Weather Photos blog

Today could be an interesting day over southern Alpine region as looks like a weak mid-level disturbance/front will cross the Alps, extending from the weakening frontal boundary crossing the north-central Europe during the afternoon hours.

Morning skew-t diagrams are quite impressive as regarding the kinematics, though DL shear is rather borderline and just slightly higher than 20kt. As can be seen on the Ljubljana 03z sounding below, a loaded-gun type feature is on the temperature profile. A quite strong capping inversion will hold most of convection until the late afternoon hours, convective temperature should be around 35°C today. Looks like almost reachable in the afternoon given the expected Tmax values in this range. Also quite high PWAT values and somehow a dry layers above the inversion can be seen as well, which could be supportive for large hail and strong winds with any storm taking this environment.

However, models have a quite strong near-surface wind convergence across central Slovenia, it can be well visible also on the bottom ALADIN H925 wind map. Storm intiation is indeed quite possible along this convergence as soon surface temperatures will be close to +35. As regarding the BL moisture today, its not much different from the previous days, whole Slovenia is experiencing SFC dewpoints from 19 to 22°C while coastal areas and Friuli have even higher values up to 26°C! Definatelly conditions which are quite unusual for our region during heat waves. Usually, its much drier during summer.

Such dewpoints will result in extremely high amount of SBCAPE later in the afternoon, likely in range from 2000 to more tha 3000 j/kg locally. So the convection is likely to be pretty explosive at times. If storms indeed fire along the aforementioned convergence, some orgazation with DLS of 10-15m/s is expected. Though, only weak mid-level winds do not favour any well organized severe storms, as their storm motion will be pretty slow. The main threat seem to be very intense to extreme rainfall, resulting in local flash floods. With such amount of CAPE and relatively dry mid-levels, some marginally large hail and strong winds are indeed possible, even the storms will have pulsating nature. But will likely tend to cluster into multicells in such extreme instability and weak mid-level winds.

Its interesting to see the above’s H925 wind map from ALADIN model, which shows a good convergence over central Slovenia. A sign, that a near-surface convergence is quite strong and not only a minor one. East of this convergence, an easterly flow could enhance SREH and therefore support some rotation in the cells. The fact about weak mid-level winds cannot be ignored, but maybe such instability would help to provide robust dynamics in the updrafts.

At ESTOFEX, a level 1 was issued for mostly Alpine region, read more details below:

Storm Forecast
Valid: Thu 15 Jul 2010 06:00 to Fri 16 Jul 2010 06:00 UTC
Issued: Wed 14 Jul 2010 23:05
Forecaster: GATZEN

A level 1 was issued for the northern Allpine region, Czech Republic, and western Poland mainly for severe hail.

SYNOPSIS

At the periphery of a trough centred over the British Isles a strong south-westerly jet affects western and northern Europe. A negatively tilted short-wave trough moves northward into Scandinavia during the period while another short-wave trough crosses the British Isles. At lower levels, waves form along the frontal boundary over north-western Europe. To the south-east of this front, a warm and unstable air mass is present.

DISCUSSION

Alpine region to western Poland

While the low-level cold front has moved away from the trough, it slows down over the Alpine region and western Poland. Model output suggests some frontal waves moving northward. Given low-level convergence along the front, storms will likely go on during the day. Latest model output suggests that rather strong vertical wind shear will overlap with the instability in the range of the front as least over the Alps. As a consequence, storms that develop during the day may become organized with the main threat being large hail.

Source: ESTOFEX outlook from July 15th 06z until July 16th 06z 2010

Afterall, behind this activity today, winds will turn into more easterlies which could mean that tonight temperature will not drop a lot, we will likely experience Tmin values near or just slightly below +25°C. Certainly a record breaking values are possible, depends how strong these winds (Bora) will be. And they seem to be continuing tomorrow during the day which will favour very high to extreme temperatures over western Slovenia. Daytime maximum tomorrow over 37°C cannot be ruled out.

Stay tuned for future updates!

A quite unusual situation today across western Europe. Unseasonably strong trough moves from UK towards Scandinavia and creates a very robust conditions, favorable for a large outbreak of severe storms over much of France, Benelux and western Germany. High instability overlaping with impressive shear values up to 25m/s in the 0-6km layer and backing flow ahead of a strong surface cold front will create explosive environment fur numerous severe storms along the main frontal boundary. An outbreak of strong supercells and powerful squall lines (bow echo possible) seem very likely, starting over central France and rapidly shifting NE-wards across Benelux and towards western Germany. A serious threat for a severe wind gusts in excess of 30-35m/s is possible along the leading bow echo, as well as a couple of tornadoes and large hail where discrete and more isolated intense storms/supercells could form.

