Global Overview
The December 1998 - February 1999
season was dominated by the mature La Niña (cold water)
conditions in the east equatorial Pacific. This corresponded to
colder than normal temperatures in southern Europe, northwestern
Africa and western Australia. The eastern two thirds of North
America was warmer than normal, as a result of fewer than normal
Arctic outbreaks. A persistent ridge over northeastern Africa and
the Middle East kept that region warmer and drier than normal. It
was also warmer and drier over China throughout the season. A broad
ridge over Asia resulted in above normal temperatures over most of
Russia except for the eastern-most portion. A persistent trough off
the northwest coast of North America produce near record rain and
snow cover in the area. Drought was widespread in southern Africa
during the season.
Global mean land temperature anomalies for the period December-February (using a base period 1880-1998) are shown in the above figure. The anomaly for the 1998-1999 season was well above the mean value. In fact, this 1998-1999 season was the third warmest since 1880. The curved yellow line is a nine-point binomial filtered value which shows the decadal-scale variations.
Globally averaged precipitation for the three-month period December 1998 through February 1999 was 31.12 millimeters above the 1900 - 1998 mean. Although precipitation averaged well above normal for the globe as a whole, there were many areas that were drier than normal during the period. (See the Global Analysis page for more information.) |
US National Overview
The predominant upper-level flow
pattern for the Winter Season was a Pacific onshore flow which
provided copious amounts of rainfall for the Pacific Northwest and
record snowfalls for the higher elevations. Once these storm
systems crossed the Rockies, they were much weaker and provided
only limited amounts of moisture. This near-persistent zonal flow
permitted only a few intrusions of arctic air, most notably during
late December and early January. The lack of arctic air masses
allowed for the second warmest winter since 1895 for the contiguous
United States.
Extreme Events
Numerous weather related natural
disasters occurred during the December 1998-February 1999 period.
Major highlights included outbreaks of tornadoes in the U.S. in
January 1999 with a record number of tornadoes (169), which is
about ten times the January climatological norm, and heavy
precipitation (over 60 inches in some locations) across portions of
the U.S. Pacific northwest. Heavy snowfall lead to numerous and
fatal avalanches across portions of Europe during this period as
well..