Extended Forecast

050617 Extended ForecastWell, it’s been a while since I posted. I usually try to start posting to my blog by March, but it’s been a slow tornado season for the plains so far and I’ve been real busy so here we are into May and I’m just getting my first post of the season up. I’ve chased a handful of times already this year. Most of them were just opportunities to test out some new equipment, but I’ve gotten on a couple tornado warned storms so it hasn’t been totally worthless. It is starting to look like we may be transitioning into an active weather pattern next week though so it’s time to dust off the blog and starting posting regularly again.

We are currently in what’s known as an Omega block pattern, which is where a somewhat stationary high pressure resides over the central US with west and east coast troughs. Upper air charts in this pattern look like the greek letter Omega, hence the term omega block. We will begin to transition out of this pattern early next week as the west coast trough transitions to a cutoff low that will slowly meander into the plains around Tuesday/Wednesday of next week. The gulf has been scoured of quality moisture by the east coast trough, but by tomorrow morning favorable trajectories will start to advect better quality moisture back into the western gulf. This moisture advection will continue through the first half of the week, with dewpoints in the 60’s expected back in the central plains by Tuesday, which is the first day of interest.

Right now I think moisture quality is a big question mark for Tuesday as the closed low first starts to nose into the western plains. The GFS tends to be a little fast with moving troughs into the plains, so between that and the fact that there won’t be much time for good quality moisture to tighten up again the dryline. Given those concerns, for now we’ll skip over Tuesday, but it is something to keep an eye on and there may be some potential for severe storms over the west central plains.

By Wednesday good quality moisture is expected along a tightening dryline. Mid level winds are forecast to veer a bit more as the closed low starts to transition to more of an open wave style trough. Right now the threat area looks to be located along a dryline that will run from a surface low near SE Colorado down towards the far SW corner of Oklahoma. This setup doesn’t look like anything major, but there may be a lower end tornado threat. My main concerns at this point with regards to tornado potential is directional shear. With closed lows, the low pressure tends to get stacked through the atmosphere and you get little turning with height/poor directional shear due to mid/upper winds backing. The mid/upper level winds should be tending to veer more as the closed low moves into the plains and transitions to more of an open wave as it phases back into the polar front jet over the eastern part of the country, but directional shear still doesn’t look real good Wednesday with only modest turning in the 0-6km layer. I’ll get into a little more detail in the coming days, but I think we are only looking at a lower end tornado threat Wednesday. It kind of seems like a big deal though given the slow stretch we’re coming out of.

By Thursday it looks like we could have high quality moisture back into the plains ahead of a dryline running north-south across central Kansas/Oklahoma, but the closed low will be tying back in with the east coast trough/polar front jet and it is forecast to take on a positive tilt as it does. This will veer low level winds and be detrimental to any tornado potential on Thursday. It’s still a long ways out so I don’t think it’s worth discussing mesoscale details too much. It is worth mentioning that slight differences in timing or the tilt of the trough can easily take the tornado potential up for Thursday and the models have not been at all reliable with details this far out, so Thursday is definitely worth keeping an eye on for chase potential. It’s just not the type of setup you’d expect a big tornado day out of though.

After Thursday we’ll get a bit of a break in the plains as the closed low/trough pushes off to the east and high pressure takes over the plains again. The pattern is forecasting to change pretty quickly though as southwest flow could return to the plains as early as Tuesday May 16th. Long ways out still, but the models have been showing a higher amplitude west coast trough taking shape and ejecting into the plains the second half of the week. Climatology would back that up, given the fact that we are coming into the historical peak of tornado season in the plains during the second half of May, so I’m optimistic.

Well that’s it for today. I’ll be keeping a close eye on the models and updating regularly so check back if you’re interested.

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