September 2014

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Latest Monthly Assessment - During the past 9 days (since the SDO release on Aug. 21), a highly-amplified pattern produced warm and dry weather across the Far West, unseasonably cool conditions in the Intermountain West and Rockies with excessive precipitation in northern sections (central Montana measured 5-9 inches) and little or no rainfall in the south, oppressive heat in the Midwest and southern half of the Plains with ample rains in the Midwest and more widely-scattered precipitation in the Plains, and near to below-normal readings and minimal rainfall in the eastern quarter of the Nation. Another week of heavy rain fell on parts of Puerto Rico due in part to developing Tropical Storm Cristoval, while mostly light windward showers fell across Hawaii. During the past 30 days, drought relief (according to the U.S. Drought Monitor) had occurred in parts of the Southwest (especially AZ and NM) mostly due to a robust summer monsoon, in the northern and central Plains, Tennessee Valley, and most of the mid-Atlantic and Carolinas. In contrast, subnormal rainfall and occasional heat has developed or expanded abnormal dryness and drought in the Southeast, the southern Plains, and parts of the Great Lakes region. By the end of August, showers and thunderstorms enveloped most of the western and central Gulf northward into the Great Lakes region, with widely-scattered showers in the northern Rockies and Plains. Mostly dry weather prevailed in the West and Southwest, and along the East Coast. Above-normal readings covered most of the country east of the Rockies.

During September, drought persistence is strongly favored in the Far West as September is normally dry and warm, especially in California, with some possible development in the Northwest where current D0 areas exist due to growing short-term deficits and favorable odds for subnormal precipitation and above-normal temperatures. In the Southwest, although a brief lull in the summer monsoon is forecast early in the period, activity is gradually expected to increase, especially in Arizona and western New Mexico. Unfortunately, the model forecasts show mixed signals to the east, hence the persistence in eastern New Mexico and eastward into northern Texas and central Oklahoma. With the monsoonal moisture expected to move northeastward, it should interact with frontal activity moving southward from Canada, hence the improvement in the central Plains. In southern Texas, short-term and monthly forecasts favor enhanced rainfall, and with September normally one of the wettest months, significant rains could provide some improvement. Across parts of Southeast (southern Georgia, central Alabama, northern Florida), short-term drought from 90-day deficits accumulating to 6-10 inches and occasional heat have some potential for improvement based upon moderate 7-day totals, weeks 1 and 2 odds tilting toward above-median rainfall, and possible tropical system rains during the normal peak of the Atlantic and Caribbean hurricane season (Sep. 10); however, recent rainfall forecasts have overdone the amounts, and with good odds for well-above normal temperatures, a lackadaisical 2014 Atlantic hurricane season, and conflicting model forecasts do not favor either improvement or development - hence status-quo. Favorable rainfall forecasts during the first half of September should be enough for improvement in the Tennessee Valley (western Kentucky). In Hawaii, seasonal dryness favors drought persistence over Molokai, while the expected continuation of suppressed convection during the peak Atlantic and Caribbean hurricane season favors expanding drought across eastern Puerto Rico, although favorably-placed storms Bertha and Cristoval have provided recent relief, mainly to western locations.

Tools used in the U.S. Monthly Drought Outlook (MDO) included the official Climate Prediction Center (CPC) updated temperature and precipitation outlooks for September 2014, various short- and medium-range forecasts and models such as the 5-day and 7-day precipitation totals from the Weather Prediction Center, 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 (CAS) moisture, dynamical models (CFSv2, NMME, IMME, and IRI), the 384-hour total precipitation forecasts from several runs of the GFS, climatology, historic modeled soil moisture changes during the month, August 26-30 observed precipitation (not included in the August 26 Drought Monitor analysis), and initial conditions.

