Convective Outlook: Sun 12 Jun 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sun 12 Jun 2016 - 05:59 UTC Mon 13 Jun 2016

ISSUED 17:18 UTC Sun 12 Jun 2016

ISSUED BY: Dan

UPDATE 17:18 UTC SLGT marginally extended N across Midlands, MDT shrunk in size to concentrate on northern convergence zone where best environment for thunderstorms persists. Given notable CAPE and low-level convergence, funnels or weak tornado will be possible with these storms. Southern convergence (sea breeze) so far is less obvious on surface observations and cells here have been struggling to maintain their identity - hence the removal of these areas from the MDT. That said cluster of heavy showers across Dorset sliding east to Hampshire *may* have the potential to turn thundery during next 1-2 hours

UPDATE 13:05 UTC SLGT added over Ireland / Northern Ireland, while expansions made to existing SLGTs over England. Expectations remain more-or-less as depicted in original forecast below, with convection subject to saturated profiles across N England this afternoon, and therefore uncertain as to how much lightning activity there will be. Farther south, SMZ showing signs of cloud breaking to produce insolation, which should enable isolated to scattered thunderstorms to develop here, especially late afternoon and through the evening. Have added a MDT to highlight areas where thunderstorms may be a little more organised / have better coverage (but not guaranteed given uncertainties in cloud cover). These could produce quite frequent lightning. A further expansion of this SLGT over S England into more of the Midlands / East Anglia may be required - trends will continue to be monitored

Another messy day is expected on Sunday with a mix of dynamic and convective precipitation across the country. Various weak(ening) frontal boundaries will attempt to nudge NE-wards, but eventually decaying before becoming the required forcing for deep convection given some strong surface heating. In particular, wind convergence along a cold front will allow a line of scattered heavy showers and perhaps a few thunderstorms to form from Norfolk curving NW-wards to Cumbria - however, instability is marginal (couple hundred Jkg-1 CAPE) with hints in forecast soundings of slight capping in the mid-levels, which may limit overall convective depth.
Given PWAT in the order of 26-28mm, scope exists (like Saturday) for slow-moving heavy showers capable of local flooding, but perhaps with limited amounts of lightning. Have issued a SLGT for now to highlight main zone at risk, but may require a downgrade if lightning potential continues to look marginal.


Farther south, heating of the shallow moist zone (SMZ) with surface dewpoints of 13-15C coupled with falling heights and the approach of a dry intrusion will yield an environment with 700-1,000 Jkg-1 MLCAPE. Provided there are sufficient cloud breaks to allow some insolation to develop, then a few isolated to well-scattered thunderstorms may form during the afternoon/evening hours, capable of producing hail locally over 1.0cm in diameter and quite frequent lightning. DLS begins to increase from the SW towards evening, and so this may enable storms to become better organised later in the day with perhaps late initiation in some areas.

Low LCLs, wind convergence (especially close to the south coast from sea breeze, and perhaps also in a separate zone across the south Midlands / Home Counties towards evening as a small surface low forms) and modest CAPE may produce one or two funnels or weak tornadoes. Cells that develop earlier on in the afternoon will pose the greatest threat of excessive rainfall / minor flash flooding given reducing PWAT values as the afternoon/evening progresses. This zone will need monitoring for perhaps either an expansion of the SLGT farther north into the central Midl