Storm Forecast
Valid: Mon 03 May 2010 06:00 to Tue 04 May 2010 06:00 UTC
Issued: Sun 02 May 2010 18:42
Forecaster: TUSCHY

A level 2 was issued for the Balearic Islands mainly for excessive rainfall.

A level 1 was issued for parts of the western Mediterranean for excessive rainfall, strong to severe wind gusts and tornadoes.

A level 1 was issued for parts of Hungary / Slovakia / Poland and the Ukraine mainly for large hail and tornadoes. The latter risk probably maximized over SE Poland, W-Ukraine and SW-Belarus.

SYNOPSIS

A complete breakdown of the westerlies is well underway over the NE Atlantic during the forecast with omega blocking now pushing the blocking index south of Iceland to at or above 200 m/deg lat in respect of latest 00Z GFS run analysis. The eastern trough over the Bay of Biscay and the Iberian Peninsula rapidly amplifies with the tendency of its southern part to drip off the coldest core structure of the trough further north. Downstream of this trough -e.g. parts of east-central Europe and E-Europe - an undisturbed access to an hot African air mass exists with best WAA overspreading the eastern Mediterranean. WAA continues well to the NE due to a favorable surface pressure configuration.

Along the eastern fringe of the major upper trough, an extensive surface pressure channel becomes established from NW-Africa all the way to Estonia. Various more or less defined surface low pressure areas are forecast by the models with two more prominent ones tracking from:

- Poland to Estonia (06-06 UTC)
- Balearic Islands to the Ligurian Sea (15/18 - 06 UTC).

Those depressions are embedded in a wavy surface front, which runs from SE-Spain to Estonia and further to the NE with gradual modification due to differential temperature advection, surrounding the depressions.

Using EFS data, actuated by NOGAPS, the main spread in respect of height/QPF anomalies exists over the Iberian Peninsula with respect to the major trough and the evolving surface depression. In fact, large scale features are captured quite well with ongoing discrepancies concerning of how fast the trough splits, but - not surprisingly -, the differences with the strength of the developing surface depression over/east of the Balearic Islands is still substantial. For this forecast, a combination of GFS/WRF, ECMWF,GME, NOGAPS and COAMPS were used to handle this depression with GFS/WRF used for the rest of the outlook.


DISCUSSION

... E-Croatia, Hungary, E-Austria, parts of Slovakia, SE-Poland, N-Romania and W-Ukraine ...

The NE-ward moving surface depression over Poland intensifies step by step, pushing a warm front to the north (06Z central Poland to central Belarus / 18Z Latvia to N-Belarus). Cold front remains ill defined during the fist part of the forecast, becoming more organized over NE Poland and Lithuania after 00Z, as differential thermal advection becomes stronger due to the intensifying depression. This scenario leaves behind a broad warm sector, confined to the east by a dome of dry, continental air beneath a strong high pressure. At upper levels, weak ridging overspreads the area, as the upper trough over SW Europe starts to drip off, but attendant geopotential height rises are negligible for the expected thunderstorm probabilities.

GFS seems to be on track concerning BL quality with dewpoints alread at or above 10°C and 11-15°C present south of Hungary. A streak of steep mid-level lapse rates overspreads the warm sector and the immediate area of the slowly eastward moving cold front, so 500 to isolated up to 1000 J/kg MLCAPE are a reasonable guess with maxmized values, where BL moisture maxes out.

Shear beneath weak ridging is moderate to strong with DLS of 15-20m/s, the same for the 0-3km shear along the front and good veering with height (250m^2/s^2 in the lowest 3km), so multicells and an isolated supercell are well possible. Latest data hints on highest probabilities over SE-Poland/W-Ukraine due to favorable overlap of shear/instability with an attendant large hail and severe wind gust risk. Also in this area, environmental conditions improve throughout the afternoon for LL mesocyclone development with an isolated tornado risk, especially along the front where dewpoints max out. The severe risk diminishes to the south, although an isolated large hail event is possible with 15m/s DLS and locally up to 500 J/kg MLCAPE. Overall thunderstorm coverage decreases after sunset but keeps going well into he night along the front, over SE-Poland/W-Ukraine and SW Belarus.

