DECEMBER 2012 30-DAY OUTLOOK
400 PM EST Fri. Nov. 30, 2012
Atmospheric patterns, 700 MB and 500 MB height anomalies, statistical data, and various models such as the ECMWF monthly, NMME, IMME and the CFSv2, along with observed cases from past such months, data from 1895-2000 were used in this forecast.
Oceanic and atmospheric indices continued to indicate a neutral ENSO conditions. Sea surface temperature anomalies remained fairly steady across much of the central Pacific Ocean. Anomalies between 0 and +0.5 C throughout the east-central equatorial Pacific Ocean,. Anomalies west of the Date Line along the equator are mostly between +0.5C and +1.5C. Atmospheric observations over the tropical Pacific Ocean remained consistent with a neutral ENSO conditions, with convection mostly near normal. Models indicate a continuation of neutral anomalies along the equator in the east-central Pacific through the end of the year.

The NAO index is weakly negative and is forecast to be positive by day 7 and then trend negative through day 14. The PNA index is negative and is forecast to remain negative through day 14. The AO index is negative and is forecast to remain mostly negative through day 14.

Enhanced convection was evident across parts of Southeast Asia, South America and Africa as well as south of Hawaii. Suppressed convection developed over parts of the Indian Ocean. The MJO showed signs of weakening during the past week as several observational indicators became considerably less coherent. Most of the dynamical model MJO index forecasts currently indicate little, if any, coherent MJO signal during the next two weeks. Some models and a few statistical forecast tools, however, do indicate the potential for renewed enhanced convection in the western Pacific during the period. Atmospheric Kelvin and Equatorial Rossby wave activity is weak or not well defined and in combination with a potentially weakening MJO, the forecast has high uncertainty with little forecast coverage.

Models and ensembles are in agreement on the pattern for December forecast a ridge from eastern Siberia northward towards the pole, a positively tilted trough over Alaska, a polar vortex centered over northeastern Canada, a ridge over the West and a trough in the East. The operational GFS differs from the ensemble means. The GFS forecasts a deep trough over the nation, a ridge over the Gulf of Alaska, and a weaker trough over Alaska.

Above normal temperatures are forecast for the western Rockies to the West coast. The probability of above normal temperatures across this area is 56 percent. The NMME and the ECMWF monthly model guidance favor above normal temperatures in the West and near normal temperatures elsewhere. The IMME depicts near to above normal temperatures across the nation. The CFSv2 forecast above normal temperatures across much of the nation except for the week Dec. 20-26 with above normal temperatures in the West and below normal temperatures in the East. The remainder of the nation is not expected to exceed climatological values.

Above normal precipitation is forecast for the portion of the Northwest primarily for the start of the month. The probability of above normal precipitation across this area is 52 percent. Below normal precipitation is forecast for California, the central and southern Rockies and the southern Plains. The probability of below normal precipitation for this area is 53 percent.. Above normal precipitation is forecast for the Southeast and the lower mid-Atlantic. The probability of above normal precipitation for this area is 56% The ECMWF monthly is forecasting below normal precipitation along the West coast and from the central gulf coast into the lower mid-Atlantic. Near normal precipitation is forecast elsewhere. The CFSv2 forecasts below normal precipitation for the most of the nation except at the part of the month when above normal precipitation is forecast for the Northeast and the mid-Atlantic. The remainder of the nation is not expected to exceed climatological values.
Jim Munley Jr.
http://www.jimmunleywx.com