VALID 06:00 UTC Sun 19 Sep 2021 - 05:59 UTC Mon 20 Sep 2021

ISSUED 07:49 UTC Sun 19 Sep 2021

ISSUED BY: Dan

The main focus on Sunday will be associated with slow-moving heavy downpours in east/southeast England. Low-level convergence is expected to develop through the day as surface heating encourages strengthening onshore flow, coupled with a frontal boundary advancing from the west. If sufficient heating can occur, likely yielding up to 500 J/kg CAPE, heavy showers may begin to develop from early afternoon onwards. Forecast profiles suggest warm air aloft (above 500mb) may limit the height of most convective cloud to around 20,000ft, but this is still sufficiently tall for some lightning strikes. While CAPE of <500 J/kg may appear rather low, this is in part due to it being confined to the lowest ~6km of the troposphere (rather than all the way to the tropopause) and so this in conjunction with PWAT of up to ~33mm will still likely lead to efficient rain producers and the threat of locally large rainfall totals - especially when coupled with generally weak flow through the cloud-bearing layer leading to slow-moving downpours. Showers/storms will tend to pulse up and down, with daughter cells developing nearby on outflow boundaries. The main hazard is local surface water flooding, hence a SVR has been introduced, even if not much lightning occurs. Chances are numerous heavy showers develop, perhaps with limited lightning activity - but a low-end SLGT has been introduced. Showers will slowly weaken in intensity during the mid-late evening, but further outbreaks of showery rain are likely in similar locations through the night hours, exacerbating already wet ground in places.