A Study of Severe Arctic Outbreaks at Little Rock, Arkansas 1822-2014

1. A Study of Severe Arctic Outbreaks at Little Rock, Arkansas1822-2014 Brian Smith National Weather Service Little Rock, Arkansas

2. Background • Arctic air reaches Arkansas frequently during the late fall, winter, and early spring • This air has usually modified considerably by the time it reaches central Arkansas. • On rare occasions, Arctic air does reach central Arkansas without a high degree of modification.

3. Questions • How frequently does severe Arctic cold reach central Arkansas—specifically, Little Rock? • What can prolong the severity of the cold?

4. Study Definition • For the purpose of this study, “severe Arctic cold”, or “severe Arctic outbreak” refers to minimum temperatures at Little Rock of 10° F or colder.

5. Background • About every four to eight years, low temperatures at Little Rock fall to 10° F or colder. • By comparison, the average low temperature during winter is 33° F.

6. Methodology • Data was obtained from official temperature records for the Little Rock area, which began July 1, 1879, when the Signal Service office opened. • Data before July 1, 1879 was obtained from archived newspapers. Temperature data was mentioned frequently in daily and weekly newspapers in the area. This practice became common when Little Rock became the capital of Arkansas Territory in 1821.

7. Methodology • Example… An account of the first December 1822 outbreak, given in the December 3, 1822 edition of the Arkansas Gazette. Up until the Signal Service/Weather Bureau office opened in 1879, at various times, the newspaper office took observations or published observations taken by locals who had a genuine interest in weather.

8. Methodology • Locations of weather observing sites in Little Rock 1822-2014

9. Methodology

10. Methodology • For this study, 132 outbreaks were analyzed where low temperatures fell to 10° F or colder. • For seven outbreaks, temperature data was unavailable. Therefore, the particular outbreak was taken into consideration when single-digit or below zero temperatures occurred within Arkansas or in states bordering Arkansas near or below the same latitude as Little Rock.

11. Data • Severe cold at Little Rock usually results from strong high pressure following Arctic fronts. • These outbreaks have occurred as early as November 18th (1880), and as late as March 17th (1838).

12. Data • For temperatures to be this cold, there is usually extensive snow cover from northern Arkansas to Canada

13. Data • Severe cold outbreaks at Little Rock are most frequent during the month of January

14. Data • Severe cold outbreaks have occurred in each decade since the 1820s. • There was a peak during the 1910-1919 decade, and a lull 1990-1999

15. Snow Cover • The combination of strong high pressure and snow cover works in concert to bring temperatures down into the single digits and even below 0° F. • However, snow cover at Little Rock is not always required for temperatures as cold as 0° F.

16. Snow Cover • Low temperatures have only fallen to 0° F or colder twice with no snow cover. -January 1940, December 1989 -During these outbreaks, the extent of snow cover was limited to southern Missouri. • In both cases, the maximum central pressure of the Arctic high was abnormally strong.

17. Maximum Central Pressure • Severity/longevity of the outbreak is largely dependent on the maximum central pressure of the Arctic high. -Outbreaks with the highest pressures were February 1899 (1063 hpa) and January 1962 (1059 hpa). Both lasted five consecutive days. • Temperatures at or below 0° F occurred with high pressure systems exhibiting a maximum central pressure at or above 1035 hpa. -One notable exception…late December 1876/early January 1877. In this case, snow was likely on the ground for a prolonged period of time before the outbreak’s peak on December 30th.

18. Maximum Central Pressure

19. Strong Temperature Gradient • Temperatures usually hit 10° F or colder after a fall of several days. • There have been two big exceptions -January 12-13, 1916 – from a maximum of 72° F on the 12th to a minimum of 10 ° F the morning of the 13th. There was a drop of 60° F in 24 hours during this period. Only a trace of snow fell. -January 19-20, 1985 – from a temperature of 60° F just before 3 PM the 19th, to -2° F at 9 AM on the 20th, with one inch of snow on the ground.

20. Record Pressures • These outbreaks often see record high sea level pressures at Little Rock. Six of the highest pressures ever recorded have been set during these outbreaks.

21. Outbreak Severity • For the purpose of the study, severity of the outbreak is measured in the number of days with low temperatures at or below 10° F

22. 500 mb Height Anomaly • For the purpose of this study, 500 mb height anomaly was calculated from NCEP reanalysis data for 1200 UTC of the date of the observed low temperature. • Severe cold outbreaks in Arkansas have occurred where the mean 500 mb height anomaly is strongly negative east of the Rockies (on the order of 25 meters or higher), and positive west of the Rockies or over the Desert Southwest (about 25 meters or higher). • The coldest temperatures occur when there is a strong mean positive height anomaly on the order of 25-100 meters centered over the northern Rocky Mountains, in concert with a strong negative anomaly on the order of 25-200 meters east of the Rockies.

23. 500 mb Height Anomaly • For temperatures at or below 0° F, the mean anomaly should be around 200 meters below normal over at least the northeast half of Arkansas.

24. Recap • Severe Arctic outbreaks at Little Rock occur most frequently in January. • Arctic air moving over snow cover between the Canadian border and Arkansas usually yields frigid temperatures with readings, at times, falling to the single digits or below zero. • The outbreaks tend to be short-lived and last only for a day or two. However, there have been longer events lasting up to five days.

25. Recap (continued) • The prolonged events usually are associated with abnormally high sea level pressures in excess of 1040 hpa, as well as snow cover at Little Rock itself. • Severe cold episodes occurred most frequently in the 2nd decade of the 20th century, and only once during the last decade of the 20th century.

26. THE END • Questions? Brian.D.Smith@noaa.gov