Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EST Thu Feb 15 2024 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS While El Niño conditions continue with equatorial sea surface temperatures (SSTs) above average across the central and eastern equatorial Pacific Ocean, the total subsurface ocean heat content anomaly across the equatorial Pacific has dropped to near zero, signaling an imminent transition to ENSO-neutral conditions. The atmospheric circulation across the global tropics is consistent with the ongoing El Niño though, and El Niño is forecast to impact the circulation for the March-April-May season with a transition to ENSO-neutral conditions later in the spring and early summer. La Niña conditions are forecast to likely develop later in summer or by autumn. The March-April-May (MAM) 2024 temperature outlook favors above-normal temperatures across the northwestern Contiguous United States (CONUS), including the Pacific Northwest, northern to central California, parts of the Great Basin, and the northern Rockies. Above-normal temperatures are favored across the northern CONUS, including the northern Great Plains, Great Lakes region, and Northeast. The highest forecast confidence for above-normal temperatures is for the Pacific Northwest, where the probabilities of above-normal temperatures exceed 60 percent. Above-normal temperatures are favored across Alaska. Equal chances (EC) of above-, near- or below-normal seasonal mean temperatures are indicated across the southern CONUS, where El Niño impacts increase the chances of below- and near-normal temperatures, while decadal time scale trends increase the likelihood of above-normal temperatures. The MAM 2024 precipitation outlook favors below-normal precipitation for parts of the Pacific Northwest and northern Rockies, while above-normal precipitation is favored from the eastern Central Plains across the middle and lower Mississippi Valley into the mid-Atlantic and Southeast, associated with lingering impacts due to El Niño. Above-normal precipitation is also favored for parts of northwestern and southern Mainland Alaska. Below-normal precipitation is favored for much of the Rio Grande Valley. EC of below-, near- and above-normal total seasonal precipitation are depicted by remaining areas in white. 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 Above-average SSTs persist across the equatorial central and eastern Pacific Ocean, with a slight weakening of positive anomalies over the east-central Pacific in recent weeks. The most recent weekly SST anomaly for the Niño 3.4 region is at +1.7 degrees Celsius. Equatorial subsurface temperature anomalies (averaged over 180-100W longitude and 0-300 meters depth) are near zero degrees Celsius after a decrease from a peak of +1.5 degrees Celsius in late November. Negative subsurface temperature anomalies at depths greater than 50 meters have spread from the Western Pacific eastward across the east-central Pacific in recent weeks. In the latest four-week period, outgoing longwave radiation (OLR) anomalies at the top of the atmosphere are negative to the west of the Date Line near the equator and extend over the Eastern Pacific to the north of the equator. Low-level (850-hPa) wind anomalies were near long-term averages over most of the equatorial Pacific in recent weeks. Upper-level (200-hPa) wind anomalies were easterly over the eastern equatorial Pacific. These atmospheric and oceanic conditions reflect a continuation of El Niño into the current month. The Madden-Julian Oscillation (MJO) continues to propagate with active convection across the Western Pacific ocean, constructively interfering with El Niño. However, dynamical model forecasts indicate large uncertainties in the Realtime Multivariate MJO (RMM) index and a likely decrease in the amplitude of the MJO in the coming weeks. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC SST consolidation forecast for the Niño 3.4 region predicts rapidly decreasing positive anomalies in spring with a near zero median anomaly by the April-May-June (AMJ) season. Dynamical and statistical models are in good agreement that a transition to ENSO-neutral conditions is likely to occur either in spring or by early summer. The CPC ENSO Outlook indicates that the probability of ENSO-neutral during AMJ is nearly 80 percent. The probability of a La Niña increases to greater than 50 percent beginning in the June-July-August (JJA) summer season. By the August-September-October (ASO) three-month period and later into autumn, there is more than a 70 percent chance of La Niña. