Storm Forecast
Valid: Wed 30 Aug 2017 07:00 to Thu 31 Aug 2017 06:00 UTC
Issued: Wed 30 Aug 2017 07:34
Forecaster: PUCIK

A level 1 was issued for SE Spain mainly for excessive precipitation.

A level 1 was issued for N Spain and S France mainly for excessive precipiation, severe wind gusts and to the lesser degree for large hail.

A level 1 was issued for Central / E France, N Switzerland and Germany mainly for severe wind gusts and large hail.

A level 1 was issued for Greece mainly for excessive precipitation.

A level 1 was issued for S Turkey mainly for large hail and severe wind gusts.

SYNOPSIS

A deep trough is stretching across Atlantic towards the Iberian penninsula. At the water vapour imagery, a short-wave trough approaching France is clearly visible. According to the forecast models, it will reach the coastline only during the night and early morning hours. Ahead of the deep trough, strong southerly to southwesterly flow is simulated from S Spain towards Denmark and Sweden. This suggests a strong vertical wind shear regime across much of W Europe. Another trough is currently residing over E and SE Europe with the base over Greece.

Tstm activity will be associated with the aforementioned troughs and also with the cold front that will cross France and parts of Germany during the forecast period.

DISCUSSION

... SE to N Spain ...

Satellite imagery reveals numerous thunderstorms that developed offshore of SE Spain and also a large anvil over the Pyrennees. Surface observations show moist airmass over E coastline with dewpoints generally around 18 deg C. In the early forecast hours, thunderstorms are expected especially over SE Spain with their cores and cell regeneration staying mostly offshore in moderately unstable airmass. Heavy rainfall may be associated with these storms. Towards the evening and night hours, with the approach of the trough, destabilisation is simulated over N Spain with MLCAPE values up to 1000 J/kg. At the same time, increasing southerly flow will result in DLS up to 20 m/s, providing conditions sufficient for well organised DMC. Boundary parallel shear suggests quick clustering of storms and forecast soundings reveal potential for heavy rainfall with saturated profiles overnight and towards the morning. This will likely be the primary threat, followed by large hail. Severe wind gust threat will be marginal, especially at first, when lower tropospheric flow will be quite weak.

... France ...

Considerable cloudiness is currently present over much of France, putting question mark on how much surface heating will occur during the daytime. Surface observations show dewpoints ranging from 15 to 22 deg C with the highest moisture north of the Pyrenees. A covergence zoneis currently residing over Central France, but models simulate its decay during the day. A quite complex scenario is forecast over this area with numerous rounds of storms possible. With the lack of steep lapse rates, CIN will be rather low, but so will CAPE values. Nevertheless, 20 m/s of DLS will allow for well organised multi- or supercells. The highest chance for destabilisation will exist over E part of the country and supercells accompanied by large hail and severe wind gusts will be most likely there. Towards the night, with the approach of the trough an MCS will likely develop over Pyrenees and will move N-wards, affecting S France with heavy rainfall and possibly severe wind gusts.

... Germany ...

An advection of moister airmass is anticipated in the following hours with dewpoint values increasing to around 18 deg C over W part of the country. With lapse rates around 6 K/km in mid-troposphere, MLCAPE values over W part of the county may reach around 1000 J/kg. Unidirectional shear profiles are forecast as the surface wind, currently SE-ly, veers to SW during the day. Nevertheless, 15 to 20 m/s of DLS is forecast over much of the area, allowing for well organised multicells and even some transient supercells. East of the surface low, over N Germany, enhanced SRH is forecast around 12 to 15 UTC with DLS approaching 25 m/s, but storm initiation is questionable there atm. Large hail and severe wind gusts will be possible with stronger storms, but threat of both is limited, i.e. by lack of steep lapse rates in case of hail and by lack of stronger lower tropospheric flow in case of severe wind gusts. Storm coverage is forecast to increase in the late afternoon and evening hours with the approach of the cold front.

... W Greece ...

In the core of the trough, thunderstorms are currently developing along the coastline or along the convergence zone that is readily visible over the Ionian Sea. With moist airmass at the lower troposphere, isolated excessive precipitation may occur with these storms.

... S Turkey ...

Surface observations show dewpoints of around 20 deg C. Combined with the lapse rates of 7 K/km, MLCAPE values over 2000 J/kg will be possible along the coastline, decreasing inland due to the lesser moisture content. With 10 to 15 m/s of DLS over the area, well organised multicells may pose a threat of large hail and/or severe wind gusts.