Storm Forecast
Valid: Thu 19 Jul 2018 06:00 to Fri 20 Jul 2018 06:00 UTC
Issued: Wed 18 Jul 2018 15:41
Forecaster: TUSCHY

A level 2 was issued across parts of extreme N Spain and SW/W France mainly for large to very large hail and severe wind gusts.

A level 1 surrounds the level 2 for similar risks with lower probabilities.

A level 1 was issued for parts of N Italy mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Sweden mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for E/SE Europe mainly for excessive rain and a few large hail and severe wind gust events. In addition a low-end tornado risk can't be ruled out.

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


SYNOPSIS

A subtropical ridge over the W Mediterranean is framed by a positively tilted trough over the far W/NW Europe and an extensive but weakening cyclonic vortex over E/SE Europe. Aforementioned ridge coalesces with an eastward moving anticyclone over Finland, the White Sea and Kola Peninsula, which causes quiescent conditions convective-wise for many areas over CNTRL and N Europe.

Despite a trailing cold front over N Sweden/Norway (which probably becomes a wavy cold/warm frontal zone during the forecast) no substantial synoptic-scale boundaries will impact today's outlook. The mesoscale will once again play an important role regarding CI and thunderstorm coverage.

DISCUSSION

... Far N Spain and parts of France ...

Placed along the eastern fringe of the positive tilted upper trough, subtle mid-level waves and a tightening geopotential height gradient add both lift and shear to today's setup. Forecast surface dewpoints in the mid to upper tens look reasonable compared with today's reports. With LL mixing ratios in excess of 11 g/kg in place, robust CAPE build-up beneath moderate mid-level lapse rates is anticipated. MLCAPE of 1000-1500 J/kg is likely with local higher amounts just north of the Pyrenees. Cap should be an issue as seen in forecast soundings (especially over the E-Pyrenees) due to a warm layer in 850 hPa. This layer cools towards the SE Bay of Biscay and so does the strength of the cap.

A broad area of weak PVA encompasses the E fringe of the trough, so a prolonged period of weak to moderate synoptic-scale lift impacts the Bay of Biscay towards NW France - shifting east during the night. Hence expect early and scattered CI over the Bay and NW France. Forecast soundings support manly elevated convection offshore but confidence in surface based convection increases during the day over far W/NW France with ongoing diabatic heating (probably excluding far NW France due to early CI with limited diabatic heating). Further south (W-CNTRL France), well mixed BL/inverted V profiles and overlap of 15-20 m/s DLS with 1000 J/kg CAPE indicate a chance for organized multicells/isolated supercells with severe wind gusts and large hail the main hazard. This activity either initiates or evoles out of the activity over NW France. With 15 m/s 0-3km shear in place, a forward propagating MCS to the east is possible with swaths of severe wind gusts. Hence the level 2 was broadened a bit to the north.

Further south (far SW France), most models are a bit more reluctant with CI, but we don't see a good reason why mountain convection should not evolve. 15-20 m/s DLS, some mid-level veering (assisting in pockets of enhanced SRH-3) and fat CAPE profiles in the hail growth layer point to well organized multicells and supercells with a large to very large hail threat. Severe wind gusts are also well possible. Despite uncertainties in model data we also upgraded that area due to the expected overlap of ingredients with expected confidence in CI.

During the night, falling surface pressure, ongoing synoptic-scale forcing / DMC activity and overlap of 15 m/s DLS with .5 kJ/kg to aoa 1kJ/kg MUCAPE keep scattered to widespread thunderstorm activity alive in the shape of a growing MCS and semi discrete supercells. To the south with increasing cap and weakening forcing, conditions for discrete supercells improve. In general all kind of hazards are possible with large hail and severe wind gusts. Isolated very large hail is still possible with robust MUCAPE (Massif Central) but we kept this area in a high-end level 1 for now.

Parts of the level 2 may see either late or very spotty CI. Despite that expectation a broad level 2 was preferred due to ongoing (model) uncertainties, which can't be solved that far out.

... N-Italy...

Ongoing ridging prevents CI away from orography. There, isolated CI is possible during the late afternoon and evening. 500-1000 J/kg MLCAPE (regionally even more) and 10 m/s DLS point to strong/severe pulsating convection with large hail and severe wind gusts (the latter risk mainly due to well mixed BL). A confined level 1 should cover that threat, which diminishes during the night. Isolated activity continues until 06Z however.

...Parts of Sweden...

Along a wavy and quasi-stationary boundary enhanced BL moisture beneath moderate mid-level lapse rates result in a tongue of roughly 800 J/kg MLCAPE. Enhanced mid/upper level flow over N Sweden pushes DLS in the 15-20 m/s range and hence a few organized multicells with large hail and strong to isolated severe wind gusts are possible.
Further south, DLS weakens with heavy rain becoming a risk over CNTRL Sweden. Nevertheless, healthy updrafts could produce large hail and a few severe wind gusts after initiation.
The activity weakens during the evening and night with loss of CAPE.

...E/SE Europe...

Beneath the broad cylconic vortex BL moisture features rather high values with BL mixing ratios in excess of 11-13 g/kg. Despite weak lapse rates (partially due to convective overturning from the previous days), MLCAPE in excess of 1000 J/kg is forecast with some diabatic heating (especially over N Romania, N Moldova, Ukraine, S-Ukraine and far SW Russia). DLS of less than 10 m/s, a moist troposphere and numeorus convergence zones create a favorable setup for slow moving convection with excessive rain the main hazard. Despite the weak DLS, the amount of CAPE should support a few large hail events with initiating and more discrete storms. Low LCLs and regionally enhanced LL shear along boundaries or forced by the orography could result in an isolated tornado threat. Despite the definite chance for level 2 conditions on a regional scale (e.g. due to serious flash flooding), only a broad level 1 area was issued as mesoscale features are either not handled well by current model data or depend on convection from the previous night.

... N/S Turkey and Georgia ...

20 m/s DLS, weak forcing and roughly 800 J/kg MLCAPE in the highlighted area create favorable conditions for a few organized multicells with large hail and severe wind gusts. Ongoing forcing ahead of a trough over the Black Sea keeps thunderstorm probabilities ongoing during the night but with less coverage.

Similar activities are expected over S Turkey but with more uncertainties in thunderstorm coverage. Hence only a low probabilitiy lightning area was added.