Storm Forecast
Valid: Fri 24 Jun 2016 06:00 to Sat 25 Jun 2016 06:00 UTC
Issued: Thu 23 Jun 2016 22:42
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for the Alpine region, the Balkans and N Italy mainly for excessive convective precipitation and large hail.

A level 1 and level 2 were issued for parts of France, Germany, Denmark and S Sweden mainly for large hail, severe convective wind gusts, excessive convective precipitation and to a lesser extent for tornadoes.

A level 1 was issued for N Spain mainly for large hail and severe convective wind gusts.

A level 1 was issued for Ireland mainly for excessive convective precipitation and (non-supercellular) tornadoes.

A level 1 was issued for S Italy mainly for large hail.

SYNOPSIS

A pronounced warm ridge extends from the W Mediterranean region across central Europe towards Belarus. Low geopotential is present over N Scandinavia and the British Isles. In-between, a SW-erly mid-level jet runs from Portugal towards Denmark, where it turns W-erly and fans out over the Baltic States.
Steep mid-tropospheric lapse rates continue to spread over the continent, owing either to the advection of an elevated mixed layer from NW Africa and Spain, or to an in situ "production" over the Alps, Carpathians or Dinaric mountains. Daytime heating and plentiful evapotranspiration over the lowlands allow the development of moderate to high CAPE (1000-2000 J/kg, regionally even more) over wide areas.

DISCUSSION

... Balkan peninsula into greater Alpine region ...

The body of hot air is characterized by weak vertical wind shear, a lack of large-scale lift and dynamics, and a distinct capping inversion that has been left by the recent warm air advection, as many of Thursday's 12z soundings showed. The (primary) initiation of scattered afternoon storms will be almost entirely tied to mountains where thermally driven upvalley and upslope winds converge. Strong pulse storms are expected which pose a risk of moderate hail (2-4 cm) and localized flash floods.
Highest storm coverage is again foreseen from Romania across Serbia to Bosnia-Herzegovina. Thermodynamic conditions will not change a lot compared to Thursday, when impressive 2m dewpoints from 20 to 25°C were recorded in this area and the 12z Belgrade sounding showed CAPE around 3500 J/kg. The density of severe events is expected to become high enough to justify a level 2. Otherwise, a level 1 suffices due to the uncertainties of where and when the cap will be broken.
It is not ruled out that secondary storms or storm clusters may propagate some distance onto adjacent plains towards evening with an ongoing risk of large hail, excessive precipitation and one or two severe downbursts. The inconspicuous low-level wind field does not allow to pinpoint any exposed area yet, though.

... France across Germany, Denmark into S Sweden ...

The western rim of the hot air mass is overspread by an increasing mid-level flow, yielding 0-3 km vertical wind shear between 10 and 15 m/s. These values are a little lower than on Wednesday and Thursday, but still sufficient for organized storms.
Geostrophic lift due to strong warm air advection will exit into S Sweden on Friday. Otherwise, it is mostly absent as the current Rossby wave pattern has lost most of its baroclinicity. Smaller-scale sources of lift include a slowly eastbound cold front, sea breezes moving inland, or outflow boundaries left from overnight's convection.

Any storm forming in this belt can soon turn multi- or supercellular. Large hail is the dominant risk and may locally exceed 5 cm in size. Severe wind gusts and excessive precipiation may occur as well, especially in case convection organizes into a linear system.
Two foci emerge from the NWP forecasts:
(1) Convective initiation is likely from noon onwards over N Germany and Denmark. S Sweden may also be affected later on, as the subtropical air with abundant moisture (forecast 2m dewpoints around 20°C) even makes its way that far north. Next to the above-mentioned risks, a tornado is not ruled out in coastal areas and generally in S Sweden, where the cloud bases will be lower and 0-1 km shear will regionally be enhanced to 10 m/s.
(2) Another round of storms is expected to form over central France, though it will turn quite late - possibly 15 UTC - before the cap can be broken. These storms may travel into W Germany in the evening and overnight. Convection may turn elevated betimes, but plentiful CAPE and increasing vertical wind shear even across lower levels keep the severe weather risk enhanced all night long.

... N Spain ...

A sea breeze front - it may also be interpreted as the tail of the above-mentioned cold front - moves inland in N Spain and cuts underneath the dry and hot inland air. A belt of CAPE will develop as the sea breeze regime gets exposed to daytime heating, though it appears a bit fragile in extent and magnitude. Wind profiles would be adequate for supercells. If storms form, large hail and severe downbursts are the main risks.

... British Isles ...

Weak CAPE will develop in response to daytime heating of the well-mixed maritime air. Vertical wind shear will quickly relax in the postfrontal environment. A low-end level 1 is issued for Ireland, where numerous thundery showers suggest a risk of localized heavy rainfall and one or two nun-supercellular tornadoes near the cyclone's coreor along sea breeze fronts. Otherwise, no severe weather is expected.

... S Italy and surrounding sea waters ...

A compact upper-level low continues a slow eastbound travel to the south of Sicily. CAPE up to 1000 J/kg and enhanced 0-3 km shear (~15 m/s) with veering wind profiles suggest a potential for organized storms.
Forecast models show some precipitation signals, but it is unlikely that convection will be able to root down into the maritime boundary layer. Current thinking is that storms may manage to become surface-based over the mountains of Calabria, Basilicata and Campania, where even a splitting supercell was observed under similar conditions on Thursday. Large hail is the dominant risk with such well-organized and surface-based storms.
Otherwise and similar to Thursday, convection - if any - will likely be elevated over Sicily, and in general over coastal and maritime areas, where a massive cap sits on top of the maritime boundary layer (refer to Thursday's 12z Trapani and Brindisi soundings). It does not pose an enhanced severe weather risk.