An unseasonable depression will bring significant rainfall to much of the UK on Monday. The SE will escape the worst of the heavy rain crossing the rest of the country further NW in a deepening LOW moving NE across the country through Sunday night and Monday. Many parts of the UK will experience large rainfall totals exceeding 40mm and with flooding in places. The SE is blessed by the late arrival of any fronts (weakening too) and the movement of the system to the NE driven by a brisk southerly jetstream 20,000 feet overhead. It simply passes us by. Nevertheless, Reigate could see 5mm or so of rain, mostly arriving in the form of showers in the afternoon, some heavy: so there is a twist in the “tail” of this depression.
The sounding shows the instability of the air as the front crosses. Any warm parcels of air lifting through the atmosphere (dashed line) as the cold front approaches will be warmer than the surrounding incoming cool airmass (red line) so will continue to lift and this is a so-called “unstable” situation likely to form tall cumulonimbus clouds and heavy rain and possibly fast moving thunderstorms. The dew point (blue line) is also very near to the air temperature so the atmosphere to a great altitude is humid / near saturated and likely to condense large amounts of water vapour into cloud droplets. Moderate rainfall totals on a brisk air flow are possible.
The far corner of the SE may see no rain at all during most of Monday but an area of instability and rapid upward velocity (lift) associated with the passage of a cold front approaching from midday could spark off heavy showers and thunderstorms in the SE and Reigate.
Temperatures in the SE will be higher than anywhere else in the UK as we enjoy a warm southerly flow of upper air in the warm sector of this depression with a strong southerly jetstream starting the day over Reigate (>70mph). Temperatures at 10,000 feet will fall below freezing over Reigate during the course of the day as the cold front brings in much cooler NW air flow across the UK. The rest of the week looks less unsettled, temperatures recovering to normal though with more rain at times. Typical August!