Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EST Thu Dec 21 2023 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS El Niño conditions are currently observed, and equatorial sea surface temperatures (SSTs) are above average across the central and eastern Pacific Ocean. The El Niño Advisory remains in effect, and tropical Pacific atmospheric anomalies are consistent with El Niño. El Niño is expected to continue through the Northern Hemisphere winter, with a transition to ENSO-neutral favored during April-June 2024. The Jan-Feb-Mar (JFM) 2024 Temperature Outlook favors above-normal temperatures for Alaska, the west coast of the Contiguous United States (CONUS), and from the Northern Rockies across the Great Lakes to the mid-Atlantic and Northeast. The highest odds for above-normal temperatures are across the Northwest, northern New England, and much of Alaska. Near normal temperatures are favored for parts of the Central and Southern Plains and parts of the Southeast. The overall pattern favoring above-normal temperatures across the northern CONUS with near-normal or below-normal temperatures favored for the southern tier of the CONUS persists through Mar-Apr-May (MAM) 2024. The JFM 2024 Precipitation Outlook depicts elevated probabilities for below-normal precipitation over southwestern Alaska and elevated chances of above-normal precipitation over northern and southeast mainland Alaska. Below-normal precipitation is favored from the Northern Rockies to the Great Lakes and into the Ohio Valley. Above-normal precipitation is favored over much of the West Coast, with a slight tilt toward above-normal over parts of the central CONUS. The highest confidence for above-normal precipitation is over the Southeast CONUS where teleconnections from El Niño are strongest. For both temperature and precipitation, areas depicted in white and labeled "EC" (Equal-Chances) are regions where climate signals are weak or inconsistent, and thus equal odds for either above-, near- or below-normal seasonal mean temperatures and seasonal total precipitation amounts are favored. 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 conditions are observed in both the atmosphere and ocean. In the last 4 weeks, equatorial SSTs were above average across most of the Pacific Ocean and near-average in the western Pacific Ocean. Above-average SSTs strengthened in the Central Equatorial Pacific but cooled in extratropical Pacific near the Date Line. SST anomalies over the recent week were 1.5 degrees C in the Niño1+2 region, and 2.0 degrees C in the Niño 3.4 region. Upper ocean heat anomalies for 180-100 degrees West have been decreased from mid-November into mid-December, but remain among the highest levels observed in 2023. Positive (negative) subsurface temperature anomalies dominate much of the equatorial Pacific Ocean east (west) of the Date Line, with little change to the anomalies amplitude in the early parts of December. The above normal subsurface temperatures support persistence of the El Niño event. Positive Outgoing Longwave Radiation (OLR) anomalies (suppressed convection and precipitation) were evident around Indonesia. Negative OLR anomalies (enhanced convection and precipitation) were observed around the Date Line, and extend into the eastern Pacific, remaining just north of the equator. Low-level (850-hPa) wind anomalies were westerly over the eastern half of the equatorial Pacific. Upper-level (200-hPa) wind anomalies were easterly along the equatorial Pacific. An anomalous anticyclonic couplet straddles the equator around the east-central Pacific. The Madden Julian Oscillation (MJO) appears active, however, given the strength and persistence of the El Niño event, El Niño remains the dominant influence on the forecast. Outside of the Equatorial Pacific, SST anomalies are broadly positive in the North Pacific and Atlantic basins. Positive SST anomalies are also present around the western and southern Alaskan coastlines, though much weaker than in prior months. SST anomalies are also positive along the west coast of the U.S., in the Gulf of Mexico, and up much of the East Coast. Negative and neutral to weakly positive SST anomalies are present off the coast of New England and the northern mid-Atlantic states. Some modeling systems are indicating the potential for a sudden stratospheric warming during mid to late January. Uncertainty is high for such events as they are relatively rare, and forecasting impacts at the surface adds another layer of uncertainty. Should stratospheric warming occur, intraseasonal variability over some parts of the central and eastern CONUS could be increased. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC Niño3.4 SST consolidation forecast shows a steady signal for Nino3.4 standardized anomaly values above 1.0 deg C through MAM, then a drop to near average values for the remainder of the outlook period. Data from the NMME suite of models have a similar pattern, though many of those models have individual members indicating some values less than -0.5 deg C by late in the Northern Hemisphere summer of 2024. The official NOAA CPC ENSO probabilities indicate that El Nino is favored through MAM, with ENSO-Neutral through JJA, and about equal probabilities for ENSO-Neutral and La Nina in JAS 2024. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Dynamical model forecasts from the Copernicus (C3S) and NMME multi-model ensemble systems are used for the first three and six leads respectively, as well as individual model forecasts from the Climate Forecast System version 2 (CFSv2) and European Centre for Medium Range Weather Forecasts (ECMWF) models as available. The objective, historical skill weighted consolidation that combines both dynamical and statistical forecast tools was also considered throughout all leads. El Niño impacts were considered given the strength and forecast persistence of the El Niño event through the MAM 2024 season, and El Niño composites and correlation/regression of temperature and precipitation with the Niño3.4 region were used to determine typical El Niño impacts. Statistical guidance such as the global SST based Constructed Analog (CA) and long term temperature and precipitation trends played a role in many of the outlook seasons, particularly