A Story Of Sleet and Drought

Oklahoma can be a state of extremes, particularly in the winter, with springlike heat followed just hours later by bitter cold, or torrential, flooding rains followed by literally weeks of sunshine. This month we'll be doing a double-header that pays tribute to these extremes, chronicling two opposite ends of the weather spectrum that we've seen here in Oklahoma over the last month. On one end, a winter storm that left several inches of sleet (or, if you prefer, ice pellets) on the ground. On the other, a developing drought across much of the state.

Let's begin with that winter storm, which moved through central Oklahoma during Jan. 26th and 27th. Though winter storms are not uncommon in this region (as evidenced by last winter's major ice storm ), seeing such a large accumulation almost entirely of sleet is rather rare. To receive long-lasting, accumulating sleet requires atmospheric conditions to remain within a narrow range for a relatively long time. Because atmospheric conditions often change rapidly during the passage of a strong storm system, it is more common to see sleet mixing with other precipitation during a transition from rain (or freezing rain) to snow, or vice-versa.

For sleet to fall, there must be a layer of above-freezing air aloft, but sub-freezing air near the surface. Precipitation which begins as snow melts into rain as it passes through the warm layer. The raindrops then re-freeze as they pass through the cold near-surface layer, forming the small, icy balls characteristic of sleet. Also, the cold air near the surface must be relatively deep (on the order of a kilometer or more), or the raindrops will not have time to re-freeze fully, and freezing rain will result instead of sleet. Relatively deep arctic air was in place over most of Oklahoma at the time of the sleetstorm, with warm air being drawn over it – perfect conditions for sleet.

Though up to three inches of sleet accumulation was observed in regions near and to the south and east of Norman, just 0.43 inches of liquid equivalent was measured in Oklahoma City (OKC). Liquid equivalent is simply the amount of water you get when you melt down frozen precipitation. The liquid equivalent can vary widely – for snow, it can be as little as 1 inch for every 20 inches of snow, while for freezing rain, the ratio is near 1 to 1. This 0.43 inches of liquid is the only precipitation that has been recorded in OKC this year, leading us nicely to our next point – the developing drought.

Looking back, 2007 was the wettest year on record for much of central Oklahoma. OKC, for example, received 56.95 inches of rain during the year – a full 20 inches above normal! Early 2008 was more of the same; by mid-August, OKC was running nearly 10 inches above normal for the year. At this point, the pendulum abruptly swung the other way; in the 6 months since then, only a total of 4 inches of rain have been observed in OKC.

Because of the recent dearth of precipitation, much of central and southern Oklahoma has been thrown from the frying pan into the fire – or in this case, from flood to drought. OKC, which we've been using as an example, is now classified as “D0”, or “abnormally dry”, in the National Drought Mitigation Center's Jan. 29th analysis. Farther to the south, “D1 (Moderate drought)” and “D2 (Severe Drought”) are seen as you approach the red river, as shown in the image above. Under the NDMC system, drought is rated in a manner similar to the F-scale for tornadoes, increasing from D0, the least severe, up to D4 (Exceptional drought), the most severe.

Looking ahead, the future remains uncertain. There is hope to break the drought as the spring storm season approaches, as spring is climatologically the wettest season in Oklahoma. However, the Climate Prediction Center's seasonal outlooks paint a hazy picture, calling for “equal chances” of below normal, normal, or above normal precipitation. One thing is for certain – with the growing season fast approaching, and wildfire danger elevated by the dry conditions, it won't just be thunderstorm enthusiasts who have their eyes to the skies looking for signs of rain this spring.





Story is © Nate Snook, 2009