Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Aug 20 2020 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS The September-October-November (SON) 2020 temperature outlook favors above normal temperatures across all of the contiguous U.S. and Alaska. The SON precipitation outlook favors above normal precipitation from the Pacific Northwest to the northern plains, portions of the Southeast, Southwest Alaska, and along the north coast of Alaska. Elevated odds for below normal precipitation stretch from the Southern Sierra Nevada mountains to the central Rockies and Southern Plains. Equal chances (EC) of below, near and above normal precipitation are indicated for central Alaska, and for all remaining areas of the contiguous U.S. 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 Oceanic and atmospheric observations across the equatorial Pacific indicate ENSO-neutral conditions persisted into August. Observed sea-surface temperatures (SSTs) for the previous 30 days showed positive anomalies west of 170E and negative anomalies from about the Date Line to the coast of South America. The equatorial central pacific cooled while the eastern Pacific warmed during the past 30 days, though the eastern Pacific still remained below average. The four Niño indices were negative during the latest week, with the Niño-3.4 value at -0.5C. Positive subsurface temperature anomalies continued to be confined to the western Pacific, with negative temperature anomalies to a depth of 150 meters in the eastern Pacific. Integrated upper-ocean heat anomalies declined during the second half of July and early August. Easterly low-level wind anomalies were present over the western low-latitude Pacific during the previous 30 days, with westerly upper-level wind anomalies along the equator over the western equatorial Pacific Ocean. Positive outgoing longwave radiation (OLR) anomalies, during this same period show suppressed convection across the western and central equatorial Pacific. Negative OLR anomalies were noted over portions of the Maritime Continent. A La Nina watch is in effect. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC SST Consolidation for the Nino 3.4 region indicates an SST anomaly declining to near -1.0C during late autumn. The CCA and Markov model inputs CPC SST Consolidation remain in ENSO-neutral territory. Among the NMME models, there is a range from far below normal (-2.0C) to cold-neutral. The C3S models all get close to, or below, -0.5C for at least one month. The CPC/IRI consensus forecast favors a La Niña development during Northern Hemisphere fall 2020 (60% chance) and continuing through winter 2020-21 (55% chance). PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Tools used for the seasonal outlooks included dynamical model guidance such as the North American Multi-Model Ensemble (NMME), including the Calibration, Bridging, and Merging (CBaM) post-processing applied of the NMME. The projected state of ENSO is considered generally, and specifically through a regression of temperature and precipitation as included in the seasonal consolidation tool. Additionally, various statistical tools such as CCA and constructed analogs on SST patterns were viewed but considered quantitatively as part of the consolidation. In the further leads, beyond winter 2020-21, decadal trends were the primary source of climate signals. PROGNOSTIC DISCUSSION OF OUTLOOKS - SON 2020 TO SON 2021 TEMPERATURE During SON 2020, the outlook favors above normal temperatures for the CONUS, based on model outputs, trends, and relationships with SST patterns. Uncertainty is higher across the Pacific Northwest and from the Northern Great Plains to the Southeast. The outlook therefore calls for slightly lower odds in the Pacific Northwest than the SON outlook issued last month, and higher odds for above normal temperatures in the Northeast and portions of the Southwest. The changes in odds are small, and related to slightly more certainty about the projected state of La Nina. Similar reductions in odds for above normal temperatures in the Pacific Northwest were carried through DJF 2020-21, with below normal temperatures now favored from Washington to North Dakota in JFM 2021. The changes are again related to more certainty of impacts related to projected state of ENSO, and more recent dynamic and statistical guidance. The shift toward colder outcomes across the northern tier of the CONUS was carried through MAM 2021. Minor or no changes were made in the outlooks for Spring and Summer of 2021, as the core guidance for those leads is trend. Outlooks across Alaska remained largely unchanged as the impacts for La Nina were already largely accounted. Some below normal temperatures were indicated across Southeast Alaska in some of the C3S models and statistical tools, but other tools had little to no signal, so EC remained until there is more certainty. PRECIPITATION The status of ENSO and likelihood of La Nina developing through the winter of 2020-21 played a role in the outlook. The SON 2020 outlook is similar to the outlook from last month for the same target period. The odds for above normal precipitation in the Southeast were extended up the Carolina coast while the odds for below normal precipitation were trimmed back slightly along the western Gulf Coast. Both related to the updated model guidance and potential for late season tropical cyclones to develop in the Gulf of Mexico, aligned with the seasonal hurricane outlook. Outlooks covering the autumn through winter were modified to reflect more certainty with respect to La Nina and potentially a more intense impacts, so the odds for above normal precipitation were increased across the Pacific Northwest with a drier signal into the Great Basin. Outlooks for the core winter seasons of DJF and JFM do include a signal for below normal precipitation into portions of California and higher odds for below normal precipitation across the entire southern tier of the CONUS. Outlooks for the spring of 2021 reflect a little more uncertainty across the Northern Plains as the transition period from a La Nina is uncertain. Enhanced odds for below normal precipitation were added to the Pacific Northwest for late spring and through the summer to reflect trends, while the rest of the outlook was largely unchanged. FORECASTER: Matthew Rosencrans 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 Sep 17 2020 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$