beyond lead 6 when dynamical model guidance is unavailable. Coastal SST and sea ice anomalies are considered at early leads. Snow cover was also considered for the Northern Plains. PROGNOSTIC DISCUSSION OF OUTLOOKS - JFM 2024 TO JFM 2025 TEMPERATURE The JFM 2024 Temperature Outlook favors above-normal temperatures for Alaska, the west coast of the Contiguous United States (CONUS), and from the Northern Rockies across the Great Lakes to the mid-Atlantic and Northeast. The highest odds for above-normal temperatures are across the Northwest, northern New England, and much of Alaska. Near-normal temperatures are favored for parts of the Central and Southern Plains and parts of the Southeast. The overall pattern favoring above-normal temperatures across the northern CONUS with near-normal or below-normal temperatures favored being restricted to the southern tier of the CONUS persists through Mar-Apr-May (MAM) 2024. The trend signals are weak during JFM for much of the CONUS and Alaska, so the forecast is largely based on patterns associated with El Nino and dynamical model guidance. Some of the guidance is indicating a warmer signal across portions of the Southeast, so some of the areas where near-normal temperatures were indicated last month for JFM are removed. Odds for above-normal temperatures were lowered across the Ohio Valley and Lower Great Lakes due to additional uncertainty associated with the potential for an SSW and to reflect recent model guidance. Near-normal temperatures are favored for the Central and Southern Plains and parts of Southeast during JFM 2024, supported by a consolidation of dynamical and statistical models. These areas of favored near-normal temperatures represent regions where there is interplay in the tools between expected impacts from El Niño, decadal trends, and dynamical model guidance, leading to a forecast of enhanced near-normal probabilities. El Niño composites tilt weakly toward below normal in the Southeast during winter months, which is balanced by above normal trends and a less confident but still above normal tilt in dynamical model forecasts, which overall supports near normal temperatures. The tilt toward above normal temperatures is maintained over Alaska, the West Coast, and northern parts of the CONUS until March-April-May (MAM) 2024 when El Niño impacts are expected to begin to wane, and higher uncertainty (EC) is introduced in the Northern Plains. Though the season(s) may tilt above normal in the mean, some periods of cold may occur during winter and early spring due to intraseasonal variability. During FMA through AMJ 2024, below-normal temperatures are favored over parts of the Central and Southern High Plains, reflecting the associated impacts of ENSO. That impact is seen in regressions of temperature and Nino3.4 as well as the SST-CA. From spring (May-June-July, MJJ) into summer and the remaining forecast leads, El Niño impacts are expected to decrease and decadal trends become the primary driver of the forecasts. Predictability decreases in spring and more EC is depicted in the forecasts in the central CONUS. Above-normal temperatures are favored over much of the CONUS through summer 2024 and shift toward the east coast by winter 2024-2025. Over Alaska, above-normal temperatures are likely through MJJ 2024 given expected impacts from El Niño, and a mixture of EC and above-normal temperatures are forecast over the state for the remainder of the forecast leads, mainly driven by trends. PRECIPITATION The JFM 2024 Precipitation Outlook depicts enhanced probability of below-normal precipitation over southwestern Alaska and above-normal precipitation over northern and southeastern Mainland Alaska. The southwestern below-normal precipitation area is consistent with expected El Niño teleconnections, and the northern above-normal precipitation area is supported by El Niño and decadal trends. Below-normal precipitation is favored from the Northern Rockies to the Great Lakes, including parts of the Ohio Valley, where there is a stronger signal for below-normal precipitation compared to the prior outlook for JFM. Above-normal precipitation is favored over much of the West Coast, across the Great Basin, Central Rockies, and into the Central Great Plains to reflect likely impacts from El Niño and the potential for southerly shifted storm track. Given a strong connection to El Niño over the Southeast CONUS, the highest confidence for above-normal precipitation is over the Gulf States and along the East Coast to South Carolina (60 to 70 percent), with probabilities decreasing toward New England. Probabilities of at least 50 to 60 percent for above-normal precipitation, remains along parts of the Gulf coast through MAM 2024. El Niño impacts dominate forecast evolution through MAM 2024 for the precipitation outlook, and are modified based on other tools such as NMME, C3S, and decadal trends, as well as a consolidation of statistical and dynamical models. El Niño impacts are less pronounced in the forecasts beginning in AMJ 2024, and dynamical model guidance (where available) and decadal trends become the primary considerations. A tilt toward above- normal precipitation is depicted over parts of the southeastern CONUS and shifting northward to the Ohio Valley to the Northeast in AMJ through ASO 2024, and re-emerges for parts of the eastern CONUS by winter 2024 as decadal trends tilt toward above-normal precipitation. Based mainly on decadal trends, below-normal precipitation is favored over parts of the Northwest and Northern Plains through fall 2024. For Alaska, above-normal precipitation is favored over northern parts of the state through MAM 2024, owing to both El Niño impacts and precipitation trends. Below-normal precipitation is favored for the southwest coast in JFM 2024, mainly driven by expected El Niño impacts, before above-normal precipitation begins to dominate the forecasts by Spring 2024, supported by NMME and C3S. From SON 2024 onward, long term precipitation trends which favor above-normal precipitation across much of northern mainland Alaska dominate the forecast over Alaska. FORECASTER: Matthew Rosencrans 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 Jan 18 2024 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$