Storm Forecast
Valid: Fri 19 Oct 2012 06:00 to Sat 20 Oct 2012 06:00 UTC
Issued: Thu 18 Oct 2012 22:03
Forecaster: TUSCHY

A level 1 was issued for E/NE Spain and far SW France mainly for heavy rain and an isolated tornado and hail event.

A level 1 was issued for the area east of the Balearic Islands and west of Sardinia mainly for isolated large hail, strong to severe wind gust, an isolated tornado event and heavy rain.

SYNOPSIS

A strong long wave trough is placed over W Europe with WAA downstream, enforcing ridging over S/C Europe. This results in quiescent conditions for most of Europe.

DISCUSSION

... NE Spain and SW France ...

Between a wavy frontal boundary over E Spain and W France and high pressure over Italy, strong S/SE-erly gradient flow is forecast.
Calculated forward trajectories show a favorable connection to a plume of rich low tropospheric moisture over the C-Mediterranean which advects rapidly to the NW and affects the SW coast of France. Further west, BL flow backs and veers somewhat during the forecast period with mainly a southerly component (offshore flow of dry air from N-Algeria). This keeps LL moisture over NE Spain on the lower-end side for most of the day. With lowering surface pressure E of SE Spain during the night, surface winds back more significantly to the SE and assists in the advection of better moisture towards the E/NE coast of Spain. This results in marginal BL moisture for most of the forecast period despite the 06-15Z period over far NE Spain/SW France and 00-06Z for E/NE Spain.

With warm air moving in from the SW at 500 hPa during the forecast, lapse rates at mid-levels weaken especially over Spain and France. Also no EML plume affects the area of interest, so with only temporarily existing more favorable BL moisture values, only moderate CAPE of 500-locally 1000 J/kg is forecast.

Shear remains strong with DLS of 20 m/s and marginal weaker shear in the lowest 3 km.

Various short waves eject out of the base of the upper trough and affect the highlighted area, assisting in a prolonged period of lift. Strength of those waves however remains weak at best. Therefore, scattered showers and thunderstorms repeatedly evolve over NE Spain, far SW France and just offshore until the early afternoon hours. A few well organized multicells are expected with isolated large hail, strong wind gusts and an isolated tornado event. Heavy rain however remains the predominant risk. Overall severe risk gradually diminishes after noon. As short waves pass the forecast area to the west, a temporarily enhanced wind field may also assist in either slow moving or back-building thunderstorm activity along the coastal areas with locally excessive rain.

A heavy rainfall upslope event evolves along the S Pyrenees with sporadic, embedded thunderstorm activity. A level 1 with a low prob. lightning area was expanded far to the west. The rainfall risk temporarily decreases during the overnight hours, before increasing again after the end of the forecast period.

Back-building storms might impact the Balearic Islands and areas west of Sardinia. It probably needs some type of mesoscale boundary where storms build south (e.g a convergence zone east of the Balearic Islands, pointing to the SE). Forcing at mid-levels is weak, so coverage probably remains on the lower-end side (depending on cap stregnth and LL lift) . Any storm however resides in a high CAPE/strong shear environment and can produce heavy rain, large hail, strong wind gusts and an isolated tornado. Despite questionable coverage issues, we went with a 50-% lightning area due to the chance of regionally concentrated thunderstorm clusters. Latest data points to a peak activity after sunset.

... W France ...

A plume of unstable air advects far inland beneath a strong southerly mid/upper jet. LL shear remains strong with 15 m/s speed shear and SRH-1 in excess of 200 m^2/s^2. If the thick cloud shield of the warm conveyor belt breaks open for some time, diabatic heating assists in locally enhanced SBCAPE with a more extensive MUCAPE plume covering most of W-France. Embedded thunderstorms are likely and an isolated surface based event is possible. The latter activity is able to produce an isolated tornado and strong to severe wind gust event. Of interest is the strong wind field in the lowest 2 km AGL (25 m/s southerly LLJ), which could offest the propagation component. Slow moving storms could therefore produce locally heavy rainfall amounts. Models diverge in the degree of diabatic heating which also influences the thunderstorm risk. Hence only a low probability lightning area will be issued despite the aforementioned sporadic severe risk.


... Far W-Mediterranean and E-coast of Spain ...

A strong impulse moves out of Morocco/NW Algeria and approaches the region from the south after midnight. Isolated to scattered CI is likely and a large cluster of well organized thunderstorms along the E coast of Spain is forecast. With impressive diffluence atop and a very unstable air mass in place, slow moving and heavy to excessive rain producing storms are forecast. It remains still unclear, if the main activity starts before 06Z, so for now a level 1 should cover that risk.