Tuesday, 14 May 2013

Ramping up to departure!

Hello! It's beginning to feel a lot like....no, not Christmas, but the time of year when we head over the pond to the USA, for our annual storm chasing holiday. There are good and bad points about having to do it the pre-booked way: the obvious bad point is that we're at the whim of the weather, and its fickleness. Those who have the luxury of being to make a last minute choice before heading over can pick and choose as weather systems dictate. Of course, that could mean not going at all, or having to make a fairly crucial decision about whether to go, as you never know whether the next system might be more productive.

The good thing about going at a fixed time it lends itself perfectly to being a proper holiday, and gives us something to look forward to. OK, so we're at the weather's whim, but so what? There's plenty of other stuff to do, and it's (almost, touch wood!) unheard of to go for 2 weeks and not see anything. This year has, though, been extremely quiet on the severe weather side of things in the USA. Of course, fundamentally this is great news for the residents - storm chasers have been climbing the walls, though!

There is a more active weather pattern setting up later this week and into the weekend as a trough moves in from the Pacific, and sharpens as it enters the Plains. The image below depicts 500 hPa heights and winds for 00Z Sunday (1900CDT Saturday), from the excellent http://weather.cod.edu  site


Notice the powerful jet nosing into California, and the trough to its east. This jet should start to collapse into a more pronounced trough through Sunday and into early next week, as it moves eastwards, eventually becoming a closed low towards eastern states, although the eventual outcome is not clear at the moment.

Lower down, the mass response will be a southerly 'return flow' bringing rich Gulf moisture into the Plains. The stage is set for several days of severe weather, especially from the central Plains into the mid-Missouri valley, and then perhaps points further east.

Those chasers already there or about to head out will no doubt be looking at this with interest. For those of us going next week, this is a situation to whet the appetite but also one which will likely make us feel we just missed!

The longer term outlook appears to favour, at least for a time, the development of a ridge across south-western and then southern states through next week. This is likely to be detrimental to severe weather on a regional scale. There appears to be a trend within ensemble products of the ridge flattening later in the week, with a faster flow in the upper atmosphere developing across central and northern states. With the indications suggesting that this weekend's system should not send a cold front crashing right through the Gulf of Mexico, moisture return may be able to commence from mid-week, especially across western parts of the Plains. This all depends, crucially, on where surface high pressure sets up, and whether or not ridging close to the Gulf coast prevents this flow.

Thus, severe storms look likely this weekend across the Plains and Midwest, and then probably further east and south on Monday, before quieter weather develops for a while into mid-week - a chance of storms may then return later in the week, but confidence obviously becomes very low by that stage.

For those of us who make it a holiday, there's always a Plan B, and C, and quite usually a D too! Keep tuned to this blog to see which plan we follow!

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