85th AMS Annual Meeting

Tuesday, 11 January 2005: 1:30 PM
An evaluation of January temperature anomalies in the United States utilizing a synoptic climatological approach
Melissa Lynn Malin, Center for Climatic Research, Newark, DE; and K. L. Frank, S. Quiring, and L. S. Kalkstein
For several decades a phenomenon known as the ‘January Thaw’ has been investigated by climatologists. To date, a causal mechanism for the Thaw has not been determined, which consequently has led to the widespread dispute and uncertainty of the singularity’s existence. The lasting intrigue surrounding the thaw is attributed to the apparent recurrent, anomalous warm spells associated with an event that invades during the coldest time of year. The goals of this research are to identify a Thaw signal across the country and to utilize a synoptic climatological approach to explain the singularity. Specifically, the inter- and intra- regional variability of the January Thaw’s presence will be addressed through an assessment of changes in air mass frequency. This will determine if changes in air mass types over a several day period are contributing to systematic Thaw conditions. Initial observations also indicate that a cold singularity, a ‘January Freeze,’ occurs in the western United States. These anomalous cold events will also be examined.

This analysis provides evidence for the existence of a regionally coherent January Thaw across the northern United States. These events are the most prominent in the East, where seasonal temperatures are naturally more variable. Further, a regionally cohesive January Freeze, often experienced just prior to the Thaw, occurs in the western United States. The trend of both warm and cool singularities is generally consistent inter-regionally, moving eastward across the country and appearing to follow typical mid-latitude circulation.

Thaw and Freeze events are related to variable synoptic conditions across the United States rather than the advection of a single air mass type. Perhaps as expected, in some areas of the country Thaws are associated with more frequent warm air mass types, like MT in the Midwest. In other regions like the Great Plains, Thaws occur while the character of cold air masses change, exhibiting fewer of the very coldest days. These situations are sometimes accompanied with overall less frequent cold air mass types. Freeze singularities are often associated with more frequent DP- air masses, though in general are related to more frequent cold air masses and less frequent warm air masses.

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