Storm Forecast
Valid: Mon 23 Jun 2014 06:00 to Tue 24 Jun 2014 06:00 UTC
Issued: Sun 22 Jun 2014 21:01
Forecaster: BEYER / PUCIK

A level 1 was issued for Northern towards Southeastern Spain mainly for large hail, severe wind gusts and to the lesser extent for excessive precipitation

A level 1 was issued for Southern to Central France mainly for excessive precipitation and large hail.

A level 1 was issued for the Alpine region towards the northern parts of the Balkan mainly for large hail and excessive precipitation.

SYNOPSIS

A wide upper level trough influences most parts of Northern and Eastern Europe as well as Western Russia. Its center is situated mostly stationary over the Baltic States. Thanks to the trough cold airmasses of polar origin are present over most parts oft the named region.

An upper level low is situated near Portugal travelling slowly to the east showing only a loose connection to the frontal zone to its north. During the forecast period it starts to weaken. Downstream of the upper level low warm and humid airmasses are advected to the north influencing most part of southern Europe from the eastern half of Spain and the Central Mediterranean area until the Balkan peninsula.

While Southeastern Europe benefits from a shallow ridge, short wave features are forecasted to travel on the forward flank of the upper level low easterward all the way to the crest of the weak ridge.

DISCUSSION

... Northern to Southeastern Spain...

...an elevated mixed layer is present over most parts of Southwestern Europe. It was advected with a southwesterly flow from northern Africa and Central Spain and leads to steep mid level lapse rates that become even steeper troughout the day due to diurnal heating. This instability overlaps with mixing ratios of 9 to 11 g/kg, advected inland from the sea. This will lead to the development of 1000 to 1500 J/kg in the afternoon hours.

Due to the approaching upper level low, shear values will increase in the afternoon and evening hours. 0-6 km shear is forecast to lie between 10 and 20 m/s at 18 UTC. The best overlap of CAPE / shear is forecast for SE and NW part of the Lvl 1. Here, storms with the best organisation are forecast, including some supercells.

Thanks to the steep lapse rates and rather dry profiles, large hail and isolated downbursts will be the main threat for this region. Weak storm motion in combination with ppws of 25 to locally more than 30 mm may also result in enhanced excessive precipitation threat, especially in the eastern parts of the Spain area.

Storm activity should increase in the late afternoon hours not only due to diurnal heating but also due to the approaching upper level low that induces QG lift.

... Central to Southern France...

Similar airmass is present as in case of Spain but with a higher amount of low-level humidty and less steep mid-level lapse rates. Mixing ratio is forecast to have values of 10 to locally 13 g/kg in the afternoon hours. Therefore, CAPE values should therefore reach values of 1500 to 2000 J/kg. Shear values will be rather weak (0-6 km shear: around 10 m/s with higher values more to the east).

Main uncertainty is how much convection will be left from the night time hours. Several local area models forecast (complex) convection that is still active at the beginning of the day. This scenario makes new developments in regions where convection was active rather unlikely. However, in neighbouring regions outflow boundaries coming from the convective complex can lead to enhanced convergence and thus lift. In addition it may influence the forecasted values of DLS. Also some small short wave features that can be found in the IPV field and travel on the forward flank of the upper level low northeastward. This should also play a role for the lift mechanism.

Current thinking is that an enhanced convective activity with threat of excessive precipitation at the beginning of the day that weakens around noon. With the above written arguments convection than will develop and strengthen in the afternoon hours in the neighbouring regions. Large hail will be the main threat at the beginning oft the development. Having ppws between 30 to 35 mm also excessive precipitation events are possible. In addition local severe wind gusts can not be ruled out with better organized thunderstorms.

... Alpine region and Balkans area...

Situation will be similar to the situation the day before. That means steep lapse rates in combination with mixing ratios of 10 to 12 g/kg should make CAPE values of 1000 to 1500 J/kg quite likely.

Shear values are a bit higher as the day before with values of around 15 m/s (0- 6 km). Over Slovenia, Croatia and Bosnia DLS up to 20 m/s is forecast with stronger mid-tropospheric flow. This will allow for a better organization of the convection, including a possiblity for supercells. Large hail and excessive precipitation will be the main threats. The convection is primarily bounded to topography owing to a lack of synoptic scale lift. With better organized storms a local severe wind event cannot be ruled out but mostly this threat should be somehow mitigated by the lack of stronger low-level wind field.

All in all convective activity should quickly weaken in the late evening and night hours as boundary layer stabilizes.

... Northern and eastern lightning area...

Similar to the days before convective activity should increase troughout the day due to diurnal heating. In combination with cool mid levels this will steepen lapse rates. Although low-level moisture will be only poor, a few tens to few hundreds J/kg of CAPE should develop in the afternoon hours. This is most likely in the belt from the Baltic states towards Belarus and N Ukraine. In this region there can additionally be found a mid level short wave trough that is approaching in the afternoon and evening hours and should also bring sufficient lift. A 50% lightning threat holds for this. Having strong quite strong DLS with values reaching from 15 to 25 m/s a few better organized storms are possible. They will have the threat of producing marginal hail and locally severe wind gusts. However, overall threat is too low to justify a level area. Convection will be weaking in the late evening hours.