SR/SSD 98-8

3-1-98

Technical Attachment

Seasonal Thunderstorm Study at Little Rock, Arkansas

Barbara Shea

NWSFO, Little Rock AR

1. Introduction

Thunderstorms impact Arkansas throughout the year and they affect not only the general public but both commercial and general aviation. Accurate forecasts of onset, duration and severity of thunderstorms challenge all forecasters.

The objective of this study is to examine past thunderstorm events and statistically determine their duration, time of occurrence, the lowest ceiling and visibility MOS category observed during the event, peak wind gusts and percent of reported winds greater than 11 kt and 30 kt. Meeting these objectives would provide NWSFO Little Rock forecasters with additional tools to help them refine thunderstorm forecasting accuracy.

2. Data Retrieval, Analysis and Classification

Weather observations taken at Adams Field in Little Rock between January 1, 1991 and December 31, 1996, as recorded on MF 1-10C forms, were used for this study. If thunder was heard or if a thunderstorm with rain was recorded at any time during an hour, it was included in this study as a thunderstorm event.

The events were then separated by hour of occurrence, month, sustained winds or gusts above 11 kt (TAF amendment criteria), above 30 kt (a relatively strong gust), and peak wind events. The total number of hours that each MOS ceiling and visibility category occurred were counted by month. The data were then entered into a Lotus 1-2-3 spreadsheet for further analysis. Graphs of the number of thunderstorms reported each hour were constructed for each month (Fig. 1). Graphs for the number of lowest MOS ceilings and visibilities were also constructed (Figs. 2 and 3). A summary of winds and total observed hours is found in Fig. 4.

MOS CATEGORIES

CEILINGS
(feet)
VISIBILITY
(miles)
1 &le 200 1 &le 1/2
2 200 to 400 2 1/2 to 7/8
3 500 to 900 3 1 to 2-3/4
4 1000 to 3000 4 3 to 5
5 3100 to 6500 5 > 5
6 6600 to 12000

3. Summary and Conclusions

March had only 2% of possible hours with thunderstorms and showed a relatively even distribution across the 24 hour day for time of occurrence (Fig. 1). Average duration was 3.7 hours. Category 4 ceilings were most common while Category 4 visibility was closely followed by Category 3 (Figs. 2 and 3). During 26 periods of thunderstorms, only 38% had winds over 11 kts but a 55 kt gust was reported.

Figure 2: Hours of Each MOS Ceiling Category

April had 4% of possible hours with thunderstorms and an average duration of 4.5 hours per period. Peak thunderstorm frequency occurred during the nocturnal period: from 1700 to 0400 local time (Fig. 1). Category 4 ceilings were most frequent while Category 3 visibilities predominated (Figs 2 and 3). During 39 periods of thunderstorms, 69% had winds over 11 kt , 23% had winds over 30kt and a peak gust report of 52 kt was reported (Fig. 4).

May had 3% of possible thunderstorm hours with an average duration of 3.5 hours, the overall yearly average. Peak occurrence was during the 1800 to 2400 local time period (Fig. 1). Category 4 ceilings and Category 3 visibilities were once again most common (Figs. 2 and 3). During 41 periods of thunderstorms, winds over 11 kt occurred 56% of the time with a peak wind of 40 kt (Fig. 4).

June had 4% of possible thunderstorm hours with an average duration of 3 hours. Peak thunderstorm frequency occurred from 1300 to 2100 local time. Category 5 ceilings and Category 5 visibilities were most frequent. During 64 periods of thunderstorms, the second highest month behind July, winds over 11 kt were observed 66% of the time with a peak gust of 50 kt.

July had the highest thunderstorm occurrence, with 5% of possible hours, and an average duration of 3 hours. Peak thunderstorm frequency occurred from 1200 to 1800 local time. Category 5 ceilings and Category 5 visibilities were again most common. During 68 periods of thunderstorms, the highest monthly report, winds over 11 kt occurred 49% of the time with a peak gust of 45 kt.

