Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Mar 21 2024 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS During mid-March 2024, an El Niño Advisory remains in effect, but El Niño ocean conditions are rapidly weakening and ENSO neutral is most likely to be in place by the end of the April-May-June (AMJ) season. A La Niña Watch is also in effect as La Niña conditions are favored to develop by summer and continue through autumn 2024. The AMJ 2024 temperature outlook favors above-normal seasonal mean temperatures for Alaska, much of the Far West, the southern Plains and the eastern half of the CONUS. Greatest odds for warmer than normal temperatures is shown for northwest Alaska, the Pacific Northwest and in proximity to the Great Lakes. No areas of favored below-normal temperatures are forecast for Spring 2024. The AMJ 2024 precipitation outlook favors above-normal seasonal total precipitation amounts for portions of southern Alaska and for a region from the northern High Plains southeastward across the central Plains to include the Southeast and mid-Atlantic. Below-normal precipitation is most likely for parts of the Pacific Northwest and Southwest. Equal chances (EC) are indicated for areas where seasonal mean temperatures and 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 El Niño is rapidly weakening as equatorial Pacific Ocean sea surface temperatures (SSTs) continue to cool with anomalies in SST near zero for parts of the equatorial eastern Pacific. The most recent weekly value of the Niño3.4 SST index is +1.1 degrees C. Below the surface along the equator, the 0-300 meter depth temperature anomaly (i.e., integrated heat content) is now negative. Significant areas of cooler than normal water temperatures are observed at depth across much of the basin, and especially in the eastern Pacific, where a reservoir of anomalously cold water is present just below or at the surface from 120 W eastward to the South America coast with the maximum anomaly surpassing -6.0 degrees C. Although the oceanic indicators for El Niño have decreased in the Pacific Ocean, there does remain some atmospheric El Niño signature, and it will likely take some time for this response to completely disappear. For example, in the past 30-day average, suppressed convection remained present for some parts of the Maritime continent, and enhanced convection was present in proximity to the Date Line along and near the equator. Moreover, upper-level anti-cyclonic circulations, symmetric about the equator in both hemispheres, are evident. Trade winds are increasing in magnitude with weakening El Niño conditions. An expansive area of below-normal snow cover and depth is present from the northern Plains eastward to the Great Lakes and Northeast. Above-normal soil moisture is present in many areas this March - including areas in California and other parts of the West as well as for the eastern seaboard. Drier than normal soils are evident near the far northern Plains, upper Midwest, Great Lakes, Ohio Valley and Southwest. PROGNOSTIC DISCUSSION OF SST FORECASTS Forecasts of the Niño3.4 SST index from the NMME are in good agreement for a rapid transition to near zero (i.e., no anomaly) during AMJ 2024 and increasingly negative anomalies less than -1.0 degrees C by ASO 2024. The NMME ensemble mean forecast enters La Niña territory (less than -0.5 degrees C) approximately during July 2024. The CPC Niño3.4 SST consolidation forecast follows a similar trajectory as the NMME ensemble mean crosses into La Niña territory at a similar season. The CFS and the CA models show the most negative values and potentially a stronger La Niña event. The CFS model would be considered an outlier at the current time however, as it predicts a strong event with a Niño3.4 index value less than -2.0 degrees C by SON 2024. The official CPC ENSO outlook favors ENSO neutral through the MJJ season with La Niña favored thereafter increasing to near 85% likelihood by SON 2024. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Given some likely remaining El Niño atmospheric response during April 2024, typical El Niño impacts are considered to some degree very early in this set of outlooks (i.e., AMJ 2024). Current anomalous soil moisture and snow cover/depth are considered and contributed substantially to the outlook in some locations during the AMJ season - primarily across areas of the north where well below-normal snow cover and depth are present and in parts of the Midwest and Southwest where soils are dry. Dynamical model forecasts from the NMME and C3S multi-model ensemble systems are utilized, as is the Calibration, Bridging and Merging (CBaM) tool anchored to the NMME forecasts and "bridged" with the Niño3.4 index - primarily for temperature outlooks. The CA statistical tools based on SST and soil moisture respectively along with the ENSO-OCN forecast tool that targets impacts from ENSO as predicted by the CPC consolidation Niño3.4 SST forecast and long term trends played a large role in many of the outlooks. La Niña impacts are considered in the outlooks from JJA 2024 through JFM 2024-2025. Natural analogs for years that underwent a transition from El Niño to La Niña during a similar portion of the seasonal cycle are also explored. PROGNOSTIC