DECEMBER 2017 30-DAY OUTLOOK
405 PM EDT Tue. Oct. 31, 2017
Atmospheric patterns, 700 MB and 500 MB height anomalies, statistical data, and various models such as the ECMWF monthly and weekly, NMME, IMME and the CFSv2, along with observed cases from past such months, data from 1895-2000 were used in this forecast.
During the past month, La Nina conditions developed as negative sea surface temperature anomalies continued across the eastern and central equatorial pacific along with suppressed convection across the central tropical Pacific. Conditions typically associated with a cold episode were considered in making the December 2017.
During the past month, a robust MJO signal circumnavigated the global tropics, starting from and ending over the Maritime Continent region. This signal was on the high-frequency end of the conventional 30-60 day MJO period. All that remains now is a very weak intraseasonal signal over the Maritime Continent, as confirmed by both CPC's velocity potential based index and the RMM index. Dynamical model MJO forecasts display significant differences in both the forecast amplitude and phase of a potential developing MJO signal over the next one to two weeks. A general consensus of solutions favors an amplifying signal over the eastern Indian Ocean and western Maritime Continent during Week-1, followed by eastward propagation of this signal during Week-2. A modal decomposition technique that attempts to identify various sub-seasonal modes of variability reveals the most significant players during this period are likely to be the low-frequency base state with likely modulation by eastward-moving Kelvin waves, and westward-moving equatorial Rossby waves, with the MJO portion of the intraseasonal continuum expected to play a more subdued role.
The AO is negative and is forecast to remain negative over the next two weeks. The PNA is positive and is forecast to remain positive through the sam period. The NAO is also currently negative with positive height anomalies across Greenland. The ridges are predicted to extend northward covering the entire Arctic region with positive height anomalies and maintain a negative AO. Cold temperatures are predicted downstream. Troposphere-stratosphere coupling has been relatively quiet, allowing for the stratospheric polar vortex to strengthen. However predicted increase in energy transfer from the troposphere to the stratosphere is predicted to dislodge the polar vortex form the Pole towards Eurasia.
This displacement opens up the Polar Vortex to subsequent disruptions and could eventually result in a more significant disruption of the vortex in the coming weeks. In addition, the October Eurasian snow cover extent was above normal, Arctic sea ice extent is well below normal and will continue to be below normal for the entire winter. Also, strong blocking was observed this fall across the high latitudes. All three factors favor colder temperatures across at least parts of the Northern Hemisphere mid-latitude continents during the winter months. Above normal October Eurasian snow cover extent and below normal Arctic sea ice extent are both correlated with a negative winter AO. Often following a polar vortex disruption severe winter weather is more frequent across the mid-latitude continents for a period of typically four to six weeks.
The ECMWF Monthly depicts above normal heights in the West and near normal heights in the East. The QBO becoming negative with minimum solar influence. The CFS weeklies is more aggressive with the cold air in the East and the warmth in the West by the second week. The CFS monthly depicts above normal heights in the West and South.
The North American Multi-Model Ensembles, the CFSv2 and the ECMWF monthly indicate above normal temperatures across much of the nation with below normal temperatures across the upper Mississippi Valley and the Great Lakes in January.
Above normal temperatures are forecast for the western states. Anomalies near 4F are forecast. The probability of above normal temperatures for this region is 56 percent. Below normal temperatures are forecast for the middle and upper Mississippi Valley to the Atlantic Coast. Except for northern new England. Anomalies of 3F are forecast. The probability of above normal temperatures is 56 percent. The remainder of the nation is not expected to exceed normal climatology.
Below normal precipitation is forecast for the southern Rockies and the southern Plains. The probability of below normal precipitation for these regions is 55 percent. Above normal precipitation is forecast for the eastern Gulf Coast States, the Ohio and Tennessee Valley, the Southeast and the Mid-Atlantic. The probability of above normal precipitation is 56 percent. Below normal precipitation is forecast for the eastern Gulf Coast States and the Southeast. The probability of below normal precipitation for this region is 57 percent. The remainder of the nation is not expected to exceed normal climatology.
Jim Munley Jr.
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