August had 3% of possible thunderstorm hours with an average duration of 3.5 hours. Peak thunderstorm frequency occurred from 1100 to 1900. Category 4 and 5 ceilings were almost equally reported, as were Category 3, 4 and 5 visibilities (Figs. 2 and 3). During 40 periods of thunderstorms, winds greater than 11 kt were observed 50% of the time with a peak wind of 58 kt.

September had 3% of possible thunderstorm hours with an average duration of 3.3 hours. Peak thunderstorm frequency occurred from 1200 to 2300 and from 0100 to 0400. Category 5 ceilings were most frequent. Category 5 visibilities were most frequent, but Category 3 and 4 visibilities were also common. During 34 periods of thunderstorms, only 35 % were over 11 kt and the peak gust reported was 28 kt (Fig. 4).

October had 4% of possible thunderstorm hours with an average duration of 4.5 hours. This duration tied April's as the longest. Peak thunderstorm frequency occurred from 2400 to 0800, with a second peak at 2000 to 2100. Category 4 ceilings were most common while Category 3 and 4 visibilities were about equal. During 36 periods of thunderstorms, 60% had winds greater than 11 kt, with a peak wind reported of 52 kt.

November had only 2% of possible thunderstorm hours with an average duration of 3 hours. Peak thunderstorm frequency occurred between 1300 to 0500. Category 4 ceilings were most common, with Category 3 visibility the most frequent. With only 22 periods of thunderstorms, 58% had winds over 11 kt and the peak gust reported was 40 kt.

December, January and February, the winter season, were grouped together due to the small number of reports for these months. Only 0.6% of the possible hours during this season recorded thunderstorm activity. The average duration was 3 hours. Peak activity occurred from 0200 to 0700 local time, with a second peak at 1500 to 1600. Category 4 ceilings were by far the most common ceilings observed, while Category 3 visibilities were most common. During 26 periods of thunderstorms 58% had winds over 11 kt and a maximum wind of 40 kt was reported.

It should be noted that even in the month with the highest frequency of thunderstorms (July), they were reported only 5% of all possible hours (Fig. 1). This fact should be reflected in TAFs. Winds greater than 11 kt occurred on average 56% of the time, while winds over 30 kt occurred 11% of the time overall. During the summer months, thunderstorms tend to occur once the convective temperature has been reached during the afternoon to early evening. During the fall to winter season, thunderstorms occur more often at night. During the spring months, thunderstorms can occur throughout the day and night.

Despite the fact that the months of March through May are considered Little Rock's severe weather season, the two months during which the greatest amount of thunderstorm activity occurred are June and July.

Figure 4. Observed Wiind Speeds and Duration with Thunderstorms

WINDS

>11 KTS % >30 KTS % PEAK
GUSTS
PERIODS
MAR 27 38 1 4 55 26
APR 27 69 9 23 52 39
MAY 23 56 6 15 40 41
JUN 42 66 7 11 50 64
JUL 33 49 10 15 45 68
AUG 21 50 4 10 58 40
SEP 12 35 0 0 28 34
OCT 21 60 5 14 52 36
NOV 13 59 1 5 31 22
DEC TO FEB 15 58 3 12 40 26
OVERALL 222 56 46 11 58 396

Figure 4. Observed Wiind Speeds and Duration with Thunderstorms

SUMMARY

TOTAL
OBSERVED
HOURS
POSSIBLE
HOURS
% NUMBER
OF
PERIODS
AVERAGE
DURATION
HOURS
MAR 95 4464 2 26 3.7
APR 173 4320 4 39 4.5
MAY 149 4464 3 41 3.5
JUN 183 4320 4 64 3.0
JUL 206 4464 5 68 3.0
AUG 141 4464 3 40 3.5
SEP 114 4320 3 34 3.3
OCT 163 4464 4 36 4.5
NOV 66 4320 2 22 3.0
DEC TO FEB 74 13008 0.6 26 3.0
TOTALS 1364 52608 2.6 396 3.5