In just a few days time we'll be heading across to the USA for our 2013 storm chasing trip. Sadly, the last 48 hours has brought some powerful and deadly tornadoes to portions of the USA, namely north Texas. The region to the west of the Dallas-Forth Worth metroplex was hardest hit. It was a day where severe storms were expected but the coverage was likely to be fairly low, and this turned out to be the case. For those of you who read this and wonder what I'm on about when I speak of the Storm Prediction Centre's risk levels of Slight, Moderate, and High, those risk levels are to give an indication of the overall coverage of severe storms. Thus, a slight risk does not mean that storms will only be slightly severe, etc, it just means that they are not expected to be numerous and/or especially widespread.
The environment was one where rich tropical moisture had spread in from the Gulf of Mexico and a disturbance in the upper air was moving in from the south-west. As evening approached, winds in the lower atmosphere became much more favourable for low-level rotation, which aided in tornado development. One large tornado took a sharp left turn late in its life, which seems to be something bigger/more powerful tornadoes are want to do. This is something chasers should bear in mind.
The risk from severe storms increases this weekend as a large scale upper trough moves in from the west and Gulf moisture moves northwards. However, the strong SW flow aloft will advect some very warm air eastwards atop the Gulf moisture, providing a strong capping inversion. Across the southern Plains on Saturday this could prevent storms from forming but further north, lift from the approaching system should cool the air aloft enough to erode the cap. Any storms which form will do so in a region of strong vertical wind shear, and will likely be severe - very large hail the main threat although with deep Gulf moisture in place, tornadoes are possible too, perhaps more likely in the evening as the cloud bases drop and the low-level jet cranks. Sunday and Monday also look like bringing severe weather - hopefully residents will be aware as the season has been very quiet thus far and a false sense of security may have crept in.
Several UK chasers are already there, along with many others from around the world, including Australia. This is in addition, of course, to the US chasers. Watching the developments this weekend from here in the UK will just add to my desire to get out there! Next week should see the trough easing eastwards into the central USA as a closed low, before it opens again and drifts further east. Beyond that, a new trough moves into the west but models can't then agree about downstream developments. It does look like a ridge will try to form in the central USA at least for a time, but some models, e.g. the GFS, have trended to a flatter ridge. I suspect the closing days of May may well see a general westerly flow established.
Severe weather should be less widespread into the middle of next week but still possible, especially across the western Plains as an easterly upslope develops beneath a modest westerly. This could yield some picturesque storms over open country.
Of course, this is all an age away meteorologically speaking - however, it doesn't look like, at this stage, a strong cold front will barrel into the Gulf of Mexico, so moisture will be in place across the Plains.