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679 FXUS07 KWBC 191331 PMD30D Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 830 AM EST Thu Dec 19 2024 30-DAY OUTLOOK DISCUSSION FOR JANUARY 2025 Following a few months of weakly below normal sea surface temperatures (SSTs), SST departures reached -0.6 degrees Celsius in the Nio3.4 region over the past week. The current El Nio Southern Oscillation (ENSO) Alert System Status is a La Nia watch. The extended period of weakly below normal SSTs and recent drop to -0.6 degrees Celsius may lead to some La Nia-like impacts over the contiguous United States (CONUS) during January and the upcoming season, however, we expect that any impacts may be weak and variability to be high, leading to uncertainty in some of the typical impacts. Though the Madden-Julian Oscillation (MJO) has been a significant player in the tropics in recent weeks and dynamical models depict continued eastward propagation of the MJO envelope with a slowed phase speed, the emerging La Nia has the potential to interfere with propagation and amplitude of the MJO. Should the MJO continue into January, this may lead to cooler than average temperatures for the northern parts of the CONUS and Northeast, but the potential interference from La Nia and slow phase speed add to the uncertainty. In addition to the large-scale drivers of La Nia and the MJO, coastal or local SSTs, sea ice, and snow cover are taken into account for this forecast where appropriate. Monthly forecasts of temperature and precipitation by dynamical models from the North American Multi-Model Ensemble (NMME), Copernicus Climate Suite (C3S), and CFSv2 were utilized in preparing this Outlook. Week 3-4 predictions for the first part of January from GEFSv12, ECMWF, and CFSv2 and the expected transition in the atmospheric pattern from the Week 2 period were also considered. Enhanced ridging is forecast over most of the CONUS toward the end of December, leading to the potential for above normal temperatures to end the year. However, this strong ridging is expected to moderate by the beginning of January, giving way to ridging over the West and neutral to above normal heights over the remainder of CONUS. Week 3-4 models forecasting the beginning of January favor weak ridging over the West and troughing over the East, though the position, exact timing, and strength of the pattern is uncertain. Given the atmospheric pattern leading into January and the forecasts for early January, we expect a warm start to the month followed by a transient pattern. Uncertainty is high due to the potential for this transient pattern, particularly for temperatures. The January 2025 Temperature Outlook features above normal temperatures over the southern half of the western CONUS, Southern Plains, and the East Coast. This pattern is fairly typical of La Nia, and reflects dynamical model predictions that favor a La Nia like response for the month. However, given the expected transient height pattern during the month, probabilities are overall low for temperatures. Probabilities are enhanced over the Southern Plains where there was the best agreement among available tools. Some models such as the C3S suite and CFSv2 favored higher probabilities of above normal temperatures over the Southwest, however, NMME and statistical tools that include decadal trends which are below normal in parts of the Southwest more strongly favor near-normal temperatures. Given this discrepancy, a weak tilt toward above normal temperatures is favored despite some of the model results showing stronger probabilities in the region, which also aligns with forecast below normal precipitation in the region. Similarly, some models indicate stronger probabilities for above normal temperatures over the East Coast relative to the Outlook, but given Week 3-4 models that tilt toward below normal temperatures over the East, probabilities are again weakened. Equal chances (EC) of above, near, and below normal temperatures are favored over the northern tier of the CONUS and Central Plains where tools had weak or uncertain signals, and, moreover, both MJO and La Nia may lead to cold air intrusions into the region, though both influences are currently somewhat uncertain. In addition, given the forecasted warm start to the month of January, it is uncertain if periods of colder temperatures will be strong or long enough to tilt the probability to below normal over the northern tier. Over Alaska most models favored above normal temperatures, particularly CFSv2, and as such a tilt toward above normal temperatures is indicated in the Outlook. Despite the transient pattern, precipitation signals were more consistent in tools than temperatures. While SSTs in the Nio3.4 region just recently dropped below -0.6 Celsius, it is possible that tools are picking up the extended period of weakly below normal SSTs and forecasts of a weak La Nia as many of the tools are showing a La Nia like precipitation pattern over the CONUS. For example, both NMME and C3S probabilistic multi-model ensemble probabilistic forecasts show a general pattern of below normal precipitation over the southern tier of CONUS, and above normal precipitation over the Northwest and Great Lakes and interior Southeast, which are hallmarks of a La Nia pattern. The January 2025 Precipitation Outlook thus resembles a La Nia like pattern, featuring enhanced probabilities of above normal precipitation over the Intermountain West and parts of the Northern and Central Plains, the Great Lakes and interior Southeast, and a tilt toward below normal precipitation from the Southwest along the southern CONUS and parts of the coastal Southeast. Over Alaska, above normal precipitation is favored for western and northern parts of the state, with a small area of below normal precipitation over its southern coast. Some uncertainty exists along the West Coast where EC is indicated. NMME and C3S favor below normal precipitation over the southern West Coast and above normal precipitation over the northern West Coast, while CFSv2 tilts toward below normal over the northern West Coast. Given the potential for periods of above normal precipitation over the West Coast during winter months, EC is favored despite some of the tools leaning toward below normal. Models and tools also had mixed signals along the coastal northeast and New England, and there is potential for variability during the month due to uncertainty in storm tracks, leading to the favored region of EC. Finally, over Alaska, below normal precipitation is favored over its southern coast given La Nia teleconnections, and above normal is favored over the western and northern parts of the state given La Nia teleconnections, dynamical model agreement, and above normal decadal trends over the northern coast. FORECASTER: Johnna Infanti The climatic normals are based on conditions between 1991 and 2020, following the World Meteorological Organization convention of using the most recent 3 complete decades as the climate 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. An updated monthly outlook... for Jan will be issued on Tue December 31 2024 These outlooks are based on departures from the 1991-2020 base period. $$Fetched URL: http://weather.uky.edu/ldmdata/firewx/FXUS07.KWBC
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