Convective Outlook: Mon 08 Apr 2019
What do these risk levels mean?
Convective Outlook

VALID 06:00 UTC Mon 08 Apr 2019 - 05:59 UTC Tue 09 Apr 2019

ISSUED 06:23 UTC Tue 09 Apr 2019


UPDATE 19:19 UTC As mentioned in main forecast below, convergence line developed slightly further south than earlier expectations, and so the SLGT has been adjusted southwards accordingly

UPDATE 13:02 UTC Increasing coverage of AcCas over Suffolk drifting inland from the North Sea needs monitoring for potential elevated instability release. LOW threat level extended to cover this risk, but the SLGT may need extending further into East Anglia (and perhaps also westwards across north Wales)

A quasi-stationary warm front will straddle Ireland - Wales - SE England for much of this forecast period, resulting in a broad corridor of modest ThetaW air. Extensive low cloud and fog is expected initially on Monday morning due to the very moist low-level airmass, while showery bursts of rain will occur in a rather random fashion within this ThetaW zone due to subtle forcing aloft coupled with some mid-level instability. While a few isolated lightning strikes cannot be ruled out, very little is expected given the weak forcing, marginal instability and fairly saturated profiles.

There is good multi-model agreement for cloud cover to become more broken during the day, allowing increasing amounts of insolation and hence surface heating. Temperatures of 16-18C seem plausible on Monday afternoon to the north of the frontal boundary (i.e. East Anglia, the north Midlands and southern parts of northern England) with dewpoints of 10-13C. Profiles will become increasingly unstable, with 500-800 J/kg CAPE possible over these areas.

In a rather slack pattern with weak forcing aloft, low-level convergence and associated moisture pooling will serve as the primary forcing for ascent, with a few isolated to well-scattered thunderstorms possible from mid/late afternoon onwards into the evening hours - particularly within the SLGT area. Generally weak shear (winds both similar in direction and speed through the profile) will result in pulse-type convection, where an individual thunderstorm may only last an hour or so. An easterly steering flow will result in storms drifting to the W or NW with time, their slow motion bringing the risk of some localised surface water issues. It is worth noting that due to some uncertainty over the exact position of the warm front, this SLGT may need shifting north or south slightly.

Depending on sunshine amounts, sea breeze convergence may also aid the development of one or two isolated heavy showers/thunderstorms over East and West Sussex during the late afternoon/early evening, especially combined with additional forcing over the High Weald. An additional, small SLGT may be introduced here if confidence improves on this potential.

During Monday evening and night, increased forcing aloft approaching from France will result in more widespread outbreaks of showery rain developing across southern England / south Midlands / south Wales and across into Ireland. This will tend to become a messy mix of convective and increasingly dynamic precipitation, and while some embedded lightning is possible - especially at first - marginal instability and saturated profiles precludes the need for any SLGTs to be issued for this part of the forecast.