September - November 2014

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Latest Seasonal Assessment - During the past 30 days, widespread rainfall over the Southwest (robust monsoon), central and northern Plains, parts of the Mississippi Valley, and most of the eastern third of the Nation (except portions of the Southeast) improved drought conditions across parts of Arizona, most of New Mexico, portions of the south-central Plains, and the Tennessee Valley and Carolinas. Locally heavy rains (more than 4 inches within 24-hours) fell at several locations, including almost 8 inches north of Phoenix, AZ, and over 13 inches near Islip, NY, producing flash flooding. Increased rainfall, partly due to tropical systems (Tropical Storm Iselle for Hawaii, Tropical Storm Bertha for Puerto Rico), also diminished dryness, but mainly in Hawaii and less so in Puerto Rico. In contrast, drought conditions worsened across sections of the Northwest, Great Basin, and parts of Texas. Abnormal dryness expanded across the Southeast and scattered areas of the north-central Plains and Midwest due to short term dryness. Unseasonable heat over the Western States exacerbated ongoing drought conditions. Except for the formation of Bertha, suppressed convection over the Caribbean basin generally persisted, with abnormal dryness and moderate drought remaining in southern and eastern Puerto Rico.

The seasonal drought outlook valid for August 21 through November 30, 2014 is based on analysis of initial conditions, short-, medium-, and long-range outlooks, and climatology. Climate anomaly composites based on the forecast of a weak El Niño (65% chance this fall and early winter) were also considered, though these impacts typically become more pronounced during the winter, after the end of this outlook period. Decreased soil moisture due to below normal rainfall increases the potential for drought development across parts of the Southeast and Midwest, but subnormal temperatures have kept evaporative levels low. An anticipated wet pattern during late August in these regions, however, may halt the development of drought and even erase abnormal dryness, so no areas of development were included here. Drought development is favored across the Northwest based on below average precipitation the past 90-days and a tilt towards dryness and/or warmth in the 1-5, 6-10, and 8-14 day forecasts, monthly and seasonal outlooks. Persistence is forecast for the Far West and Great Basin as September and October are climatologically dry and warm (and peak fire season), especially in California, although November typically is the start of the wet season in the Pacific Northwest. A continued active monsoon across the Four Corners states is anticipated to bring additional localized drought relief - with the moisture expected to spill northeastward into the Plains which brings favorable odds of above-normal rainfall and drought amelioration. Equal chances of SON rainfall, changing to below-normal odds later in the year and into 2015, favors persistence of the remaining small D1(L) in central Molokai, Hawaii (low reservoir), while a low frequency tropical convective climate signal (trending towards El Niño conditions) is anticipated to continue a regime of suppressed convection across the Caribbean, favoring continued drought degradation across Puerto Rico.

Tools used in the U.S. Seasonal Drought Outlook (SDO) included the official Climate Prediction Center (CPC) temperature and precipitation outlooks for September 2014 and September-November 2014, various short- and medium-range forecasts and models such as the forecast 5-day and 7-day precipitation totals from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC forecasts, the NAEFS precipitation outlooks, the soil moisture tools based on the Constructed Analog on Soil Moisture (CAS), dynamical models (CFSv2, NMME, IRI, and IMME), the 384-hour total precipitation forecasts from several runs of the GFS, the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions. An El Niño Watch is currently in effect, with the August 18 ENSO update indicating about a 65% chance of El Niño during the fall and early winter.

