Tools used in the U.S. Monthly Drought Outlook (MDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for August, various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) totals from the Weather Prediction Center (WPC), the 6-10 and 8-14 day CPC extended-range forecasts (ERFs), the Weeks3-4 outlook from CPC, dynamical models at the monthly time scale, the 384-hour total precipitation forecasts from several runs of the GFS, climatology, and initial drought conditions (the July 24, 2018 U.S. Drought Monitor).
During the past few weeks (but especially last week), large parts of the Northeast received heavy rainfall (2-6 inches, locally to 10 inches). Soil moisture conditions from both CPC and NLDAS (North American Land Data Assimilation System) showed marked improvement during the period. ACIS (Applied Climate Information Service) Departure from Normal Precipitation (DNP) for the past 30-days ranged from 2-10 inches above normal across portions of the Northeast. ACIS Temperatures were about 2 to 6 degrees F above normal across the region. The 30-day Standardized precipitation Index (SPI) map depicts wetter conditions (values between 0 and +1.5) in most of this area. USGS stream flows are currently within the highest quartile of the historical distribution in parts of Pennsylvania and southern New York, but near to above normal most parts of the Northeast. The latest U.S. Drought Monitor (USDM) depicts parts of New England and New York in D0 (abnormally dry) and D1 (moderate drought) conditions. According to the GFS model, soil moisture anomalies are likely to improve in the region for the next two weeks. For temperature and rainfall forecasts out to one month in advance, prospects for drought relief in the Northeast are likely. The one exception to this is coastal Maine where recent precipitation has been lighter (less than 1 inch), and improvement may take longer (hence persist) as interior sections have received much greater totals.
Forecast confidence for the Northeast is moderate (coastal Maine) to high (interior New England).
Currently, the Southeast is drought free, with just a few small areas in the Carolinas and Alabama with D0. August precipitation normally contributes 10-20% of the annual precipitation for coastal sections of the Southeast due mainly to tropical systems, while interior Southeast locations generally see less of their annual total during August (1/12, or about 8%). Soil moisture, stream flows, and the 30-day SPI all show near to above normal values across most of the Southeast. With above normal rainfall forecast for most time scales within August, drought development is considered unlikely across the Southeast.
Forecast confidence for the Southeast is moderate to high.
Recurring thunderstorm clusters (Mesoscale Convective Systems or MCSs) have dropped surplus amounts of rain over most of the northern Mississippi and Ohio Valleys, preventing the onset of dryness and drought in the region. Southern Iowa and northern Missouri, however, have consistently missed out on the MCS-related rainfall, with USDM categories currently ranging between D0 (abnormal dryness) and D3 (extreme drought). U.S. Geological Survey stream and river flows in this region are near to below normal. During the past two weeks, ACIS DNPs ranged between -2 and -4 inches. Soil moisture anomalies of negative 2-6 inches are currently indicated by NLDAS and CPC's Leaky Bucket in northern and central Missouri, and the GFS model predicts a continued deterioration in soil moisture conditions for the next two weeks, although Week3-4 outlook hinted at above-median precipitation. Development was not forecast for the area surrounding the core drought area of southern Iowa and northern Missouri as there was no clear signal from the various precipitation forecasts (some above, some below-median). Drought improvement is favored in northeastern Michigan due to 1.5-3 inches of rain since July 24 (which created 30-day surpluses) and good odds for above-median totals in CPC’s extend range forecasts; however, persistence is more likely in southeastern Michigan as late July rains were less and 30-day deficits were larger (2-3 inches).
Forecast confidence for the Midwest is moderate.
Across the South, the USDM depiction generally shows dry conditions in most area, culminating in widespread D2 and D3 (severe and extreme drought) over northeastern and western Oklahoma, western and central Texas, and the Arklatex area. During the past 7-days, decent rains (1.5-5 inches, locally greater) fell on western and south Kansas, central and eastern Oklahoma, western Arkansas, and northern Louisiana. 30-day ACIS temperatures across the South range from near normal (South Texas) to 2-6 degrees F above normal in northern and central Texas. Negative soil moisture anomalies of 2-5 inches are indicated by both NLDAS and CPC Leaky Bucket model across much of the South, excluding Mississippi and Tennessee, where soil moisture values are either near normal or above normal. Looking forward, drought improvement and/or removal is possible over parts of Arklatex region, western and northeastern Oklahoma and south-central Kansas in the short-term based heavily on rainfall since July 24 which may be offset in early and mid-August by drier and warmer condition. In contrast, drought persistence and drought development is favored in southwestern and south-central Texas as recent rains missed the area, and all forecasts out to a month pointed toward EC or sub-median precipitation. A wild card for this area could be moisture from Pacific tropical storms that make landfall in northwestern Mexico and travels northeastward. In addition, the heavy mid-June rains along the western Gulf Coast should be enough to stave off D1 development by the end of August in south and southeastern Texas.
