1. A Statistical Analysis of Trends of Cold Air Outbreaks and Anomalously Cold Day in the United States

2. Previously…
The Northeast region was explored to understand general trends in both
anomalously cold day and CAO frequency.
Trends appeared to show a decrease in both anomalously cold days and CAO frequency for the northeast.

3. This Time:
The analysis performed in the northeast has been expanded to the remaining NCEI regions within the United States to determine regional differences in anomalously cold day and CAO trends.
This particular presentation will group the nine NCEI regions into those that lie west or east of the Rocky Mountains and scrutinize all nine of their statistical trends of cold days and CAOs individually.
The east-west division follows the one seen in Scott Runyon’s work.

4. Definitions Used Throughout This Presentation (For Reference)
Anomalously Cold Day: Any day where the minimum temperature recorded at a station is less than or equal to a 31-day centered moving mean of the climatological 5th percentile of that day’s minimum temperatures.
Cold Air Outbreak (CAO): A sequence of three or more anomalously cold days recorded at a station.
Regional CAO: Two or more stations within an NCEI U.S. Climate Region share the same sequence of 3+ anomalously cold days with a minimum of one overlapping day.
Statistical Significance: When the coefficient of determination (R2) for the frequency plots is greater than or equal to 0.2. This is the value that Scott Runyon used in his heat wave study.

5. U.S. Regions West of the Rockies

6. Regions and Stations in the Western U.S.
Northwest Region Stations
West Region Stations
Southwest Region Stations
Seattle, WA
San Francisco, CA
Salt Lake City, UT
Spokane, WA
Los Angeles, CA
Denver, CO
Medford, OR
Las Vegas, NV
Phoenix, AZ
Boise, ID
Reno, NV
Albuquerque, NM
Ely, NV

7. Southwest Region: Anomalously Cold Days
Negative trends of frequency evident across all seasons.
Trends are statistically significant for all seasons.
Summer (fall) shows the largest (smallest) R2.

8. Southwest Region: CAOs
Decreasing trends of frequency visible across all seasons.
None of the trends are statistically significant.
Highest (lowest) R2 values seen in Spring and Winter (Fall)

9. West Region: Anomalously Cold Days
Overall, trends of frequency appear to be decreasing across all seasons.
All trends are statistically significant.
Summer (Winter) shows the largest (smallest) R2 though

10. West Region: CAOs
Decreasing trends of frequency visible across all seasons.
All trends (except for winter) are statistically significant.
Highest (lowest) R2 values seen in Spring (winter).

11. Northwest Region: Anomalously Cold Days
Negative trends of frequency evident for all seasons.
Trends statistically significant for spring and summer.
Trends not statistically significant for fall and winter days.
Summer (winter) shows the largest (smallest) R2.

12. Northwest Region: CAOs
Decreasing trends of frequency visible across all seasons.
None of the trends are statistically significant.
Highest (lowest) R2 values seen in spring (summer).

13. U.S. Regions East of the Rockies

14. Regions and Stations East of the Rockies

15. Regions and Stations East of the Rockies (Continued)
Northeast Region Stations
Southeast Region Stations
Central Region Stations
East North Central Region Stations
West North Central Region
Stations
South Region Stations
Caribou, ME
Washington, D.C.
Chicago, IL
International Falls, MN
Great Falls, MT
Topeka, KS
Albany, NY
Norfolk, VA
St. Louis, MO
Minneapolis, MN
Billings, MT
Dodge City, KS
Boston, MA
Raleigh-Durham, NC
Cincinnati, OH
Milwaukee, WI
Bismarck, ND
Oklahoma City, OK
Erie, PA
Charleston, SC
Nashville, TN
Des Moines, IA
Pierre, SD
Little Rock, AR
Pittsburgh, PA
Atlanta, GA
Cheyenne, WY
Amarillo, TX
New York City, NY
Tallahassee, FL
North Platte, NE
El Paso, TX
Tampa, FL
San Angelo, TX
Miami, FL
Waco, TX
Port Arthur, TX
Corpus Christi, TX
New Orleans, LA
Meridian, MS

16. Northeast Region: Anomalously Cold Days
Trends are decreasing in frequency across all four seasons.
Statistically significant trends evident for summer and fall seasons.
Statistically insignificant values for fall and spring.
Summer (winter) have largest (smallest) R2 values.

17. Northeast Region: CAOs
Trends are exhibiting very slight decreases in frequency across all four seasons.
Statistically insignificant trends evident for all seasons.
Summer (spring) have largest (smallest) R2 values.

18. Southeast Region: Anomalously Cold Days
Trends are decreasing in frequency across all four seasons.
Statistically significant trends evident for summer and fall seasons.
Statistically insignificant values for fall and spring.
Summer (winter) have largest (smallest) R2 values.

19. Southeast Region: CAOs
Decreasing trends in frequency visible across all seasons.
None of the trends are statistically significant.
Highest (lowest) R2 values seen in summer (winter).

20. Central Region: Anomalously Cold Days
Decreasing trends in frequency evident across all seasons.
None of the trends are statistically significant.
Summer (winter) have the largest (smallest) R2.

21. Central Region: CAOs Decreasing trends visible across all seasons.
None of the trends are statistically significant.
Highest (lowest) R2 values seen in Spring (summer and winter).

22. East North Central Region: Anomalously Cold Days
Decreasing trends visible across all seasons.
None of the trends are statistically significant.
Highest (lower) R2 values seen in Spring (winter).

23. East North Central Region: CAOs
Decreasing trends visible across all seasons.
Spring trends are statistically significant. The rest of the trends are not.
Highest (lowest) R2 values seen in spring (summer).

24. West North Central Region: Anomalously Cold Days
Very slight decrease in frequency of cold days across all four seasons.
Trends for all seasons are statistically insignificant.
R2 largest (smallest) for winter (summer).

25. West North Central Region: CAOs
Very slight decrease in frequency trends visible across all seasons.
None of the trends are statistically significant.
Highest (lowest) R2 values seen in winter (fall).

26. South Region: Anomalously Cold Days
Trends show that there is a very slight decrease in the number of cold days across all four seasons.
Trends appear to be statistically insignificant across all four seasons in this region.
Winter (spring) shows the largest (smallest) statistical significance.

27. South Region: CAOs
Slightly decreasing trends of frequency visible across all seasons.
None of the trends are statistically significant.
Highest (lowest) R2 values seen in winter (spring).

28. Summary and Future Work
All nine regions appear to be experiencing a decrease in both anomalously cold days and regional CAO frequency. This trend is noticeable across all seasons.
Statistically significant decreasing trends more common in the three western regions for all seasons. Cold Days/CAO frequency peaks early, in the late 1940’s-1950’s
Seasonal trends for CAOs and anomalously cold days for all 6 regions east of the Rockies are statistically insignificant for the most part. The seasonal trends in the southeast region, though statistically insignificant, possesses the highest R2 values for its decreasing anomalously cold day and regional CAO trends. Cold days/CAOs frequency generally tend to peak in the 1960’s-1970’s.
Future Work:
Composite all of the eastern and western NCEI regions together to determine trends in CAOs and anomalously cold days across each.
Should we also break down the period into two time periods (for example, use , per Scott’s work and DeGaetano and Allen (2002)) and compute trend lines for each individual period?
Bin regional CAOs by duration and perform a statistical analysis on trends of CAO duration based on how frequent CAOs of certain durations appear.
Construct CAO-centered composites to determine large-scale and synoptic-scale CAO evolution patterns.