DECEMBER 2017-FEBRUARY 2018 90-DAY OUTLOOK
200 PM EST Fri. Dec. 1, 2017
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.
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.
Positive height anomalies are forecast across Greenland and Alaska. 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 and the CFS month depicts below normal heights across the northern Rockies into the Plains in January. Otherwise, near to above normal heights are forecast across much of the nation. The ECMWF weeklies depicts below normal heights in the East and above normal heights in the West for the first week of January. The QBO becoming negative with minimum solar influence.
The North American Multi-Model Ensembles and the CFSv2 indicate above normal temperatures across much of the nation. Some of the NMME depict below normal temperatures in parts of the West. The ECMWF monthly depict above normal temperatures in the West and near to below normal temperatures in the East from mid-month period onward as does the CFS.
Above normal temperatures are forecast for California, the southern Rockies, the southern Plains, the lower Mississippi Valley, the Gulf Coast, the Tennessee Valley and the Southeast. Anomalies of 3F are forecast. The probability of above normal temperatures for this region is 57 percent. Below normal temperatures are forecast for the northern and central Rockies, the northern Plains, and the upper Mississippi Valley. Anomalies of 3F are forecast. The probability of below normal temperatures for this region is 57 percent. The remainder of the nation is not expected to exceed normal climatology.
Below normal precipitation is forecast for California and the southern Rockies. The probability of below normal precipitation for these regions is 50 percent. Below normal precipitation is also forecast for the central Plains and the mi-Mississippi Valley. The probability of below normal precipitation for this area is 56 percent. Above-normal precipitation is forecast for the lower Mississippi Valley, the Tennessee Valley, the Southeast and the Mid-Atlantic. The probability of above normal precipitation for this area is 57 percent. The remainder of the nation is not expected to exceed normal climatology.
Jim Munley Jr.
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