Storm Forecast
Valid: Sat 19 Jul 2014 06:00 to Sun 20 Jul 2014 06:00 UTC
Issued: Fri 18 Jul 2014 19:35
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for England, France and BeNeLux for large hail, severe wind gusts, excessive precipitation and to a lesser extent for tornadoes.

A level 1 was issued for E Spain for large hail and severe wind gusts.

A level 1 was issued for the Switzerland mainly for large hail.

SYNOPSIS

Main feature is an Atlantic trough which slowly proceeds eastward over the British Isles and the Bay of Biscay. A mid-level jet streak rounds the tip of the trough over Spain and circles northward over France into England. In these regions, a 500 hPa flow between 20 and 30 m/s and travelling vorticity maxima set the stage for another day of organized convection. The main focus for convective initiation is provided by a wavy frontal zone, which separates a tongue of very hot air of African origin to the East from cool maritime air to the West.
In the rest of Europe, warm and partly moist air is present under weak geopotential gradients. A diffuse, V-shaped upper-level low stretches from Southern Sweden via Western Russia into Turkey, while a highly amplified ridge extends from the Central Mediterranean across Central Europe and the North Sea to Northern Scandinavia.

DISCUSSION

... England, BeNeLux, N France ...

Remnants of nocturnal convection will likely affect Western and Northern France and especially England in the morning hours. Since these storms will mostly be elevated, the severe weather risk should temporarily be low, though some heavy rainfall may still extend into the current forecast period. Activity will move northward into Scotland and onto the North Sea while it gradually decays.
Forecast models largely agree on clearing in the wake of these storms. With mostly sunny late morning and noon hours, a quick recovery of the air mass can be expected with CAPE values on the order of 1000 to 2000 J/kg (even higher per GFS) under deep-layer shear around 20 m/s. From the early afternoon onwards, more and more storms are forecast to initiate in the vicinity of the main frontal zone or along the mess of old outflow boundaries laid out by overnight's convection. They can quickly turn into multi- and supercells with a threat of large hail and severe wind gusts. Backbuilding and training activity may also easily produce a few flash floods, as the moisture content is exceptionally high (precipitable water around 40 mm).
Of particular concern are areas of reduced vertical mixing, which allow a moisture accumulation and the maintenance of enhanced low-level shear and helicity. This can be expected along the sea breeze front in Northern France and Belgium, and in general in Eastern England, where warm air advection re-intensifies on the forward flank of a weak surface low which is forecast to track from Northwestern France (06 UTC) into Northern England (18 UTC). If diurnal heating and/or the lift impulse by outflow boundaries suffice to break the capping inversion in these regions, particularly well-organized supercells could spawn a few tornadoes, possibly even including a strong one. However, mesoscale developments need to be awaited before the degree of this threat can further be specified.
Despite remaining uncertainties due to downstream effects of overnight's vigorous storms, a level 2 seems to be warranted.

... Central France into E Spain ...

Further South, the main frontal zone will slowly move eastward as a diffuse cold front. Large-scale lift ahead of the trough axis, weak cold air advection at 850 hPa and strong diurnal heating of near-surface air will erode the strong cap, and convective initiation becomes more likely than on Friday. Depending on the qualitiy of low-level moisture which may be mixed out in some places, 500-1500 J/kg of CAPE seem realistic. Deep-layer shear increases from 20 m/s over Central France to 35 m/s and beyond over parts of Spain.
At least scattered storms are expected to form from the early afternoon onwards in a belt across France. In Eastern Spain, activity will be more isolated and tied to orographic features, as the maritime moisture will likely be diluted before it can move too far onshore.
The French storms will organize into multicells, supercells and short bowing lines while moving NNE-ward. Large hail (especially in the early stages) and severe downbursts are the primary risks, and even isolated extreme events (hailstones larger than 5 cm and wind gusts stronger than 32 m/s) are possible. The motion of these storms almost parallel to the frontal zone seems detrimental to the development of a larger bow echo but enhances the risk of excessive precipitation especially towards evening, when one or several large MCSes may crystallize.
Activity will move into BeNeLux and possibly Westernmost Germany overnight while it gradually becomes elevated. The main focus of severe weather should then shift to Southernmost France, where a weak cyclogenesis over the Gulf of Perpignan will promote further storms with a possibility of all kinds of severe weather all night long. This includes in particular a risk of flash floods along the Southeastern rim of the Massif Central, which experiences sustained strong upslope flow.

... Portugal, W Spain ...

Scattered thundery showers will form in the well-mixed to slightly unstable postfrontal air mass. Towards the South, storms may still benefit from deep-layer shear up to 20 m/s beneath the departing jet streak. Marginally severe hail or wind events are not ruled out, but the risk seems to be too low for a level 1.

... other thunderstorm areas ...

Mostly diurnally-driven storms are forecast. With low to moderate CAPE and weak vertical wind shear, severe weather is unlikely, though isolated large hail and heavy rainfall events are possible.
The hail risk seems to be somewhat enhanced over Switzerland, where afternoon storms over the Alps and the Jura mountains can benefit from increasing vertical wind shear as the French jet approaches.