Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Sep 16 2021 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS ENSO-neutral conditions are present, as represented in current oceanic and atmospheric observations. A transition from ENSO-neutral to La-Niña is favored in the next couple of months, with a 70 to 80 percent chance of La Nina during the Northern Hemisphere Winter 2021-22. Conversely, the chances of El Niño are exceedingly small, with probabilities less than 5 percent through the upcoming winter. The October-November-December (OND) 2021 temperature outlook favors above normal temperatures for the southern two-thirds of the Contiguous United States (CONUS), the eastern third of the CONUS, and western Alaska, with the largest probabilities (greater than 50 percent) forecast across the Southwest and New England. The OND 2021 precipitation outlook depicts enhanced probabilities of below normal precipitation amounts across the Southern Tier and much of the central CONUS. Above-normal precipitation is more likely for the northwestern CONUS, the Eastern Great Lakes, parts of the interior Northeast, and parts of southwestern Alaska. Equal chances (EC) are forecast for areas where probabilities for each category of seasonal mean temperatures and seasonal accumulated precipitation amounts are expected to be similar to climatological probabilities. BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS Note: For Graphical Displays of the Forecast Tools Discussed Below See: http://www.cpc.ncep.noaa.gov/products/predictions/90day/tools/briefing CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS The coupled oceanic and atmospheric observations reflect ENSO-neutral conditions. In the last month, ENSO-neutral continued with near-to-below average sea surface temperatures (SSTs) persisting in the central and eastern equatorial Pacific. In the last week, the Niño 3.4 index value stood at -0.4°C. Negative subsurface temperature anomalies (averaged from 180-100°W) remained steady in August reflecting below-average temperatures that extended from the surface to ~250m depth in the eastern Pacific Ocean. Low-level wind anomalies were easterly over the western Pacific Ocean, while upper-level wind anomalies were westerly over the western and east-central Pacific. Tropical convection was suppressed near and west of the Date Line and enhanced over Indonesia. Given these conditions, the ocean-atmosphere system reflects ENSO-neutral, but is edging toward La Niña. The Real-time Multivariate Madden Julian Oscillation (RMM) index depicts a strengthening signal over the eastern Indian Ocean, where enhanced convection has persisted for the past several weeks. There is considerable spread among dynamical models on the future evolution of the Madden Julian Oscillation (MJO). Models generally depict an MJO event crossing the Maritime Continent but differ in the strength and evolution of this feature. Thus, uncertainty is relatively high on the future evolution of the MJO at this time. PROGNOSTIC DISCUSSION OF SST FORECASTS The IRI/CPC plume average of forecasts for the Niño-3.4 SST region from the last month favored borderline or weak La Niña during the fall and winter 2021-22. The current forecast favors the latest predictions from the NCEP CFSv2 and the North American Multi-Model Ensemble (NMME), which suggest higher chances for the emergence of La Niña. At this time, La Niña is anticipated to be of weak strength (seasonal average Niño-3.4 index values between -0.5°C to -0.9°C). In summary, a transition from ENSO-neutral to La Niña is favored in the next couple of months, with a 70-80% chance of La Niña during the Northern Hemisphere winter 2021-22. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Dynamical model forecasts from the NMME and Copernicus (C3S) multi-model ensemble systems are used extensively for the first six leads when they are available, as was the objective, historical skill weighted consolidation, that combines both dynamical and statistical forecast tools. Additionally, the official ENSO forecast depicts probabilities of La Niña that are significantly higher than climatological probabilities through the upcoming winter. This anticipated La Niña signal played a role in the construction of these outlooks. At later leads, decadal trends in temperature and precipitation were increasingly relied upon in creating the seasonal outlooks. PROGNOSTIC DISCUSSION OF OUTLOOKS - OND 2021 TO OND 2022 TEMPERATURE Above normal temperatures are favored throughout a majority of the southern and eastern CONUS and western Alaska during OND. Conversely, below normal temperatures are more likely for southeastern Alaska. Equal chances (EC) of below, near, or above normal temperatures are forecast for the northwestern and north-central CONUS. This EC area is due to weak or conflicting signals among temperature tools. Probabilities of above normal temperatures are increased relative to last month across the East due to stronger support among the temperature guidance. Above normal temperatures are especially likely (greater than 50 percent) for eastern New England due to good agreement among both dynamical and statistical guidance. Guidance has trended colder across much of Alaska relative to last month. Thus, increased probabilities of below normal temperatures were introduced to southeastern parts of the state, which