Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Oct 15 2020 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS La Nina conditions are present across the equatorial Pacific Ocean, as represented in current oceanic and atmospheric observations. La Nina conditions are likely to continue through the Northern Hemisphere winter and into Spring 2021. The November-December-January (NDJ) 2020-2021 temperature outlook favors above-normal seasonal mean temperatures for a majority of the CONUS and for northern and western parts of Alaska. The greatest probabilities (larger than 60 percent) are forecast for parts of the Southwest. Below-normal temperatures are most likely for areas of southeast Alaska, the Alaska Panhandle and parts of the far Pacific Northwest. The NDJ 2020-2021 precipitation outlook depicts enhanced odds for above-normal seasonal total precipitation amounts for Alaska and parts of the Pacific Northwest, northern Rockies and northern Great Plains. Below-normal precipitation is most likely for much of California (slight tilt in the odds), stretching eastward to include the Southwest, south-central Great Plains, lower Mississippi Valley and Southeast. Equal-chances (EC) are forecast for areas where probabilities for each category of seasonal mean temperatures or seasonal total precipitation amounts are favored 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 La Nina conditions. The latest monthly mean Sea Surface Temperatures (SSTs) through October 10, 2020 continue to indicate a strengthening and westward expansion of the Pacific equatorial cold tongue with SST negative anomalies on or near the equator stretching from 160E eastward to the South America coast. Some negative departures reach as large as -1.5 degrees C in some areas. The latest Nino3.4 index value is -1.2 degrees C. Negative subsurface ocean temperature anomalies continue to persist at a depth ranging from approximately 100-200 meters from just west of the Date Line to 80W. Suppressed convection continues over the west-central Pacific while enhanced convection continues to increase in coverage in the Maritime Continent region. Trade Winds have also been enhanced over the past 30 days across the central Pacific Ocean. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC Nino3.4 SST consolidation forecast depicts negative anomalies increasing in magnitude to near -1.5 degrees C by DJF 2020-2021, followed by a gradual increase to near normal by AMJ 2021. The North American Multi-Model Ensemble (NMME) ensemble mean forecast for the Nino3.4 SST anomaly has a similar evolution during the next several months through May 2021. Based on the latest observations and model forecasts as of early October, the official CPC/IRI ENSO outlook favors La Nina conditions persisting through the Northern Hemisphere winter into Spring 2021 with La Nina conditions remaining the most likely ENSO phase through FMA 2021. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Given the current ENSO conditions and predictions, the seasonal outlooks through MAM 2021 utilized typical impacts during past observed La Nina events as the guide for many areas of the forecast domain. This included standard composite analysis, regressions anchored to forecasts of the Nino3.4 region and via "bridging" techniques utilizing relationships between the Nino3.4 index and observed temperature and precipitation. Dynamical model guidance from the North American Multi-Model Ensemble (NMME) and the model suite from the Copernicus program are also used heavily in the outlooks. Beyond AMJ 2021, the consolidation of various statistical tools including long-term trends was the primary basis for the outlooks from this point forward given little reliable evidence of the influence from ENSO and other reliable large-scale climate variability for these forecast leads. Also, based on current and potential drought conditions in many areas across the southern tier of the CONUS, low soil moisture conditions influenced the temperature outlooks during the spring and early summer 2021. At later leads, decadal trends in temperature and precipitation were the primary tool used in creating the seasonal outlooks. PROGNOSTIC DISCUSSION OF OUTLOOKS - NDJ 2020 TO NDJ 2021 TEMPERATURE The November-December-January (NDJ) 2020-2021 outlook favors above-normal seasonal mean temperatures for a majority of the CONUS based on La Nina base state conditions along with consistent and considerable dynamical model guidance support from the NMME and Copernicus program participant models and their respective ensemble means. Highest odds are forecast for the Southwest where long-term trends are strongly positive and dynamical model output probabilities are the greatest. Less forecast confidence as depicted by lower forecast probabilities for the Ohio Valley and Southeast are associated with uncertainty in the phase of the Arctic Oscillation / North Atlantic Oscillation and other mid-latitude variability especially during the months of December 2020 and January 2021. There is a slight tilt in the odds for below-normal temperatures for parts of the far Pacific Northwest consistent with La Nina conditions especially later in the NDJ season. Lack of or below-normal coverage in sea ice in waters in proximity to Alaska favor above-normal temperatures for areas in northern and western Alaska. Anticipated impacts associated with La Nina conditions, however, favor below-normal temperatures for areas in southeast Alaska and the Alaska Panhandle. Divergence in forecast tools and other factors and so high uncertainty result in a forecast of Equal-Chances (EC) of below-, near-, or above-normal seasonal mean temperatures for a region from the Pacific Northwest to the western Great Lakes for NDJ 2020-2021. The evolution of the temperature outlooks from DJF 2020-2021 through MAM 2021 are consistent with typical impacts associated with La Nina conditions spanning the winter and spring seasons displayed in various statistical, dynamical and hybrid (statistical-dynamical combined) guidance products. This typical historical evolution is augmented by the latest dynamical model guidance and considerations from the latest long-term trends and shows a gradual expansion of favored below-normal temperatures for areas along the northern tier of the CONUS, primarily for parts of the northern Plains and western Great Lakes as well as portions of the Pacific Northwest and southern Alaska. Later in Spring 2021, probabilities for above-normal temperatures across parts of the Southwest and southern Great Plains are influenced by potential worsening or developing low soil moisture conditions and its warming influence on surface temperatures during the warm season after favored below-normal precipitation associated with La Nina winters. PRECIPITATION Similar to the NDJ 2020-2021 temperature outlook, the NDJ 2020-2021 outlook for seasonal total precipitation amounts are strongly influenced by typical impacts associated with a La Nina base state during the winter and spring seasons. The greatest confidence in the precipitation outlooks for the NDJ 2020-2021 through FMA 2021 seasons are for elevated odds of below-normal precipitation first for a region centered across Texas during NDJ 2020-2021 evolving to include the Southwest, parts of the Gulf Coast and Southeast during DJF 2020-2021 through FMA 2021. Above-normal precipitation is most likely for the northern Rockies, parts of the Pacific Northwest and northern Plains during the NDJ 2020-2021 season. This favored region of above-normal precipitation expands eastward to include the Great Lakes and Ohio Valley during the DJF 2020-2021 through MAM 2021 seasons. Above-normal (below-normal) seasonal precipitation amounts are favored for western Alaska (southeast Alaska and the Alaska Panhandle) during similar seasons - consistent with typical La Nina impacts. The precipitation outlooks in subsequent seasons are driven primarily by long-term trends and areas labelled EC indicate a forecast of climatological odds for either below-, near- or above-normal seasonal total precipitation amounts. FORECASTER: Jon Gottschalck The Climatic normals are based on conditions between 1981 and 2010, 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 Nov 19 2020 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$