MODERATE RISK OF SEVERE STORMS

Issued 2014-07-19 08:25:26
Valid: 19/07/14 0900z to 20/07/14 0600z
THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FORECAST

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Synopsis
Long-wave trough just west of western Europe continues to slowly advance eastward today, southerly mid-upper jet streak continues across France and British Isles ahead of this advancing trough, with a plume of warm moist air continuing to advect north, though beginning to be displaced east later in the day. A slack area of surface low pressure covers the UK, with a frontal zone lying SW Scotland down to Dorset at 12z (noon), with a warm/moist surface airmass that will be very unstable to surface heating to the east of frontal zone.

… ENGLAND and WALES …

Overnight storms will continue to spread north across England and Wales this morning, with further elevated storms moving north out N France in next few hours. Dependent on insolation breaking through cloud cover from morning convection later, any surface heating will allow temperatures to soar to the mid or upper 20s deg C this afternoon across S England. Combine this with dew points of 19-20C (already 19C observed in the SE) – large amounts of CAPE are indicated by models. GFS shows up to 1600 j/kg MLCAPE and up to 2000 j/kg SBCAPE this afternoon across SE England.

Breeze convergence ahead/near frontal zone and large scale ascent of airmass with approaching upper trough will likely erode any CINH/cap in place this morning, to allow storms to develop should surface heating take place. Strong mid to upper southerly jet and winds backed SE towards the surface will generate sufficiently strong vertical shear for any storms that develop to quickly organise into muticell and even supercell structures given large CAPE too. Such storms may produce large hail and damaging wind gusts.
Given potential for surface-based storms, any old outflow boundaries from morning storms and influence of backed winds near convergence zones, areas of strong low-level shear may develop – which may be sufficient for rotating mesocyclones on supercells that develop – which may produce one or two tornadoes too.

Therefore, have issued a Moderate risk across southern, central and eastern England – where surface heating is most likely and thus risk of severe thunderstorms.

Moderate risk of Severe Storms

Netweather.TV has issued a moderate risk for severe storms.

From Netweather.TV:

Issued 2014-07-18 14:37:16
Valid: 18/07/2014 1200z to 19/07/2014
THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FORECAST

Synopsis
Long-wave trough extending south to west of British Isles and into Iberia remains slow-moving whilst becoming increasingly aligned NW-SE (negatively tilted). A strengthening southerly flow continues to transport, warm, moist and unstable flow north across British Isles. A thundery trough in the upper S’erly flow, that brought morning storms, will continue NE across northern Britain reaching N Scotland by midnight. Another thundery low developing over N France Friday PM will move N into S England by midnight, before spreading north across Midlands and Wales Sat morning.

… MDT RISK FOR S ENGLAND, E WALES, MIDLANDS, E ANGLIA …

Plume of very warm and dry air at 850mb, evident on 12z Bordeaux and Trappes (Paris) radiosonde ascents above hot but moist surface airmass advecting north is providing a strong cap (lid) to convection this afternoon across France, though 850mb plume is creating steep lapse rates which combined with high temps approaching the mid 30s degrees C, is likely to yield MLCAPE of 1500-2000 j/kg. Cooling of mid-upper level temps with approach of upper trough to the west and convergence over N and W France is forecast to erode this cap Friday evening.

Model forecasts indicate thunderstorms breaking out this evening across N/W France, already there are storms across W France, which will then track north towards S England tonight. A strengthening southerly jet stream moving up from France and low-to mid level flow backing more SE’erly, as shown on Bordeaux and Trappes ascents, will increase vertical shear – allowing storms to organise upscale into an MCS, which may exhibit bowing line segments on its forward side, bringing the risk of damaging wind gusts, and also one or two supercells could be embedded. Large amounts of MLCAPE and strong directional shear shown by GFS suggests threat of large hail. The storm system is also likely to produce frequent cloud-to-ground lightning. PWAT (precipitable water) values of 39mm indicated by GFS are very high- so excessive rainfall and flash flooding are a real threat too. Given the likely organisation of an MCS bringing widespread threat of wind damage, large hail and flash flooding – have issued a MODERATE risk of severe storms.

The situation will be monitored, given storms have still yet to form, with updates possible to the categorical risk of severe weather.

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