Storm Forecast
Valid: Fri 01 Jun 2018 07:00 to Sat 02 Jun 2018 06:00 UTC
Issued: Fri 01 Jun 2018 07:23
Forecaster: PISTOTNIK

A level 2 is issued from NW Germany to E Croatia mainly for excessive convective precipitation and to a lesser degree for large hail and severe convective wind gusts.

A level 1 and level 2 are issued for parts of Poland and Slovakia, Bosnia-Herzegovina, and S to SE Turkey for excessive convective precipitation, large hail and severe convective wind gusts.

A level 1 is issued for parts of Belgium, France, SW Germany, parts of England and Ireland mainly for excessive convective precipitation.

A level 1 is issued for N Spain mainly for excessive convective precipitation and large hail.

SYNOPSIS

An ill-defined omage pattern covers the European sector. Between a deep polar trough over W Russia and cut-off lows over the north-central Atlantic and Portugal, a broad and shallow ridge stretches from the central Mediterranean region into Scandinavia. Near the surface, an anticyclone moves from Belarus into the Ukraine, while pressure gradients are weak everywhere else.
Warm and moist air remains in place across wide areas of the continent.

DISCUSSION

... level 2 belt from Germany to Croatia ...

Daytime heating will once more create CAPE on the order of 500 to 1500 J/kg NE of the level 2 axis. A convergence zone moves slowly eastward into this airmass. It is already marked by scattered storm clusters at the time of writing (07z), which will soon become more numerous and intense. Next to this synoptic feature, convergence over hills and mountains and outflow boundaries act as further triggers.
Under weak vertical wind shear, convection will stay disorganized but will soon grow into large clusters. Numerous flash floods are expected across the level 2 area. In addition, marginally large hail is possible in iniating stages and severe wind gusts later at the leading edges of MCSs.
Convection will persist into the night while slowly diminishing.

... from Poland to Romania, from Bosnia-Herzegovina to W Greece, S and E Turkey ...

Better mixed, drier and deeper boundary layers are present in this place. Vertical mixing of moisture will keep CAPE limited to some hundred J/kg. Under weak vertical wind shear, scattered singlecells and multicells will form from noon onwards and persist into the late evening. Isolated marginally severe hail, wind and flooding events are possible. Higher CAPE and convergent winds indicate more widespread initiation with an accordingly more robust threat in NW Poland, warranting a (low-end) level 2.

... Belgium, W and S Germany, Switzerland, France, Spain, Portugal, Uk, Ireland ...

Slightly cooler but still moist air is present behind the main convergence zone. Around 500 J/kg of CAPE will form after some hours of daytime heating. Scattered, disorganized afternoon storms pose a risk of isolated flash floods. Slightly enhanced deep-layer shear is present (1) beneath a further weakening jet-streak from S Germany into Belgium and (2) in N Spain ahead of the Portuguese cut-off low. Marginally large hail is not ruled out in these areas.
Particularly low cloud bases over the British Isles indicate a possibility of one or two non-supercellular tornadoes, in particular if sea breeze fronts move some distance inland.

... NE Algeria ...

Similar to the previous days, NE-Erly sea breezes and upslope flow advect Mediterranean moisture towards the Tell Atlas mountains. Some CAPE (with unclear magnitude) is predicted but the capping is very strong. In the strongly sheared and helical flow beneath the subtropic jet, kinematic conditions would be excellent for organized storms with a hail and downburst risk.
In the past few days, convective initiation either failed or happened very late, leaving only elevated evening and night storms. Therefore no level 1 seems necessary at the moment.