Convective Outlook: Thu 15 Sep 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Thu 15 Sep 2016 - 05:59 UTC Fri 16 Sep 2016

ISSUED 19:44 UTC Thu 15 Sep 2016

ISSUED BY: Chris/Dan

UPDATE 19:44 UTC Thundery outbreaks of rain likely to become more widespread after midnight across southern England, some uncertainty how much activity may extend up into the south Midlands. However, of greatest concern is the persistent signal for a quasi-stationary cluster of torrential downpours that may develop over London and environs down to Hampshire later in the night and into Friday morning. This may cause some significant disruption in places from flooding (and perhaps also lightning strikes) and hence the issuance of a SVR - rare to be issued for rainfall alone

UPDATE 15:17 UTC Line of TCu from NW London - Dorset, along marked convergence zone, is now developing isolated thunderstorms as it continues to nudge WNW-wards next few hours. MDT has been modified based on latest guidance and trends (still some uncertainty as to how far west convergence line will move before retreating back east later in the evening), with an extension across the London area primarily for later tonight, during which some disruptive rainfall totals will be possible, but (as is often the case) with large uncertainty

UPDATE 12:48 UTC No changes at this stage - breeze convergence now in evidence from Swanage NE-wards to Medway. Latest high-res modelling suggesting potential for further SE shifts in the areas at greatest risk (such as roughly along the position of the convergence zone currently), which will be considered. Still a concern for parts of London and SE England later in the night for significant flooding, but  a lot of uncertainty.

UPDATE 08:17 UTC Some adjustments to the shape of the MDT based on latest guidance - but still room for some margin of error W / E of depicted area. Some models beginning to pick up on the idea of thundery outbreaks developing towards London area late evening/overnight, which might have some disruptive impacts - treated with caution for now, but will be monitored

Elevated convection over the English Channel and environs on Thursday morning should gradually weaken with time - however, there is some uncertainty as to how far north precipitation (and more importantly the cloud shield) may extend, and this may have some impacts on evening developments if cloud clearance is too slow. Some sporadic/isolated lightning from embedded medium-level instability release will also be possible in Northern Ireland / western Scotland from showery bursts of rain.

The upper pattern on Thursday will gradually become more progressive as an Atlantic upper trough approaches from the west, causing the cut-off low over Biscay to nudge eastwards across France, while elongating - and this process may be responsible for the advection of some elevated convection into parts of SE England and coastal East Anglia on Thursday night. 

Elsewhere, capping will persist for much of the day. That said, strong heating and increasing low-level wind convergence, as the surface low over Brest peninsula migrates NNE to southern England by evening, may provide sufficient forcing for scattered thunderstorms to develop late afternoon into Thursday evening, particularly across the W Country / Cen S England NE to the south Midlands. Forecast profiles are quite dry, and so initial convection may struggle to develop at first, but should any storms form then the biggest concern will be local flash flooding given PWAT in the mid-high 30s mm and slow storm motion. Gusty winds and hail to 1.5cm in diameter will also be possible, which falls just shy of our SVR criteria, hence no SVR issued at this stage. CAPE values near 1,000 Jkg-1 and steep mid-level lapse rates (although these gradually weakening (in a relative sense) through the evening) suggest lightning will be quite prolific in the most active storms.

During the evening and night, the approaching Atlantic front will likely increase low-level convergence and provide some forced ascent, with perhaps an increase in coverage of showery precipitation in a N-S zone (highlighted by the elongated SLGT), only slowly migrating eastwards. 200-400 Jkg-1 MLCAPE overnight suggests that some occasional lightning will be possible with this activity, albeit in a rather sporadic nature, and likely to not be as frequent as initial surface-based storms on Thursday evening.

The exact positioning of the zone of wind convergence, and hence greatest thunderstorm potential, is subject to a little doubt and hence the MDT may need adjusting slightly as conditions become clearer through the day on Thursday.