Weather Photos blog

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…

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

An extremely dangerous setup is shaping up for northern Adriatic sea due to high potential for hurricane force Bora winds during the next 48 hours period. Tightening pressure and temperature gradient will create conditions for very extreme wind gusts, locally exceeding 220km/h.

Not so unusual situation for Europe for the last winter decade, but quite rare setup if looking more into the details. A large ridge is placed over west-central Europe while a deepening surface low is located over western Mediterranean and moving NE-wards towards the northern Adriatic sea. As can be seen on the latest satellite image (see the Water Vapour image below), the surface low is quite impressive while forming a nice cyclonic shape just NE of Balearic islands.

The following maps show this surface low pushing NE-wards towards the large high pressure area and resulting in tightening pressure gradient all the way from central Italy towards the northern Balkans. Ahead of this system, a warm front will be pushed northwards and rapid moisture return with strong WAA from the south will be on going.

Its expected that snowing will arrive later in the evening tomorrow and night on Wednesday until Thursday’s night will be quite snowy over Slovenia and surrounding areas. Depends how north the surface low will be pushed, the snow accumulations will be likely from 10-30cm. More likely even higher in southern Slovenia on the eastern side of Dynaric mountain range (SE Slovenia) where strong SE-erly winds will be favorable for strong orographic lifting with persisting flow. There, accumulations even higher than 40cm until Thursday noon cannot be completely ruled out. But once again, all depends how north the surface low will be. Only minor changes could have big effect on evolution of extreme Bora as well as snow accumulations. Given the very cold temperatures at all levels, it will be snowing also along the seaside and probably also over the sea towards Venice and Istria. The extreme winds will be creating snow drifts and will be pushing the driving conditions to very unfriedly, as zero visibility will be locally possible.

But what is the most impressive with this setup is, as already said, an extremely dangerous potential for very strong to hurricane force Bora winds. Such model simulations that can be found today were surprisedly never seen before, as especially local ALADIN model has 850mb and 925mb level average wind speed as high as 35 to more than 40m/s along the Dynaric mountain range. Sometimes values up to 35m/s can be seen on the maps, but this time it seems like a record breaking values!

At surface level winds are also incredibly impressive with values even above 30-35m/s average speeds locally, which becomes extremely dangerous for lowlands where people live. A special ALADIN products for simulated wind gusts at 10m heights are predicting gusts near or above 180km/h in SW Slovenia and Trieste gulf, while Kvarner area has values near 220km/h. This has never been seen before! And what is even more impressive is the fact, that even a very good models like ALADIN for wind speed simulations are usually underestimating values for max gusts possible, so its is quite likely that is this setup realizes like it shows now, gusts could be even 15-20% higher than currently forecasted.

For this reason of expected very extreme winds gusts, our national meteorlogical agency ARSO has issued a Red Alert for SW Slovenia and Primorska region.

It has to be noted that as can be seen on the maps above, the wind direction will be mostly from the east, which is usually very favorable for extreme gusts over Karst region into Trieste gulf, along with classic well known Bora places. I do believe that record breaking wind gusts could occur locally, if not over mostly all places.

To repeat once more; issued Red Alert for SW Slovenia tomorrow and day after means that this will be extremely dangerous and life threatening situation where a lot of damage could be done on buildings as well as to people lives. When such winds are blowing, a lot of rocks, trees and other objects are flying in the air and can kill people, so be prepared for such dangerous events during the next 48 hours.

And stay tuned for possible updates or analysis later!

I am in time shortage today, so this will only be a quick update/outlook for tomorrow. Winter period is returning into Alpine region again, as the blocking pattern has established over western Europe with strong ridge forming. This will allow polar/arctic airmass to blast into central Europe and Mediterranean and lead into colder than average temperatures during the next 5-7 days at least. First, some snowing is expected tonight and tomorrow as the surface depression approaches from the SW. Light to moderate snowing is likely over most of Slovenian inland areas, except the lowlands in SW Slovenia. Only 5-10mm are forecasted by the models, with snow mixing down to near 300m or slightly lower. Ground accumulations are quite questionable given the warm period in the last days, so the ground has temperatures several degrees above zero. However, up to 15cm is still possible locally, especially a bit higher, above 500m or so. Lower places might have only few cm deck according to the most optimistic scenarios.

Additionally, very strong to extreme Bora is forecasted along the whole eastern Adriatic coast as well, as tightening gradient and increasing temperature difference along the Dynaric mountain range creates perfect conditions for strong gusts. Locally, gusts will be reaching above 150km/h (e.g. Vipavska valley).

I hope I’ll find some time tomorrow afternoon to edit this post and atttach some of the interesting model maps for this event. Stay tuned for future updates!