During the previous 30 days, portions of the Southeast (central Alabama, Georgia, and the extreme northern Florida Panhandle) missed heavy showers and thunderstorms that yielded above-average rainfall to the west (in Louisiana), to the east (in Florida), and to the north (from Tennessee eastward into the Carolinas). This continued a dry pattern going back to 90-days that have accumulated deficits of 6-12 inches in the same areas. The dryness plus occasional heat during the main growing season (high evapotranspiration rates) has been enough to develop and expand D0-D2 in central Alabama, southern Georgia, and the northern Florida Panhandle; however, the drought is only short-term as surpluses existed at 180-days due to a very wet spring. Farther to the north, moderate to heavy rains have reduced or eliminated drought and abnormal dryness in central Kentucky, northern Tennessee, southwest Virginia, and the Carolinas, but the rains missed western Kentucky, expanding drought there. During the upcoming week, showers and thunderstorms are likely from the lower Mississippi Valley northward into the Great Lakes region and in the Southeast, although the focus for heaviest rainfall is expected to be from Louisiana into the Ohio Valley. This expected heavy rain plus favorable 6-10 and 8-14 day CPC precipitation outlooks should be enough to improve conditions in western Kentucky. Farther to the south, the WPC 7-day QPF outlook indicates a potential for about an inch of rainfall across southern Georgia and Alabama, but with temperatures forecast to be above normal, this is not likely to provide much, if any, help. However, the CPC week 1 and 2 outlooks favor above median precipitation across the Southeast, although they also tilt the odds toward above median temperatures. Recent past short-term forecasts, however, have overdone the rains, and deterioration is quite possible with the expected heat. The updated CPC monthly outlook has equal chances for precipitation and above median probabilities for temperature. The wild card is any tropical system from the Gulf or Atlantic during normally active September which could quickly erase any deficits, although this season has been and is forecast to be rather tranquil. Based on several factors and forecasts which could either improve or expand drought in the Southeast and the lack of a clear precipitation signal during September, this outlook keeps conditions status-quo (persistence) for the existing drought areas.
Forecast confidence for the Tennessee Valley and Southeast are medium (TN Valley) to low (Southeast).

Although July brought drier conditions to the northern half of the Plains after a wet late spring, widespread heavy rainfall returned during August in the northern and central Plains, eliminating all drought in the Dakotas, nearly all in Nebraska, and improving conditions in Kansas. Light to moderate rainfall is forecast during the upcoming week in eastern sections (accumulations generally around an inch), with lower amounts (less than 0.5 inches) expected in western areas. The CPC 6-10 and 8-14 day outlooks indicate above median precipitation, with temperatures forecast to be close to or slightly above normal. The updated monthly precipitation outlook slightly favors above median precipitation in the central Plains. This is anticipated from ample Southwest monsoon moisture flowing northeastward into the central Plains that interacts with southward moving cold fronts from Canada. In addition, September temperatures typically decrease as does evapotranspiration as crops mature, which combined with favorable September precipitation odds, should help to further improve (or at least stabilize) drought conditions.
Forecast confidence for the central third of the Plains (Nebraska and Kansas) is moderate to high.

During August, Texas and Oklahoma both observed scattered pockets of moderate to heavy rainfall along with areas of minimal precipitation. Surplus 30-day rains fell on extreme southern, southwestern, south-central, and extreme northern and eastern Texas, and in parts of the Oklahoma Panhandle and along the Red River Valley. 30-day deficits were the greatest (1-3 inches) in central Oklahoma, Texas Panhandle, and southeastern Texas. During the upcoming 5-days, decent rainfall (more than 1.5 inches) is forecast along the western Gulf and in northeastern Oklahoma, with about an inch anticipated in southern Texas and northern Oklahoma. Little or no rain is predicted for southern Oklahoma and northern Texas. The CPC 6-10 day outlook maintains enhanced chances for a dry and warm pattern over the southern Plains, while the 8-14 day outlook odds have near normal precipitation and above median temperatures. The updated September precipitation outlook has favorable probabilities for above-median rainfall in southeastern Texas, but no tilt either way (EC) elsewhere in the two states due to weak monsoon flow forecasted for early in the period in eastern New Mexico and western Texas, and then mixed signals from the models later in the month. With September climatology being the wettest month of the year in extreme southern Texas and favorably wet odds this month, chances are good for some improvement there. Elsewhere, however, with short-term forecasts looking dry and warm, medium-range outlooks generally unfavorable, September precipitation climatology in Texas somewhat wet, and monthly rainfall predictions having no odds either way, the rest of Texas and Oklahoma lean toward status-quo (persistence or deterioration) instead of improvement.
Forecast confidence for the southern third of the Plains (Oklahoma & Texas) is low to moderate.

