Department of Geography, University of Delaware Center for Climatic Research Melissa Malin

1. An Evaluation of January Temperature Anomalies in the United States Utilizing a Synoptic Climatological Approach Department of Geography, University of Delaware Center for Climatic Research Melissa Malin Dr. Katrina Frank Steven Quiring Dr. Laurence Kalkstein 85th AMS Annual Meeting January 15, 2005

2. The January Thaw has roots in New England weather folklore “…a characteristic meteorological condition that tends to occur on or near a specific calendar date.” -American Meteorological Society an anomalous warm spell invading during the coldest time of year

3. i. identification of a January Thaw signal across the United States ii. assess the inter- and intra- regional variability of the January Thaw Can the Thaw be explained synoptically? …through an assessment of air mass frequency The Goals Of This Investigation....

4. US Regions West Mountain East Great Plains Midwest

5. Weather Data National Climatic Data Center twice daily 4 a.m. & 4 p.m. Air Temperature Dew Point Temperature Air Mass Data Spatial Synoptic Classification daily Dry Moderate Dry Moderate + Dry Polar Dry Polar - Dry Tropical Moist Polar + Moist Moderate Moist Polar Moist Tropical Transition 1948 – 2001 December 1 – February 28 Data

6. Philadelphia, Pennsylvania average daily temperatures plotted at each station Methods and Analysis • standardized using a five-day moving window • identified by date of the third day

7. Philadelphia, Pennsylvania Upper Bound Winter Trendline Lower Bound the identification of singularities singularity at January 24 -25 • a second-order polynomial curve fit for winter trendline • upper/ lower bounds set at two standard deviations

8. Cheyenne, Wyoming (Thaw) December 3 Upper Bound (Thaw) January 16 - 18 Winter Trendline Lower Bound (Freeze) January 2 - 4 the identification of singularities an example of singularities found at a mountain region station

9. synoptic analysis Three techniques were used to detect changes in air mass frequency during Thaw and Freeze events. Linear Method Second-order Method Mean Method

10. Philadelphia, Pennsylvania linear method • useful for air masses that exhibit a general frequency trend across a season and for stations with winter seasons that extend beyond the period examined here

11. Philadelphia, Pennsylvania second-order method • useful for air masses that do not display such general, linear frequency tendencies and may be less (more) frequent in early and late winter though more (less) prominent throughout the middle of the season

12. mean method Station Thaw Observed DM Frequency DM Monthly Mean Frequency PHL 24 Jan 22.5% 21.5% PWM 23 Jan 16.0% 14.5% PVD 23 Jan 20.8% 19.8% RIC 24 Jan 25.5% 26.5% SYR 24 Jan 11.6% 10.5% • useful here because singularity windows are scrutinized with only frequencies typical of that month and seasonal trends do not contribute to calculating the strength of that departure

13. finding correlations between temperature differences and frequency differences Eastern Region Thaws and Linear Method Output

14. 14-17 January 15-17 January 26-29 December 22-24 January 14-16 January 23-26 January 15–17 January 24-25 January 29 December Results Inter- and Intra-Regional Variability of the Thaw • found rather cohesive January thaw signal in every region • appears to move somewhat systematically across the country

15. found less cohesive January Freeze signal in western US 28-29 January (all stations) 5-8 January 26-28 January 4 January 3-4 January 1 January 2-7 January Results Inter- and Intra-Regional Variability of the Freeze • appears to also progress somewhat systematically eastward

16. Results of Synoptic Analysis.... January Thaw It appears there is no single synoptic explanation for the Thaw across the country. East and West ….. No Relationships Mountain……. Significantly more frequent warm air masses (all) Significantly less frequent cold air masses (L,M) Great Plains………. Significantly less frequent DP- (S) Significantly more frequent DP (S) Midwest………… Significantly more frequent MT (all) Significantly less frequent DT (all)

17. Results of Synoptic Analysis.... January Freeze It appears there is no single synoptic explanation for the Freeze across the country. West ………. Significantly less frequent warm air masses (all) Significantly more frequent MP (L,S) Mountain……… Significantly more frequent DP- (L) Significantly more frequent MP (M) Great Plains…… Significantly more frequent DP- (all) Significantly less frequent DP (all) Significantly more frequent DT* (all)

18. Conclusions this research provides evidence for the existence of a regionally coherent January Thaw and January Freeze both show signs of west- east advancement across the US The Freeze often occurs just prior to the Thaw the Thaw and Freeze are related to variable synoptic conditions rather than a single air mass type most conditions observed are generally intuitive the relationship between synoptic conditions and singularities is most apparent in the Central US the air mass-based methodologies used here did not detect a cause for the Thaw in the East this needs further investigation…