Storm Forecast
Valid: Sun 15 Jul 2018 06:00 to Mon 16 Jul 2018 06:00 UTC
Issued: Sat 14 Jul 2018 21:18
Forecaster: TUSCHY

A level 2 was issued across the far E-Black Sea and surroundings for excessive rain.

A level 1 was issued for a broad swath of SE/E Europe mainly for heavy to excessive rain and isolated large hail.

A level 1 was issued for parts of N Portugal and NW Spain mainly for heavy rain.

A level 2 was issued for SE France, S Switzerland and NW Italy mainly for very large hail and severe wind gusts.

A level 1 was issued for NE Spain, parts of France, Switzerland, S Germany and NW Italy mainly for large hail (an isolated very large hail event over S France, Switzerland and NW Italy can't be excluded), heavy rain and severe wind gusts.

SYNOPSIS

Numerous troughs and mid/upper vortices impact SW/W/CNTRL and E Europe. They divert the polar front jet towards far N Norway and the subtropical jet to the Mediterranean area and feature a classic and already long lasting split flow regime over Europe.
The upper trough, which was more or less quasi-stationary west of Portugal during the past few days, finally lifts NE and becomes more mobile. Interaction with a subtropical ridge over N-Algeria/Tunisia causes a rapidly strengthening mid/upper jet which extends from N-Morocco towards the W Mediterranean.
Another vortex over N-Germany steers a mid-level wave from E-France to S-Germany with a third one over the Ukraine, circled by numerous impulses. The latter two ones feature only weak and diffuse boundary layer responses in form of a broad LL depression or an open LL channel of low pressure. The main interest in the BL arises over and E of Spain with the approaching upper wave. Interaction with a baroclinic zone could result in a temporarily strengthening LL vortex over NE Spain which moves rapidly east during the night.

DISCUSSION

... Spain and the W Mediterranean ...

Eastward moving upper trough (positive to neutral tilt) slides east and covers all of Portugal/NW Spain until 18Z. Thereafter rapid NEward movement towards NE Spain is forecast by all models. Tightening height gradients result in intense DLS with 20-30 m/s over E Spain and the W Mediterranean with less shear to the west.

During the day the main focus for thunderstorm activity exists over Portugal and N-Spain. Weak DLS with less than 10 m/s and moderate low tropospheric moisture combined with abundant lift result in scattered CI and numerous slow moving storms, which tend to grow upscale rapidly betimes. Heavy rain will be the main hazard. Further east CI will be suppressed by strong cap (caused by the tilt of the upper low and ongoing intense WAA downstream of the trough).

During the late afternoon and onwards, focus for more organized DMC activity shifts to NE Spain and beyond midnight also offshore of E/NE Spain. Overlap of 1000-locally 1500 J/kg MLCAPE and 15-20 m/s DLS favors organized updrafts with large hail and severe wind gusts. Beyond 03Z a risk of near surface based activity with isolated supercells is possible over far NE Spain and offshore. Large hail and severe wind gusts are well possible. This threat could extend towards the Balearic Islands. We thought about a small level 2 but uncertainties in storm initiation and coverage precluded an upgrade for now.
South of the consolidating LL vortex over NE Spain, strengthening offshore flow advects a dry and stable air mass from Spain towards the Balearic Islands and despite elevated thunderstorms no severe risk is forecast south of the Balearic Islands.

... S-France, Switzerland, N-Italy and S Germany ...

Eastbound moving mid-level wave over E-France increases the mid-level flow over S-France, Switzerland and NW Italy. DLS of 15-25 m/s overspreads a seasonable moist BL with meager mid-level lapse rates. Resulting 800-1500 J/kg MLCAPE are enough for robust updrafts which, interacting with the mentioned shear, could be long-lived. Forecast hodographs show rather straight signatures, favorable for splitting. The only inhibiting factor could be weak CIN and ongoing convection from the previous night, so early and scattered CI is anticipated, limiting overall diabatic heating and fragmenting the CAPE plume. However, where air mass remains avoid of early convection, multicells and a few supercells are well possible. Large hail and severe wind gusts are forecast and an isolated very large hail event can't be ruled out with the expected overlap of ingredients. An upgrade was performed, where ingredients and storm coverage look most promising for isolated long tracked supercells with very large hail and severe wind gusts.

With the approach of the Iberian wave and the second one over Germany/Switzerland, numerous overnight clusters are possible over S-France and N-Italy with heavy rain, isolated large hail an strong to severe wind gusts.

Over S-Germany DLS increases from N to S from 10 to 15 m/s with MLCAPE aoa 1000 J/kg. GFS remains an outlier with 20 m/s along the Swiss/Austrian border. Nevertheless, shear/CAPE overlap supports organized multicells with large hail the main hazard. Isolated severe wind gusts and heavy rain are also possible with the latter risk mainly due to overall slow storm motion. With the approaching wave during the evening, a cluster may move out of Switzerland/far E-France and crosses S-Germany to the E. Forecast soundings show most of that activity to stay elevated with a limited severe risk. Heavy rain and isolated large hail remains still possible.

Over S Austria and towards the Dinaric ALps ingredients also support a few isolated severe events (mainly large hail), but rising heights due to NE ward builidng ridge should keep both activity and the overall severe risk in check. Still parts were covered in a level 1.

... E and SE-Europe ...

Beneath the extensive upper low, conditions are favorable for scattered to widespread CI with upscale growing slow moving convection. Heavy to excessive rainfall will be the main risk and this hazard was highlighted by level 1 areas. In addition, MLCAPE of regionally more then 1000 J/kg also points to an isolated large hail threat. A small level 2 was issued for the extreme E Black Sea for excessive rainfall with repeatedly onshore moving/slow moving convection.