Extended Forecast
Valid: Sun 18 Sep 2011 06:00 to Mon 19 Sep 2011 06:00 UTC
Issued: Fri 16 Sep 2011 17:13
Forecaster: KOROSEC

A level 2 was issued for parts of N Italy, W-CNTRL Slovenia and N Adriatic sea mainly for large hail, excessive convective rainfall, severe wind gusts and tornadoes.

A level 2 was issued for western parts of central Italy mainly for excessive convective rainfall and lesser extend for large hail and strong winds.

A level 1 was issued for north-central Mediterranean mainly for large hail and strong to severe wind gusts, excessive convective rainfall and isolated tornadoes.

A level 1 was issued for areas covering Austria, Slovakia, Czech Republic, Hungary and Poland mainly for large hail, strong wind gusts and excessive convective rainfall.

A level 1 was issued for parts of North Sea and coastal areas of Netherlands and Belgium mainly for water/landspouts and excessive convective rainfall.

SYNOPSIS

A complex scenario unfolds across the northern Mediterranean on Sunday into early Monday. A powerful upper trough/low with well-defined DCVA couplet ejects from western Europe and crosses the Alps and unsettled conditions result over the region.

At surface, a secondary low forms over N Italy during the day, while a strong cold front moves across central Europe and the Alps. Front slows down a bit overnight to Monday due to strong WAA on going ahead on the surface low. This enhances a rainfall threat across the northern level 2 area.

Elsewhere, a high pressure area builds up over NE Europe and Scandinavia while a rapidly building ridge is spreading into the western and SW Europe.

DISCUSSION

... parts of N Italy, W-CNTRL Slovenia and N Adriatic sea ...

Activity should start quite early over the Alps due to persisting WAA ahead of an approaching trough/low and a developing secondary low over N Italy. Strong WAA will be in place with healthy BL moisture across the northern Mediterranean. Mixing ratios around 10-12 g/kg and SFC dewpoints maintaining from upper 10s to low 20s will advect from the Adriatic sea northwards, providing source for moderate instability. Different models have some troubles with correct simulations, but its likely that AOA 1000 J/kg of MLCAPE can reasonably be expected in the afternoon. Once the DCVA approaches, strong QG forcing ahead of it will favour storms development along the pre-frontal convergence zones and organize into numerous multicells given the moderate to strong deep layer shear in place. GFS suggests around 20-25 m/s bulk shear and up to 10 m/s of LL shear, while local models has slightly more enhanced shear in the lowest levels. With a SFC low over N Italy, enhanced SREH should support supercells as well. Threat for large hail, strong to severe wind gusts and very intense rainfalls will exist. With low LCLs and enhanced LL SREH/shear, a couple of tornadoes cannot be excluded with the most discrete cells.

Areas more inland towards the mountainous terrain should experience persisting intense rainfall throughout the day and high accumulations could result from an orographic rainfalls, combined with storms in strong shear and unstable airmass. Activity will slowly advance eastwards towards Slovenia. In the evening hours when the cold front ejects from the Alps, it slows down due to stronger advection from the Adriatic sea. This result in intensifiction of precipitation and enhanced flash floods risk will take place across the level 2 area. This might result in a large MCS or a few big clusters along the Adriatic sea affecting the NW Balkans overnight. Combined with strong SSW-erly flow in mid-levels, very warm SSTs and moderate instability, high rainfall accumulations are expected with these storms. With persisting strong southerly LL winds, training cells effect appears possible in these areas, resulting in high amount of rainfall in short period of time.

... North-central Mediterranean including western parts of central Italy ...

Moderate to strong instability is forecast for central Mediterranean under the strong mid-level jet rounding the base of the Alpine trough/low. This will result in favorable kinematics and dynamics for organized storms. However, due to stronger capping, activity will be more isolated than further north. With at least 25 m/s of shear, locally 100-200 m^2/s^2 SREH3 and 1000-1500 J/kg of MLCAPE in place, well organized multicells and supercells will be possible near the moving front. Storms should cluster into larger system or two later on. Parts of central Italy have been placed into a level 2 threat given the favorable environment there for persisting upslope flow, causing intense orographic rainfalls as well as some organized severe storms. High rainfall amounts and local flash floods will be possible. Activity will be spreading SE-wards towards the Monday morning as the front moves further across the central Mediterranean.

... areas covering Austria, Slovakia, Czech Republic, Hungary and Poland ...

A kind of classic setup for areas across the eastern Alpine region and further NE. This region will be mostly affected by the main cold front crossing central Europe, placed in moderately sheared environment but with rather limited instability. A developing cold pool and persisting SE-erly flow ahead of the front will bring the main threat into strong wind gusts, excessive rainfall and large hail. Later overnight, activity will spread eastwards along the moving front with mostly elevated storms expected across parts of Poland and Slovakia.

... parts of North Sea and coastal areas of Netherlands and Belgium ...

In the wake of the main upper trough/low, very cold mid-levels will overspread parts of NW Europe and several hundreds of MLCAPE will result from a daytime heating. A rather widespread activity should take place along the coastal front as still good forcing remains in place. The center of the upper low will be placed over the Netherlands and Belgium. A weak pressure gradient with limited wind field, steep LL lapse rates and high LL CAPE suggest that a couple of waterspouts/landspouts can be possible across the level 1 area. Given the slow moving cells and high moisture content, excessive rainfalls could locally be possible as well.