Tuesday, 14 April 2009

First warm air storms of the year?

Well, we sometimes have to wait until well into May, but it does look like we could see the first of what I would call 'warm air' thunderstorms of the year across parts of the UK overnight tonight and into tomorrow.
Overnight tonight, models do indeed break out precip across south-eastern parts of England, thence advecting/developing it NW'wards into parts of central England and Wales. This appears to be in response to isentropic lifting of 850 hPa high WBPT air atop a relatively cool boundary layer, as the 850 flow increases overnight, and differential thermal advection, as the 850 WBPT increases below fairly cool upper air. Note that this means that we may not see much moving NW from the near continent this evening - rather, it should develop here. There is a reasonable chance of thunder with this too. Shear in the cloud layer is not too high - around 20-25 knots, with a little veering - thus, rotating updraughts seem unlikely, but any stronger cores could bring gusty winds and hail, perhaps to 15 mm diameter or so. 

Tomorrow, this lot should have cleared away to the NW, with sunshine developing across southern and some central parts of England. Somewhat lower 850 WBPT air will move into parts of SE England and E Anglia later in the morning and through the afternoon, meaning storms are unlikely here, or at least, if any develop, they will soon have moved away to the WNW. Further west and north-west, there will still be a moist and unstable airmass in place at peak heating - any storms which initiate seem most likely to do so 
(in the absence of any larger scale low-level forcing mechanism) in regions of small-scale forcing, such as elevated heating over the likes of the North Downs around the Berks/Hants border, or the Berkshire Downs/Salisbury Plain, and then moving NW'wards - others may develop over the Chilterns. Deep layer shear will tend to increase through the afternoon as stronger 500 hPa flow begins to arrive, and will increase further into the evening, reaching around 40 knots. Low-level shear will also tend to increase in the early evening. Given the lack of a major boundary to initiate storms, any which do form could remain fairly discreet. Parts of central southern England, the Midlands, and east Wales have a chance of some severe weather, primarily in the form of strong winds (up to 60 mph) and hail (20-30 mm dia). However, there is also the chance of a tornado or two, especially towards early evening in parts of the Midlands and east Wales. 
The shear is certainly sufficient for organised multicell storms, and these could contain rotating updraughts/supercell type structures, augmenting the severe threat.

Sunday, 5 April 2009


Testing from email

Wednesday, 1 April 2009

More US severe weather

Well, Spring is well and truly here now, so it should come as no suprise to see a series of powerful troughs pushing through the USA, bringing rounds of severe weather. Some of these in recent days have not had huge amounts of Gulf moisture to feast on, meaning fairly low-end severe events. However, on Thursday, such a trough will be accompanied by plenty of moisture, as strong return flow from an open Gulf streams into portions of the south and south-east.

Basically anywhere from around the Mississippi Delta region eastwards, and northwards towards Tennessee is at risk from severe weather as the potent upper trough and associated surface low move through the area. 500-1500 J/Kg of CAPE are expected, along with strong 0-6 and 0-1km shear values - more than sufficient for supercell structures, although given the intense forcing, a fairly complex scenario is expected. Also, surface winds are expected to be a little veered - however, I recall seeing something similar on the Super Tuesday outbreak of Feb 2008, and there were numerous strong tornadoes, and very long-track tornadoes.

The warm front currently looks like lying through central Alabama (E-W, ish!) by around 21z/close to peak heating. Should this occur, low-level helicity would be maximised just ahead of the front. Storms crossing this front would have an enhance chance of producing tornadoes, although could become more elevated as they continue to move NE into the cooler air.

At this stage, although a large area is at risk, I would place the Birmingham - Montgomery - Columbus area of Alabama in the highest risk area for strong tornadoes Thursday afternoon/evening, although anywhere from Mississippi eastwards could be in for a rough ride.