Convective Outlook: Mon 28 Jun 2021
LOW
SLGT
MDT
HIGH
SVR
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Mon 28 Jun 2021 - 05:59 UTC Tue 29 Jun 2021

ISSUED 07:05 UTC Mon 28 Jun 2021

ISSUED BY: Dan

A rather messy setup initially, with outbreaks of rain stretching from SW England across Wales, the Midlands to Lincolnshire/East Anglia, and rather extensive low cloud and fog in places. Some elevated convection is likely in places, enhancing rainfall intensity, but the risk of lightning from this will be very low given skinny CAPE and saturated profiles. Through the day the cloud may break across southern England allowing better insolation and surface heating, the temperature differential then encouraging an onshore breeze to develop along English Channel coasts. How far inland this convergence zone gets will be dependent on the shape of the surface pattern and the degree/extent of surface heating, but reasons to believe that surface based convection could develop a line of heavy showers along some southern counties of England during the afternoon and evening. Model guidance varies over the exact location of the convergence zone - it could be as far north as the M4 corridor for example - and it should be noted that if cloud struggles to break then this will ultimately inhibit the development of heavy showers. So the two main uncertainties are (a) how many showers will actually develop (due to cloud cover) and (b) exactly how deep will the convection be (will they actually produce much in the way of lightning).

Some drier mid-level air is also expected to drift into S/SE England, but again the exact shape of this (ie location and extent) is a little uncertain amongst model guidance. Forecast profiles suggest potential for tall, skinny CAPE but weak shear (some directional, but the strongest flow is above 500mb) which suggests showers may be rather pulse-type in nature. Steering flow may eventually push the showers/thunderstorms offshore (including Isle of Wight) later in the day. Given PWAT around 30mm, light steering winds and slow storm motion there will be scope for locally torrential downpours to cause some flash flooding, more especially should one occur over an urban area. Vorticity stretching as updrafts develop over/along convergence zone coupled with low cloud bases suggests the potential for a couple of funnel clouds or an isolated weak tornado.

There may also be scope for another wave of rain with embedded weak elevated convection to push out of France towards S/SE England on Monday night too.