This storm season has been underwhelming, to say the least. Only 153 tornadoes have been reported this year, which is a little more than half of the normal value (Image 1).
What gives? Well, during the month of March, multiple cold frontal passages removed the rich, “high octane” moisture of the tropics (necessary for explosive supercell thunderstorm development) far from the Plains. Accordingly, storm chasers lamented, with some openly wondering whether or not to stick a fork into 2013. Thus, when a big upper trough appeared in the West in the long-range models (Image 2), chasers were thrown into a frenzy of spring-time excitement.
While the details about this system remain muddy – the big picture looks quite tantalizing. In particular, the global models suggest the development of a broad, strong upper-level low over the western United States that will induce strong, southerly low-level flow and associated moisture advection over the central U.S. Unlike previous events this year, the moisture quality looks quite decent – courtesy of a nice Caribbean fetch (dewpoints at or above 60 F, perhaps over 65 F at some locations, depending on the model; Image 3).
Given the mean westerly flow this spring – and drought conditions – the elevated mixed-layer also looks primed to create strong instability; indeed, the 12Z 3 April GFS forecast shows over 3500 j/kg in western Oklahoma/western north Texas by Monday evening (April 8th; Image 4).
Nevertheless, the severity of the event remains uncertain. In particular, the timing of upper-tropospheric waves could be the difference between a decent chase day – and a major tornado outbreak. As usual, exiting waves would lead to less-than-ideal hodographs, while an entering wave would create the familiar “sickle-shaped” hodographs present during major outbreaks. In all likelihood, though, the timing of the waves won’t be resolved until the event is within 24 hours.
Also of some concern is the quality of the moisture return. While the GFS continues to insist on high-grade moisture (65 F isodrosotherm near the dryline by game time), the Euro is significantly less bullish (only lower 60s F Tds). Even if the Euro is correct, tornadoes would still be in the cards, though the event might be less significant (unless the wind fields are stronger than forecast). It will certainly be interesting to see which model is closer to reality – although we may not get a feel for this until the day before the potential event. We’ll see!