Tornadoes require clashes of air masses to form generally to include cold, dry air in the upper atmosphere and very warm and humid air in the lower atmosphere. For much of the spring, the missing ingredient for this scenario in the US was the influx of very warm and humid air from the Gulf of Mexico thanks in large part to the persistent cold pattern in the central and eastern US that acted to inhibit Gulf of Mexico air from advancing northward. The cold air masses, however, began to retreat a couple of weeks ago and subsequently, dew points, which are a true measure of moisture content in the air, have consistently climbed in the central and eastern US and this increased dramatically the prospects for severe weather.
Indeed, the combination of very warm and humid low-level air with a vigorous cold upper-level low pressure trough on Monday helped to spawn the powerful tornado that struck portions of Oklahoma with a major impact on Moore, a heavily populated (50,000 residents) suburb of Oklahoma City some twenty miles to its south. The preliminary estimate is that the one-to-two mile wide tornado that struck the town of Moore reached EF-4 status on the “Enhanced Fugita” scale (5 being the most intense) with winds of nearly 200 mph, but these estimates will be re-evaluated over the next several days/weeks as tornado experts on the ground closely assess the damage and I think it may very well end up being classified as an EF-5. The tornado lasted for about 40 minutes causing destruction over a 20 mile swath.
The overall weather pattern that helped to produce the severe weather yesterday (i.e., warm, humid low level air, strong cold upper-level low with a powerful jet streak, strong surface cold front) will continue for the next couple of days with the severe weather threat zone slowly shifting eastward. There is a threat today for severe weather from the southern Plains to the Upper Midwest, and then likely in the Ohio Valley on Wednesday, and then perhaps in the Mid-Atlantic and Northeast US late Thursday as a strong cold front approaches the east coast. Strong-to-severe thunderstorms are possible later Thursday in the region from the Carolinas to New England as all the atmospheric dynamics shifts towards the east coast. This afternoon’s video includes some time-lapse video of yesterday’s tornado and a detailed discussion as to why the previously quiet tornado season has ended.