Sunday, 18 March 2012

Great Plains season starts up - thoughts for March 18 2012

Many chasers across the Great Plains region of the USA will be getting rather excited today as severe weather is expected. To keep my 'eye in' I like to chase vicariously by 'armchair chasing' here in the UK. I use all the tools as if I were there, and try to pick out possible chase targets, etc.

The latest surface chart (below) shows a moist Gulf airmass surging north-westwards through Texas and into Oklahoma, whilst the dry-line is out of the west across New Mexico. Surface low pressure is also developing in the lee of the Rockies, in response to south-westerly flow aloft, and an approaching large upper trough.

The trough can be seen on enhanced water vapour imagery very clearly (below). The red lines show approximate streamlines in the upper flow - a large area of divergence (= lift through the low-mid troposphere = cooling) is apparent across New Mexico. A Pacific cold front is moving through Arizona.

You can also see just to the left of the right-hand red arrow line (in eastern New Mexico) and narrow dark 'stripe'. This is a short-wave trough rotating around the larger upper trough, and has caused showers to recently start to form across parts of Oklahoma and Texas. This should move through in the next 2-4 hours.

During the next 12 hours, the upper trough will continue to move eastwards towards the southern Plains. The strengthening upper flow combined with lifting will cause the lee-trough to intensify across the western High Plains. After some fairly brief complications to the low-level flow caused by the lead short-wave trough, the surface flow in the moist sector should back again to the south-east, and strengthen. The dryline will sharpen up as diurnal heating takes place, and should move eastwards into the eastern Panhandles by peak heating. One possible issue is that the winds to the west of the dryline may back due to the approaching upper trough. Thus, low-level convergence along the dryline is likely to be rather weaker than what might be hoped for.

However, cooling mid-levels as the trough approaches, 1500-2000 J/Kg of CAPE, and at least some low-level convergence should aid in the development of severe thunderstorms across portions of TX and OK later on.

Picking a target region along the dryline is always tricky in situations like this, as there is effectively a several hundred mile long area where the ingredients may come together to support severe convection. Whilst this is fine for producing an overall risk-based forecast, for chasing, you really need to be within 50-75 miles of convective initiation - well, if you want to have a reasonably un-fraught chase you do!

What might aid in initiation other than convergence along the dryline? Upslope flow can bring parcels slightly closer to their level of free convection, and the extra height means that convection can develop with slightly lower surface dewpoints. The area west of Childress, TX, where the south-easterly flow rises up the Caprock escarpment is often a favoured area of initiation.

Models can highlight risk areas which may or may not come true, as much of the atmosphere is not sampled directly, and so spurious artefacts can develop. Only by checking hourly observations can it be determined whether the simulated feature is actually developing.

With this in mind, and given I don't actually have to make the drive (!), two areas of interest are suggested, in my eyes:

1) SW Texas, to the west of  San Angelo. Several models suggest that the dryline may not move too far east in this region, allowing a good pool of low-level moisture to develop ahead of it. Fairly strongly backed surface flow looks like providing excellent wind shear. Persistent convergence along the dryline along with a slight upslope component to the low-level flow means that initiation is possible, despite the upper trough only marginally starting to impact the area by late afternoon. Models are suggestive of development here too, which adds some confidence.

2) NW Oklahoma - or more generally the eastern TX Panhandle into western Oklahoma. The dryline is progged to drift into the eastern TX and OK Panhandles by mid-afternoon, but surface convergence may not be great. The upslope flow onto the Caprock may well help initiation, before storms then migrate to the north-east into better moisture. Towards early evening, as the upper trough approaches and the dryline tends to come to a halt, further backing of the low-level flow across NW Oklahoma suggests that any storms which develop on the dryline and move into this region have a reasonable shot at producing tornadoes. Indeed, a narrow window of opportunity exists for the development of a significant tornado, as the low-level flow cranks up. The limiting factor is that with the loss of daytime heating, a low-level inversion will start to take shape.

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