Convective Outlook: Sun 14 Jun 2020
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Sun 14 Jun 2020 - 05:59 UTC Mon 15 Jun 2020

ISSUED 07:16 UTC Sun 14 Jun 2020

ISSUED BY: Dan

Upper low will reside over the Celtic Sea throughout Sunday and Sunday night, the associated cold pool generating steep mid-level lapse rates, especially in western and southwestern areas with greatest proximity to the upper low.

... SCOTLAND ...
The east-southeasterly flow aloft across the North Sea and E / SE Scotland will maintain a pronounced Theta-E plume, advecting westwards across Scotland through the day. Several subtle impulses will run westwards atop this plume, generating episodes of elevated convection atop extensive low cloud and sea fog, and consequently bringing periodic risks of a few sporadic lightning strikes over the North Sea and adjacent portions of E / SE Scotland. However, the exact timing/placement of these features is uncertain, and it is likely most lightning activity will probably occur offshore and so for now we have refrained from introducing a SLGT (unless confidence improves).

By the afternoon, cloud breaks are expected to the lee of high ground (i.e. central and western Scotland), with diurnal heating then yielding 300-800 J/kg CAPE. Low-level convergence and orographic forcing will play a key role in initiating convection, perhaps struggling to pass a slight warm nose at ~700mb initially, but potentially aided by a minor PV anomaly mid-late afternoon. As a result, a few isolated heavy showers or thunderstorms could develop and drift to the west for a couple of hours, before weakening by mid-evening. A low-end SLGT (25-30% chance) has been introduced to highlight the main area of interest.

... ENGLAND / WALES ...
On Sunday morning, a narrow tongue of marginal mid-level instability may be present over the East Midlands and East Anglia, lifting northwards with time. While a few elevated showers may be possible (potentially above areas of low cloud and sea fog), the risk of lightning is relatively low. Meanwhile, an area of showery rain will likely be stretching across the English Channel into Dorset, and this is expected to extend and migrate northwestwards into south Wales through the morning. Diurnal heating ahead and behind this feature will likely lead to convective overturning, with cloud and showery rain generally breaking up and providing the focus for scattered heavy showers to develop in the afternoon hours. Meanwhile some mixing of the boundary layer could lower dewpoints slightly through the day, potentially setting up a differential heating / quasi-dryline boundary (13-15C ahead and 10-12C behind) that will lift northwards across the Midlands and East Anglia. This boundary may also be reinforced by sea breeze convergence in the afternoon.

Either way, this boundary combined with sea breeze in other areas and orographic forcing in Wales / SW England will all contribute in generating scattered heavy showers during the afternoon and evening hours, aided also by a shortwave. Some concerns exist over the extent of cloud cover associated with earlier showery rain, which may have some impact on reducing instability due to subdued surface temperatures, and even if deep convection does occur there are also concerns about a slight warm nose at 700mb and very dry mid/upper levels which may restrict how tall such convective cloud may be able to grow. Either way, the environment could potentially yield 400-800 J/kg CAPE with 30-35kts bulk shear, perhaps aiding in cell organisation with the strongest cells producing hail up to 2cm in diameter. Low-level convergence boundaries and local topography will aid in generating some low-level vorticity, and this may be ingested to perhaps produce an isolated tornado - most likely close to the coast of west Wales. Some sporadic lightning is certainly possible, most likely in west and north Wales into the north Midlands (35% chance). An isolated heavy shower or thunderstorm may also be possible over Cumbria. Activity will slowly weaken through the mid-late evening.

During the early hours of Monday, an emerging shortwave drifting from the Channel Islands towards the south coast may bring the risk of some elevated showers, which could perhaps produce a few isolated lightning strikes.

... NORTHERN IRELAND / REPUBLIC OF IRELAND ...
Some pockets of elevated convection may be ongoing first thing on Sunday morning, particularly across central and/or southern Ireland, but probably with a weakening trend as mid-level instability gradually reduces. However, it is possible this may become rooted within the boundary layer across parts of Munster by late morning/midday, as diurnal heating increases surface-based CAPE. Either way, assuming early cloud cover breaks sufficiently to allow strong surface heating through the day, some 500-1,000 J/kg CAPE is expected to build. Highest dewpoints will be located in W and NW Ireland, and here the greatest instability is expected. A combination of sea breeze convergence inland from the west coasts, and orographic forcing, will provide the primary trigger for deep convection to develop but also likely aided by a substanial shortwave that will drift eastwards across Ireland during the second half of the afternoon into early evening. 

The net result is numerous thunderstorms are expected to erupt, perhaps initially in Munster late morning as previously mentioned, but more widely across SW / W / NW Ireland through the afternoon. General storm motion will be to the W or SW, which is likely to lead to elements of backbuilding as additional cells develop along the same sea breeze boundaries / orographic features and train over similar areas. The magnitude of CAPE, forcing and steep mid-level lapse rates, albeit with modest shear (15-25kts) suggests lightning could be quite frequent at times (40-45% chance), and since some elements of backbuilding / training will likely occur, it could also persist for some length of time in places. As a result we may consider upgrading to MDT if confidence increases in areas that are most prone. While hail up to 1.5cm in diameter may be possible, the greatest threat is from surface water flooding given PWAT ~25mm, with potential event totals of 30-50mm possible locally.

While thunderstorms will generally weaken during the evening hours and drift offshore, remnant showers from Wales may drift across the Irish Sea into eastern Ireland overnight and as such a few isolated lightning strikes may be possible with this activity, even well into the early hours.