June - August 2008

Latest Seasonal Assessment - Some Improvement is forecast for the area of protracted (though recently improved) drought across the interior Southeast, but farther south, improvement is expected in the dry areas of the Florida Peninsula. To the west, improvement is also forecast for far southern and southeastern Texas, northwestern Kansas and adjacent areas, northwestern Nebraska, and the areas of drought currently affecting the northern Rockies and Plains. Meanwhile, less significant improvement is anticipated across interior southern Texas, in the drought areas from the southern Rockies and the Big Bend of Texas northeastward into southeastern Colorado and southwestern Kansas, and across southeastern Idaho and southwestern Wyoming. Unfortunately, indications are that drought should persist in west-central Texas and in the dry areas from south-central Idaho westward into Oregon then southward through California, much of Nevada, and western Arizona, with drought expected to expand to cover all but northernmost California by the end of August. Finally, conditions should persist or intensify in the current areas of drought scattered across Hawaii from eastern Oahu southeastward through northern sections of the Big Island, and drought should expand into some other areas of Hawaii by the end of August, but at this time specific locations of development can't be pinpointed, so an enhanced likelihood of drought development was depicted for most of the island chain not already in drought (the exceptions being eastern and west-central parts of the Big Island, eastern Maui, and the northeastern half of Kauai, where upslope winds tend to wring out enough rainfall to keep drought in check even when precipitation is below normal).

Tools used in the Drought Outlook included the official CPC precipitation outlook for June 2008 and the long lead forecast for June - August 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 moisture, the CFS seasonal precipitation forecasts, and climatology.

La Niña conditions continue to slowly erode across the equatorial Pacific Ocean, and the influence of La Niña on sensible weather across the United States reaches a nadir during the summer months in any case. For this reason, the Drought Outlook continues its trend of relying less and less on La Niña composites and instead the current outlook is based primarily on climatological considerations, antecedent conditions, the official outlooks through August 2008, and the apparent potential for drought-related impacts to change, for better or worse, during the forecast period.

The drought across the interior Southeast has made slow but progressive improvement over the last few months. However, little if any precipitation is likely through June 9, 2008, and below-normal rainfall is favored for the ensuing 5-day period across roughly the northern half of the region. After that, the period through June 18, 2008 also looks drier than normal for the drought-affected sections of central North Carolina and adjacent Virginia while the odds tilt slightly toward wetter than normal conditions across the small drought area extending into northern Florida. Looking farther ahead, the updated June 2008 monthly outlook leans toward a wetter than normal month overall on the northwestern fringes of the region, but outlook is non-committal elsewhere, as is the June - August 2008 outlook for the entire drought region. Climatologically, June - August is typically one of the wetter times of the year in central North Carolina and adjacent South Carolina, and across southern Georgia and northern Florida, but only by a small amount (any 3-month period is roughly 25% of a year, but June - August typically brings 30 to 35 percent of the annual rainfall to the aforementioned regions). For other parts of the drought region, June - August typically brings about what would be expected if precipitation were evenly distributed throughout the year - about 25 percent of the annual total. In addition, the rain that does fall on the region during summer primarily comes from relatively disorganized and small-scale shower and thunderstorm activity, or less frequently but more dramatically, tropical storms and hurricanes that push into the region from the Atlantic Ocean or the Gulf of Mexico. In either case, the amount and location of rainfall produced by these phenomena are nearly impossible to predict beyond the medium range period. Given all these considerations, the bottom line is that there is a fair amount of confidence that near or below normal rainfall totals will be observed through mid-June 2008, and little confidence in any scenario thereafter. However, the purely climatological chances for heavy rains from thundershowers and/or tropical systems are too great to forecast persisting or intensifying drought, and conversely, the relative dryness through the medium range and the uncertainty thereafter preclude a forecast for widespread improvement, especially since the surface water lost to evaporation, vegetative absorption, and public water usage is at a maximum during the summer months. Thus, the forecast for some improvement is the path of least regret.
Confidence: Low

Drought in the Florida Peninsula has intensified in the last few weeks, but medium-range forecasts through mid-June and the seasonal outlook for June - August all favor above-normal precipitation. In addition, June - August is a relatively wet time of year for the current drought areas. Typically the 3 summer months, 25 percent of the year timewise, feature 35 to 50 percent of the total annual rainfall. As a result, improvement is forecast for this region with considerable confidence.
Confidence: High

Although spatially disparate, forecasts for drought conditions in the northern Plains, north-central Montana, northwestern Nebraska, and areas in and adjacent to northwestern Kansas to improve are based essentially on the same set of facts. Moderate to heavy rains are forecast in all these areas through June 9, 2008, with over 2 inches forecast to fall on south-central North Dakota and northwestern Nebraska. Medium-range outlooks favor above-normal precipitation through mid-June 2008, and the recently-updated June 2008 monthly forecast features enhanced probabilities for above-normal rainfall across the northern Plains. It should be noted that according to the seasonal forecast for the summer months as a whole, the odds favor below-normal June - August 2008 rainfall in the drought areas north of Colorado. However, this forecast was issued a few weeks ago, and given the fact that, climatologically, June - August is one of the wetter times of the year (on average 40 to 50 percent of the annual precipitation total falls during these months), and this precipitation is 'front-loaded' so to speak, with June typically a wetter month than July, and a much wetter month than August. Taking these facts into consideration, along with the recently-updated June 2008 forecast of near- or above-normal precipitation, the seasonal forecast was not given much weight in the Drought Outlook for this region. Finally, it should be noted that these indicators, when factored together, indicate more robust likelihoods for improvement across the northern Plains and northwestern Nebraska than in north-central Montana and the areas in and adjacent to northwestern Kansas. The short-range forecast is not as wet in the latter two areas, the forecast for the week centered on mid-June 2008 actually favors near- or below-normal precipitation, and in the west-central Plains, the relatively wet June - August period doesn't peak early in the period as is the case farther north, leading inherently to more uncertainty. Still, there are enough indicators to justify a reasonably-confident forecast for drought improvement in these areas.
Confidence: High in the northern Plains and northwestern Nebraska; moderate in north-central Montana and the west-central Plains

