Storm Forecast
Valid: Tue 25 Oct 2011 06:00 to Wed 26 Oct 2011 06:00 UTC
Issued: Mon 24 Oct 2011 23:45
Forecaster: KOROSEC
A level 2 was issued for parts of NE Italy mainly for excessive (convective) rainfall/flash floods threat.
A level 2 was issued for NW Apennines and north-east Ligurian coast mainly for excessive convective rainfall, strong wind gusts and tornado/waterspout threat.
A level 1 was issued for surrounding areas of level 2 mainly for isolated threat for excessive convective rain and marginal wind threat over Ligurian sea and central Mediterranean.
A level 1 was issued for extreme NW Iberia and areas further west into Atlantic sea mainly for excessive convective rain and strong winds.
SYNOPSIS
A complex scenario unfolds across parts of Europe on Tuesday. The main feature will be a large long-wave trough placed over western Europe, blocked by the extensive ridge over eastern Europe and Russia. Several disturbance are embedded on the southern flank of the strong jet rounding the base of the trough, providing focus for convective development. At surface, a well-defined frontal zone extends from the main cyclone NW of UK towards the Alps where a secondary cyclogenesis takes place. Another frontal zone develops and slowly moves across N-CNTRL Italy towards Wednesday morning.
DISCUSSION
... NW Apennines and north-east Ligurian coast ...
A rather dangerous setup is shaping up for parts of north-central Italy today, where high rainfall accumulations could result in local flash floods and flooding. All models agree in a developing secondary low west of Ligurian sea, sliding into N Italy during the forecast period. Ahead of this low, a strong WAA delivers a very humid BL airmass towards the southern Alps. With an increasing mid-level jet, moderate shear and several hundreds J/kg of MLCAPE combine into environment conductive for intense convective storms. Very high PWAT values suggest that a combination of strong orographic lifting and convective storms could easily result in serious flash floods threat across the level 2 area. Such setups usually resulted in 100-200mm of rainfall in 24hrs period locally in the past years.
The main area of concern today is the southern part of level 2, where slowly moving frontal zone with strong southerly LLJ maintains intense showers/storms ahead of it. A combination of instability and shear seem extremely favorable for intense multicell storms/clusters maintaining over the same areas for longer period of time. A training-cells effect is likely to result is heavy rain periods. Given the quite strong LL shear and SR helicity, a couple of supercells will be possible as well. Those will be capable of producing strong winds or even a tornado threat, especially along the coastal areas where conditions will be maximized. Ahead of the main activity, coastal near SFC wind convergences could produce a couple of waterspouts as well.
Activity will continue well into the night on Wednesday when the frontal zone slowly advances towards NE further inland across the northern Italy.
... the rest of N Italy ...
Despite the very limited instablity more inland and closer to the Alps, conditions will be still in place to result in excessive rainfall threat. The main focus is needed for the areas in the level 2 where models agree in the highest rainfall accumulations. As a result of a deepening low over Ligurian sea, a WAA with persisting southerly LL flow from the Adriatic sea will result in strong orographic rainfalls. Some daytime heating will yield marginal instability and combine with moderate shear. So some storms with intense rain could take place as well. As partly combined with convection precipitation, this setup could locally result in 100mm or more of total rainfall accumulation by Wednesday morning. Expect the activity to persist further overnight and continue on Wednesday while spreading east.
Areas further south were included into the threat level 1 as the cold front races across the central Mediterranean towards east. Moderate instability and around 10-15 m/s of deep layer shear seem favorable enough for some organized storms mainly along the frontal convergence. Those should mostly be multicellular and bring isolated threat for intense rain and strong wind gusts.
... extreme NW Iberia and areas further west into Atlantic sea ...
During the second half of the period a short-wave trough with frontal system approaches Nw Iberia with a sharp surface cold front. Some instability develops across the warm sector and ovelaps with rather strong shear/SR helicity. A convective line seem possible along the front, bringing strong wind gusts threat and locally intense rain. A marginal level 1 has been issued for these areas. The front will continue to rapidly race east across the Iberian Peninsula on Wednesday.
... western France and south UK ...
Areas further north across the bay of Biscay and WNW France/SE UK will be affected by maritime airmass with very cold mid level temperatures. Expect widespread low-topped showers and thuderstorms in weakly sheared environment. Those could locally produce some marginal hail and excessive rain due to their slow moving nature. Activity will be spreading from SW towards NE as the upper cold core makes its progress towards northeast.
