Storm Forecast
Valid: Mon 13 May 2013 06:00 to Tue 14 May 2013 06:00 UTC
Issued: Mon 13 May 2013 02:00
Forecaster: TUSCHY

A level 2 was issued for parts of Latvia, Lithuania and a region south of St. Petersburg mainly for large hail ... a few significant events are possible.

A level 1 surrounds the level 2 mainly for an isolated tornado, numerous severe wind gust, large hail and isolated flash flood reports.

A level 1 covers regions from W-Belarus all the way towards the Aegean Sea/Turkey mainly for a few severe wind gusts, isolated tornado and large hail events. Serious/excessive flash flooding on a local scale is possible, especially over and south of Bulgaria.

SYNOPSIS

A potent cyclonic vortex north of Scotland affects most parts of NW Europe and results in windy and unsettled conditions. A decaying mid-level trough over C-Europe reinforces the high over low blocking pattern over E-Europe. An extensive N-S aligned convergence zone over E-Europe remains the focus for scattered to widespread CI.


DISCUSSION

... S/C Spain ...

Anticipated thunderstorm activity will be a mixture of diurnal driven CI and increasing mid-level forcing beyond 18Z.

During the daytime hours, differential theta-e advection continues with rich LL moisture advection towards the north ... only increasing betimes with start of faint cooling at mid-levels from the west during the afternoon hours onwards. Diabatic heating will be strong as only some cirrus of the grazing subtropical jet affects parts of S-Spain. Forecast soundings show potential CI as early as noon along the mountainous orography. EZ is a bit slower and less aggressive with mid-level lapse rates / CAPE build-up and with yesterday's soundings in my mind and with expected temporary strengthening of a thermal ridge over Spain ahead of an approaching trough, we currently trend more to the less aggressive EZ solution, keeping MUCAPE at or below 500 J/kg. Isolated CI is expected and strong upper ventilation by the nearby subtropical jet should result in a strong pulsating thunderstorms with an isolated large hail and strong downburst risk. Coverage will be too marginal for any level area.

During the overnight hours, the second and way more active round of storms occurs ahead of a weak mid-level impulse, which approaches from the west. A plume of modest MUCAPE remains in place with 20 m/s DLS, so despite the scattered character of storms, only an isolated large hail risk is anticipated. For this event, a 50-% lightning area was added.

... Latvia, Lithuania and an area south of St. Petersburg ...

With a dissolving vorticity lobe entering the scene from the west, a N-S aligned convergence zone strengthens, as deep easterly flow continues along the southern fringe of a Russian high pressure area. Along that convergence zone, maximized BL moisture beneath steep mid-level lapse rates causes strong MLCAPE build-up in the order of 1-1.5 kJ/kg with locally higher values expected. With a 20 m/s speed maximum at mid-levels approaching from the south, DLS increases to 15-20 m/s with strongest shear confined to the lowest 4 km. Forecast soundings show a modestly sheared hail growth zone with abundant mid-level CAPE, so large hail (a few significant events possible) will be the main hazard. Dependent at which side of the boundary/convergence zone storms evolve, in addition to the hail risk, a severe downburst risk (south/east of the boundary with LCLs in excess of 1.5 km and inverted-V profiles) or an isolated tornado risk (west and north of the boundary, where moist BL and some directional shear are forecast) may accompany those storms. Betimes, a large cluster of storms is forecast to gradually shift E/NE-wards during the overnight hours. In addition, repeatedly CI is expected all night long along that convergence zone, which remains more or less stationary.
The level 2 was issued for the significant hail risk with a broad level 1 likely covering the rest of the expected hazards. We thought about expanding the level 2 more to the east to include the chance for a cold-pool driven MCS, given deeply mixed profiles south of that convergence zone, but missing forcing, which could play the focus for enhanced CI, preclude a broader level 2 for now.

... E/SE Europe ...

A weakly capped air mass with 500-800 J/kg MLCAPE covers a broad area over E/SE Europe. Incoming vorticity lobe from the west increases convergence and a mixture of weakly forced storms and diurnal driven thunderstorms is forecast. South of Romania, the influce of a broad upper-level trough also plays a role in CI and we expect a belt of scattered thunderstorms to extend from Belarus all the way to Turkey. Despite weak shear, intense pulsating storms are possible with a temporal severe downburst risk mainly from W-Belarus to N-Romania, where deeply mixed subcloud layer exists and an isolated tornado risk over and south of Romania, where enhanced LL CAPE is expected. Also, this convergence zones reveals deep convergence with very moist profiles, so slow storm motions and high PW values increase the risk of local flash-flood producing storms and/or the risk of a huge amount of small hail. Despite the bad kinematics, expected strength of pulsating thunderstorm activity caused a N-S aligned level 1 area.

W-Turkey and most of Greece were both added to the level 1 as very weak storm motion increases the risk of training showers and thunderstorms with locally excessive rainfall amounts and I would not be surprised to see 50-100 mm rain in a short amount of time beneath slow moving clusters....similar to Bulgaria. Also, a huge amount of small hail can accompany strongest storms next to a few wet downburst events.

Crete was added to the level 1 due to isolated daytime convection, which could become severe (20 m/s DLS and 500-800 J/kg MLCAPE). Large hail and strong wind gusts will be the main hazard. During the night, a cluster or showers and thunderstorms approaches from the NW with an heavy rainfall risk.

... UK, parts of the North Sea and Denmark ...

Diurnal driven convection is forecast mainly atop onshore areas, where heating beneath cold mid-levels assists in modest SBCAPE build-up. Weak LL shear and only a concentrated swath of cold EL temperatures and strong DLS (C-UK) exist, so stronger storms may contain marginal hail/sleet and strong wind gusts. Also, I don't want to rule out a few cold-core funnel (isolated short-lived tornad) reports. After sunset, convection rapidly decays over UK but sporadic storms may now occur over Denmark ... especially after midnight as mid-level temperatures start to crash.