Weather Photos blog

It looks like a weekly periodical frontal passage has setled up across Europe, as another trough with cold front affects southern Alpine region overnight from Friday to Saturday. Similar that happened exactly a week ago. Again, a strong Mediterranean ridge with very hot airmass provides stable conditions there, while rather rich boundary layer moisture is present across Northern Mediterranean and southern Alpine region. 10-15g/kg 0-1km mixing ratios are seen from the latest surface observations across Slovenia and northern Italy.

Another broad trough crosses central Europe during the next 24hrs and the weakening frontal boundary, that currently sits on the northern side of the Alps, slowly shifts into southern Alpine region overnight and tomorrow morning. Quite strong trough/frontal forcing should provide widespread convective activity mostly overnight, while a few isolated storms are possible in the afternoon hours.

A strong jet streak accompanied by this trough will support very high deep-layer shear up to 70kts in the 0-6km. Once the trough digs more southwards, a classic Lee weak surface depression should take place over northern Italy and therefore backing wind profiles in the lowest levels are likely. Therefore enhanced LL shear and SREH can be expected, GFS predicts availability of 15-30kt wind shear in the lowest 1km while enhanced SREH values up to 300m^2/s^2 are possible.

Aforementioned rich boundary layer moisture should be enough for MLCAPE release near 1000-1500 J/kg, resulted by strong diurnal heating and moderate mid-level lapse rates in the afternoon. ML lapse rates will be slightly improving later towards the evening/overnight as trough starts to dig in.

Until the evening coverage of the storms and especially storm intiation across northern Italy and Slovenia are questionable given the only weak to moderate mid-level lapse rates as well as pretty strong capping inversion placed there during the last few days. Models do suggest there is a possibility for initiation along the LL convergence across Po valley and Scirocco winds from Adriatic and dry SW-erly winds from Apennini converging together could serve enough pre-frontal forcing for an isolated cell or two. Especially GFS model is more confident with weakening capping inversion over N-ern Italy in the afternoon, but it is often mistaken as well. There, if storms manage to initiate before the evening, the potential for well organized multicells and supercells exist, given the very strong shear and helicity overlaping with moderate instability. The main threat is large hail and damaging winds. Enhanced LL shear/SREH values and rather low LCL heights also suggest LL rotating updrafts and minor threat for tornadoes.

Later on in the evening/overnight, a rapidly clustering storms are expected forming a large MCS across the Alps moving mostly ESE-wards as strong frontal forcing will provide widespread intiation. Potential for locally large hail but mostly damaging winds exists. All models are quite confident about the overnight convective activity across the Alps.

The cold front that will be likely across Slovenia in the morning hours will then move slowly SE-wards onto the Balkan states tomorrow by day and continue to weaken. Cooler conditions are expected behind the front as well as NE-erly winds (Bora), until Monday when Mediterranean ridge with warmer/hot and stable conditions returns.

At ESTOFEX, a level-1 threat level was issued for southern Alpine region until tomorrow morning. Questionable coverage of storms before the nightime MCS limits the higher threat level, but as mentioned in the forecast discussion, southern Alpine region could be upgraded into level-2 threat levels if conditions change dramatically during the day. As given the impressive shear and moderate instability overlaping together, robust severe storms would be possible, if they manage to form earlier.

A level 1 was issued for N-Italy and W-Switzerland mainly for large hail and severe wind gusts.

DISCUSSION

… N-Italy, Slovenia …

A trough passes by north of the Alps and the attendant forcing affects the area during the forecast period. At the same time, a cold front is gradually shifting southwards. Up to 1000 J/kg MLCAPE along the S-Alps are a reasonable scenario with 35m/s DLS, so multicells and supercells are possible with a large to very large hail and severe wind gust threat, the latter one enhanced by roughly 20m/s 0-3km shear. Models still disagree in the final coverage and we stay with a level 1 for now. An update may become necessary tomorrow.

Source:

I’ve had another great chase day on Friday, July 17th 2009. I was expecting storm intiation by 13-14z over central Po valley in SE Lombardia and got there in time. Two incredibly impressive supercells formed, both dropped few funnel clouds and at least one confirmed tornado, the second possible tornado was not confirmed yet. Here are just a couple of photos, the rest can be seen at the bottom, where links to the longer chase report and other photos are posted. I was also recording time-lapse video which can be found below as well.