There are fairly strong indications that drought will improve across southern and coastal southeastern Texas, at least early in the period. Light to moderate rains should fall on the region for the few days after this forecast is issued, at which point an increasingly strong influx of tropical moisture should trigger fairly widespread above-normal precipitation through at least mid-June 2008. However, forecast uncertainty increases later in June and doesn't get any better through the rest of summer, so the high-confidence in improvement through the medium range period must be tempered somewhat by the uncertainty that follows through the end of August 2008.
Confidence: Moderate

Farther northwest, limited drought improvement is forecast for south-central Texas, the Big Bend of Texas, southern New Mexico, southeastern Arizona, the northern Texas Panhandle, the Oklahoma Panhandle, northeastern New Mexico, southeastern Colorado, and southwestern Kansas while drought is forecast to persist across portions of west-central Texas from roughly the central Rio Grande eastward to the San Antonio and Austin areas and northward through the San Angelo and Midland-Odessa areas. All of these areas should be dry for the few days after this forecast is issued. After that, the outlooks are replete with uncertainty. It appears as though some of the tropical moisture expected to bring heavy precipitation to southern and coastal southeastern Texas will also bring moderate precipitation to the adjacent drought areas in south-central and interior southeastern Texas. Farther north and west, showers and thunderstorms related to the Southwest Monsoon circulation pattern typically brings an increase in precipitation during late summer to the drought areas from southeastern Colorado and southwestern Kansas southward through southeastern Arizona, southern New Mexico, and the Texas Big Bend. At this point it is unclear how much precipitation will fall during August relative to normal, but the climatological increase in precipitation is substantial enough that some drought improvement is a good bet late in the period in these areas. Finally, in the drought areas sandwiched between the expected tropical thundershowers during the medium-range period across south-central and interior southeastern Texas, and the climatological August increase in monsoon-related rainfall farther north and west, drought is forecast to persist in the absence of any compelling reason to expect improvement. However, it should be reiterated that this forecast is based essentially on climatology and a lot of uncertainty for the medium- and long-range periods.Confidence: Low

In southeastern Idaho and southwestern Wyoming, some improvement is expected in drought conditions by the end of August. Moderate to locally heavy rainfall is expected through June 9, 2008, but the period from mid-June through the end of August 2008 is rife with uncertainty. The odds tilt slightly toward below-normal precipitation during the week centered on the middle of June, but outside of that, the 6- to 10-day forecast valid through June 14, 2008, the updated June 2008 monthly forecast, and the June - August 2008 seasonal outlook are all noncommittal in their forecasts. In such cases it helps to look at climatology, especially in regard to how wet is the period in question relative to other times of the year. Unfortunately, in this case, even climatology doesn't help much, as the June - August period typically brings about 25 percent of the annual precipitation total, which is exactly what you'd expect from a period that covers 25 percent of the year. In the final analysis, the forecast is based on the good confidence for significant precipitation in the short-range period, and the fact that June - August typically is, at the very least, not a disproportionately dry time of year.
Confidence: Low

Farther west, drought is forecast to persist in the remainder of southern Idaho and in the areas of ongoing drought across Oregon, much of California and Nevada, northwestern Utah, and western Arizona. In addition, the odds favor drought expanding to cover all but the northernmost sections of California by the end of August. Northwestern Utah could see up to 0.5 inch of precipitation in the few days after this forecast is issued, but little or no rainfall is expected during the short-range period elsewhere. After that, below-normal rainfall is expected across the northern tier of this region during the week centered around mid-June 2008, and drier than normal weather is also expected in areas from central Nevada and northeastern California northeastward into east-central Oregon and through south-central Idaho for the month of June 2008 as a whole. Farther west and south, across the remainder of California, the medium- and long-range outlooks are indeterminate, but the extreme June - August climatological dryness in this region leads to a confident forecast of persisting and expanding drought. Less than 2 percent of the annual precipitation total typically falls during this period outside of the southeastern California deserts, where the proportion is somewhat higher but the annual total considerably lower). The only exception to the confident forecast of drought persisting and expanding through the end of August 2008 across this region is in western Arizona, where the late-summer monsoonal circulation can bring substantial rainfall totals during August.
Confidence: Moderate in western Arizona; high elsewhere

In Hawaii, the odds continue to favor drier than normal weather statewide for June - August 2008, resulting in drought conditions persisting or intensifying across easternmost Oahu, western Molokai, interior western Maui, and northern sections of the Big Island. Current conditions are abnormally dry throughout the remainder of the state, and when antecedent dryness is followed by (if the forecast verifies) 3 months of below-normal precipitation, drought expansion is a pretty likely outcome, and outside of the climatologically-wet windward areas of the Big Island, Maui, and Kauai (where even below-normal rainfall is usually sufficient to stave off drought development this time of year), drought development is forecast. It should be noted that drought is not expected to cover all of the highlighted areas by the end of August, but should expand beyond its current extent during the forecast period, and all of these areas are at risk.
Confidence: High