Discussion for the Seasonal Drought Outlook

[Gman]

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, climatology, and to some extent La Niña composites.

La Niña conditions continue across most of the equatorial Pacific Ocean, though weaker than earlier this year. In addition, the influence of La Niña on sensible weather across the United States is typically on the wane from mid-May through August. 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 medium- and long-range outlooks through August 2008, and the apparent potential for drought-related impacts to change, for better or worse, during the 3.5 month forecast period.

Short- and medium-range forecasts all indicate near or wetter than normal across the interior Southeast, but longer-term outlooks favor neither above- nor below-normal precipitation, and with the seasonal increase in temperatures comes an increase in water demand by residential, commercial, and agricultural interests, as well as an increase in soil moisture usage by all manner of plants, and an increase in evaporative water losses. On the other hand, heavy showers and thunderstorms (difficult to forecast very far in advance) can deliver large amounts of precipitation quite quickly in this region during summer. Based primarily on the short- and medium-range forecasts, and the potential for local to widespread heavy precipitation during this time of year, drought improvement is forecast for the region, but water usage and evaporative considerations during this warmest time of the year make the forecast anything but a sure bet. In addition, the protracted nature of the hydrologic drought means that base streamflows and large reservoirs will be relatively difficult to pull closer to normal.
Confidence: Moderate

Moderate precipitation during the medium range and official forecasts favoring wetter than normal June - August conditions were the primary reasons behind the improvement forecast for the drought area in the Mississippi Delta and adjacent locations. In addition, dryness has not been entrenched for as long a time as in the interior Southeast. However, medium range outlooks are drier here than in the larger drought area off to the northeast, and the inherent uncertainty in 3-month forecasts, along with the same water usage and evaporative considerations as in the interior Southeast, brings down confidence in the forecast somewhat.
Confidence: Moderate

Southern Florida has a good chance for drought-related impacts actually to worsen over the next few weeks, but official forecasts favoring above-normal June - August 2008 precipitation, in addition to the rather pronounced climatological increase in precipitation that occurs during June - August, make this forecast a relatively confident one.
Confidence: High

Medium-range forecasts favoring wetness, along with the fact that June - August tends to be one of the wetter times of the year, led to the forecast for drought improvement in eastern Colorado and nearby sections of the High Plains. However, even the models and indicators on which the medium-range forecasts are based were rife with uncertainty, and the 3-month forecast through August is indeterminate.
Confidence: Low

In the southern High Plains and adjacent New Mexico, southeastern Arizona, and southern Texas, most forecasts and indicators are weak or indeterminate, and in some cases contradictory. Neither the medium- nor long-range forecasts substantially favor drier or wetter than normal conditions. Some improvement is forecast because June - August based on the fact that this is one of the climatologically wetter times of the year in the region, in addition to the fact that monsoonal rainfall should begin to overspread New Mexico and southeastern Arizona late in the period, though the strength of the monsoon is another source of uncertainty.
Confidence: Low

This is one of the wetter times of the year for the northern and west-central Plains while June - August tends to be a rather typical 3-month period (relative to other 3-month periods during the year) in the northern and central Rockies. Generally near- to below-normal precipitation is anticipated in these areas for the last half of May, and June - August 2008 outlooks favor below normal precipitation in these areas. As a result, drought is expected to persist where it already exists, with expansion expected into the remaining areas of northeastern Montana. Still, the relative wetness of this time of year, and the inherent uncertainty of precipitation in mountainous regions, precludes this forecast from being an exceptionally confident one.
Confidence: Moderate

Across the drought areas in the Intermountain West, Great Basin, and West Coast states, conditions are expected to persist given little or no precipitation forecast in the medium range, and the fact that June - August tends to be one of the regions drier times of the year, especially across California. Since the last two months have been exceedingly dry and the mid-May through August 2008 period should be quite dry for climatological reasons alone, drought is expected to expand through all but northernmost California by the end of the forecast period. Confidence in this scenario is enhanced by the strength of the climatological signal, and the fact that dryness-related impacts are already on the increase in much of the state.
Confidence: High

In Hawaii, the odds favor drier than normal weather statewide for June - August 2008, which should keep the drought in western Molokai intact through the period. To some extent, drought development possibilities appear enhanced statewide for the forecast period since the entire state is already assessed as "abnormally dry" by the Drought Monitor, and the odds favor below-normal summer precipitation statewide. However, development probabilities seem particularly strong in central Maui and northwestern sections of the Big Island, where impacts and precipitation deficits indicate that conditions are already teetering on the brink of drought.
Confidence: High

Forecaster: R. Tinker

Next Outlook issued: June 5, 2008 at 8:30 AM EDT

http://www.cpc.ncep.noaa.gov/products/expert_assessment/DOD.html