Storm Forecast
Valid: Tue 15 Jun 2010 06:00 to Wed 16 Jun 2010 06:00 UTC
Issued: Mon 14 Jun 2010 20:55
Forecaster: KOROSEC

A level 2 was issued for northern Africa across south-western Mediterranean towards parts of central Italy and towards SW Alps mainly for excessive convective rainfall, large hail, strong wind gusts and tornadoes.

A level 1 was issued for areas surrounding the level 2 including SE France, whole north-central Italy and north-central Adriatic sea mainly for excessive convective rainfall and lesser extent for large hail and tornadoes.

A level 1 was issued for parts of western Balkans mainly for large hail.

A level 1 was issued for southeastern Spain mainly for large hail.

A level 1 was issued for parts of eastern Romania, most of Ukraine, Moldova and western Russia mainly for large hail, severe wind gusts and excessive convective rainfall.

SYNOPSIS

A large long-wave trough over western Europe, now acting like a well defined upper low, remains rather stationary during the forecast period over Pyrenees. Defined by two vorticity maxima, one over N Spain/Pyrenees and another one moving across the level 2 area NE-wards towards northern Mediterranean. At surface, a deepening low is located over Balearic sea. A cold frontal boundary stretches from SW Alps towards towards northern Africa, slowly moving NE-wards. A warm front is advecting northwards across the Alps during the late afternoon/evening hours.

Another frontal boundary is extending from western Russia SW-wards across SE Balkans states where another active convective day is expected across the unstable environment placed there.

DISCUSSION


... areas from northern Africa towards northern Italy, SE France, north-central Adriatic sea and parts of western Balkans...

A very robust and complex setup unfolds across the level 2 area today. Ahead of the large trough/upper low over SW Europe, a relatively strong SW-erly jet streak advects hot and moist airmass towards the northern Mediterranean. Strongly sheared environment (20-30 m/s of deep-layer shear and around 10 m/s of 0-1km shear) aross the warm sector, overlaping with moderate instability, will create environment for organized severe storms. Large scale ascent and strong forcing will there support widespread deep convective initiation ahead of the surface front. Numerous well organized multicells and supercells are likely, merging into large clusters with bowing segments possible. Large to very large hail, strong winds and tornadoes will be possible. Given the impressive amount of moisture throughout the troposphere and high values of LL moisture inflow, excessive convective rainfall could become an issue as well. Combined with strong orographic precipitation in NW Italy, both could result in flash floods or even larger scale flooding events.

In the late afternoon/evening hours, activity continues moving across N Italy and N Adriatic sea. Relative to the position of the surface low, ESE-erly winds will enhance the LL shear/SREH across north-central Italy and near 300 m^2/s^2 of SREH1/SREH3 should result, as well as around 10 m/s of LL shear. Given the at least marginal instability in place, supercells with threat for tornadoes could form. Limiting factor for more temperature insolation could be and extensive cloud cover due to ongoing large cluster on early morning hours.

Different models then agree in high amount of precipitation overnight on Wednesday, likely partly generated from the stratiform rainfall of past large clusters as well as additional convective activity given the still at least marginal instability in place. For this reason a level 1 was also extended across the N Adriatic sea areas and NE Italy.

A level 1 was extended also towards the western Balkans given the models agreement in rather high instability (MLCAPE of AOA 1500 J/kg) release with near-surface convergence along the eastern slopes of Dynaric mountain range. Placed beneath the near 15 m/s DLS, but with stronger capping layer aloft, a few isolated severe storms with large hail can form.

... Bay of Biscay and parts of Spain ...

Eastern and SE Spain will see storms initiation along the surface convergence as soon as mid-level lapse rates steepen with an approaching colder airmass within the low. A couple of well organized multicells or even supercells can form as good ascent seems likely to support storms initiation under the upper-level divergence in left-jet exit region right over SE Spain. Locally, marginally large hail can occur, given the overlaping of 500-800 J/kg MLCAPE and near 20 m/s of deep layer shear. The bay of Biscay and northern Spain will be placed beneath the upper low centre, where weakly sheared environment with LL steep lapse rates suggest that a few spout-type tornadoes could occur. However, limited LL instability doesn't warrant a threat level there.

... eastern Romania, most of Ukraine, Moldova and western Russia ...

A healthy environment with very warm/hot airmass and rich boundary layer moisture placed ahead of a moving surface front will rapidly become unstable towards the midday. Strong surface heating will soon overcome the weakly capped inversion layer and convective initiation will first take place around the local higher topography and then continue across the level 1 area along the surface front. Moderate deep-layer shear near 10-15 m/s and strong instability with values locally again above 2000 J/kg will support explosive convection. Threat for large to very large hail, damaging winds and locally excessive convective rainfall due to rather slow storm movement and high moisture content will exist. Bordeline shear limits the higher threat level attm, but this area will be monitored for possible upgrade into level 2 if needed.