Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for October and October through December 2021 (OND), various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC extended-range forecasts (ERFs), the Weeks 3-4 outlooks and related tools from CPC, dynamical models at the monthly and seasonal time scales, the 384-hour total precipitation forecasts from several runs of the GEFS, climatology for the OND season, and initial conditions such as soil moisture. The USDM valid on September 14 was used for initial drought conditions.
Drought remains entrenched across the western conterminous U.S. (CONUS), with most of the Western Region experiencing drought conditions (D1 or worse) according to the U.S. Drought Monitor, valid on September 14, 2021. Most drought improvements across the West have mainly been confined to the Four Corners Region, where a robust North American Monsoon (NAM) season has resulted in marked improvements to soil moisture in the months leading up to the start of the OND forecast period. A late surge of monsoon moisture in recent weeks also made it as far north as Montana and, coupled with a stretch of below-normal temperatures, led to some modest localized improvements in the Northern Rockies as well. Much of the West is entering into a climatologically wetter time of year, with the mean storm track shifting southward from the Pacific Northwest and into California during OND. The tropical Pacific Ocean is presently experiencing ENSO-neutral conditions, but leaning cooler than normal. Weak La Niña conditions (sea surface temperature anomalies of -0.5°C to -0.9°C in the tropical Pacific) are expected to develop later in the OND season, with a 70-80% chance of La Niña during the Northern Hemisphere winter 2021-22. Given the potential onset over the next couple of months, the La Niña signal may not be realized until later in the period, so its effects are mainly considered for the Pacific Northwest, where ENSO signals are strongest. As such, broad drought improvement (D2 to D4) and removal (D1) are favored across much of the Pacific Northwest, extending eastward to Montana, with persistence favored elsewhere across the West. Some drought development is also favored for portions of eastern New Mexico, as the NAM winds down during September and warm and dry conditions are favored in the extended and medium ranges. Additionally, the increased potential for La Niña conditions later in the period could tilt conditions further toward warmer and drier across eastern New Mexico, enhancing the potential for development, but timing of La Niña onset is a factor for the Southwest.
Forecast confidence is moderate-to-high for the West.
Similar to the West, the northern and western portions of the High Plains Region also remain entrenched in drought, while the southern High Plains Region is experiencing more localized drought conditions. Much of the region has experienced a mix of improving and deteriorating conditions since the September-November SDO release last month. However, many areas that experienced improvements did not receive enough rainfall to offset longer-term deficits for drought-stricken areas. For many areas not currently experiencing drought, 30-60 day deficits on the order of 2-4 inches have begun to mount. In addition, the Great Plains states are entering into a drier time of year at the start of the OND valid period, which does not bode well for any potential improvements, given the generally dry start to the period for most. Given the antecedent conditions heading into a climatologically dry time of year, drought development is favored for much of the southern High Plains Region southward from central Nebraska, with persistence favored elsewhere across the region, due to improved soil moisture and SPIs leading up to the valid period across areas to the north.
Forecast confidence is low-to-moderate for the High Plains Region.
In late August, Category 4 Hurricane Ida made landfall in Louisiana bringing high winds and flooding to much of eastern portions of the Southern Region (Louisiana, Mississippi, and Tennessee) during its transition into the mid-latitudes. A couple weeks later, leading up to the start of the OND valid period, Hurricane Nicholas made landfall in southeastern Texas near Matagorda Bay bringing heavy rainfall and flooding that extended eastward to parts of Louisiana affected by Hurricane Ida. Equal chances for above or below-normal temperatures and equal chances to above-normal chances for precipitation are favored across the Lower Mississippi Valley through October. However, with the potential onset of La Niña during the latter half of the period, this tilts odds toward warmer and drier conditions, as depicted in the CPC's seasonal temperature and precipitation outlooks for OND. Given the antecedent wetness across much of the Lower Mississippi Valley and generally near to wetter-than-normal signals during October, Louisiana, Mississippi, and Tennessee are favored to remain drought-free through the end of December. Farther west in the Southern Region, however, many locations across the Southern Plains have started to dry out over the past 30-60 days. Many locations across Texas, Arkansas, and Oklahoma are experiencing 1-3 inch deficits in the last 30 days, with pockets of 3-4 inch deficits for parts of central and northeastern Oklahoma. Oklahoma and Texas typically experience a secondary maximum in rainfall during October. However, given the warmer and drier signals in the extended and long ranges coupled with the antecedent dryness across the western and northern portions of the Southern Region, there is an increased risk for broad drought development.
