FXUS05 KWBC 211331 PMD90D Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EST Thu Nov 21 2024 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS El Niño Southern Oscillation (ENSO)-neutral conditions remain present, as equatorial sea surface temperatures (SSTs) are near-to-below average in the central and eastern Pacific Ocean. A La Niña Watch remains in effect, with La Niña most likely to emerge in October-December 2024 (57% chance) and is expected to persist through January-March 2025. Any La Niña event that develops this autumn is favored, however, to be a weak short duration event. The December-January-February (DJF) 2024-2025 temperature outlook favors above-normal seasonal mean temperatures from the Southwest across the Southern Plains to the Southeast, then northward from the Great Lakes to New England. Below-normal seasonal mean temperatures are favored from the Pacific Northwest to the Northern Great Plains. The DJF 2024-2025 precipitation outlook depicts enhanced probabilities of below-normal seasonal total precipitation amounts along most of the southern tier of the contiguous U.S. (CONUS) and for parts of southeast Mainland Alaska and Alaska Panhandle. The greatest odds of below-normal precipitation (greater than 50 percent) are forecast for parts of the Southwest and Southern Plains. Above-normal precipitation is favored for parts of the Pacific Northwest, northern Rockies, central Great Lakes, and western and northern Alaska. The strongest of those signals is over the northern Rockies. Equal chances (EC) are forecast for areas where probabilities for each category of seasonal mean temperatures and seasonal total precipitation amounts are expected 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 ENSO-neutral continued through early November 2024, with near- to below-average SSTs from about 170W to 100W along the equator and off equatorial SSTs just above normal. The equatorial central Pacific SSTs, exhibited a mix of cooling and warming during the last 4 weeks, with some portions of the Nino3.4 region, so no strong movement toward La Nina was noted. The latest weekly Niño indices ranged from +0.2°C (Niño-4) to -0.3°C (Niño-3.4), similar to values from October. When removing the global tropical mean to construct a Nino3.4 value relative to the global tropics, the relative value is -0.76°C, so that may be why we are seeing some signals consistent with La Nina but not robustly clear indications, yet. Equatorial ocean heat content for the area from 180 to 100W remained below normal, near -0.5°C. Below-average temperatures remain at depth in the east-central and eastern Pacific Ocean, while above-average temperatures prevail at depth and near the surface in the western Pacific. Above-average OLR (suppressed convection and precipitation) was observed around the Date Line. Below-average OLR (enhanced convection and precipitation) was evident over the Philippines. Low-level (850-hPa) wind anomalies were mostly near average across the equatorial Pacific Ocean, with a small region of westerly wind anomalies over the eastern Pacific Ocean. Upper-level (200-hPa) wind anomalies were mostly near average across the equatorial Pacific Ocean Then MJO was active during October and moved across the central Pacific in late October. The MJO is forecast to remain active for at least the rest of November, potentially enhancing low-level easterly anomalies across the central and eastern Pacific basins. SSTs remain warmer than normal for most areas of the Atlantic and Indian Oceans, as well as the mid-latitude regions of the Pacific. Below-normal ocean surface temperatures are evident in parts of the Gulf of Alaska and the Bering Sea, as well as the Chukchi Sea where there are some pockets of new ice at less than 4-tenths coverage. Sea ice is largely in place for much of the Chukchi and Beaufort seas (https://tgftp.nws.noaa.gov/data/raw/fz/fzak80.pafc.ice.afc.txt), so any signal from a late onset of sea ice is not applicable during DJF. Drought conditions that rapidly expanded across much of the CONUS during late summer and early autumn have been ameliorated across much of the central Great Plains while continuing to intensify across much of the Southeast, Central Appalachians, and the Northeast. PROGNOSTIC DISCUSSION OF SST FORECASTS Taken collectively, statistical and dynamical model forecast guidance of the Niño3.4 index favor the development of a weak and most likely short duration La Niña event. Some statistical model forecasts do favor a continuation of ENSO-neutral into and through winter 2024-2025. Dynamical model guidance predictions tend to support weak La Niña conditions to develop, including the majority of participant models from the NMME and C3S forecast suites. Most recent observations and the forecast guidance noted above favor La Niña to emerge during OND 2024 (57% chance) and it is expected to persist through JFM 2025. After JFM 2025, ENSO-neutral is the most likely category into the northern hemisphere summer of 2025. