Record highs! Gulf moisture returning! Talk of ... gasp ... severe weather?!?! Winter is over!
As Lee Corso would say: Not so fast, my friend. The north winds today are a stiff reminder it's still winter.
It pains me to say it, but as of yesterday, we still have one solid month of the calendar winter season. March 20th is the date to circle on the calendar. Then, we can start using that spring word a little more frequently. However, don't expect the switch to instantly flip from cold to warm. Some of Oklahoma's coldest temperatures and worst snowstorms occur through the end of March.
It's fair to say we're firmly into spring when we stop seeing 32 degrees in our 7-day forecast, right? If that's the case, un-circle March 20th because in Oklahoma, the date of the average last freeze is March 30th. Oh, and that's the average last freeze from 1891 to 2005. Just a few years. How about the best case scenario? Oklahoma's earliest last freeze was on February 21, 1905. That'd be today's date. And I can say with some confidence that we're not finished with the cold air just yet. How about the worst case scenario? You're not going to like it. Oklahoma latest last freeze was on May 3, 1954. That's close to a 2.5 month period for the last freeze to occur. Sure, we may not have 32F as frequently as January or February, but that doesn't mean we won't have it at all. Since 2000, Oklahoma has never had a last freeze before March 20 and half of them have been in April.
Snowstorms, on the other hand, really seem to be ugly when they occur in Oklahoma during March. Heck, we just went through two nasty snowstorms and amounts from both storms combined still don't equal Oklahoma City's record for monthly snowfall. February 2011 (so far): 18". March 1924: 20.7".
So ... why? Here in Oklahoma, March is a transitional month, seasonally speaking. There is still lots of very cold, Arctic air in Canada and, as we've seen this past week and weekend, lots of warm, subtropical air to our south. During winter, the difference between those two air masses is more extreme than during late Spring and Summer. You can thank the uneven heating of our tilted earth. Oklahoma is in just the right spot for helpings of both air masses. When that difference in temperature (or "gradient") is especially strong, the polar jet stream develops and transports the arctic air into Oklahoma. That's why cold fronts are much sharper in winter than late spring or summer. When the northern hemisphere is tilted toward the sun during summer, the colder air stays well to our north and cold fronts bring cooler, rather than colder, air.
March is a perfect time to see both winter and spring temperatures and weather systems. So while we've had the privilege of enjoying some five-star, spring-like weather in Oklahoma over the past few days, the arctic air stays ready to strike at a moment's notice. It's dangerous to call winter "over" anytime before April in Oklahoma or, in some cases, May because as soon as everyone settles into a warmer weather mindset, mother nature surprises us.
Will Rogers wasn't a meteorologist, but he understood weather can always quickly change in Oklahoma. "If you don't like the weather in Oklahoma, wait a minute and it'll change."