Forecast confidence for the South is low (improvement areas) to moderate (drought areas).
In the High Plains region, the most intense drought conditions are currently in southern and western Colorado, and northeastern Kansas. Small-scale drought areas are indicated in central North Dakota and eastern South Dakota. Climatologically, August is typically a wet month for the region, in part related to the northward, seasonal migration of the upper-level westerlies and the mean storm track, and also to MCS activity. These nocturnal clusters of thunderstorms provide much of the rain that is needed for agriculture during the growing season. During the past week, moderate to heavy rainfall (1-4 inches) was observed in eastern Colorado, western and southern Kansas, western Nebraska, eastern Wyoming, and southwestern South Dakota, and should provide short-term improvement to drought areas while keeping drought-free locations out of development. Unfortunately, the ERF and updated 1-month precipitation forecasts favor sub-median amounts in the central Plains, so any improvements from the recent rains in southeastern Colorado and southern Kansas may be temporary by late August. In Colorado, CPC’s ERF and updated 1-month LLF precipitation forecasts favors a robust SW monsoon rainfall which should dampen drought in southwestern sections. In the Dakotas, all of the outlooks suggest drought persistence for the small D1 areas.
Forecast confidence for most of the High Plains is low (improvement areas) to moderate (drought areas).
Climatologically, much of the West is in the midst of its dry season during August, with the exception of the Southwest where the summer monsoon should be well underway. The latest USDM depicts D2 to D4 (severe to exceptional) drought conditions over most of the Four Corners States, and widespread D1 to D3 (moderate to extreme drought) in southern California and Nevada. USGS stream flows throughout most of the Southwest are quite low. The predicted summer monsoon and elevated tropical cyclone activity in the eastern Pacific (along with the potential for Gulf moisture surges) favor drought improvement, albeit scattered in nature, across a significant portion of the Four Corners region. The predicted area of drought improvement generally coincides with where the highest probabilities of above-median rainfall are indicated on all CPC's updated precipitation outlooks out to a month, with drought persistence expected in the reminder of the southern half of the West where probabilities for above-median precipitation were lower or at EC. For the northern half of the West, the current dryness and drought in interior Washington and Oregon are likely to persist given the dry climatology; however, CPC’s ERF forecasts suggest a wet period for the Pacific Northwest Coast, thus improvement is likely for Washington’s Olympic Peninsula and immediate surrounding area. With a dry August climatology in northern Washington and northern Idaho, the two small D1 areas were not expected to receive sufficient rainfall for improvement, thus persistence was drawn. In northern Montana, with a somewhat wetter August climatology, most forecasts pointed toward sub-median precipitation, so some development of the existing D0 was made.
Forecast confidence for the Southwest is moderate, moderate to high across the Northwest, and high across California.
In Alaska, D1 lingered in the extreme southern SE Panhandle with D0 in the central SE Panhandle. The latest CPC’s ERF forecasts tilt toward a wet pattern for the southeastern Panhandle, so drought development is unlikely in the central section. But with the updated August precipitation outlook favoring sub-median totals along the southern and southeastern Alaskan coast, drought was left to persist in the extreme southern section of the southeastern Alaskan Panhandle. In Hawaii, the odds favored above-median rainfall for the windward locations (currently no drought), but since the leeward sides (D0 or D1) are in their typically dry season, not enough rain is expected for improvement. It is expected that the above-median precipitation odds for the windwards will keep development from occurring across the leewards. One exception to this is the western Big Island (Kona) where their wet season is summer. Normally this forecast would point toward improvement there, but drought is expected to persist through the Kona wet season due to reduced precipitation efficiency caused by excessive volcanic emissions from the ongoing Kilauea rift zone eruption. In Puerto Rico, drought development is favored in the existing D0 area as the Atlantic/Caribbean tropics are expected to be quiet this summer (August) according to dynamical models.
Forecast confidence is moderate for Alaska, and moderate to high for both Hawaii and Puerto Rico.
Forecasters: David Miskus and Yun Fan
Next Monthly Drought Outlook issued: August 31, 2018 at 3 PM EDT