2004/05 Winter Season Outlook

By: Victor Nouhan, Climate Focal Point

The Climate Prediction Center (CPC) is calling for equal chances of a colder, normal, or average temperatures and precipitation this coming winter for northern and eastern Maine. Forcing factors for this coming winter include a weak to moderate El Nino similar in magnitude to the El Nino that occurred during the 2002/03 winter. If you recall, that winter began relatively early (mid November) and continued relatively late in the spring (early May), if one used permanent snow pack as a measure, and featured relatively high persistence of cold and wind. In fact, the winter of 2002/03 was within the top 20 percent of coldest winters for our area. So one might ask, what are the uncertainties regarding making a similar outlook for our area to what verified for the 2002/03 winter?

The answer to this is the exact strength and location of the current El Nino, knowing that no two El Nino’s are exactly the same. Recently, climate scientists have divided El Nino’s and their impact into two different camps: 1) strong El Nino’s and 2) Weak to Moderate El Nino’s. Strong El Nino’s tend to stretch from the dateline eastward across the equatorial Pacific to the South American coastline with warmer than average tropical waters and enhanced thunderstorm/tropical rainfall activity. When this occurs, a strong subtropical jet merges with a strong polar jet across the entire north Pacific which continues eastward across the mid/high latitudes of North America during winter, bringing enhanced storminess to the west coast and southern U.S., but little in the way of cold air from Canada.

Most weak to moderate El Nino’s tend to reside over the central equatorial Pacific well west of the South America coast, with warmer than average tropical waters and enhanced thunderstorm/tropical rainfall activity. Historically, these El Nino’s have a more variable effect on winter weather across North America. First, California rainfall tends to be quite variable from each weak/moderate El Nino event, and often vary within the winter season of an individual event. For example, the winter of 1976-77, which featured a weak El Nino event, resulted on the driest and least stormy winter on record for California, since a strong, very persistent upper ridge over the west coast sheltered California from strong Pacific storms. Other weak/moderate El Nino’s feature a split flow pattern during the winter where the upper north Pacific jet stream splits before reaching the western U.S. coast. More often than not in a split flow regime, the southern jet stream results in above average rainfall and storminess to California and the desert southwest while the northern branch of the Jet is deflected northward into western Canada or even southern Alaska, leaving the Pacific northwest of the U.S. less stormier and drier than average. An example of this was the winter of 1986-87.

Since a significant portion of the jet stream is deflected northward to high latitudes of western North America and then moves southeast to eastern North America during many weak to moderate El Nino winters, some of these winters can be characterized by frequent outbreaks of arctic air from north central Canada into the eastern third (up to half) of the U.S., resulting in colder and often stormier than average conditions for this portion of the country. A significant fraction of this increased storminess includes a class of storms called “Nor’easters”, which initially form over the northeast Gulf of Mexico or off the southeast U.S. coast, and then intensify as they move northeast just off the mid Atlantic and New England coastline. The contrast of arctic air masses inland with maritime tropical air along and east of the Gulf Stream make these storms prodigious snow producers when they occur from the mid Atlantic states through New England.

CPC’s winter outlook shows equal chances of each category of precipitation along the immediate coast of the mid Atlantic states and New England just east of a large area of more weighted below average precipitation chances over the Midwest, Ohio Valley, and the Great Lakes, implying a potentially active east coast storm (Nor’easter) track. Lastly, even if this upcoming winter experiences a greater frequency of Nor’easters, there is no guarantee that every storm would affect northern and eastern Maine, since some are occasionally diverted south and east of our region by north Atlantic blocking. This blocking factor varies in strength from winter to winter and even within the same winter. For more details concerning the CPC outlook, check to web at:

www.cpc.ncep.noaa.gov/products/predictions/90day

and select the outlook for Dec-Jan-Feb 2004/05.

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