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS The seasonal temperature and precipitation outlooks for MAM 2024 through MAM 2025 were based on dynamical model guidance through the first five leads, including the North American Multi-Model Ensemble (NMME). Statistical models, such as the ENSO-OCN tool that combines the probable temperature and precipitation impacts of the predicted median Nino 3.4 SST anomaly, as forecast by the CPC SST consolidation, with the decadal temperature and precipitation trends, were used at all leads. A skill-weighted consolidation of NMME dynamical models, a consolidation of statistical models, as well as a full consolidation of all available models were primary tools in the seasonal temperature and precipitation outlooks. El Niño impacts were considered explicitly, using regression of the Niño 3.4 index on temperature and precipitation, for only the MAM 2024 outlook. Beginning in ASO 2024 through the final lead outlook for MAM 2025, the impacts of La Niña were considered, using temperature and precipitation signals from regression to negative values of the Niño 3.4 index. PROGNOSTIC DISCUSSION OF OUTLOOKS - MAM 2024 TO MAM 2025 TEMPERATURE The MAM 2024 temperature outlook predicts elevated probabilities of above-normal temperatures for Alaska and across the northern tier of the CONUS, supported by NMME forecasts, forecast tool consolidations, and typical impacts due to El Niño. The highest probabilities for above-normal cover northwestern parts of Oregon and much of Washington, where the teleconnection to El Niño has its strongest correlation. Weaker probabilities still favoring above-normal temperatures are forecast over the Northern Plains, due to uncertainty in the Arctic Oscillation (AO) phase. A persistent negative phase of the AO would lead to near- or below-normal temperatures in this region. Above-normal temperatures are likely for much of the Great Lakes region and Northeast with probabilities exceeding 50 percent, where the Great Lake surface temperatures are anomalously warm and decadal trends in the Northeast are strongly positive. As a rapid transition to a neutral ENSO state is predicted, the seasonal temperature outlooks for AMJ through JJA 2024 rely primarily on signals related to decadal trends. The outlooks are supported by the NMME forecasts through July-August-September (JAS) 2024 and by the consolidation of available tools through ASO 2024. Uncertainty for the northern and central Great Plains increases through spring into summer where decadal trends are weaker, while higher probabilities for likely above-normal temperatures expand across the western CONUS. Above-normal temperatures are favored across most of Alaska through MJJ 2024, and primarily eastern Mainland Alaska in the summer seasons. By the autumn and the September-October-November (SON) 2024 outlook, probabilities for above-normal temperatures decrease for the northwestern and north-central CONUS, while elevated probabilities for above-normal temperatures increase for northern Mainland Alaska and decrease for southern Mainland Alaska, related to potential impacts of a developing La Niña. For the CONUS, areas forecast to have EC of below-, near- and above-normal temperatures expand from the Northwest in October-November-December (OND) 2024 down the west coast by December-January-February (DJF) 2024/25. Below-normal temperatures are slightly favored for parts of the Pacific Northwest for the DJF 2024/25 season through MAM 2025, due to the more likely impacts of La Niña. Forecasted areas of EC expand into the Central Plains, Central Mississippi Valley, and Ohio Valley in the winter season and later leads through MAM 2025. These longer lead forecasts are supported largely by the ENSO-OCN forecast tool and the statistical model consolidation. PRECIPITATION The MAM 2024 precipitation outlook favors below-normal precipitation for the Pacific Northwest, consistent with possible impacts of El Niño during the first half of the season. Below-normal precipitation is favored for much of the Rio Grande Valley, while above-normal precipitation is favored for much of the eastern CONUS, supported by the consolidation of available forecast tools. Above-normal precipitation is favored for northwestern and southern Mainland Alaska, consistent with the NMME consolidation forecast. Through spring into summer, an area of favored above-normal precipitation expands across most of Mainland Alaska, largely supported by the NMME and the consolidation of precipitation forecast tools. Below-normal precipitation is slightly favored across much of the western half of the CONUS (excluding most of California and western areas of the Great Basin and Southwest) through JJA 2024, supported by the NMME and the consolidation. The areas where above-normal precipitation is favored over the eastern CONUS shift eastward towards the Atlantic coast by MJJ 2024, and then are confined to parts of the northeastern CONUS by summer, largely consistent with signals related to decadal precipitation trends. Below-normal precipitation is favored for parts of the Southwest from ASO 2024 through MAM 2025, and for parts of the Southeast from OND 2024 through MAM 2025. Above-normal precipitation is favored for parts of northern Alaska in the autumn seasons and for northern and eastern interior Alaska in the winter through MAM 2025, consistent with the statistical model consolidation. Above-normal precipitation is favored over parts of the Midwest during the winter seasons. The precipitation outlooks for autumn through winter are largely related to potential impacts due to likely development of La Niña. FORECASTER: Dan Collins 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 Mar 21 2024 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$