Monthly and Geographic Distribution of Heavy Snow Events in Oklahoma, 1951-2001
Mike Branick National Weather Service Norman OK
1. Introduction
Heavy snowfall events are rare in Oklahoma relative to other parts of the
United States, but they occur with sufficient frequency to create a significant
forecasting challenge. The purpose of this study is to quantify the frequency of
heavy snow events in Oklahoma, in terms of monthly and seasonal trends as well
as geographic distribution within the state.
2. Methodology
Monthly climatological data summaries, compiled by the National Climatic Data
Center, include daily 24-h snowfall totals from available first-order and
cooperative observer stations in Oklahoma. Summaries typically include daily
reports from 100 or more stations. The actual number varies from month to month,
due to missing or incomplete data, relocation of observing sites, and/or changes
in active status of some sites over the years.
Heavy snow events, for purposes of this study, are defined as those which
produce at least one observed 24-h total of six inches or more in the state, or
at least two observed 24-h totals of four inches or more. Once either of these
criteria were met, snowfall totals were examined for every available observing
site from that date and the dates immediately preceding and following it. Storm
totals were determined at each site by adding all measurable snowfall totals on
consecutive days including and surrounding the date identified. Each event was
identified using the first and last calendar dates in which measurable snowfall
was reported, e.g., 5-7 January 1988. (Note that some events did not occur
entirely within a single calendar month, e.g., 31-December 2000-1 January 2001.
Such events were assigned to either a] the month with the most calendar days
affected, or b] the month in which they began. So the above example was a
December event, while one occurring over, say, 31 January - 2 February would be
a February event. Surprisingly, there were very few such "end-of month"
events.)
Geographic distribution of snowfall for each event was determined by
identifying the counties which were wholly or largely affected by storm totals
of four inches or more and eight inches or more. A certain amount of
subjectivity was necessary, due to variations in the number and distribution of
available reports. For each event, there were enough data points to develop a
reasonably accurate subjective analysis of geographic snowfall distribution for
that event. But there were very few data points for which data were available
consistently throughout the entire 50-year period of study. Many sites either
closed, opened, relocated, or had missing reports at some point during the
period. As such, it was not possible to develop long-term objective numbers of
events for each specific observing site.
All available storm totals were plotted and subjectively analyzed for each
event, providing a map of the geographic distribution of snowfall for that
event. Counties affected by the event were then identified based on the
analysis. An example is shown in Fig. 1. Note that in many cases there were
counties which did not contain any data points, so it became necessary to
evaluate the analyses to determine whether those data-void counties were
affected by the event. (The process of determining counties affected is
qualitatively similar to determining which counties are to be included in a
severe thunderstorm or tornado watch. The process arguably is subjective in many
cases, but should yield meaningful data given a sample size of over 200 events.)
Seasonal and monthly frequencies of events for each county were then determined
by simply adding up the total number of events affecting the given county.
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Figure 1. Storm-total snowfall (inches), 25-27 December 2000. Contours are drawn every two inches. Counties which received totals over four (eight) inches over all or a large part of their area are shaded in light (dark) gray.
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3. Monthly and seasonal distribution
A total of 225 heavy snow events were identified between January 1951 and
April 2001, a period of just over 51 years. This results in an average of
between four and five events per season. There have been only two winter seasons
without a heavy snow event in Oklahoma: 1962-63 and 1973-74. In other words,
heavy snow has fallen somewhere in Oklahoma in roughly 95 percent of all winter
seasons. Of the 225 events which produced storm totals of four inches or more,
189 produced these amounts somewhere in the main body of Oklahoma; heavy
snowfall in the remaining 36 events was confined to the three counties of the
Oklahoma panhandle.
Heavy snow events have occurred as early as October and as late as May. The
earliest event occurred on 8-9 October 1970, while the latest occurred on 3 May
1978. (Both were limited to the far western panhandle.) The overall frequency of
events increases gradually to a peak in January, then remains relatively
constant through February and March, before dropping much more abruptly in April
(Fig. 2).
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Figure 2. Total number of heavy snowfall events in Oklahoma by month.
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Of the 225 events, 102 produced maximum totals of 8 inches or more (two per
year), 48 produced a foot or more somewhere (nearly one per year), and 14
produced a maximum storm total of 16 inches or more (about once every three
years). The monthly distribution of eight- and 12-inch events (Fig. 2) shows a
relatively constant frequency throughout the peak months of December through
March. But the distribution of 16-inch events shows an interesting late-season
peak, with a maximum in March. This apparent preponderance of "mega" snowstorms
late in the winter season is further demonstrated in the list of ten heaviest
snowstorms in Table 1 (as ranked by maximum storm total): Five of the ten
heaviest events occurred in March.
Table 1. Heaviest snowfall events in Oklahoma, 1951-2001. Rankings are based
on maximum storm totals (Max, inches). Location of the maximum reported storm
total for each event is given.
Rank | Date(s) | Max | Location |
1. | 21-22 February 1971 | 36 | Buffalo |
2. | 24-25 Nov 1992 | 22 | Laverne |
3. | 16 March 1970 | 20 | Bartlesville |
| 16-17 January 2001 | 20 | Kenton |
5. | 8-9 March 1994 | 19 | Stillwater |
| 12-14 March 1999 | 19 | Medford |
7. | 4-5 March 1989 | 18 | Kansas |
| 18-19 January 1990 | 18 | Goodwell |
| 22-24 December 1997 | 18 | Laverne |
| 18-19 March 1999 | 18 | Kenton |
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4. Geographic distribution
The numbers in Fig. 3 were obtained for each county by tallying the number of
times the given county was "hit" by heavy snowfall during the period. They thus
represent the number of heavy snow events which produced four-inch or greater
storm totals over at least a part of the county. With the period of study being
just over 51 years, one can divide the numbers shown by 51 to obtain an
approximate annual (or seasonal) frequency of four-inch or greater events. These
frequencies are depicted by contours representing "turnaround times," i.e., the
average time between four-inch snowfall events in the given area.
