Southeast in general, which will likely result in very localized, spotty relief. Another consideration for this Drought Outlook in the lower Mississippi Valley is the NOAA forecast of an active tropical cyclone season in the Atlantic basin, with the first named storm of the season (Alex) already affecting the region. The small area of moderate drought in southwestern Oklahoma is expected to see some improvement as well.
Forecast confidence for the lower Mississippi Valley is moderate, but low for southwestern Oklahoma.

Above-median precipitation across the northern Rockies in recent weeks has resulted in the removal of the drought area in the eastern panhandle of Idaho, and nearby western Montana. Although somewhat less precipitation has fallen over the central Rockies recently, the USGS network of stream flow gauges shows streams and rivers in this region are running near or above normal for this time of year. CPC's Calculated Soil Moisture Ranking Percentile (CSMRP) chart indicates soil moisture values falling within the 70th and 90th percentiles (above normal tercile of the historical distribution) for the northern Rockies, and falling within the 30th and 70th percentiles (near normal tercile) for the central Rockies. The July 2010 and July-September 2010 official CPC precipitation outlooks are noncommittal for this region, with no discernible tilt in the odds for abnormally wet or dry conditions. It will be difficult to overcome dry summertime climatology, therefore persistence of drought conditions in the central Rockies is thought to be the best bet.
Forecast confidence for the northern and central Rockies is moderate.

In the Southwest, very little rain has fallen over the moderate to severe drought area encompassing northern and central portions of both Arizona and New Mexico. With the climatological onset of the summer monsoon only days away, the region can expect to see some improvement in the drought conditions. In addition to the light to moderate rains predicted in the short term over New Mexico, HPC also indicates showers near the Arizona-New Mexico border, a feature commonly observed in the higher elevations near monsoonal onset, as the humidity gradually increases and measurable precipitation is finally able to reach the parched ground. Early in the monsoon season, while the moisture available at lower levels is still very limited, there is the concern for dry thunderstorms, lightning-induced brushfires, and strong, gusty surface winds. With time, the storms have more moisture to work with, and potential hazards shift toward torrential rainfall and flash flooding of arroyos and other low-lying areas. Longer range rainfall predictions from CPC do not show a discernible tilt in the odds toward abnormally wet or dry conditions during the month of July, or for the July-September season as a whole.
Forecast confidence for the Southwest is moderate.

In the far West (west-central Great Basin), little if any rain has fallen in the past two weeks. Given we are now in the hot, dry summer season, it will be extremely difficult to overcome regional climatology. Therefore, persistence of drought conditions is anticipated for nearly all of this area, with the exception of south-central Oregon, where some improvement is expected in the shorter-term.
Forecast confidence for the far West is high.

Moderate to exceptional drought (D1 to D4 designations) continues across the leeward sides of the Hawaiian Islands. Forecasts for these areas call for below-median rainfall during the July-September 2010 season, leading to drought persistence. In addition, with this typically being a drier time of the year, it is difficult to see how drought amelioration may come about, excluding of course, any tropical storms or hurricanes.
Forecast confidence for Hawaii is high.

A relatively small area of moderate drought persists in the central Interior of Alaska. After an early and vigorous start to the wildfire season, conditions have become much more tranquil. Localized summer thunderstorm activity in the Interior is often accompanied by little rain, lightning, gusty winds and small hail, the first two of which are particularly dangerous in heavily wooded conifer forests. In addition, drought is expected to expand westward across most of the central Interior during this period. The CPC Precipitation Outlooks valid for the month of July, and the July-September season, do not show a tilt in the odds toward below, near, or above-median precipitation for this area.
Forecast confidence for Alaska is moderate.