Forecast Update
Valid: Fri 09 Sep 2011 11:00 to Sat 10 Sep 2011 06:00 UTC
Issued: Fri 09 Sep 2011 11:05
Forecaster: ESTOFEX

A level 1 was issued for Ireland, parts of UK and Scotland mainly for an isolated tornado/severe wind gust and marginal hail risk.

SYNOPSIS

Please refer to the outlook issued at Thu 08 Sep 2011 17:43 Z.

DISCUSSION

... Parts of UK and Scotland, 21 Z onwards...

Not much has changed regarding confidence in thunderstorm initiation over the area of interest. Models still diverge significantly regarding magnitude of CAPE (surface based or elevated) and potential initiation (even significant run-to-run inconsistency ). Nevertheless, GFS is the model, which persistently showed a build up of some SBCAPE after 21 Z over UK and Scotland. Forecast soundings from this area hint at a very moist lower troposphere with weak lapse rates below 800 hpa and gradually steepening ones atop that layer. Not much T/Td bias needed for surface based initiation and we now think that any residual boundary or mesoscale convergence zone may allow for adequate moisture pooling to assist in surface based initiation. Initiation may be fostered by a prefrontal convergence zone, which moves in from the west after midnight, but strength of this boundary remains vague at best.
Also initiation in the southern part of the level 1 may be more sporadic due to increasing WAA/capping.

The kinematic environment improves through the period with LL shear exceeding 15 m/s and SRH-1 approaching 200 m^2/s^2. Forecast hodographs also reveal strong loops, so any more persistent updraft may organize/rotate. The LCL remains at or below 600 m, so an isolated tornado event will be possible and hence we decided to expand the level area to the northeast/southeast.

A more certain risk will be the severe wind gust risk, as 850 hPa winds increase to 20 m/s and more. Although LL lapse rates remain weak, a few severe wind gusts are well possible (which also justifies a larger level 1 area). Marginal hail accompanies stronger storms and if expected MUCAPE values verify, a few large hail reports are in the realm of possibility.

Only cosmetic changes were performed for the rest of the outlook. Not much thunderstorm activity expected in those areas. Also, the level 1 for Ireland remains unchanged.