Recent spotty rainfall in parts of the Midwest, Ohio and Tennessee Valleys, eastern New England, mid-Atlantic, and Southeast have produced short-term deficits and decreased soil moisture, promoting the expansion of abnormal dryness (D0) across parts of Wisconsin, northeastern Iowa, southern Minnesota, eastern Indiana and western Ohio, parts of Kentucky, a few areas in Virginia, West Virginia, and the Carolinas, and in most of Alabama, Georgia, and northern Florida. Small areas of D1 existed in western Kentucky, central Alabama, and southwestern Georgia, with a bit of D2 in the latter region. Not surprisingly, the largest 90-day deficits (6 to 12 inches) existed in northern Florida and southern Georgia. Fortunately, subnormal temperatures have tempered the effects of the precipitation deficits by reducing agricultural demands and lowering evapotranspiration rates which are normally high during the summer. During the next 5 days, widespread precipitation is forecast from the northern Rockies southeastward into the mid-Atlantic, with decent odds for above-median rainfall across the eastern half of the U.S. in both the 6-10 and 8-14 day outlooks. The September and SON outlooks, however, provides no signal (Equal Chances). However, peak activity for the Atlantic and Caribbean tropical season typically occurs during late August, September, and early October, so it is possible that heavy rainfall could fall along the Gulf and Atlantic Coasts - albeit that this year is predicted to have below normal activity. On the other hand, since climatological precipitation decreases over the eastern half of the Nation during the fall, any drought areas that develop during late August may persist through the remainder of the forecast period - yet the fall provides excellent soil moisture recharge with lower temperatures and little or no agricultural demand. Therefore, based on the potential for widespread precipitation during the remainder of August into early September, possible tropical rainfall, lower temperatures and moisture demands, improvement or removal is indicated for existing drought across western Kentucky, central Alabama, southwestern Georgia, and northern Florida, along with expected removal of many D0 areas.

Forecast confidence for the Southeast, mid-Atlantic, eastern New England, Ohio and Tennessee Valleys, and the Midwest is moderate.

During the past 30 days, surplus rains fell across southwestern, central, south-central, far eastern, far northern Panhandle, and along the Red River valley of Texas, while generally subnormal precipitation was observed across the rest of the Panhandle, north-central, extreme south, and southeastern Texas. At 60-days, however, much more of Texas was at or above-normal, and even more so at 90-days. Over the past month, the state has seen both improvements and deteriorations depending on where the largest short-term rainfall totals versus where the greatest long-term deficits lingered (a balancing act). During the upcoming week, most of Texas is expected to be dry, with moderate totals forecast along the western portions and light amounts in the east. The CPC 6-10 and 8-14 day outlooks indicate enhanced odds for below median precipitation in the south and a slight tilt toward above-median rainfall in the north. The September monthly outlook continues the enhanced chances for above median precipitation across western Texas (active southwest monsoon), while maintaining equal chances for near, below, or above-median precipitation for the rest of the state. The SON seasonal outlook tilts the odds towards above median precipitation for nearly all of Texas (via active monsoon and frontal activity), while maintaining equal chances for extreme southern Texas. Climatologically, September and October are the first and second wettest months of the year in extreme southern Texas due to expected fronts, Gulf tropical systems, and southwest monsoon moisture. But with a subdued tropical Gulf season thus far (and predicted for SON), the odds for less rainfall than usual via the Gulf tropics are increased. Based on these outlooks, drought improvement is anticipated across most of Texas, while persistence is maintained in extreme southern sections.

Forecast confidence for western and northern Texas is moderate to high, low to moderate elsewhere in Texas.

Across the remainder of the southern and central Plains, spotty rainfall during the previous 30 days promoted areas of drought relief across southern South Dakota, central Nebraska, parts of Kansas, western Missouri, and northern Oklahoma. In contrast, small areas of abnormal dryness or drought developed across western South Dakota, western Nebraska, central Missouri, and southwestern Oklahoma. During the upcoming week, widespread moderate to heavy (more than 3 inches) rainfall is forecast from western Montana and northeastern Colorado eastward to the mid-Atlantic. The forecasted 5-day totals, enhanced chances of above-median precipitation at the 6-10 and 8-14 day, monthly, and seasonal outlooks, and recent moisture increases are anticipated to result in continued drought reduction across the central and southern Plains. Although the precipitation climatology rapidly dries out as the fall progresses, the gradual reduction in temperatures and agricultural demands during the autumn should aid with soil moisture recharge.

Forecast confidence for the central and southern Plains is moderate.