Forecast confidence is moderate for the Southern Region.
Western areas of the Midwest Region experienced marked improvements to drought conditions, particularly northern Iowa and central and southern Minnesota, where many locations received more than 150% of normal precipitation in the last 30 days. However, as is the case for the Northern Plains, the precipitation was not enough to overcome longer-term (beyond 30-day) deficits. Odds tilt toward above-normal precipitation over the next 6-10 days for much of the Midwest, with the largest above-normal probabilities (greater than 40%) over the Great Lakes. Equal chances of above, near, or below-normal precipitation are also favored during the month of October, with above-normal probabilities diminishing in magnitude and coverage across the Upper Midwest and Great Lakes beyond October. Additionally, evaporation and evapotranspiration rates are likely to drop off dramatically across many areas in drought in the Midwest as the OND season progresses. As such, short-term improvements (D2-D3) and removal (D1) are favored across the Arrowhead of Minnesota and removal (D1) across remaining pockets of drought in the Great Lakes. Persistence is favored elsewhere with diminishing evaporation rates during the season, despite an increased potential of above-normal temperatures.
Forecast confidence is moderate for the Midwest Region
Presently, the Southeast is drought-free. However, some D1 (moderate) drought was removed across portions of the Blue Ridge Mountains of Virginia and West Virginia since the September-November SDO release due to heavy rainfall from the remnants of Hurricane Ida. Farther south in the Carolinas, some short-term dryness has warranted an increase in abnormally dry (D0) coverage in the U.S Drought Monitor. Despite the recent dryness in the Carolinas, precipitation is expected to remain near to above-normal at least through the end of October. Additionally, the potential exists for tropical activity during the first half of the period, especially with ENSO-neutral conditions already in place across the tropical Pacific, which typically leads to more favorable upper-level conditions for tropical cyclone activity across the Atlantic. La Niña conditions are favored to develop in the latter portions of OND, which could signal a shift to a warmer and drier regime. However, despite the potential for La Niña conditions to develop, the Southeast is forecast to remain drought-free through the end of the year.
Forecast confidence is high for the Southeast Region
In the Northeast, drought is confined to a small area on Cape Cod and northern New England. Parts of northern New England are experiencing 15-20 inch deficits going back to one year ago in D2 (severe) drought areas. However, above-normal precipitation is favored for parts of the Northeast to varying degrees throughout OND. Although above-normal temperatures are favored throughout OND, evaporation typically dwindles toward the end of the period. Conditions have also trended wetter during more recent La Niña episodes in the Northeast Region. Given the long-term antecedent dryness and the tilt toward above-normal precipitation, modest improvement (D2) and removal of drought are likely in the Northeast.
Forecast confidence is moderate-to-high for the Northeast Region.
Precipitation signals across southwestern Alaska are opposing at the monthly and seasonal timescales, with weak signals elsewhere for precipitation. Meanwhile, equal chances to below-normal temperatures are favored for central and eastern portions of Alaska, with above-normal temperatures likely along the western Mainland coast and the Aleutians. However, much of the state is expected to enter its annual freeze during the OND period. Therefore, Alaska is likely to remain drought-free through the end of the year. Hawaii is likely to experience near-normal temperatures and below-normal precipitation through October. However, with the increased potential for La Niña development in the latter half of OND, this historically results in increased trades. Given La Niña is likely to remain relatively weak during its initial onset later during OND, this increases chances for weakened trades and increased precipitation along the leeward slopes (in areas with ongoing drought conditions). In addition, Hawaii is transitioning into a climatologically wetter time of year. Given the favored dry start to the season, the transition into the rainy season, and the potential for weak La Niña conditions, drought improvement (D2-D3) and removal (D1) are favored across Hawaii. Puerto Rico has seen an increase in D1 (moderate) drought coverage since the September-November SDO release across southern and northern parts of the island. Despite warmer and drier signals at the monthly and seasonal timescales, ENSO-neutral to La Niña conditions in the tropical Pacific increases the threat of tropical cyclone activity in the Atlantic Ocean, offsetting those warm, dry signals in the forecast tools. As a result, drought persistence is favored across Puerto Rico.
Forecast confidence is high for Alaska, moderate-to-high for Hawaii, and low-to-moderate for Puerto Rico.
Forecaster: Adam Hartman
Next Seasonal Drought Outlook issued: October 21, 2021 at 8:30am EDT