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Figure 3. Number of four-inch or greater snow events by county, 1951-2001. Contours are frequencies, given as average turnaround times for a single event.
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(Note: It is reasonable to expect that larger counties would have more events
than smaller ones, simply by virtue of the fact that they are larger "targets."
However, the data do not show a distinct size bias in most cases, probably
because the counties of Oklahoma are, for the most part, similar in size. One
exception was made for Osage county in northeast Oklahoma, which is noticeably
larger than surrounding counties. In order to minimize any potential size bias,
Osage county was split into north and south sections for this study. The
dividing line is an eastward extension of the border between Kay and Noble
counties, the two counties immediately west of Osage county.)
Figure 3 shows a marked preference for heavy snow in northwest Oklahoma. Much
of the northwest one-fourth of the state experiences heavy snow more than once
per year, on average. In the western and central panhandle, the average is
closer to two per year. These frequencies are more than five times greater than
those along the Red River counties in far south central and southeast Oklahoma,
where heavy snow falls on average only about once every three to four years. For
much of the rest of Oklahoma, including the larger population centers around
Lawton, Oklahoma City, and Tulsa, heavy snow events occur, on average, once
every one to two years.
Figure 4, obtained in the same manner as Fig. 3, shows the frequency of
eight-inch or greater snowfall events. Geographic distribution generally is
similar, with greatest frequencies in the panhandle and northwest. Snow events
of eight inches or more occur on average more than once every other year in the
panhandle. Lowest frequencies extend along the entire Red River region, where
the data suggest eight-inch snowfall events occur on average only once every 20
years or so. Most of the counties in west central, central, and northeast
Oklahoma average around 5 to 10 years between eight-inch snowfall events.
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Figure 4. As in Fig. 3, except for eight-inch or greater events.
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In order to look at geographic variations by month, the numbers in Figs. 3
and 4 were broken down by month. (Actually, the monthly totals were derived
first; Figs. 3 and 4 were obtained later by adding up the respective monthly
totals.) The monthly results constituted another eight pairs of figures similar
to Figs. 3-4. In the interest of condensing the presentation of all these data
into a more manageable form, it was decided to divide the state into a number of
regions, and calculate monthly numbers for each region by averaging the monthly
totals from each of the counties in that region. These numbers then could be
plotted in the form of bar graphs for each region, and displayed on a single
figure which would show frequency variations by month and by region. The result
is shown in Fig. 5.
Subdivision of the state began with the nine standard geographic divisions
used by the Oklahoma Climatological Survey (OCS) in their data publications. A
further subdivision was performed by splitting eight of the divisions in half
again (all but the southeast division). The final result is 17 subdivisions as
shown in Fig. 5, with solid lines outlining the OCS regions and dashed lines
indicating the subdivisions.
Looking at the distribution of four-inch or greater events (light gray bars
on the graphs in Fig. 5), it is clear that January is the peak month across
southern and east central Oklahoma. But the peak month shifts to February from
west central to north central Oklahoma. In between, much of southwest, central,
and northeast Oklahoma is equally split between January and February. In the
panhandle and far northwest, the peak month is March. In fact, the highest
frequencies for any month and any region are in the panhandle region in March.
But there is evidence of a bi-modal distribution in the far northwest, which
includes a secondary earlier peak in December or January.
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Figure 5. Monthly distribution of four-inch (light) and eight-inch (dark) snow events by region.
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Evidence for a dual peak in frequencies in far northwest Oklahoma is more
evident in the distribution of eight-inch events (darker bars in Fig. 5). A
relative minimum in January or February is present in the three
northwestern-most regions, and in fact extends eastward into northeastern
Oklahoma. In general, frequency distributions for eight-inch events are
consistent with those for four-inch events. But the data show that in
northeastern Oklahoma, the peak time for eight-inch snow events shifts from
January/February to March. These findings are consistent with the fact that a
larger number of "big" snow events occur later in the season (section 3 and
Table 1).
5. Summary
Oklahoma averages between four and five heavy snow events per season. They
are most likely to occur in northwest Oklahoma, where most counties can expect
one to two snowfalls of four inches or more per season. Far south-central and
southeast Oklahoma are the least likely areas to receive heavy snow, but even
these areas can expect a four-inch or greater snowfall about once every three to
four years. On a seasonal basis, heavy snow can occur in the state as early as
October or as late as early May (at least in the panhandle).
For the state as a whole, the frequency of heavy snow events increases
gradually through fall and early winter to a peak in January, then levels off
through February and March before dropping sharply in April. But the peak
frequency varies by region - January in southern and east-central Oklahoma, to
February in west central and north-central Oklahoma, to March in the panhandle.
The most likely time and place for a heavy snow event in the state is in the
panhandle in March. The panhandle area may experience a dual peak in heavy snow
frequency, with the maximum in March preceded by a weaker maximum in December or
January and a relative minimum in January or February.
The monthly frequency of "big" snow events, which produce eight inches or
more, generally is similar to that of four-inch events - except in northeast
Oklahoma, where the peak frequency shifts to March. Data on even larger
"mega-snow" events, producing storm totals of 16 inches or more, suggest that
March is the most likely month of occurrence in Oklahoma.
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