DISCUSSION OF OUTLOOKS - AMJ 2024 TO AMJ 2025 TEMPERATURE The AMJ 2024 temperature outlook favors above-normal seasonal mean temperatures for most of the U.S. with the exception of some areas of the Southwest U.S. and the northern and central High Plains. The greatest odds of above-normal temperatures is for parts of the Pacific Northwest, northwest Alaska and the Great Lakes region. The AMJ outlook does not highlight any favored areas for below-normal seasonal mean temperatures. Residual El Niño impacts, positive temperature trends, above-normal coastal SSTs and dynamical model guidance from both the NMME and C3S systems support warmer than normal conditions in the Pacific Northwest as well as much of the eastern CONUS. Odds are highest in the Great Lakes area due to additional likely impacts from antecedent surface conditions such as below-normal snow cover extent and depth, dry soils in some areas, and warmer than normal Great Lakes. Dry soils also result in a local maximum in probabilities for above-normal temperatures for parts of Texas and New Mexico. Residual impacts of El Niño (potential troughing and cooler conditions) primarily early in the season, and conflicting or weaker climate signals result in a forecast of equal chances (EC) for areas in the Southwest and northern and central High Plains. Negative trends in sea ice coverage and thickness and so more open water earlier than normal for ocean areas along the western and northern coasts of Alaska and statistical and dynamical model guidance favor above-normal temperatures for most of the state of Alaska for AMJ 2024. Progressing through the MJJ through JAS 2024 temperature outlooks, odds for above-normal temperatures increase across the western and southern CONUS as there is nearly unanimous and consistent agreement from NMME and C3S dynamical model guidance. There remains more uncertainty for parts of the north central U.S. through JJA 2024 so EC is forecast in this area. The anticipated transition to La Niña entering summer 2024 supports gradually increasing forecast coverage and odds for above-normal temperatures for the Midwest during JAS 2024 and ASO 2024. Thereafter, considerations for the slow development of typical La Niña impacts (supported by La Niña composites, regressions, and statistical forecast tools) during the autumn and winter months are the reasons for the forecast evolution through FMA 2025. This includes decreasing odds for above-normal temperatures and then EC for much of the northern tier of the U.S. through OND 2024 and the introduction of favored below-normal temperatures for the Pacific Northwest in NDJ 2024-2025 through MAM 2025. Moreover, above-normal temperatures are favored for the southern half of the U.S. and along the eastern seaboard over this period. For Alaska, the transition from El Niño to favored La Niña conditions results in decreasing odds for above-normal temperatures from west to east through JAS 2024 with the introduction of favored below-normal temperatures for the southwest part of the state. This orientation is adjusted north to south through OND 2024 with above-normal temperatures most likely for the northern part of the state and below-normal temperatures favored for southern coastal areas. PRECIPITATION The AMJ 2024 precipitation outlook favors above-normal seasonal total precipitation amounts for the Southeast, lower Ohio Valley, mid-Atlantic and parts of the central and northern High Plains. Residual El Niño influence, long term positive precipitation trends as defined by OCN, dynamical model guidance from the majority of the participant models of the NMME and C3S ensemble systems support this forecast. Below-normal precipitation is favored for parts of the Pacific Northwest due to residual El Niño influence and dynamical model guidance. Dynamical and statistical forecast tools support elevated odds for below-normal precipitation for areas in the Southwest. Thereafter, there is a rapid transition to strong dry signals from MJJ through ASO 2024 for the eastern Southwest monsoon region to the interior of the contiguous U.S. This is consistent with developing La Niña conditions and associated potential impacts and is supported very strongly by strong signals of favored below-normal precipitation from the NMME and C3S dynamical forecast guidance. The outlooks highlight rather large forecast coverage of below-normal precipitation for many areas of the western and central CONUS. Thereafter, favored drier than normal conditions shift to the southern tier of the U.S., associated with a La Niña response during the cool season from SON 2024 through FMA 2025. Over the same forecast period, above-normal precipitation is most likely for the Pacific Northwest and areas in the Ohio Valley and Great Lakes region with maximum coverage and odds during the NDJ 2024-2025 and JFM 2025 seasons respectively. For Alaska, above-normal precipitation is favored during the warm season through JAS 2024. Later in the set of outlooks, a slight tilt toward below-normal precipitation is forecast for the southeast areas of the state including portions of the Panhandle from SON 2024 through NDJ 2024-2025. 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 Apr 18 2024 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$