Storm Forecast
Valid: Mon 22 Jul 2013 06:00 to Tue 23 Jul 2013 06:00 UTC
Issued: Sun 21 Jul 2013 23:15
Forecaster: TUSCHY

A level 2 was issued for NE Algeria and N-Tunisia mainly for very large hail and severe downbursts. Local flash flooding is possible.

A level 1 was issued for far N-Spain, W-France, UK and parts of Ireland mainly for large hail, strong to isolated severe wind gusts and heavy to excessive rainfall amounts. An isolated very large hail event over W-France is possible. An isolated tornado event over NW-France and SW UK is possible.

A level 1 was issued for extreme SE France, S-Switzerland and extreme NW-Italy mainly for isolated large hail.

Numerous level 1s were issued for parts of C-Italy, Corsica and Sardinia mainly for isolated excessive rainfall amounts.

A level 1 was issued for parts of far W-Russia mainly for an augmented tornado risk.

SYNOPSIS

An omega-like blocking pattern continues over Europe although the ridge in-between does not look healthy at all. In fact a gradual weakening trend is forecast during the day and thunderstorm probabilities increase along its western/southern fringe.
Two upper troughs flank the high along its western and eastern fringes. Especially the western one (over the far E-Atlantic) provides increased thunderstorm probabilities during the forecast and also for the following days.
Numerous vortices with marginal cooler mid-level air cross the C-Mediterranean and the Aegean Sea from west to east and result in isolated to scattered thunderstorm activity over Italy and Greece.

DISCUSSION

... N-Spain, W-France, UK, Ireland and Scotland ...

A slowly improving set-up for DMC is forecast over an extensive area over far W-Europe with a peak overlap of most supportive ingredients during the overnight hours.

A convergence zone is forecast to become better organized during the forecast period and will be the focus for active thunderstorm development. This convergence zone evolves due to the coincidence of a drier NE-erly surface flow (steered by the strong high pressure area, which extends from Norway to the Alps) with dewpoints in the mid tens and a strengthening westerly flow ahead of an approaching Atlantic depression. This westerly flow advects a very moist air mass from the S/SE Bay of Biscay to the NE and E with MIMIC-TPW showing values in excess of 40 mm! This air mass evolved during the past few days over that region and is now ready to spread to the NE. Therefore we would not be surprised to see surface dewpoints in the upper tens and lower twenties to affect far W-France with a gradual advection to the N/NE towards UK. To summarize all that, we expect a sharpening convergence zone from SW-UK to the western Pyrenees. This boundary offers marginal eastward motion over W-France with a faster motion to the NE over UK during the day. As a consequence of the persistent convergent flow, BL moisture deepens and becomes less vulnerable to diurnal mixing.
A northward spreading Spanish Plume becomes washy betimes with mid-level lapse rates of around 7 K/km forecast. Very healthy BL moisture however offsets that trend and we expect a belt of 1500 to 2500 MLCAPE to evolve along that north-south aligned convergence zone with less CAPE over Ireland/N-UK. GFS and EZ disagree a bit regarding the placement of that convergence zone, but other than that, both models forecast a very healthy looking CAPE environment.

6 km bulk shear along the convergence zone remains weak with 5 to 10 m/s during the daytime hours, increasing to 10 to 15 m/s during the overnight hours. The reason for that is the ongoing diffluent mid/upper level flow regime during the daytime hours with no potential forcing mechanism seen in model data. During the night however, an upper level trough rotates from the SW-Bay of Biscay to Ireland and Scotland. Attendant tightening of the mid-level height gradient results in an increase of DLS during the night.

As mentioned above, no appreciable forcing is forecast to affect the area of interest until 18Z. After sunset, a modest PVA maximum approaches from the west and induces enhanced lift in the highlighted area.

Initiation will occur within numerous time frames. First, showers and thunderstorms probably evolve over Ireland and UK before noon. This activity is forecast to grow upscale during the afternoon hours and numerous northward moving thunderstorm clusters affect Ireland, UK and Scotland.
During the afternoon hours, initiation also occurs over NW/W France and far N-Spain, but more on an isolated scale. Nevertheless, abundant CAPE and expected increasing upper level divergence during the evening hours also indicate a good chance for numerous thunderstorm clusters to evolve with that activity. Those clusters also tend to move to the N/NE. Finally after sunset, widespread thunderstorm development is expected from N-Spain into Scotland with numerous large thunderstorm clusters moving to the N/NE.