During the past 30 days, enhanced monsoon activity, especially early in the period, brought widespread moderate to heavy rainfall to southwestern and central New Mexico, southeastern and central Arizona, extreme southeastern California, and parts of Colorado and Utah, reducing drought conditions that have persisted for years in some areas. Temperatures also averaged below normal across much of the region. Monsoon thunderstorms were more widely-scattered and lighter across northeastern Arizona, southeastern Utah, northwestern New Mexico, west Texas, and Nevada where drought conditions persisted or slightly worsened during the past four weeks. A complicated precipitation pattern, both temporally and spatially, is forecast during September. Little or no rain is expected the next 5 days, but the CPC 6-10 day outlook favors wetness across Arizona and western New Mexico but dryness in eastern New Mexico and west Texas, and the 8-14 day period odds slightly lean toward wetness throughout the region (except northern Utah). Meanwhile, the updated September outlook tilts the odds towards above-median precipitation from extreme southeastern California eastward to eastern New Mexico, but EC in southeastern New Mexico and west Texas. Based on these various outlooks, further drought improvement is forecast across most of Arizona, southeastern Utah, western New Mexico, and southwestern Colorado, while persistence is more likely for eastern New Mexico, west Texas, western Utah, northwestern Arizona, and southeastern California. Even if September precipitation is below normal, deterioration or development is not likely in New Mexico and west Texas as surpluses existed at 30-, 60-, 90-, and 180-days. Unfortunately, long-term (several years) hydrologic drought remains at many of these areas, and it will take a prolonged period of surplus precipitation (summer and winter) to overcome the impacts.
Forecast confidence for the Four Corners region is moderate across Arizona and western New Mexico and low to moderate across eastern New Mexico and west Texas.

Seasonably dry and slightly above-normal temperatures prevailed across the Far West, maintaining extreme to exceptional drought conditions in California and producing some degradations and development over the Pacific Northwest. Widely-scattered thunderstorms developed over the Great Basin and Sierra Nevada, but rainfall was inconsequential for making an impact on the drought. However, increased thunderstorm activity and cooler temperatures did provide some assistance to firefighting activities. Little change to the pattern is anticipated during the first half of September, with near-median temperatures favored and seasonal dryness continuing. The CPC update September outlook maintains enhanced odds for above-median temperatures across the Pacific States, and added below median precipitation for the Pacific Northwest. Based on these outlooks and dry September climatology, drought persistence or intensification is anticipated, with further expansion possible across parts of Oregon, Washington, and central Idaho where D0 already exists.
Forecast confidence for the West is high.

Although a general pattern of suppressed convection continued across much of the Caribbean and western Atlantic, the near proximity development of Tropical Storms Bertha and Cristoval dumped heavy rains (10-20 inches) on portions of Puerto Rico during August, creating 30-day surpluses and eliminating D0-D1 in southwestern sections while improving conditions farther east. However, with such a dry start to the late spring and summer, deficits still lingered at 60-days and beyond across eastern sections of the island. A low frequency climate signal evolving towards El Niño conditions favors suppressed convection over the Caribbean and reduced tropical cyclone activity across the central Atlantic, with September normally the most active month. However, only one September storm developing or tracking near Puerto Rico (like Bertha and Cristoval) is needed to negate a monthly forecast for dryness. But based on the potential for continued dryness during the normally wettest time of the year, drought development of existing D0 areas is anticipated across eastern Puerto Rico (with confidence reduced due to the tropical storm caveat).
Forecast confidence for Puerto Rico is moderate.

A small drought area remains on Hawaii’s central Molokai Island due to a low reservoir. September is a climatologically dry time of year which would promote drought persistence, and the monthly precipitation forecast slightly tilts toward below median rainfall. There is a potential, however, for tropical cyclone activity in the central Pacific (like Tropical Storm Iselle did) to bring enhanced rainfall to parts of Hawaii, but the potential is non-existent during the next week, and low later in the month.
Forecast confidence for Hawaii is moderate.