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A Study of Severe Arctic Outbreaks at Little Rock, Arkansas1822-2014Brian SmithNational Weather ServiceLittle Rock, ArkansasBackgroundArctic 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. QuestionsHow frequently does severe Arctic cold reach central Arkansasspecifically, Little Rock?

What can prolong the severity of the cold? Study DefinitionFor the purpose of this study, severe Arctic cold, or severe Arctic outbreak refers to minimum temperatures at Little Rock of 10 F or colder.BackgroundAbout 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.

MethodologyData 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. MethodologyExampleAn account of the first December 1822 outbreak, given in the December 3, 1822 edition of the Arkansas Gazette.

Up until the Signal Service/WeatherBureau 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.

MethodologyLocations of weather observing sites in Little Rock 1822-2014

MethodologyObserving Sites in Little Rock, Arkansas 1822-PresentTypeDate BeganDate EndedRemarksArkansas Gazette Office01/20/182206/30/1879Periodic temperature and snowfall observationsU. S. Army Arsenal07/01/183906/30/1873Periodic temperature and rainfall observationsPrivate CitizenAbt. 187006/30/1879Periodic temperature observationsU. S. Army Signal Corps Rainfall and River04/21/187307/01/1879Snowfall and rainfall observationsU. S. Army Signal Corps Office07/01/187903/01/1887Daily temperature observations beganU. S. Army Signal Corps/ Weather Bureau Office03/01/188711/16/1892Weather Bureau Office11/16/189207/01/1898Weather Bureau Office07/01/189810/08/1907Weather Bureau Office10/08/190707/01/1920Weather Bureau Office07/01/192012/29/1932Weather Bureau Office12/29/193208/07/1942Weather Bureau Office12/02/193804/15/1942Weather Bureau Office/Weather Service Office/Weather Service Forecast Office04/16/194212/06/1975Flight Service Station12/07/197504/30/1991Federal Contract Weather Observation Site04/30/199110/28/1998Automated Surface Observing System 10/28/1998PresentContract02/01/2004PresentSnowfall observationsMethodologyFor 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.

DataSevere 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).11DataFor temperatures to be this cold, there is usually extensive snow cover from northern Arkansas to Canada

DataSevere cold outbreaks at Little Rock are most frequent during the month of January

DataSevere cold outbreaks have occurred in each decade since the 1820s.

There was a peak during the 1910-1919 decade, and a lull 1990-1999Snow CoverThe 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. Snow CoverLow 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.

Maximum Central PressureSeverity/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 exceptionlate December 1876/early January 1877. In this case, snow was likely on the ground for a prolonged period of time before the outbreaks peak on December 30th.

Maximum Central PressureStrong Temperature GradientTemperatures 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. Record PressuresThese outbreaks often see record high sea level pressures at Little Rock. Six of the highest pressures ever recorded have been set during these outbreaks.

Sea Level Pressure in inches Hg and (hectopascals)OutbreakDate(s)30.98 (1049.1)Dec 1989 #2Jan 192423 Dec 198905 Jan 192430.97 (1048.8)Jan 196210 Jan 196230.95 (1048.1)Dec 1989 #223 Dec 198930.94 (1047.8)Dec 1989 #2Jan 196823 Dec 198907 Jan 196830.93 (1047.4)Dec 1989 #2Jan 1905 #223 Dec 198926 Jan 190530.92 (1047.1)Dec 1989 #2Dec 1983 #2Dec 192422 Dec 198909 Feb 198930 Dec 198329 Dec 1924Outbreak SeverityFor the purpose of the study, severity of the outbreak is measured in the number of days with low temperatures at or below 10 F OutbreakDatesNumber of days with lows at or below -12.5C (10F)Coldest TemperatureDate(s) of Coldest TemperatureFebruary 18999th-13th5 days-12F12th January 19629th-13th5 days -4F12thDecember 18221st-6th4 days -9F3rd January 18868th-11th4 days -5F9thDecember 198921st-24th4 days -1F23rdDecember 198323rd-26th4 days 0F25thFebruary 18237th-10th4 days 2F8thJanuary 19888th-11th4 days 7F9th, 11th500 mb Height AnomalyFor 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.

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

RecapSevere 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. 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. THE ENDQuestions?

Brian.D.Smith@noaa.gov