Probably the least severe risk exists with the first thunderstorms, which evolve before noon as this activity is still slightly elevated with forecast soundings showing a weakening cap between 800 and 900 hPa. Still, 500 to 1000 J/kg elevated CAPE likely result in an isolated large hail and strong wind gust event. The later severe risk could become bimodal: existing WAA thunderstorms move to the N ahead of the uncapped CAPE/thetae-plume over S/C-UK and remain elevated with an isolated severe risk. The southernmost storms however could start to build into the weakly capped and very unstable air mass to the south (over S/C-UK) and could become severe due to MLCAPE now approaching 1000 to 1500 J/kg and more surface based activity due to a weakening cap. Other thunderstorm clusters, which evolve during the afternoon and evening hours over S-UK will also contain an enhanced severe risk due to large hail and strong to isolated severe wind gusts. The heavy to isolated excessive rainfall risk also increases during that time frame.

Isolated thunderstorm activity over W-France and extreme N-Spain during the afternoon hours features rapid thunderstorm growth due to strong CAPE signals. Once again, large hail and strong to isolated severe downbursts will be common with that activity. PWs of 35 to 40 mm, slow storm motion and rapid clustering also point to an excessive rainfall risk. Scanning through a few forecast soundings from that region, fat CAPE profiles within the hail growth layer might result in an isolated very large hail event (hail diameter in excess of 5 cm). Weak kinematics and therefore lower confidence in an extreme event preclude a level 2 for now.

We included an isolated tornado risk mainly for NW France and SW-UK especially during the late afternoon and evening hours, when LL shear increases a bit and LCLs drop to less than 1 km along/just west of the convergence zone.

Once again, a similar severe risk (large hail, strong wind gusts and heavy to excesive rainfall amounts) is forecast with the widespread overnight activity within the complete level 1 area. Stronger shear might support a few well organized multicells, which continue until 06Z while drifting to the NE. Those storms continue to remain surface based from W-France into S-UK and become more elevated further north.

... Pyrenees, SE France, Switzerland, parts of Austria, parts of Italy, Corsica and Sardinia ...

Daytime driven thunderstorms are forecast. 500 to 1000 J/kg MLCAPE but DLS less than 10 m/s support numerous intense pulse storms, which grow upscale into temporarily better organized small thunderstorm clusters. Those clusters show an erratic forward propagation, driven by expanding cold pools. A few large hail, strong to isolated severe downburst and heavy rainfall events are likely. Would not be surprised to hear from an isolated excessive rainfall event over Sardinia/Corsica and C-Italy, where BL dewpoints remain in the lower twenties and PWs stick around 30 mm. Overall thunderstorm coverage decreases after sunset, but an isolated event can't be ruled out until 06Z ... especially along the S-Alps.

... N-Algeria and N-Tunisia ...

No change compared to the past few days is expected. 1500 to 2000 J/kg capped MLCAPE and 20 to 25 m/s DLS are forecast. Confidence in initiaion is higher compared to the previous days due to the passage of a stronger mid-level wave to the N. Therefore a level 2 was issued for NE-Algeria and N-Tunisia as numerous high-based supercells with very large hail ( hail large than 5 cm in diameter) and severe downbursts are forecast. Local flash flooding might occur despite forward propagating thunderstorms.

... Parts of extreme W-Russia ...

A surge of a warm and unstable air mass along the backside of a strong cyclonic vortex is underway and spreads west during the day. Scattered thunderstorms are forecast during the afternoon and evening hours. Shear and CAPE fields remain displaced during the forecast and even enhanced LL shear can be discounted due to increasing LCL height. Therefore expect a few large hail and strong wind gust events.

An exception is the level 1 area, where significant SRH-1(in excess of 200 m^2/s^2) and lowering LCLs overlap. A few tornado events are possible.