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Active severe weather day finally across southern Alpine region tomorrow

Finally, an active severe weather is expected tomorrow across southern Alpine region. A quite robust deep trough with a strong cold front blasts into central Europe. Lee surface low takes place over N-ern Italy, where moderate instability overlap with quite strong shear. Multicells and a few supercells with threat for large hail, damaging winds, excessive rainfalls and tornadoes are possible.

See more details below in the convective outlook I have prepared for ESTOFEX…

H85 theta-e + MSLP

H85 theta-e + MSLP

H85 theta-e + MSLP

Storm Forecast
Valid: Tue 30 Mar 2010 06:00 to Wed 31 Mar 2010 06:00 UTC
Issued: Mon 29 Mar 2010 23:57
Forecaster: KOROSEC

A level 1 was issued for Northern Italy towards Adriatic sea mainly for large hail, severe winds, excessive rainfall and lesser extend for tornadoes.

A level 1 was issued for Northeastern Italy and western Slovenia mainly for excessive rainfall.

SYNOPSIS

An active deep upper-level trough will continue spreading eastwards into central Europe while becoming negatively tilted during the late Tuesday/early Wednesday. Accompanied by this trough, a strong surface depression with an active double-cold front moves across central Europe and northern Mediterranean. Additionally, a surface depression takes place in the Lee side of the Alps over northern Italy during the day.

Early in the morning hours, a relatively strong developing sting jet behind the trough axis will likely pose a significant wind threat to the coastal areas of northern Spain and extreme SW France. Another developing sting jet situation will bring wind threat towards the morning hours on Wednesday across the English Channel as the main surface low deepens while crossing UK. However, both of these activities will not be related significantly to the convection.

DISCUSSION

The main focus for convective activity will be the aforementioned deep trough moving into central Europe where southern Alpine region will experience the most of deep convection. Different models simulated this setup quite confidently during the last few days while only minor changes were made the position of the trough. However, two spots in the large level 1 area over northern Italy has to be pinpoint:

i) … North Italy towards Adriatic sea …

Significant heights falls and quite impressive jet streak will be spreading into Alpine region, while a Lee surface low forms beneath the rather strong upper-level divergence in the left-exit jet region over NW-ern Italy during the early afternoon hours. Ahead of the system, rapid warm air advection will be spreading northwards from Adriatic sea, recovering the BL moisture with mixing ratios up to near 7-8 g/kg. Height falls and cooling mid-levels aloft will result in marginal instability release. With a developing secondary surface low, curving wind profiles from southerly to more easterly directions should be sufficient enough for formation of several low-level convergences zones across the Po valley. There, storm initiation is likely which is simulated by several models as well.

Models are predicting surface dewpoints near 10-12°C which is actually not too far away from the reality as observed values on Monday were not much different. Depends on cloud cover, instability build-up will be on going during the daytime hours. Between 500 and 800 J/kg of MLCAPE seem reasonable given the current observations. Within the jet axis, relatively strong wind shear at all levels will be present which will be supportive for organized convection once storms manage to initiate. Multicells and a few supercells will form, posing a threat for large hail and damaging winds. Despite the favorable veering winds profiles with height and quite impressive LL shear/SREH, some uncertainities are present for a tornado threat, as rather limited BL moisture limits the higher chances for tornadogenesis. Although, better LL moisture seem located toward the Po valley delta where tornadic supercells cannot be completely ruled out.

Later in the night, convection will be spreading towards the central Adriatic and western Balkans along the moving front, but instability slowly diminishes while moving more inland. With an exception of costal areas of eastern Adriatic sea, where overlaping instability and moderate shear seem maximized, no organized storms are expected.

ii) … Northeastern Italy into western Slovenia …

As the strong SW-erly flow will be persisting through much of the day, upslope flow will bring an continuous rainfall northwards towards the Alps, combined with forced convection as well as additional statiform rain from the possible storm clusters emerging from the plains and moving into the mountains. This activity will be slowly spreading eastwards during the late afternoon/evening hours. Similar situations in the past, where strong S/SW flow is present across Adriatic sea, intense rainfalls are occuring over NE Italy and parts of western Slovenia. Despite the fact, that instability slowly vanishes in the evening hours while activity moves east, embedded deep convection may allow excessive convective rainfall and therefore threat for local flash floods.

Further south towards the coastal areas of N Adriatic sea, strong southerly LL flow will create a relatively strong LL shear and isolated strong cells are possible. Given the strong shear in place, chances for a well organized multicells or even an isolated supercells exist.

… eastern France, north-central Germany …

Within the deep trough, a cool mid-level temperatures will be spreading eastwards, while at surface two fronts move across France and Germany. Ahead of the main big surface depression moving into central Europe, a weak surface low forms over central Germany, moving NE-wards during the afternoon hours. The first, weaker cold front, will cross Germany in the afternoon/evening hours, while the second one, the stronger main cold front, moves across eastern France later in the night. Only marginal shear and instability are simulated, mostly aligned to the both frontal zones, so therefore no organized deep convection is expected. However, a large hail event or two over Germany cannot be excluded given the at least slightly better conditions ahead of the surface low there.

Source: ESTOFEX outlook for March 30th, 2010

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