Read more about the extreme conditions in place today in the outlook prepared by my ESTOFEX coalegue Tomas Pucik below. A higher threat level, a level 3, has been issued across the areas where a robust threat for extremely severe weather seems the most likely.

Storm Forecast
Valid: Wed 14 Jul 2010 06:00 to Thu 15 Jul 2010 06:00 UTC
Issued: Tue 13 Jul 2010 22:54
Forecaster: PUCIK

A level 3 was issued for Benelux, small area of Northern France and extreme Northwestern Germany mainly for severe wind gusts, tornadoes, excessive precipitation and large hail.

A level 2 was issued for Southern to Central France, extreme Northwestern Switzerland and Southern Germany mainly for large hail, severe wind gusts and to the lesser extent for excessive precipitation.

A level 2 was issued for Western Germany, Denmark mainly for severe wind gusts, tornadoes, excessive precipitation and to the lesser extent for large hail.

A level 1 was issued for the regions surrounding the Level 2 mainly for severe wind gusts, large hail and excessive precipitation.

SYNOPSIS

The main macro-synoptic feature at mid levels will be a trough with axis over the Eastern Atlantic, with an embedded cold-core low centered over Ireland by Wednesday 12 UTC, moving slowly to northeast. Strong flow surrounds the fringes of the trough with windspeeds up to 40 m/s at 500 hPa level. A short wave trough will translate with this flow in the evening and early night hours over Northern France, Benelux countries towards Scandinavia. To east, a large ridge will stretch from the Central Mediterranean, across much of Central Europe into Northwestern Russia.

At the surface a centre of the deep low will reside close to or over the British Isles. Its trough will extend with the frontal system to the southeast and will accelerate quickly eastwards during the day. Mesoscale low will probably form within the frontal wave over Benelux and might become a significant factor in scenario. Ahead of the frontal system, high Theta-E airmass is advected at lower levels to northeast.

DISCUSSION

… France, Benelux, Western half of Germany …

***Potentially dangerous setup with severe weather outbreak is forecast over this region***

Ahead of the frontal zone, high Theta-E airmass is advected at lower levels and as cooler mid-level temperatures overspread this airmass a belt of moderate latent instability is forecast to form. Models are in fairly good agreement regarding the CAPE values, which should be mostly in the range of 1000-1500 J/kg.

With strong southerly to southwesterly flow aloft, high values of wind shear are forecast, with over 25 m/s of DLS and 20 m/s of MLS, values sufficient for very well organised convection, including supercells. With weak relative upper level flow, convection will probably tend to organise into quasi-linear system rathern than into long-lived supercells. Another factor speaking for such system is the orientation of the mid and upper level flow to the boundary, which is almost parallel and therefore, quick clustering is expected. Nevertheless, the possibility of isolated, steadier supercellular convection, especially in the earlier stages of scenario can not be discounted.

Forcing should be very favorable, in the form of the cold front and as the above mentioned short wave trough , which might induce or support cyclogenesis in the axis of the frontal trough. Such low would probably contribute to the backing of the surface winds (increasing SREH values) ahead of it and to the strenghtening wind field at lower levels. Models simulate 30 m/s 700 hPa flow during evening and night hours in the proximity and behind of the surface front.

With such strong wind field at lower levels, quickly travelling mid-level short wave trough and strong tendency for storm clustering, severe quasi-linear MCS might travel at high speeds to northeast along the front and produce a swath of severe wind gusts, including some gusts over 32 m/s. This will be the prime threat and Level 3 is issued especially for the area, where the passage of severe MCS is most probable. In the earlier stages, when isolated form of convection will prevail and over the southern parts of Level 2, large hail will be a threat, especially if supercells form. In that case, hailstones over 4 cm might be observed. Especially to the northwest of the mesoscale low, excessive precipitation might occur, at the northern end of large MCS with possibility of “train effect”. With high effective PW values, at least short term excessive precipitation can not be ruled out elsewhere in Level 2. Last, but not least, tornadoes might occur and more so in the environemnt with higher SREH and LLS ahead of the mesoscale low and lower LCLs during the late evening and night hours - therefore BENELUX and Northwestern Germany will probably be the region with the highest probability of tornado occurence. The fact that the convection will tend to occur mostly in linear clusters limits the potential for tornadoes

…

Source: ESTOFEX outlook from July 14th 06z until July 15th 06z 2010

However, southern Alpine region will likely be rather stable today, given the higher H85 temperatures and quite strong capping inversion seen on the morning soundings over Udine and Zagreb. The more interesting and serious flash floods threat again comes tomorrow when another weak upper-level disturbance crosses central Europe and the Alps, which should result in more activity over at least the NNE-ern parts of Slovenia. I’ll have more details about this potential setup tomorrow morning.

