Convective Outlook: Wed 22 Jun 2016
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Wed 22 Jun 2016 - 05:59 UTC Thu 23 Jun 2016

ISSUED 18:43 UTC Wed 22 Jun 2016

ISSUED BY: Dan

UPDATE 18:43 UTC Current highly-sheared thunderstorms across the English Channel continue to move NE-wards. It then seems likely the flow will back a little through the evening, with a new round of thunderstorms expected to grow into an MCS late evening into the early hours and affect a more wider area, potentially followed by another area of showery rain (with perhaps less lightning activity due to exhaustion of instability by preceding storm clusters) late in the night into Thursday morning. Have extended SLGT farther W to cater for backing flow later this evening (although still some inevitable uncertainty as to true westward extent of thunderstorms). MDT left untouched at this update, although some guidance would suggest a slight westward shift/extension may be required.

UPDATE 12:03 UTC No changes made to the forecast for now - still large amount of uncertainty over spatial extent of thunderstorms tonight, as is to be expected given the setup, but overall current thoughts remain in-line with previous forecast issuance. If anything, recent trends in guidance so far today has shifted forecast activity marginally farther SE, and so a slight trim to the western side of the MDT may be required. Still also a low risk of an isolated thunderstorm developing over Lincs/E Anglia this afternoon

A sharpening upper trough to the west of Ireland will continue to dig southwards towards Portugal, backing the flow over western Europe and allowing advection of a notably warm, moist airmass northwards into parts of the British Isles. As is often the case in thundery plume-type setups, there is some uncertainty as to how much of (primarily) England will be affected by destabilisation of this rich low-level airmass - at least parts of SE England and East Anglia seems likely, but this may extend farther W across Cen S England / Midlands perhaps. The situation will be monitored closely during Wednesday and updates may be issued if necessary.

In either case, during this forecast period there are 3 main aspects to consider:

(1) Some isolated / sporadic lightning activity may occur across parts of Scotland associated with heavy showers, given reasonable DLS and weak CAPE. This is a fairly low risk, but high enough (e.g. >5%) to warrant the inclusion of a LOW threat level.

(2) Should cloud begin to break in the wake of morning showery rain (associated with a frontal wave), then given modest surface heating of an airmass with Tds of 16-18C widely, there may be scope for isolated thunderstorm development over East Midlands / East Anglia / SE England during the late afternoon and early evening, especially close to the frontal boundary (so this would place parts of Lincolnshire / The Wash / Norfolk etc with a slightly better chance, and hence the inclusion of a SLGT here). Given capping from warm nose aloft, this is treated as a fairly low probability for now.

(3) Forced ascent during the evening and night hours from the approach of a shortwave trough over N France / English Channel will likely lead to thunderstorm development that eventually grows into an organised cluster / MCS given strong DLS, being steered generally to the NE through the night across at least the highlighted MDT area, if not some parts of the surrounding SLGT also. In fact, there may be several rounds of thunderstorms that affect parts of SE England and East Anglia - current thinking is early initiation may occur around 18:00-20:00BST, but with a more organised MCS-type feature arriving late evening and continuing into the early hours (but this is all subject to change given the complexity of the situation).

Given modest instability, lightning could be quite frequent - especially on the eastern flank of any thunderstorm complex that does develop. Heavy rain, causing local flash flooding, and hail up to 2.5cm in diameter will be the primary hazards. This is borderline SVR category, and an upgrade may be required.