Storm Forecast
Valid: Fri 08 Sep 2017 06:00 to Sat 09 Sep 2017 06:00 UTC
Issued: Thu 07 Sep 2017 07:08
Forecaster: TUSCHY

A level 2 was issued for an area between Malta-Sicily to W-Greece and the far S-Adriatic Sea mainly for excessive rain, severe wind gusts, large/very large hail and an isolated tornado threat.

A level 1 surrounds the level 2 with similar hazards but lower probabilities.

A level 1 was issued for parts of E/NE-Spain mainly for isolated large hail and excessive rain. A low-end severe wind gust and tornado risk exists.


SYNOPSIS

A large-scale cold-core vortex becomes established over NW-Europe. Its center is analyzed over Scotland or just to the east. The frontal zone frames that vortex with a strong mid- and upper-level jet. This jet drives a potent wave along the western fringe of the vortex into its base. The result is an amplifying trough over the Bay of Biscay. The vortex extends all the way down to the surface with a broad sub-990 hPa depression centered over Scotland, as the whole vortex remains vertically stacked. This depression shifts a warm and cold front E/SE during the forecast, crossing parts of W- and CNTRL- Europe.

As the upper-level trough approaches N-Spain during the night, it interacts with a pronounced moisture and temperature gradient over far E-Spain. Increasing upper divergence and the baroclinic nature induce cyclogenesis along the E-coast of Spain during the night in a plume of very unstable air. A sharpening warm front will evolve north of the Balearic Islands with a structuring cold front over CNTRL-Portugal and -Spain.

An upper level trough over the W-CNTRL Mediterranean drifts east and lifts out to the NE towards the Adriatic Sea ahead of the deepening trough to its west. This impulse crosses the S-Tyrrhenian Sea and the S-Adriatic Sea during the evening and overnight hours. A LL depression evolves along its southeastern periphery and consolidates east of Tunisia and north of Libya.


DISCUSSION

... From E-Tunisia to the Adriatic and Ionian Sea ...

As the LL depression over Tunisia/Libya intensifies and moves offshore, a confluent flow regime pushes LL mixed-layer mixing ratios to values in excess of 16 g/kg with TPWs of 40 mm below an EML. This moisture plume expands NE all the way to the Ionian Sea and outruns the EML which results in lower CAPE. A plume of 1.5 to 2.5 kJ/kg moderately capped MLCAPE resides south of Sicily, although degree of LL moisture content seems to limit cap strength at least along the northern fringe. CAPE decreases to 1 to 1.5 kJ/kg MLCAPE over the Tyrrhenian and Ionian Sea.

Strong synoptic-scale forcing accompanies the upper-level trough and spreads out to the north and south during the lifting stage (affecting an area from the S-Adriatic Sea to Libya).
Therefore expect CI all day long with convection ongoing from the previous night (especially between Sicily and Tunisia).
However, models like GFS suppress substantial DMC activity for most of the time until 12 UTC over Sicily. A reason for that could be lowering BL moisture from the NW as very dry air from a Mistral event over S-France (TPWs between 10-15 mm spreading SE) seems to approach Sicily before noon with GFS inidcating strong diurnal mixing and hence a lack of CAPE. A tight moisture gradient evolves just south of Sicily and with some backing of the LL flow in response to the evolving depression the very moist air mass should be pushed back to the north to S- and E-Sicily during the afternoon hours - adequate for CI.

