Introduction

Note: The following information is based on a long-term snowfall study conducted by WFO Norman staff. The study was used to write a technical memorandum entitled "Monthly and Geographic Distribution of Heavy Snow Events in Oklahoma, 1951-2001."

After sifting through more than 50 years of daily snowfall reports in Oklahoma, forecasters at WFO Norman have identified 225 heavy snow events that occurred within the Sooner State between January 1951 and April 2001. "Heavy" snow events are defined here as those which produced at least one 24-hour snowfall report of 6 inches or more, or at least two 24-hour snowfall reports of 4 inches or more. (Note that there may have been additional events that were not reported. Heavy snow sometimes falls in very localized areas, in which case the heaviest amount may not fall at a reporting station.)

For the entire state, the average number of events per season is somewhere between 4 and 5. But the range is from none at all (in 1962-63 and 1974-75) to a maximum of 11 (1972-73).

The 225 events were sorted by calendar month to produce a graph of the relative monthly distribution of Oklahoma heavy snow events. Heavy snow has fallen in the state as early as the second week of October (8-9 October 1970) and as late as the first week of May (3 May 1978). The frequency increases gradually from October through December, to a peak in January. But from there the frequency levels off through February and March, before falling sharply in April. An interesting note is that "major" snowstorms, producing maximum storm totals of 16 inches or more, show a marked preference for March. In fact, five of the top ten snowstorms (based on maximum reported storm totals) in Oklahoma since 1951 occurred in March. These storms are listed in the following table.

Each of the 225 events was mapped by plotting all available storm-total snowfall amounts, and highlighting the counties which were affected - wholly or in large part - by snowfall of 4 inches or more. An example can be seen here, which shows the distribution of snowfall during the major winter storm of late December 2000. (Note: This event will be remembered most for the crippling ice storm across much of southern and eastern Oklahoma. But the map only shows snowfall, not ice accumulation.)

Once we identify and map each event, the same way we did for the late-December 2000 event, we can obtain the total number of events in each county, by month and by season, over the entire 51-plus year period. The results will give us a reasonably good idea of things like: How often we can expect heavy snowfall, what month (or months) it is most likely to fall in, and how the frequencies vary across different parts of the state.

The total number of events in each county shows that most Oklahoma heavy snowstorms affect the panhandle and northwest. Counties along the Red River in south central and southeast Oklahoma received the fewest events, but even in these counties there were more than a dozen events over the 51-year period. The numbers can be divided by the period of record (51 years) to obtain an estimate of the annual frequency of heavy snow events in a given county. For example, the numbers for Oklahoma and Tulsa counties are 38 and 44 respectively, which when divided by 51 yield numbers which are less than 1 but greater than 0.5. This translates into an average frequency of less than one event every year, but more than one every two years. Thus, heavy snow events occur in both the Oklahoma City and Tulsa areas roughly once every 1 to 2 years. For the panhandle and most of northwest Oklahoma, the frequencies are higher, averaging something between 1 and 2 heavy snow events per year. This is more than 5 times higher than the frequency in the counties in far southeast Oklahoma, where heavy snow occurs on average about once every 3 to 4 years.

We also looked at the county-by-county frequency of storms which produced maximum snowfall totals of 8 inches or more. The county totals for 8-inch events show a similar distribution to that of 4-inch events, except that the numbers are, of course, much lower (8-inch events are far less frequent). The numbers suggest that 8-inch snowstorms affect the panhandle counties about once every 1 to 2 years on average, but that the Red-River counties average only one such event every 20 years or so.

The monthly frequency of heavy snow varies across different parts of the state. To look at this variation, we divided the state into 17 regions and calculated the average number of events by month in each region. This resulted in 17 monthly-distribution graphs, each similar to the one seen earlier for the entire state. (We left May off of these graphs, since there was only one May event and it was confined to the western panhandle.) Comparison of the graphs shows that January is the peak month across most of southern and east-central Oklahoma, but February is the peak month across most of west-central and north-central Oklahoma. In between, peak frequencies generally are split between January and February across much of southwest, central, and northeast Oklahoma (including the Lawton, Oklahoma City, and Tulsa areas). In far northwest Oklahoma, including the panhandle, the peak month is March. But there are slight indications of two peaks, with an earlier peak suggested in December or January, followed by a relative minimum in January or February. This suggests that heavy snow may be more likely either early or late in the cold season than it is in mid-winter in far northwest Oklahoma.

The double peak also shows up in the distribution of 8-inch events (darker bars in the 17 regional graphs ) across northwest Oklahoma, and also appears in northeast Oklahoma. In fact, March emerges in northeast Oklahoma as the most likely month for 8-inch snowfalls.

Summary:

• The probability of heavy snow in Oklahoma increases gradually through fall and early winter to a peak in January, then remains high through March before dropping sharply in April.


• The panhandle and northwest counties are the most likely areas to receive heavy snow, averaging one to two 4-inch or greater snow events per year, and one 8-inch event every 1 to 2 years.


• Counties along the Red River in southern Oklahoma are least likely to experience heavy snowfall, but still can expect one 4-inch or greater snowfall on average every 3 to 4 years. Snowfalls of 8 inches or more occur on average in these areas only about once every two decades.


• The three largest population centers in Oklahoma - Oklahoma City, Tulsa, and Lawton - all average one 4-inch or greater snowfall event every 1 to 2 years. Eight 8-inch or greater events average roughly one every 5 years in Tulsa, one every 5 to 10 years in Oklahoma City, and one every 10 to 20 years in Lawton.


• The peak month for 4-inch or greater events varies from January across southern and east central Oklahoma, to February in west-central and north-central Oklahoma, to March in the panhandle and far northwest. There may be a secondary early-season peak (December or January) in the far northwest.


• Peak months for 8-inch or greater events are generally the same as those for 4-inch events - except in northeast Oklahoma, where the 8-inch peak shifts to March.


• The most likely place and time to experience heavy snow in Oklahoma: The Panhandle in March.