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](http://www.cpc.ncep.noaa.gov/products/predictions/90day)

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