Weather Photos blog

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.