... E-Spain, the Balearic Islands, Corsica, Sardinia and SE-France ...

The main story for the forecast will be the developing surface depression and the approach of the strong upper vorticity maximum from the WNW, as geopotential height drops by a rate of 10gpdm/12h or more. The combination of a stalling surface boundary somewhere around the Balearic Islands, the placement beneath the left exit of a strong upper speed maximum and the approaching forcing all support the development of a quite healthy depression just south of the Balearic Islands with a constant ENE-NE-NNE motion throughout the night hours. As mentioned in the SYNOPSIS, model discrepancies are still too significant to go into too many details, so we decided to stick with a broad level area.

First interest will be the baroclinic zone close to the Balearic Islands (probably just to the north). The quasi-stationary baroclinic zone is forecast to slowly shift to the south around noon/afternoon and therefore, it will affect the Islands for a prolonged period. Increasing frontogenesis, the influx of a moist air mass (POES soundings indicate well above 20 mm PW, which coincides well with local sounding data) and a favorable divergent upper streamline pattern will result in numerous clusters of showers/thunderstorms, probably training over the Balearic Islands from east to west, producing copious amounts of rain. In fact, WRF places a 100 mm QPF maximum /12h just NW of the Balearic Islands with a gradual SE-ward progression throughout the day. A level 2 was placed along the convective southern part of the baroclininc zone.

The second area will be further east, where the surface depression undergoes modest strengthening with a rapidly consolidating warm front moving to the north, affecting Corsica after 18Z and the Ligurian Sea thereafter. Postfrontal air mass in the warm sector is characterized by high PW values and rich LL moisture but not excessively steep mid-level lapse rates. 500-1000 J/kg SBCAPE likely evolve offshore with scattered showers/thunderstorms in this weakly capped air mass increasing in coverage am strength....probably in form of a gradually strengthening cluster of showers/thunderstorms. For now, we expect the following risks with that activity:

- excessive rainfall over SE France/NW Italy, as convection moves ashore beside ongoing isentropic lift during the day. Confidence increases that most intense rainfall - convectively augmented - may start as late as 03-06Z - and hence we won't see rainfall amounts matching the level 2 criterion. Excessive rainfall risk keeps going after 06Z

- a risk for organized convection along the coast of SE France and extreme NW Italy, as warm front moves ashore. Shear, especially at low levels and mainly the directional component, increases tornado probabilities well into the level 1....next to severe wind gusts

- a risk for organized convection along an impressive shear zone between full-blown Mistral event to its west and intense WAA to its east (west of the center of the surface depression). SRH 3 values exceed 300-400 m^2/s^2 during the morning hours with deep convection well possible. Storms, moving along this boundary will ingest high-helical inflow air with a tornado and severe to damaging wind gusts risk (30m/s shear in lowest 3km).

Due to ongoing uncertainties, a broad level 1 was issued, but parts of this level may be upgraded, most likely the western Ligurian Sea and SE France.

... SW-Germany ...

A few thunderstorms may manage to develop around noon/ during the afternoon hours with somewhat colder EL temperatures indicating at least the chance for an isolated "deeper" thunderstorm. Strength and vertical extent of updrafts will be crucial in respect of a potential hail risk due to a lowering WBZ level and a 50 m/s 300 hPa speed maximum atop of the highlighted area at 15Z. Confidence in more than a very isolated stronger storm is too low to go with a level 1, but at least an isolated large hail risk can't be completely ruled out, especially when CAPE fields are better than currently anticipated. Thunderstorms decay after sunset.

... Rest of general thunderstorm areas ...

Weak shear and/or weak instability limit the risk for organized convection. Marginal hail and/or strong wind gusts remain the main hazard. This activity is mainly daytime driven, with a rapid decrease after sunset.