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS The development of potential La Niña conditions in the atmosphere-ocean system in the Pacific contributed to the outlooks from DJF 2024-2025 through FMA 2025. Although typical La Niña impacts are less likely to occur during a weak event (currently favored), La Niña still tilts the odds in the outlook forecast probabilities. Coastal SSTs and constructed analogue forecasts keyed to current SSTs are utilized in preparation of the outlooks. Soil moisture is not largely considered for the outlook during this time of year as correlations with soil moisture are quite low for initial conditions during the cool season. Dynamical model guidance from the NMME and C3S prediction suites (first 5 and 3 leads respectively), statistical forecast tools, long-term temperature and precipitation trends, and an objective, historical skill weighted combination of much of the above guidance strongly contributed to the final outlooks. PROGNOSTIC DISCUSSION OF OUTLOOKS - DJF 2024 TO DJF 2025 TEMPERATURE The December-January-February (DJF) 2024-2025 temperature outlook favors above-normal seasonal mean temperatures from the Southwest across the Southern Plains to the Southeast, then northward from the Great Lakes to New England. Below-normal seasonal mean temperatures are favored from the Pacific Northwest to the Northern Great Plains. The DJF outlook has slightly lower odds for below-normal temperatures across North Dakota and Minnesota than the DJF outlook released in October due to the continued delay in La Nina and slightly increased odds for a weak event. Additionally, the NMME suite was judged to be too warm over the Northern Great Plains despite a shift toward cooler outcomes in the model suite, many of the models were still showing above-normal temperatures. Based on a weak La Nina and models overdoing trends, observed trends become more of the signal. Furthermore, higher frequency patterns (AO, MJO, and stratospheric variability) that result in increased uncertainty can also play a larger role. Those modes are largely not predictable on seasonal timescales, though La Nina events and the westerly phase of the QBO can result in less polar vortex splits or shifts. Similar patterns (above-average temperatures favored largely across the southern tier and below-normal temperatures across the northern tier) are present in the outlooks through Feb-Mar-Apr 2025, with below-normal temperatures favored from the Northwest to the Northern Great Plains. A band of equal chances from central California to Wyoming, and across much of the Great Plains where higher frequency modes can often have their largest impacts. Across Alaska, the DJF outlook for temperature favors above-normal temperatures across northern Mainland Alaska while favoring below-normal temperatures across southern Mainland Alaska and the Alaska Panhandle. The northern signal is related to trends and outlooks from the NMME and C3S, while the southern signal is more from La Nina, statistical tools, and the NMME model suite. Those two areas remain in the outlooks, with some minor modifications, through Mar-Apr-May, where the signal for below-normal temperatures recedes as trends become the dominant predictor. During late Northern Hemisphere spring and early summer of 2025, trends become the dominant climate forcing. Areas in the Great Plains have the lowest odds being the areas with the weakest trends and higher variance. Coming out of a La Nina winter, dry signals across the southern tier can reinforce some of the trends in temperatures, especially across the western CONUS, so the odds of above-normal temperatures are relatively the highest in the Rockies and Southwest through the summer and autumn. PRECIPITATION The DJF 2024-2025 precipitation outlook favors above-normal seasonal total precipitation amounts for portions of the Pacific Northwest, northern Rockies, Michigan, and northern and western Alaska. Drier-than-normal conditions are most likely for the southern tier of the U.S. with the highest odds forecast for the Southwest and Southern Plains. For Southeast Mainland Alaska and the northern Alaska Panhandle, a slight tilt toward drier than normal conditions is forecast through MAM 2025. As noted above, the favored development of La Niña conditions approaching the winter and consistent dynamical model forecast guidance is the basis for the outlook. The odds for above-normal precipitation are increased during DJF 2024-2025 relative to the outlook for DJF from October 2024 as the tools are showing more certainty. The most uncertain area in the precipitation outlook for DJF 2024-2025 is across the northern Great Plains, as signals are weak and mixed (NMME favors above-normal precipitation, where as calibrated versions remove the signal, and trends vacillate season to season). Moving through the FMA 2025 season, the enhanced likelihood of above-normal precipitation in the Great Lakes and Ohio Valley regions expands in coverage and probabilities increase maximizing in the southern Great Lakes and northern Ohio Valley area in JFM 2025. Positive precipitation trends also contribute strongly to these outlooks for some areas in the Midwest and Great Lakes. The precipitation outlook is highly uncertain east of the Appalachians in the mid-Atlantic and Northeast, as the storm track during La Nina years is typically more inland, but a weak La Nina will likely let other, shorter-lived climate forcings play larger roles in the overall storm track. Favored below-normal precipitation across the south through FMA 2025 remains generally consistent with the forecast coverage and odds greatest for JFM 2025. The majority of the dynamical model guidance from the NMME and C3S model suites highlight substantially elevated odds for below-normal precipitation for the Far West (i.e., California/Nevada) for the winter seasons through FMA 2025. Given the anticipated generally weak ENSO forcing, this seems overdone, especially since some of the dynamical model guidance can sometimes overemphasize ENSO impacts during weaker events. Given the lack of a clear, reliable ENSO signal after this winter/early spring, longer lead outlooks are primarily based on long term precipitation trends and depict low forecast coverage. The longer lead precipitation outlooks during the warm season generally have low forecast skill. 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 Dec 19 2024 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$