Monsoon convection was variable during June and July, bringing some relief to drought areas across New Mexico, western Texas, Colorado, and southern Arizona, but maintaining or worsening conditions in southern California, Nevada, northern Arizona, Utah, northwestern New Mexico, and southern Colorado. More recently, however, monsoon activity has expanded westward and northward, bringing scattered showers, some locally heavy with flash flooding, to southeastern California and the Sierra Nevada, much of Arizona and Nevada, Utah, southern Colorado, and even into parts of eastern Oregon, Idaho, eastern Washington, southwestern Montana, and western Wyoming. The 7-day HPC QPF brings decent rains (more than 2 inches) to most of Montana, along with much lighter amounts to the Southwest. CPC 6-10 and 8-14 day outlooks tilt the odds towards below median precipitation in the Pacific Northwest, along with above-median temperatures. In the Southwest, above-median rainfall is favored in the 8-14 day outlook, with near-normal odds in the 6-10 day forecast. Longer range tools, however, continue the recent robust monsoon convection during the remainder of the season. The month of September may be crucial toward the SON forecast as the southwest monsoon typically wanes in October, with November normally dry. The CPC monthly and seasonal outlook both maintain enhanced chances for above median precipitation across the Southwest monsoon regions, and even expand them into the central Plains. In addition, above-normal sea surface temperatures across the eastern Pacific and Gulf of California may promote enhanced tropical cyclone activity which can aid surges of Pacific moisture into the Southwest. Therefore, drought improvement is indicated from southeastern California northeastward into southern Utah and eastward to the Plains, close to where the CPC September and SON outlooks maintain a greater than 40% chance of precipitation falling in the upper tercile. Since the forecast is based on convective monsoonal precipitation, drought improvements are likely to be localized in nature, including some areas that fail to show any improvement by Nov. 30, and with a wide variability of impacts across the region. Overall, however, a majority of the region should show some improvement by the end of November.

Forecast confidence for the Southwest is moderate to high.

Mostly dry weather continued across the West - which is seasonable; however, temperatures remained above normal, exacerbating the impacts of the long-term drought. In the Northwest, however, the heat and minimal rains (where somewhat higher normal totals exist) led to expansion of abnormal dryness and some drought into the Northwest. During the upcoming week, mostly dry weather is forecast to continue, although lower temperatures should envelop most of the region. Below-median precipitation is anticipated during the 6-10 day, 8-14 day, and 3-month periods in the Pacific Northwest, while above normal temperatures are favored to return to the Far West during weeks 1 and 2. The CPC 1- and 3-month outlooks both tilt the odds towards above median temperatures across the Far West, with the September outlook maintaining odds greater than 50% for monthly temperatures in the upper tercile in California and western parts of Oregon and Washington. The only area with slight odds of above-median precipitation at 1- and 3-months is in southeastern California and southern Nevada (where improvement is shown). Based on these temperature and precipitation outlooks, a dry climatology (except for November in the Northwest), and long-term drought conditions, persistence is expected, especially in much of California (it cannot get much worse), with further drought degradation and development possible in the Northwest.

Forecast confidence for the West and Great Basin is moderate - to high in California.

Recent rains associated with Tropical Storm Iselle effectively removed most D0 areas, but a small area of moderate drought - D1(L) - remained on Hawaii's Molokai Island based upon a low reservoir. The summer months are climatologically dry for this region, and reservoir levels have begun to drop. The CPC September monthly outlook indicates no shift in precipitation probabilities, while the SON outlook is the same. Although there is a correlation between El Niño conditions and below-normal rainfall across Hawaii, this relationship is more pronounced during the winter months than in the fall. Based on climatology, current conditions, and outlooks, drought persistence is anticipated.

Forecast confidence for Hawaii is moderate to high.

Suppressed convection has persisted over the western and central Caribbean during the past 90 days, and in some places, even longer. The dry weather has promoted development of abnormal dryness and moderate drought (D0-D1) across much of southern and eastern Puerto Rico. Weak Tropical Storm Bertha brought decent rainfall to western and northern Puerto Rico, enough to briefly diminish D0 in those areas, but dryness has returned to the island recently. As a result, D0 and D1 has expanded in the south and northeast as the summer and fall months are normally the wettest time of the year (which results in large deficits when dry). CFS precipitation anomaly forecasts for the upcoming three months indicate a potential for continued near to below normal precipitation across the Caribbean basin. This forecast is consistent with the low frequency tropical convective signal that is gradually evolving towards El Niño conditions. However, even though the Atlantic and Caribbean basins have been and are forecast to have below normal tropical activity, one or two well-placed storms could dump copious rain on the island during the SON period and improve conditions. With that said, however, drought persistence and development is still anticipated across southern and eastern Puerto Rico.

Forecast confidence for Puerto Rico is moderate.