I’ve observed both tornadic supercells, seen 4 funnel clouds and got blasted by large hail 3-4cm in diameter 4 times. It was falling without a drop of rain almost 15km away from the main tower! The structure were totally insane, one of my best so far in Europe! I got some good memories of those similar structures in Tornado Alley…

Many more photos can be seen here:

You can read a chase report here:

Here is the time-lapse video:

A very strong and potentially dangerous negatively tilted trough crosses the Alps in the next 24-36hrs and severe storms with threat for very large hail, damaging winds, excessive rainfalls and tornadoes, some of them may be strong, seems likely to occur. Impresively rich boundary layer moisture with SFC dewpoints from 20-24°C are observed over the Po valley, with well-mixed EML overspreading the region during the afternoon hours. Steepening lapse rates will yield high CAPE and create explosive environment. Unusually strong mid-level jet streak accompanied with this trough will overlap with high instability and dangerous storms can be expected. Models have a clear near-surface strong convergence over the Po valley once the EML and SW-erlies eject in from the Ligurian bay and initiation is likely along the quasi-dryline extending from western Piemonte into western Veneto region. Given the forming surface depression over N Italy at the same time, which will provide backing wind profiles, supercell storms are the primary mode and given the low LCL and high PWAT values, HP supercell type should occur. Risk for very large hail and tornadoes exists as storms will be pretty isolated before evening, when a strong squall-line will enter NW Italy, connected to a large MCS trailing along the eastwards moving cold front. One of two large convective clusters will then move into NE Italy and Slovenia overnight and tomorrow morning. Locally, large amounts of rainfalls can therefore be expected given the high instability, strong forcing, shear and high PWAT values.

As for tomorrow, strong cold front will continue to race ENE-wards across Slovenia and severe storms with strong winds and hail are likely over eastern half of the country and further NE-wards into Hungary and eastern Austria in the afternoon. Elsewhere hail threat is lower, while excessive rainfalls and damaging winds are higher threat.

I’ve put some more thoughts into the outlook I prepared for ESTOFEX, see the graphics below. A detailed forecast as regarding the situation over the southern Alpine region can be found as well. It has to be noted that this outlook was prepared on yesterday evening and is based on yesterday’s model runs. But today’s 00z and 06z runs are even more robust and more confident about the storm initiation over the northern Italy later today. Milano morning 06z sounding already showed impressive instability and shear overlapping together…

Test Forecast
Valid: Fri 17 Jul 2009 06:00 to Sat 18 Jul 2009 06:00 UTC
Issued: Thu 16 Jul 2009 22:01
Forecaster: KOROSEC

A level 2 was issued for Switzerland, W Austria, extreme W Slovenia and N Italy mainly for tornadoes, large hail, excessive rainfalls and severe wind gusts.

SYNOPSIS

An unusually robust weather pattern for mid-July will affect central Europe during the forecast period. A strong trough which produced numerous severe storms yesterday across western Europe will be transforming into a negatively-tilted trough while rapidly crossing central Europe. Accompanied with this trough, a strong cold front is extending from North Sea across Benelux southwards into SE France by Friday 06z. This frontal boundary will be rapidly trailing eastwards during the next 24 hours and should be placed from western Poland southwards towards NE Italy by Saturday 06z. Numerous severe storms with risk for isolated tornadoes, large to very large hail, excessive rainfalls and damaging winds will take place given the high instability and favorable wind shear overlaping together. Another cold front will affect parts of NW Russia with severe storms possible mostly during the daytime.

DISCUSSION

Switzerland, W Austria, extreme W Slovenia and N Italy…

Frontal boundary, placed over extreme eastern France in the morning hours, will first affect western Alps where models are in high agreement for excessive rainfalls given the high PWAT values and strong orographic lifting expected. Rich boundary layer moisture with dewpoints near 20°C is placed ahead of this trough which will result in high instability with MLCAPE 1000-1500J/kg over N Italy once steeper lapse rates overspread the region. Strong QG forcing will initiate storms quite early in the forecast period, likely before 10z already in the Switzerland, but northern Italy will stay well capped until late afternoon. meanwhile, a surface low pressure will develop supporting backing and increasing LL winds in the afternoon.

Models simulate precipitation mostly only in NW Italy where also strong shear is placed. Storms that will manage to initiate there, will rapidly organize into multicells and supercells given the high instablity and shear there. GFS model predicts near 25m/s of 0-6km shear while there is near 10m/s of shear available in the lowest 1km, as well as enhanced SREH up to 200m^2/s^2 given the backing wind profiles. Therefore a threat for tornadoes will exist along with damaging wind and large hail. Some models also simulated an isolated cell or two more to the east over the Po valley, but the initiation is quite questionable as pretty strong capping could surpress any development until the evening. However, models have a well defined near surface wind convergence across Po valley moving NE-wards during the afternoon once SW-erlies cross the Apennini mountains. If any storm initiates there, the conditions for an isolated tornado or large hail are as favorable as those more to the NW, especially because of lower LCL and higher instability.

Later storm evolution seems to be connected to the cold front and a large MCS will likely form over west-central Alps. Models are in good agreement of strong squall-line ejecting into the NW Italy in the evening hours. This complex will be then moving ESE-wards overnight and bring mostly excessive rainfalls and damaging winds as the main threat over northern Italy.

Depends on the timeframe of MCS formation, which could end up more eastwards than predicted by models, a level-2 area was extended slightly more eastwards into the extreme western Slovenia. Given the increasing LL winds and shear up to 10m/s with enhanced SREH over Northern Adriatic in the night hours, a weak tornado is not completely ruled out. Also because LCL heights will be pretty low there.

Here are a couple of maps for this setup…

500mb heights…

300mb jet streak…

850mb theta-e…

850mb winds…

MLCAPE…