Wintertime Severe Weather!

Those of you not inclined to care about weather much may get bored with some posts here, or may not fully understand everything, but now that we are heading closer to Springtime, that means thunderstorm season is drawing closer, which personally is my favorite time of the year, thus I may focus on this more in the coming months. But it is also a time when people have to really keep a close eye on things, given that Spring is prime season for severe thunderstorms and tornadoes. These phenomena can occur at any time of year though, and late today / tonight may be one example of an "out of season" event, though we've seen more outbreaks in the winter season in recent years.


Today's area of concern is in central / eastern Oklahoma, northeast Texas, and over into Arkansas primarily. It's warm for this time of the year, and with a potent storm on the way this evening, may spell trouble. Predicting severe storms can be very tricky, as it is more a matter of how very small scale features interact with one another. But there are some large scale indicators that can at least offer clues as to more likely target areas. The first thing you need is to have a warm, moist air mass in place. Dewpoints ideally need to be in the 60s, which we should have in much of the affected area later this evening (take my word for it since I'm not posting a map of this here). Without this, there is not sufficient "juice" for a large outbreak typically. Warm, moist air is more "unstable", meaning it rises easier, and it is this rising motion the fules thunderstorm development. Next, consider this evening's modeled surface map of the southern Plains (click to enlarge):















The black lines indicate sea level pressure. The colors are temeprature. Winds are shown by the small wind barbs. Low pressure is seen in western Oklahoma here, with a value around 997 millibars (mb). A "rule of thumb" (in quotes because no "rule" ever works all the time in this business) is that, once you have established enough moisture (high dewpoints) in place, look for pressures of 1004 mb or lower. As you can see by this map, that covers most of Texas and all of Oklahoma. Next let's look at what's known as the low level jet. This is at roughly 5000 ft above sea level:














The colors here represent wind speed in knots, and again the wind barbs are shown to show the direction of the wind. Note the strongest winds (which is the low level jet) found in eastern Oklahoma and Texas, with speeds over 50 knots, which is well over what is necessary to generate severe weather. Also note the winds are out of the south or even southwest, whereas the surface winds in the first map are out of the southeast primarily in this region. This turning of the winds with height (wind shear) is a very key component especially in the formation of tornadic storms. Also keep in mind that if you are looking in the direction that the low level jet is blowing, the bulk of tornadic activity would usually be to your left, placing central / eastern Oklahoma and eastern Texas most at risk. Finally, to keep things sort of simply, here is the same map but for the upper level jet (now up 30,000+ feet above sea level):














There is a very strong jet here from central Texas northeastward into Arkansas and Missouri. So in looking at the last two maps, there is solid "coupling" (or intersecting) of the upper level jet and low level jet in southeastern Oklahoma and northeast Texas. Another "rule of thumb" is that when you have this coupling of the jets, areas near the region of intersection and to the left of the general direction the jets are blowing (again assuming you are facing that direction) is the primary point of concern, so given that sufficient moist air should be in place, tells me that the primary area to watch in this case is far northeast Texas and the eastern half of Oklahoma.

So we have a lot of large scale factors coming together for some rough weather this evening into tonight, with the most likely target for severe, possibly tornadic storms being in eastern Oklahoma and far northeast Texas, spreading into Arkansas late (the maps are static pictures, and of course all of this moves east and the night wears on). Keep in mind though I mentioned how important the small scale features really are, which I haven't gone into. This is more just to analyze the big picture and say, okay, here is the general region to watch for problems. We'll see how that goes. Hopefully everyone in the area is prepared just in case.

I may get more technical with some of the weather stuff in the future if enough weather saavy people check in, and heck, might be able to get back to true winter-like weather and talk about snow chances for the eastern U.S. soon!