Storm Forecast
Valid: Tue 09 Jul 2013 08:00 to Wed 10 Jul 2013 06:00 UTC
Issued: Tue 09 Jul 2013 08:19
Forecaster: PISTOTNIK

A level 1 was issued for NE Spain (parts of Catalonia) for large hail and severe wind gusts.

A level 1 was issued for Central Spain for severe wind gusts.

A level 1 was issued for parts of S Norway and S Sweden for severe wind gusts and to a lesser extent for large hail and tornadoes.

SYNOPSIS

Flanked by an extensive surface high centered off the British Isles, weak geopotential gradients persist over much of the continent. Cooler-than-usual upper levels and relatively rich low-level moisture allow low to moderate instability across the Southern half of Europe.
The main frontal zone is deflected to the far North, where a short-wave trough and an attendant low pressure system cross Scandinavia.

DISCUSSION

... Spain ...

After days of unimpeded insolation and no air mass exchange, forecast models get more confident in scattered convective initiation over much of Spain, aided by weak vorticity advection from the Northwest. However, since the maritime boundary layer with rich low-level moisture (dew points around 18°C along the North coast) is too cool to break the cap, storms will likely be confined to the hot and very dry inland areas. Deeply mixed profiles will keep CAPE to a minimum and entrainment may impede more persistent updrafts, but strong evaporative cooling beneath the very high cloud bases points to a risk of localized severe downbursts with any deep convection that forms. A level 1 is issued to cover that risk.
Similar to yesterday, storm formation is more likely over the Pyrenees. Advection of coastal moisture up the Ebro valley with Southeasterly surface winds can enable stronger instability release and enhance deep-layer shear to 15 m/s. Therefore, storms that manage to detach from the Pyrenees and to move/propagate to the South can organize into multicells or even marginal supercells. A level 1 was issued for a chance of large hail and severe wind gusts.

... other high probability thunder areas ...

Widespread afternoon storms will form again over orographic features. CAPE is expected to be on the order of 1000 J/kg (refer to Monday 12 UTC's Payerne, Milano and Cuneo soundings) but vertical wind shear is near zero. Strong pulse storm pose a primary risk of localized heavy precipitation and a secondary risk of marginally large hail. However, the lack of any clear centers with a maximization of these threats precludes a level 1.
In case of merging and clustering storms with a more organized cold pool formation, an isolated severe wind event may occur as well (refer to yesterday's F1 downburst in Kraljevo, Serbia). In that case, storms can also propagate a little distance into the forelands. Otherwise, flat areas won't see a lot of activity in the absence of other trigger mechanisms apart from upvalley/upslope circulations.

... Scandinavia ...

A pronounced short-wave trough crosses Scandinavia from the West and overruns a cold front. Forecast models agree on the formation of a few hundred J/kg of CAPE in a limited region of Southern Norway and Southern Sweden. Latest surface observations (Tue 6 UTC) show dew points around 10°C over the area of interest, but moister air may soon be advected inland with prefrontal Southwesterly winds, which is why the model forecasts are thought to be on track. Vertical wind profiles exhibit strong deep-layer shear (20-25 m/s) and also some low-level veering. Scattered multicells or supercells are possible in the afternoon hours, which will likely soon merge into a line, given the strength of linear forcing. A swath of severe wind gusts is well possible along the line, while more isolated storms in an initial stage also pose a secondary risk of tornadoes (0-1 km wind shear in excess of 10 m/s) and marginally large hail. Activity will taper off over the Baltic Sea in the evening hours.