A substantial increase in DMC activity is expected from S-/E-Sicily to the S-Tyrrhenian Sea, S-Italy and the Ionian Sea ahead of the upper trough beyond 12Z to 15Z. 20 m/s DLS is enough for organized multicells and isolated supercells with large hail (a very large hail event is possible), severe wind gusts and excessive rain. The severe wind gust risk increases during the night over the Ionian Sea and W-Greece both due to enhanced probabilities of an upscale growing thunderstorm cluster and 0-3 km shear in excess of 20 m/s. A few damaging wind events can't be excluded.
The same for the excessive rainfall risk due to a strengthening low-tropospheric wind regime, increasing the chance for temporal training with a V-shaped cluster along the W/NW-coast of Greece. Further north over the S-Adriatic Sea, slow moving convection poses an excessive rainfall and isolated waterspout risk as shear relaxes in that area. A waterspout risk also exists over the Tyrrhenian Sea (with hail and wind hazards increasing over the far S-Tyrrhenian Sea).

Finally an area between S-Sicily and Malta has to be emphasized. The aforementioned impressive BL moisture and backed LL flow north of the LL depression create a favorable environment for tornadic thunderstorms in case of CI. Discrete storm structure is more likely due to the strengthening cap, so a strong tornado event can't be excluded.

... N-France to Benelux and S-UK ...

In consequence to the amplifying and SE-ward moving upper trough over NW-Europe, strong height falls occur over N-France during the cold front's passage. An evolving upper-level coupled jet configuration with increasing upper divergence atop the cold front and the frontal circulation itself should act to increase lift along the front. In addition a tongue of dry mid-lvel air approaches from the NW and seems to interact with the front at least partially (increasing odds for potential instability release). Hence the chance for a LEWP-ish cold front structure exists during the evening and overnight hours over W-, N- and CNTRL France into Benelux. LL shear (speed and directional) combined with LCLs below 800 m and with some LLCAPE indicate a chance for an isolated tornado event. Lack of more substantial 0-3 km shear should keep the severe wind gust risk in check although an isolated event can't be excluded. The overall severe risk diminishes with SE ward motion of the front as CAPE vanishes. Nevertheless not much BL modification is needed for some MUCAPE and a localized event is also possible far inland (especially to the W with better LL moisture). Due to the diffuse nature of any severe risk, the modest nature of the 0-3 shear and meager CAPE, no level 1 was issued despite a low-end tornado risk.

S-UK will see an overlap of modest BL moisture ahead of the approaching disturbance with cooling mid-levels, so isolated to scattered thunderstorms can be expected. DLS remains in the 15 m/s range with a similar magnitude in the 0-3 km layer and 500 J/kg MLCAPE, so a few stronger pulsating storms with graupel/isolated hail and gusty winds are possible. The severe risk seems to be too marginal for a level 1.

... Denmark ...

Cooling mid-levels atop a moist SW-erly flow regime cause weakly capped 300-600 J/kg SBCAPE with some LLCAPE up to 100 J/kg possible. 10 m/s DLS with an isolated stronger updraft may cause graupel/hail with gusty winds. In addition, a low-end spout/tornado risk exists along the coasts and offshore. The severe risk is too marginal for a level 1 area.

... E-Spain ...

During the afternoon and evening, transient weak ridging clears the way to a more cyclonically coined streamline pattern during the evening and night. Onshore flow of a marine air mass and eastward fanning mixed air from the mountainous region of Spain result in a narrow CAPE plume of 1-1.5 kJ/kg MLCAPE. 15-20 m/s deep-layer shear would favor rotating updrafts in case they manage to survive for some time. Weak synoptic-scale forcing and dry mid-levels point to rather hostile environmental conditions for longer-lived updrafts. However, onshore and upslope flow may spark an isolated thunderstorm over the mountains and any stronger and longer-lived event is able to produce isolated large hail and a severe wind gust event or two.

During the night (mainly beyond 00 Z), the focus shifts north to the warm front over and east of NE-Spain. Increasing upper divergence and interaction of strong forcing with the CAPE plume (1-2 kJ/kg MLCAPE) support a rapidly growing cluster, which rides east along the warm front. Most of the activity should stay elevated, although a risk for near surface based activity exists over far NE-Spain during the end of the forecast. Large hail and excessive rain will be the main hazard with an isolated severe wind gust and low-end tornado risk for any surface based activity.