Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Oct 19 2023 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS An El Nino Advisory remains in effect as El Nino conditions remain strongly in place across the Pacific Ocean basin and its influence continues to contribute to the temperature and precipitation outlooks through the upcoming winter 2023-2024 season. In fact, odds for El Nino conditions to persist through the Northern Hemisphere Spring 2024 are currently forecast at 80%. The November-December-January (NDJ) 2023-2024 temperature outlook depicts elevated odds for above-normal seasonal mean temperatures for Alaska, the Far West, the northern tier of the contiguous U.S. as well as for the Ohio Valley and mid-Atlantic. The greatest likelihood for warmer than normal temperatures is for parts of Alaska, the Pacific Northwest and New England. For NDJ precipitation, above-normal seasonal precipitation amounts are favored for the north slope of Alaska, the southern Plains, lower Mississippi valley, Southeast and southern mid-Atlantic. Drier-than-normal conditions are most likely for areas in the Pacific Northwest, northern Rockies, northern High Plains and parts of Michigan. Areas depicted in white and labeled "EC" (Equal-Chances) are regions where climate signals are weak and so there are equal odds for either above-, near- or below-normal seasonal mean temperatures and seasonal total precipitation amounts. 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 El Niño conditions. In mid October, sea surface temperatures (SSTs) are above average across nearly the entire equatorial Pacific Ocean. Positive SST anomalies greater than +2.5 degrees C are present in the eastern Pacific with anomalies generally uniform of +1 to +2 degrees C from the Date Line to the east-central Pacific. The latest weekly values for the Nino3.4 and Nino4 SST indices are +1.5 and +1.3 degrees C respectively with the latter index continuing to increase. A strong reservoir of warmer than normal water remains observed at depth for nearly the entire Pacific basin when viewing subsurface ocean water temperatures along the equator. Largest positive oceanic heat content is located east of 140 degrees W, but substantial positive oceanic heat content remains present to a depth of at least 150 m for the majority of the remainder of the Pacific basin. The warmer than normal water at depth supports not only maintenance of this El Nino event, but the potential for some further strengthening over the next couple of months. The atmospheric state is displaying characteristics consistent with El Nino. In areas of the central Pacific, low-level winds were anomalously westerly, while upper-level winds were anomalously easterly. Convection was enhanced around the Date Line, stretching to the eastern Pacific, just north of the equator with suppressed convection observed near Indonesia. The equatorial Southern Oscillation Index (SOI) and the traditional station-based SOI are both significantly negative. Large areas of above-normal sea surface temperatures (SSTs) reside in both the north Pacific and north Atlantic oceans. More local coastal SSTs are above-normal for western and northern coastal waters of Alaska as well as off the California coast. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC Nino3.4 SST consolidation forecast shows a steady or slight increase in the Nino3.4 SST index entering late autumn and early winter 2023 at a value near or just above +1.5 degrees C. Thereafter, a gradual trend to a less positive Nino3.4 SST anomaly is depicted reaching ENSO neutral values by or near the AMJ 2024 season. Forecast spread of the consolidation members from present to the AMJ 2024 season is low. The NMME Nino3.4 SST index forecasts are similar to that written above with generally good agreement with only the GFDL-SPEAR model forecasting considerably more strengthening of the El Nino event - peaking during DJF 2023-2024 with a value slightly above +2.0 degrees C. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Potential common impacts associated with El Nino are utilized in preparation of the outlook given the current strong ongoing event. This includes standard techniques such as composite analysis and regressions anchored to Nino3.4 SST in both diagnostic and prognostic terms. Given the strength of the current El Nino event as represented in surface and subsurface ocean temperature observations, El Nino impacts are considered through the MAM 2024 season. Statistical guidance, including the global SST based CA and CCA forecast tools, played a considerable role along with long term temperature and precipitation trends (OCN) for many leads. Dynamical model guidance from the NMME and Copernicus (C3S) multi-model ensemble systems are utilized through MAM 2024. In addition, objective, historical skill-based consolidation tools contribute to the outlook and include the ENSO/OCN baseline guidance and the complete suite of statistical/dynamical forecast tool combinations. Coastal anomalous SSTs are also considered during some forecast leads. PROGNOSTIC DISCUSSION OF OUTLOOKS - NDJ 2023 TO NDJ 2024 TEMPERATURE The NDJ 2023-2024 temperature outlook favors above-normal temperatures for Alaska, the Far West, the northern tier of the contiguous U.S. as well as for the Ohio Valley and mid-Atlantic. The greatest likelihood for warmer than normal temperatures is for parts of Alaska, the Pacific Northwest and New England. The high likelihood for the continuation of a strong El Nino event through Spring 2024 and so elevated odds for associated extratropical impacts - on average over many events - is a primary driver of the