is also consistent with composites from the current ENSO state. From NDJ through FMA, impacts from the potential La Niña become more apparent as above normal temperatures continue to be favored across the Southern CONUS and the Eastern Seaboard while enhanced below normal temperature probabilities persist across southeastern Alaska and expand southward to the northwestern CONUS. Thereafter, the forecast pattern increasingly begins to reflect trends with above normal temperatures generally favored across most of the southern, western, and eastern CONUS next spring, summer, and into the fall. Probabilities of above normal temperatures peak across the West next summer and across the Eastern Seaboard next fall, consistent with trends. Across Alaska, above normal temperatures become increasingly likely with time, with the strongest warm signal emerging next summer, when probabilities of above normal temperatures exceed 50 percent for most of the state. PRECIPITATION Model guidance remains consistent from previous months depicting elevated probabilities of below normal precipitation for much of the southern and central CONUS, and increased chances of above normal precipitation for the Pacific Northwest during OND. Models are consistent in depicting an expansion of a dry signal eastward across the Southeast, with influences from potential tropical disturbances decreasing and La Niña influences increasing as we progress deeper into the fall and early winter. Conversely, precipitation guidance has trended wetter across the northwestern CONUS, resulting in an increase in above normal precipitation probabilities relative to last month. Additionally, an area of slightly increased probabilities of above normal precipitation was introduced to the Great Lakes region and interior Northeast, consistent with the latest C3S guidance. A slight tilt toward above normal precipitation is also indicated for parts of southwestern Alaska, based primarily on dynamical model guidance. EC is indicated for the rest of the forecast domain as signals for the various dynamical and statistical tools are too weak or conflicting to issue a forecast with a sufficient degree of confidence. As we progress further into late fall and through the winter, dynamical and statistical guidance persist with a dry signal across the southern CONUS, consistent with a La Niña signature. An eastward expansion of enhanced above normal precipitation probabilities is noted across the northwestern CONUS, as well as a northward expansion across the Alaska Panhandle, peaking in coverage during the winter. A wet signal is also favored to expand southward from the Great Lakes to the Ohio Valley during the winter with the strongest signal evident during JFM, consistent with La Niña composites. During next spring through next fall, trends become the dominant factor with increased chances of above normal precipitation generally indicated across parts of the Eastern CONUS and Gulf Coast Region through much of the period. Conversely, trends favor a dry pattern for much of the West next spring and summer, with increased chances of dryness for parts of the Northern Plains next summer as well. Thereafter, an area of slightly enhanced probabilities of above normal precipitation makes a brief appearance across the Pacific Northwest in ASO, based primarily on statistical guidance from trends and ENSO. A tilt toward a dry pattern persists farther to the south across parts of the remainder of the West into next fall, based primarily on trends. There exists high uncertainty across much of Alaska through the period with large areas of EC indicated. However, dynamical guidance supports increased odds of above normal precipitation across parts of the western and northern Mainland through the MAM season. Conversely, statistical guidance from trends and ENSO support a slight tilt toward below normal precipitation for parts of the South Coast early in the spring shifting toward the southwestern Mainland later in the spring. Thereafter, trends support slightly increased chances for above normal precipitation for parts of the eastern Mainland next summer and slightly increased chances of below normal precipitation for parts of the Panhandle next fall. FORECASTER: Scott Handel The Climatic normals are based on conditions between 1991 and 2020, following the World Meterological Organization convention of using the most recent 3 complete decades as the climatic reference period. The probability anomalies for temperature and precipitation based on these new normals better represent shorter term climatic anomalies than the forecasts based on older normals. For a description of of the standard forecast tools - their skill- and the forecast format please see our web page at http://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools.html (Use Lower Case Letters) Information on the formation of skill of the CAS forecasts may be found at: http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Outlook/outlook.shtm l (use lowercase letters) Notes - These climate outlooks are intended for use prior to the start of their valid period. Within any given valid period observations and short and medium range forecasts should be consulted. This set of outlooks will be superseded by the issuance of the new set next month on Oct 21 2021 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$