Good news for storm chasers this morning as Mother Nature starts to show a few of her cards for this tornado season. As I explained in my post a couple days ago there is reason for optimism with the stage getting set in early April for tornado season to ramp up. Reason being the gulf looks to be primed and the upper air pattern starts transitioning into something closer to zonal flow (as opposed to the east coast wave crap we’ve been dealing with). That was a couple days ago. Zonal flow doesn’t get you outbreaks, but it’s kind of the prep work if you will before you transition into an active pattern. The next step in transitioning into an active pattern would be high amplitude waves tapping into the gulf and that’s what the models are showing as we get into April. I haven’t spent much time forecasting this morning, but I did take a good sampling of the various model data in the extended range to get a feel for what’s coming. We’ll start in the mid range. Both the GFS and ECMWF are showing a trough digging south the middle of next week (see top picture for this coming Thursday). With good quality moisture over the NW gulf this system should be able to draw in at least decent quality moisture despite not being a real high amplitude wave. I think Thursday of next week may end up being a decent chase day, but I’ll get into that later today or tomorrow in my next forecast post. Regardless, what has me more excited is what comes in behind that. I looked over last nights run of the GFS and took a look at the GEFS (ECMWF only goes out 7 days) all of which are strongly hinting at a wave train pattern setting up for the middle of April. Thursday’s trough will kind of be the primer before some higher amplitude waves come in behind it and with it a serious ramp up in the 2015 tornado season. That initial trough isn’t forecast to dive too deep over the east coast, which should keep the gulf loaded (or at least not wipe it clean) for the wave train pattern we could transition into behind it. I don’t want to get too cute with the forecast since this is all still quite a ways out, but the next trough in line that will hopefully be a higher amplitude wave looks to come in about 8 days from now around April 6th. This is the first chance we may have at a major outbreak for the 2015 season. Below is the 500mb GFS output for 216hrs out (next Sunday) as the trough deepens and prepares to eject into the plains.
Again I don’t want to read too much into specifics since this is still quite a ways out, but one thing I do pay attention to when doing extended forecasting is the tilt of the troughs. You can see the trough above has kind of a slightly positive going neutral tilt. Most big tornado outbreaks occur with troughs that have neutral or slight negative tilt. Indications are that this high amplitude trough should have at least a somewhat favorable tilt as it ejects into the plains. So a window to keep a close eye on is going to be the beginning of the week of April 6th. My best guess is April 6-8th right now. This looks to be a slower moving system with both the polar and subtropical jets pushing into the central plains. My biggest concern right now is the tilt of the trough as it comes into the plains (positive tilts put a serious damper on tornado potential) and whether or not moisture return is going to be sufficient in the wake of the trough that will come through the middle of this coming work week. Either one of those two things could seriously inhibit tornado potential with this system. It’s something to watch though and the first setup of 2015 that has the potential to be a large scale tornado outbreak across the plains.
Beyond that system around April 6-8th, the active pattern looks to continue as we get into our wave train pattern. That’s a hell of a long ways out there so take it with a big grain of salt, but below is a couple images of GEFS ensemble members and the mean along with a global view of the long wave pattern in the northern hemisphere.
Bottom line is I wouldn’t be booking any chase vacations yet, but I think tornado season is about to really get going in April. With tornadoes the devil is always in the details so you can’t tell all that much by looking at synoptic scale details in the extended range, but all the indicators are starting to point towards tornado season getting active in April. I’m going to continue looking at the extended forecast and I’ll also start to get into specifics for this Thursday’s setup so check back for details.