evolution of the temperature outlooks from NDJ 2023-2024 through the MAM 2024. Multiple types of guidance is utilized to assess the ENSO potential impacts in preparation of the outlooks and included El Nino composites, ENSO regression / correlation information, statistical forecast tools and dynamical model guidance. The combination of ENSO (as defined and predicted by the CPC Nino3.4 SST consolidation) and OCN served as the initial baseline forecast to start for the official outlooks from NDJ 2023-2024 through MAM 2024. Favored above-normal temperatures continue along the northern tier of CONUS through the first 5 leads with a gradual decrease in coverage and relaxation of probabilities in this highlighted area. This is primarily evident in the northern Plains, Midwest and Great Lakes. It is related to greater uncertainty as represented by conflicting signals from the suite of forecast tools, potential for impacts related to changes in the stratosphere which tend to occur more often later in the winter months/early spring and long term colder temperature trends across the north central CONUS especially in FMA 2024. Favored near-normal temperatures are forecast for parts of the Rockies, Southwest and south-central Plains for DJF 2023-2024. The statistical only and total (statistical tools and NMME guidance combined) consolidation forecasts played a considerable role in this forecast. Interplay between ENSO impacts, long term temperature trends and dynamical model guidance from the NMME and C3S ensemble systems resulted in an objective forecast where near-normal temperatures are most likely for much of this area and also for the lower Mississippi valley and Southeast for JFM 2024. It is noteworthy that the ECMWF seasonal forecast favors near- to below-normal temperatures for much of the interior CONUS during the NDJ 2023-2024 through JFM 2024 seasons. There is greater consistency and a stronger signal for favored below-normal temperatures in part associated with El Nino for the southern Plains for FMA and MAM 2024. This would also be consistent with an assumed persistence - not far fetched - of a favored colder than normal signal noted above in the JFM 2024 ECMWF forecast. For Alaska, there is a strong signal for favored above-normal temperatures for the state during El Nino events and this is highlighted in the outlooks from NDJ 2023-2024 through AMJ 2024. Trends of earlier sea ice melt along the western and northern coasts of Alaska increase the odds for nearby land areas during AMJ 2024. As the forecast suite extends into summer and autumn 2024, the coverage of favored above-normal temperatures increases substantially first for areas in the western and eastern CONUS through MJJ 2024, and later expands to include the majority of the forecast domain through autumn 2024. These outlooks are heavily driven by long term positive temperature trends. PRECIPITATION For the NDJ 2023-2024 precipitation outlook, above-normal seasonal precipitation amounts are most likely for the north slope of Alaska, the southern Plains, lower Mississippi valley, Southeast and southern mid-Atlantic. Drier-than-normal conditions are most likely for areas in the Pacific Northwest, northern Rockies, northern High Plains and parts of Michigan. Similar to that written above for temperature, the evolution of the precipitation outlooks through MAM 2024 are anchored to first order to potential typically observed impacts during El Nino events in the winter and spring seasons - on average over many events. In addition, consolidation of both statistical and dynamical model guidance (NMME, C3S) modified the baseline initial El Nino and OCN forecast. The area of greatest confidence in the precipitation outlooks from NDJ 2023-2024 through FMA 2024 within the forecast domain is overwhelmingly for areas along the Gulf coast and southeast CONUS where wetter than average conditions are forecast. Elevated probabilities for above-normal precipitation is also forecast along the eastern seaboard with coverage expanding northward along the coast from NDJ 2023-2024 to JFM 2024 - which is strongly supported by most dynamical model guidance. In addition, the area of favored above-normal precipitation is expanded northward to the central and northern Plains during FMA and MAM 2024, consistent with El Nino impacts, positive precipitation trends in some of this area and dynamical model guidance from the NMME and C3S systems. Despite a strong El Nino event highly likely this upcoming winter and spring, the NMME and C3S dynamical model guidance depicts high forecast spread and seasonal precipitation outcomes - both in sign (i.e., wet/dry) and location. This is quite different from similar forecast guidance preceding the 2015-2016 El Nino event. Given this, the forecast probabilities for either above-normal or below-normal seasonal precipitation are quite modest for the western CONUS. Elevated odds for above-normal precipitation is depicted for parts of California and Nevada during DJF 2023-2024 and JFM 2024, with slightly elevated probabilities extended across the central Rockies. The best convergence of ancillary information, forecast tools and model guidance is the primary basis for the selection of this area. For Alaska, above-normal precipitation is most likely for the north slope of Alaska during NDJ and DJF 2023-2024 and later the extreme south central coast in the JFM and FMA 2024 seasons. The set of outlooks from AMJ 2024 onward are primarily based on long term precipitation trends. FORECASTER: Jon Gottschalck 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 Nov 16 2023 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$