Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Sep 17 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. The official CPC ENSO forecast indicates that La Nina conditions are likely to persist through the Northern Hemisphere winter. The October-November-December (OND) 2020 temperature outlook favors above-normal temperatures for a majority of the CONUS, with the largest probabilities (greater than 70 percent) forecast across parts of the Southwest along with northern Alaska. The OND 2020 precipitation outlook indicates enhanced probabilities of below normal precipitation amounts from southern California, across the Southwest, southern Great Plains, and Lower Mississippi Valley, to the Southeast. Above-normal precipitation is most likely across Washington, the northern Rockies, Dakotas, and Upper Mississippi Valley along with western and northern mainland 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 La Nina conditions. During the past four weeks (Aug 16 – Sep 12), sea surface temperature (SST) anomalies are below (above)- average across the equatorial east-central (west) Pacific. The negative SST anomalies have expanded since July and increased in magnitude from 120W to 90W. The latest Nino-3.4 index is -1.0 degree C. Negative subsurface temperature anomalies continue to persist at a depth of more than 150 meters from 160W to 120W. Integrated upper-ocean heat anomalies declined rapidly during July and early August with those anomalies remaining nearly steady since mid-August. Suppressed convection continues over the west-central Pacific. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC SST consolidation for the Nino-3.4 region depicts negative SST anomalies increasing in magnitude to more than -1.0 degree C by NDJ, followed by a gradual trend to near average by MAM 2021. The North American Multi-Model Ensemble (NMME) ensemble mean forecast for the Nino-3.4 SST anomaly has a similar evolution during the next six months. Based on the observational and model forecast indicators as of early September, the official CPC/IRI ENSO outlook favors La Nina conditions persisting through the Northern Hemisphere winter (~75% chance). La Nina conditions remain the most likely outcome through FMA 2021. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Given the likelihood of La Nina conditions persisting through the winter 2020-2021 and the most likely outcome into the early spring 2021, La Nina temperature and precipitation composites were a major factor through FMA 2021. Dynamical model guidance such as the North American Multi-Model Ensemble (NMME) and the Calibration, Bridging, and Merging (CBaM) version of the NMME were used. The consolidation tool, which includes NMME input and various statistical tools, was also used, especially at later leads. Also, based on the expectation of drought development during the next six months 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 - OND 2020 TO OND 2021 TEMPERATURE During OND, above normal temperatures are favored throughout the forecast domain, except for southeastern Alaska where equal chances (EC) of below, near, or, above normal temperatures are forecast. This EC area is supported by La Nina composites and CBaM. Probabilities of above normal temperatures are lowest from the Pacific Northwest east to the Northern Great Plains and Upper Mississippi Valley based on the expectation of a more variable pattern during OND. Decadal trends support elevated probabilities (above 50 percent) of above normal temperatures across the Northeast. The largest probabilities (60 to 70 percent, or more) of above normal temperatures are forecast across the Southwest, based on La Nina composites, strong support from dynamical models, and decadal trends. Probabilities of the same magnitude are also forecast across northern Mainland Alaska due to decadal trends and the likely delayed onset of sea ice. As of mid-September, the Arctic Sea ice extent is well below its 1981-2010 median coverage. The temperature outlooks during the winter 2020-2021 are consistent with La Nina composites, the latest dynamical model guidance, and a consideration of decadal trends. Beginning in DJF 2020-2021, slightly elevated probabilities of below normal temperatures are forecast for southeast Alaska, the Northern Great Plains, Northern Rockies, and parts of the Pacific Northwest. Probabilities of above normal temperatures increase below the 40th parallel. Later in the spring 2021, probabilities of above normal temperatures were increased across parts of the Southwest and southern Great Plains, compared to the previous seasonal release. This modification is related to the expectation of low soil moisture conditions and its warming influence on surface temperatures during the next warm season. PRECIPITATION The highest confidence in the OND precipitation outlook exists across the southern Great Plains and New Mexico, where more than a 50 percent chance of below normal precipitation is forecast. These relatively large probabilities are supported by multiple tools including La Nina composites. Although increased chances of below normal precipitation extend east across much of the Gulf Coast States during OND, probabilities are tempered due to the potential effect from a tropical cyclone during October and November. Based on La Nina composites and dynamical models, above normal precipitation is most likely across the Pacific Northwest, northern Rockies, and parts of Alaska during OND. The most notable changes to the previous precipitation outlooks included an increase in coverage and probabilities of below normal precipitation throughout the southern tier of the CONUS and California, beginning in OND 2020 and continuing through FMA 2021. These necessary changes are consistent with La Nina condition, the latest dynamical model guidance, and the CPC consolidation tool. The dry signal for California indicated in many tools, is strongest during JFM and FMA 2021, then diminishes later in the spring, in part due to the waning of the wet season. Elevated probabilities of above normal precipitation expand to include the northern Great Plains, Upper Mississippi Valley, and Ohio Valley. This predicted dry-wet dipole from south to north is a typical response to La Nina conditions. FORECASTER: Brad Pugh 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 Oct 15 2020 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$