Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EST Sat Nov 30 2019 30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2019 The updated temperature and precipitation outlooks for December 2019 are based on the WPC temperature and precipitation forecasts during the first week of December, the CPC 8-14 day and Weeks 3-4 temperature and precipitation outlooks, and the latest monthly forecasts from the CFS model. Although equatorial sea surface temperatures (SSTs) are above average across much of the Pacific Ocean, anomalous convection and winds continue to reflect ENSO neutral conditions. The Madden-Julian Oscillation (MJO) constructively interfered with the ongoing positive Indian Ocean Dipole (IOD) during late November. Dynamical model forecasts are in reasonably good agreement that the MJO resumes its eastward propagation across the Indian Ocean during the next two weeks. This predicted MJO evolution would favor above normal temperatures across the central and eastern U.S. during December. A rapid pattern change is likely to occur over the middle to high latitudes of the Northern Hemisphere at the beginning of December. Positive 500-hPa height anomalies at the higher latitudes are forecast to be replaced by negative 500-hPa height anomalies. The Arctic Oscillation (AO) index is forecast to become strongly positive at the beginning of the month. A positive AO index typically corresponds to above normal temperatures across the north-central U.S. Many GFS ensemble members indicate that the AO index trends back to neutral or even a negative phase by mid-December which slightly lowers forecast confidence in the temperature outlook across the north-central U.S. Despite the good agreement in recent model guidance, probabilities for above normal temperatures were not increased too much across the north-central U.S. given the poor consistency among daily CFS model runs during the final ten days of November. Multiple early season winter storms resulted in widespread, heavy snow from the Sierra Nevada Mountains east to the Colorado Rockies during late November. Anomalous snow cover and depth for this time of year may are likely to result in a cooling effect on surface temperatures across the Great Basin, at least through early December. Therefore, the updated temperature outlook calls for equal chances of below, near, or above normal temperatures across the Great Basin. The highest probabilities (above 60 percent) for above normal temperatures are forecast across the central and southern Great Plains where maximum temperatures are forecast to average 5 to 10 degrees F above normal during the first week of December with above normal temperatures likely persisting into the latter half of the month. The updated precipitation outlook expanded the favored area of above normal precipitation to include the entire West Coast. The greater chances for above normal precipitation throughout the West are mostly due to a wet start to the month along with an amplified 500-hPa trough extending from the Aleutians southeastward to near the West Coast during the second week of December. The highest probabilities (above 70 percent) for above normal precipitation are forecast from San Francisco to the Sierra Nevada Mountain range where 7-day precipitation amounts are forecast to be near the upper monthly tercile. Heavy precipitation (2 to 7 inches, liquid equivalent) is likely across northern and central California during the first few days of the month as a low pressure system interacts with tropical moisture originating from Hawaii. The revised precipitation outlook also increased the coverage for above normal precipitation to include areas from the Midwest eastward to parts of the Northeast. This expanded coverage is consistent with the upper-level trough upstream, a northward shift in the mid-latitude storm track, and likelihood of above normal temperatures for much of the central and eastern U.S. Also, heavy precipitation at the beginning of December supports elevated odds for above normal precipitation from the northern mid-Atlantic north to southern New England. Based on a dry first week of the month coupled with a consistent dry signal among recent CFS model runs, elevated odds for below normal precipitation are forecast for parts of central and southern Texas along with much of Florida. Little to no change was needed for the updated Alaska temperature outlook with the highest confidence remaining that above normal temperatures prevail along most coastal areas of mainland Alaska and the Aleutians. Based on excellent model agreement that an amplified trough becomes established over the North Pacific with enhanced onshore flow, above normal precipitation is most likely across the Kenai Peninsula. ----------- Previous message (from November 21) is shown below ------------ The December 2019 temperature outlook is based on Week-2 model guidance, Weeks 3 to 4 outlooks from the CFS and ECMWF models, recent daily CFS model output, consideration of influences from the Madden-Julian Oscillation (MJO), and long-term trends. Near to above-average sea surface temperatures were observed across the east-central tropical Pacific Ocean during October and early November. However, suppressed convection continued near the Date Line. The oceanic and atmospheric observations reflect ENSO-neutral conditions which are likely to persist through December. The Madden-Julian Oscillation (MJO) strengthened in late October and its enhanced phase quickly propagated east across the Western Hemisphere during early to mid-November. The MJO is likely to constructively interfere with the ongoing positive Indian Ocean Dipole (IOD) through the remainder of November into the beginning of December. It is unclear how this constructive interference between the MJO and IOD and the resultant enhanced convection across the western Indian Ocean will influence the mid-latitude circulation pattern. Lagged MJO composites are consistent with Week-2 model solutions that feature upper-level ridging south of the Aleutians and a downstream trough over the western U.S. The ECMWF ensemble mean maintains this anomalous ridge/trough pattern through Week-3 (early December) and then indicates a retrogression of the longwave pattern by mid-December. This evolving longwave pattern favors a relatively cold start to the month across the northern Rockies and northern Great Plains before a moderating trend or transition to above-normal temperatures occurs by mid-December. Downstream of the upper-level trough over the West, anomalous subtropical ridging is expected to result in a relatively warm start to the month across the Southeast and Gulf Coast States. The highest confidence (albeit limited at a half-month lead) in the temperature outlook across the CONUS exists for these areas due to the likely warm start to the month and good model consensus. Forecast confidence decreases north of the 40th parallel across the central and eastern U.S. since the GFS and ECMWF ensemble means continue to feature anomalous 500-hPa ridging over the Davis Strait and Greenland persisting into the first week of December. Given the persistence of this ridging at high latitudes, forecast confidence in the temperature outlook is low across the Great Lakes and New England. The precipitation outlook for December is based primarily on recent daily runs of the CFS model and the calibrated North American Multi-Model Ensemble (NMME). The area with enhanced odds for above normal precipitation across the Pacific Northwest, interior West, and central to northern high Plains is supported in part by the likelihood of an amplified upper-level trough forecast over the Rockies or near the West Coast during early December. The slight tilt in the odds for below normal precipitation forecast for south Texas is consistent with the seasonal precipitation outlook. A large area with equal chances for below, near, or above normal precipitation throughout much of the central and eastern U.S. is necessary due to weak signals among precipitation tools along with low predictability inherent with a monthly outlook at this time lead. Revisions to the December temperature and precipitation outlooks are likely with the November 30 release. The amplified upper-level ridge (trough) over the North Pacific (western North America) is forecast to become established by the end of November and likely continue into early December. This longwave pattern favors at least a slight tilt in the odds for below-normal temperatures across eastern mainland Alaska. The highest odds (above 60 percent) for above-normal temperatures are forecast for areas of the state adjacent to the Bering and Chukchi Seas where sea surface temperature anomalies are averaging as high as 2.5 degrees C above normal. The favored area of above normal precipitation across Alaska is consistent with the likelihood of enhanced onshore flow during early December and supported by many of the precipitation tools. FORECASTER: Brad Pugh The climatic normals are based on conditions between 1981 and 2010, 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 2019 These outlooks are based on departures from the 1981-2010 base period. $$