December 2008-February 2009 90-Day Forecast
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DECEMBER 2008-FEBRUARY 2009
250 PM EST Thu. December 4, 2008
Atmospheric patterns, 700 MB and 500 MB height anomalies, statistical data, various models, observed cases from past summers and sea surface temperatures in the tropical Pacific where used in this forecast.
Neutral ENSO conditions continued during across the central tropical Pacific. Sea surface temperatures were near normal across much of the Pacific Ocean, except for small areas of below-average sea surface temperatures in the east-central Pacific and off the coast of South America. Subsurface oceanic heat content anomalies became less negative due to the eastward shift of positive temperature anomalies at thermocline depth to -160°W, but anomalies remained negative in the eastern half of the Pacific. Most forecasts keep the ENSO near neutral through the Northern Hemisphere winter. However, recent CFS forecast show a developing cold episode development. There has been two strong Kelvin waves tracking out of the equatorial Indian Ocean since mid-September. These impulses could warm the waters between the Philippines and Hawaii, creating a window for a weak warm episode late in the winter. It is possible that the Pacific Decadal Oscillation could come into play. If waters are colder than normal, a moderating or zonal influence is sometimes evident over North America. If sea temperatures warm to above normal below the farthest north state, correlations exist that favor a negative EPO or positive PNA ridge formation. This often causes warm condition across the western states versus colder air to the right of the Rocky Mountains. This past autumn season ENSO has been neutral with the ONI (Oceanic Nino Index. Past autumns with this reading are 1960, 1966, 1980, and 2001. So the winters of 1960-61, 67-67, 1980-81, and 2001-02 are used as analogues. Also, when the stratospheric temperature warms it causes colder temperatures at the surface. The stratospheric temperature usually warms shortly after a ridge develops.
There has been prevailing patterns around North America during the past month or so which, through persistence, may offer clues to the trends of synoptic systems that will occur during the winter months. It was also noted that, despite the variability of cold to warm conditions over the West, strong ridging routinely occurred at higher latitudes in the past. The ongoing tendency of high latitude ridging across western North America favors Arctic air to drain into the eastern half of the nation.
A new factor being looked at for this forecast is the comparisons of years where tropical cyclone output where the number of storms exceeded seasonal climatology. This can be significant, because there is an indication of a higher rate of interaction between the tropics and higher latitudes. Therefore, the winter seasons of 1954-55, 1959-60, 1961-62, 1988-89 and 2002-03 are analogue.
Models are in fairly good agreement with the 500-HPA circulation across the nation. Above normal heights are forecast across much of western North America and below normal across the east-central part of the nation.
Above normal temperatures are expected along the West coast into the Rockies. The probability of above normal temperatures across this area is 57 percent. Below normal temperatures are expected across the upper Mississippi, Ohio and Tennessee Valleys to the East coast. The probability of below normal temperatures across this region is 56 percent. The remainder of the nation is expected to be near normal with no significant deviation from normal climatology.
Below normal precipitation is expected across California, the Southwest into west Texas. The probability of below normal precipitation in this area is 56 percent. Above normal precipitation is forecast across the Northeast and the mid-Atlantic. The probability of below normal precipitation in this region is 56 percent. The remainder of the nation is expected to be near normal with no significant deviation from climatology.
Jim Munley, Jr.;