One interesting thing to note is that after tomorrow, it seems that low level winds will turn to more easterlies which could give slightly lower relative humidity and trigger weak Bora over the SW Slovenia. With such high temperatures aloft, this could lead into extremely high temperature both at night and daytime over Vipavska Valley, Karst and all along coastal areas of Trieste gulf.

Stay tuned for future updates on this disturbance tomorrow and future activity.

Very hot, humid and partly unstable conditions continue during these days. An interesting situations unfolds today as a weak upper level disturbance crosses the Alps in the afternoon hours and will likely lead into destabilization across southern Alpine region. Given the high BL moisture, surface dewpoints from 17-20°C and 0-1km mixing ratios up to 13 g/kg, MLCAPE up to 2000 J/kg or higher will release. In rather weakly sheared environment, mostly pulse storms are likely in the beginning which will tend to cluster later when deep-layer shear slightly increases towards 10m/s. Not much of a shear, though, but enough for some organization and clustering. High PWAT (near 40 mm), extreme instability, weak shear and slow storm motion all point into serious excessive convective rainfall threat and local flash floods where storms will fire. Delta Theta-e values suggest that strong outflow gusts are possible as well.

As I was on shift today at ESTOFEX, find a more detailed discussion about conditions today across our region here below. A level 1 was issued to cover the flash floods threat:

Storm Forecast
Valid: Tue 13 Jul 2010 06:00 to Wed 14 Jul 2010 06:00 UTC
Issued: Tue 13 Jul 2010 05:51
Forecaster: KOROSEC

A level 1 was issued for south-central Alpine region including Slovenia and mountainous N Italy, parts of south-central Italy and SE France mainly for excessive convective rainfall, strong winds and marginally large hail.

SYNOPSIS

An intense trough which resulted in widespread severe storms on Monday continues tracking NE-wards across Scandinavia, while another deep trough/upper low enters western Europe and will result in another outbreak sequence on Wednesday afternoon. A weakening mid-level low over Ukraine moves slightly towards SE while slowly building omega-like ridge pushes into central Europe again.

At surface, a strong frontal system associated with the afforementioned robust trough serves as a focus for an outbreak of severe storms across Sweden. The cold front extend southwards into western Poland by the afternoon, while strong WAA ahead of the trough pushes warm front into the northern Scandinavia.

DISCUSSION

…

… SE France, north and central Italy, southern Austria and Slovenia …

Placed in the hot and very moist boundary layer, high instability with MLCAPE exceeding 1500-2000 J/kg will result. These areas will again see daytime driven activity in slightly better sheared environment around 10 m/s than previous days. Even the threat for severe storms is rather limited, a low-end level 1 was issued to cover the most intense storms. Given the pulsating nature of the storms and high PWAT values over these areas, excessive rainfall and locally small hail will be the main threat. High delta theta-e culd also support locally strong wind gusts. A small level 1 area was also issued for parts of south-central Italy where similar conditions could support a couple of flash flood events as well. A bit better shear overlaps with instability over western part of level 1 area across SE France and western Alps. There, enhanced risk for intense convective rainfalls, strong winds and large hail exists. With an approaching trough towards the Wednesday morning, strong warm advection takes place from Spain NNE-wards into southern France. An elevated convection seems likely to form along the northwards advecting warm front over SW France during the second half of the night.

…

After a severe weather outbreak yesterday over Benelux and Denmark, a robust trough today continues into Scandinavia, where level 2 was issued given the favorable conditions for numerous well organized severe storms capable of producing severe winds, intense rainfalls and a couple of tornadoes. Read more details about severe weather outlook across Europe in the source link here:

• ESTOFEX outlook from July 13th 06z until July 14th 06z 2010

This summer pattern (a classic heat-wave for south-central Europe) will continue at least until the weekend. Very moist airmass with SFC dewpoints 21-25°C all over the southern Alpine region will create environment with extreme instability, but without any disturbances, neither any decent shear. And placed right beneath the ridge axis, days with high to extreme daytime temperatures (in excess of 35°C) are likely with only isolated pulse storms possible. However, models are pushing a short-wave trough across the central Europe towards Sat/Sun period, could be a potential for a cold front crossing and changing airmass later on. But given the still 5 days out, any details would be just a wild guess attm.

Stay tuned for future updates!