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Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EST Sat Nov 30 2024 30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2024 The updated temperature and precipitation outlooks for December 2024 are based on the latest dynamical models , WPC temperature and precipitation forecasts during the first week of the month, the CPC 6-10/8-14 day temperature and precipitation outlooks, and the week 3-4 outlook (valid December 14-27). El Niño Southern Oscillation (ENSO)-neutral conditions continue with a limited area of below-average sea surface temperature anomalies observed across the east-central Pacific. During late November, the Madden-Julian Oscillation (MJO) contributed to anomalous low-level easterly winds across the equatorial Central Pacific. These enhanced trade winds may lead to a transition towards La Niña. The GEFS and ECMWF models indicate that the MJO propagates eastward from the Maritime Continent to the West Pacific during early December. However, beyond that time, ensemble spread increases on the MJO amplitude. Due to uncertainty on how much influence the MJO or the transition to La Niña has on the mid-latitude circulation pattern, the updated temperature and precipitation outlooks relied mostly upon the dynamical model guidance at varying time scales throughout December. During early December, the GEFS and ECMWF ensemble means are in good agreement and remain consistent that a highly amplified 500-hPa pattern persists over North America. These ensemble mean solutions depict a ridge over the western contiguous U.S. (CONUS) with anomalous troughing centered near the East Coast. This amplified longwave pattern is likely to result in above-normal temperatures from the West Coast east to the Rockies with below-normal temperatures across the eastern third of the CONUS. By mid-December, the GEFS indicates a retrogression of the longwave pattern with the amplified ridge along the West Coast shifting west to the North Pacific. This would eventually lead to a pattern change downstream with a 500-hPa trough (ridge) over the western (eastern) CONUS. The updated temperature outlook added a lean towards below-normal temperatures across the Mid-Atlantic and Northeast based on the anomalous cold through early December. However, probabilities are limited due to the predicted longwave pattern change which would result in a warming trend during the latter half of the month. For the western CONUS, the likelihood of above-normal temperatures through mid-December and the lack of a cold signal among the week 3-4 tools support an increased chance of above-normal temperatures. The largest probabilities (exceeding 60 percent) are forecast for southern California and the Desert Southwest where the magnitude of the anomalous warmth is predicted to be the strongest early in the month. Elevated above-normal temperature probabilities extend east to the Central and Southern Great Plains since model guidance, especially for the 8-14 day period, has trended warmer. Equal chances (EC) for below, near, or above-normal temperatures are forecast across portions of the Northern Great Plains, Mississippi Valley, and Southeast where temperatures are expected to be variable throughout the month. Following a cold start to the month across Alaska, a rapid flip to above-normal temperatures is forecast around December 4 due to enhanced onshore flow. A two-category change (favored below to above-normal temperatures) from the initial release on November 21 was necessary for southeastern Alaska as increased chances for above-normal temperatures are forecast to continue through the month. The highly amplified ridge over western North America is likely to result in a dry pattern for much of the lower 48 states through early December. A couple exceptions include areas downwind of the Great Lakes with heavy lake-effect snow and portions of Texas and the Lower Mississippi Valley as a southern stream shortwave trough tracks eastward. Above-normal precipitation is slightly favored for those areas which are forecast to have a wet start to the month. In addition, the week 3-4 outlook favors above-normal precipitation across eastern and southern Texas. Since the first week of the month is forecast to be mostly dry and there is no strong wet signal later in the month, the elevated below-normal precipitation probabilities for parts of the eastern CONUS are maintained from the outlook released on November 21. Drier-than-normal conditions through at least mid-December elevate below-normal precipitation probabilities across the Southwest, parts of the Great Basin, and California. Also, the week 3-4 outlook favors below-normal precipitation for southern California and the Desert Southwest. Although the first week of December is forecast to be on the drier side across the Northern Rockies and Northern Great Plains, the 8-14 day and week 3-4 outlook depict a wetter signal which leads to favoring above-normal precipitation across those areas. Despite a dry start to the month for the Central Great Plains, Middle Mississippi Valley, and Ohio Valley, a pattern change later in December towards a trough upstream could lead to an increase in precipitation. Therefore, EC are forecast for these areas. Enhanced onshore flow is forecast to develop across Alaska during the first week of December and tools throughout the month generally lean towards above-normal precipitation for much of the state. ***************** Previous discussion, released on November 21, is below ********************* The December 2024 Temperature and Precipitation Outlooks are based on: the Weeks 3-4 model guidance, the North American Multi-Model Ensemble (NMME), and International Multi-Model Ensemble (IMME), the consolidation (combination of statistical and dynamical tools), consideration of potential Madden Julian Oscillation (MJO) influences, and decadal trends . Although El Niño Southern Oscillation (ENSO)-neutral conditions continue, below-average sea surface temperature anomalies are observed across the east-central Pacific. La Niña is favored to develop by the end of December and La Niña composites were a factor, especially in the precipitation outlook. During late November, a major pattern change is forecast as an amplified 500-hPa ridge over Alaska results in surface high pressure with anomalous cold shifting south from Canada into the central and eastern contiguous U.S. (CONUS). By the beginning of December, the GEFS and ECMWF ensemble means are in good agreement and consistent that below-normal temperatures extend from the Great Plains to the East Coast. The latest week 3-4 GEFS (valid December 5-18) favors below-normal temperatures continuing across the Great Lakes and Northeast. Lagged MJO composites would favor a flip to above-normal temperatures across the central and eastern CONUS by mid-December. Due to an expected variable temperature pattern during December, equal chances (EC) of below, near, or above-normal temperatures are forecast across the Northern Great Plains, Midwest, and Northeast. The week 3-4 models, NMME, consolidation, and decadal trends support increased above-normal temperature probabilities across the Southeast, Lower Mississippi Valley, Southern Great Plains, and Southwest. The largest above-normal temperature probabilities (more than 50 percent) are forecast for the Rio Grande Valley and southern New Mexico where the strongest warm signal exists in the consolidation tool. EC is forecast for the Pacific Northwest, Northern Rockies, and much of California due to a weak signal in the NMME. The NMME, consolidation, and any influence from La Niña favor below-normal precipitation across the Southeast, Gulf Coast States, and Southeast. This favored dryness extends northward along the East Coast to the Mid-Atlantic and southern New England based on the NMME and daily CFS model runs. However, there is only a slight lean towards below-normal precipitation for portions of the eastern CONUS since an amplified 500-hPa trough over eastern North America early in the month would favor multiple low pressure systems tracking either along or offshore of the East Coast. Below-normal precipitation probabilities are also lower across the Florida Peninsula as an eastward propagating MJO over the Western Hemisphere could eventually lead to a more active southern stream with enhanced precipitation. In addition, the daily CFS model runs have less support for below-normal precipitation for that part of the Southeast. Week 3-4 model output, most inputs to the NMME, and La Niña composites support elevated above-normal precipitation probabilities for the Pacific Northwest and Northern Rockies. A large spatial extent of EC is forecast for the remainder of the CONUS due to a weak model signal and limited skill at this time lead for a monthly precipitation outlook. The increased chances of above (below)-normal temperatures forecast for the North Slope (southeastern Alaska) are supported by the NMME and consolidation tool. Lagged MJO composites would also favor below-normal temperatures across southeastern Alaska during mid-December. The favored wetness across western and northern Mainland Alaska is based on the NMME and also consistent with decadal trends. FORECASTER: Brad Pugh 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. The next monthly outlook...for Jan ... will be issued on Thu Dec 19 2024 These outlooks are based on departures from the 1991-2020 base period. $$
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