Storm Forecast
Valid: Mon 29 Oct 2018 06:00 to Tue 30 Oct 2018 06:00 UTC
Issued: Sun 28 Oct 2018 19:38
Forecaster: TUSCHY

A level 2 was issued across most of the CNTRL Mediterranean mainly for excessive rain, severe to damaging wind gusts, tornadoes (a significant event is possible) and large hail (a few very large hail events not ruled out).

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

A level 1 was issued for the W-Mediterranean mainly for isolated severe wind gusts and a low-end tornado risk.

SYNOPSIS

Downstream response of the intense Northeast Atlantic ridge amplification culminated in an anomalous blocking pattern over Europe. A deep and amplified trough covers most of the Iberian Peninsula and far N-Africa, where 500 hPa geopotential height anomalies tumble to significant values with a peak anomaly over far N-Algeria. A digging upper jet streak is about to circle the base of the trough and despite weakening due to the inertial-advective (ageostrophic) component, it is forecast to survive while lifting NE over the W Mediterranean (a sign of how potent the mid/upper height gradients and attendant winds are). This jet streak is accompanied by a substantial mid/upper impulse (visualized by significant drying in WV data), which races NE towards Italy and forces the complete long-wave trough to acquire a negative tilt during its lifting process (while deamplifying). The configuration of the upper flow regime and the approaching jet streak cause a broad area with significant upper-level divergence atop an extensive baroclinic zone over the W-CNTRL Mediterranean. Resulting deepening/broadening cyclonic vortex between the Balearic Islands and S France push mainly low-tropospheric winds to very anomalous values over the CNTRL Mediterranean (IFS SOT in excess of 2!).

An extensive warm sector covers most of the CNTRL Mediterranean with maximized integreated water vapor transport over Italy and the Adriatic Sea. During the forecast the N-ward lifting depression and the eastward racing cold front shift the warm sector towards E/SE Europe. The air mass in the warm sector is characterized by warm low-tropospheric air (with decreasing values and CIN to the north) and a northward fanning and gradually dissolving EML. Impressive moisture is the result of evaporation over the Mediterranean (20 C (plus) SSTs) and an ongoing elevated atmospheric river with a connection to the tropics (W of W-CNTRL Africa). This mid-/upper level moisture transport is visualized by latest sounding reports over W/NW Africa (e.g. In-Salah). This origin of the mid/upper air mass may explain the overall moderate lapse rates in this EML despite strong WAA, intense lift and trajectories off N-Africa. Nevertheless, robust CAPE is forecast to evolve in the warm sector.

Interaction of the strongly sheared warm sector with the complex orography (f.ex. the Alps) results in significant regional impacts (e.g. foehn, heavy snowfall etc.), which will be covered by the relevant national weather agencies.

Aside from the periphery of the trough, no thunderstorm risk is forecast over N Europe.

DISCUSSION

... CNTRL-Mediterranean ...

During the daytime hours (until 18Z) ...

...most of the CNTRL Mediterranean is covered by the warm sector. Maximized MLCAPE of 1 to 1.5 kJ/kg is forecast in the pre(cold)frontal air mass over the Gulf of Genoa, the Tyrrhenian Sea and the far W Ionian Sea. Beyond 12Z those peaks decrease a bit with weakening lapse rates (probably also a result of extensive convective overturning) but remain in the .5 to 1 kJ/kg range over the Adriatic and W-Ionian Sea.

The cold front, initially next to Corsica/Sardegna at around 06Z, races east and is forecast to impact N-Italy and the SE-Tyrrhenian Sea and Sicily until 18Z. Accompanied by the described dynamics in the SYNOPSIS, widespread forcing is expected for CI even to the south, where CIN remains stronger.

The most impressive parameter will be the shear with extreme values especially in the lower to mid-troposphere. 0-1/-3 and -6 km shear values remain in the 15-20 m/s / 20-30 m/s and 25-45 m/s range respectively. Responding to the deepening vortex, forecast hodographs not only feature elongated structures but also reveal substantial curvature in the lowest 2 km AGL due to the intensifying ageostrophic flow. SRH 1/3 values in the 200-400 (plus) m^2/s^-2 range emphasize the favorable kinematic environment with respect to organized convection.

The fly in the ointment (regarding a straightforward severe thunderstorm outbreak) is scattered to widespread ongoing thunderstorm activity especially over the N Tyrrhenian Sea. This may limit the chance for more discrete thunderstorm activity a bit.
Otherwise, prefrontal thunderstorms bring a significant severe risk to Corsica, Sardegna and CNTRL/NW Italy. Large to very large hail, severe to damaging wind gusts and tornadoes (a signficant event is possible) can accompany this activity. Maximized integrated water vapor transport with 30-35 m/s low-tropospheric S-erly winds also indicate a favorable environment for back-building convection along the W-facing coasts of NW/CNTRL Italy. Life-threatening flash flooding with extreme rainfall amounts can occur until the cold front passes by.
Similar conditions and attendant severe risks exist over the N-Adriaic Sea.
During the cold front passage, rapid and widespread upscale growth into a progressive MCS event is forecast, where the wind risk increases (next to an ongoing isolated tornado, large hail and excessive rainfall threat).

Further south (S-Tyrrhenian Sea and Sicily), stronger CIN keeps prefrontal convection more isolated and discrete and once again all kind of severe (with significant events) is forecast with this activity, including a signficaint tornado and damaging wind gust events. S-Sicily probably also faces an excessive rainfall event with back-building convection: life-threatening flash flooding is a distinct possibility.
The passing cold front is accompanied by an extensive V-shaped MCS with another round of severe/damaging wind gusts, isolated large hail and an isolated tornado threat.

We explicitly highlight that the favorable CAPE/shear overlap is not only confined to the offshore and coastal areas (as it is often the case during that part of the thunderstorm season) but expands well onshore with diurnal diabatic heating. Hence the severe risk also includes areas well inland over most of Italy!

The postfrontal air mass over the far W-Mediterranean features moderate instability (300-600 J/kg SBCAPE) and strong low-tropospheric flow (20-25 m/s at 850 hPa). Better mixing increases the chance for severe wind gusts and hence the level 1 was expanded far west. In addition marginal better LL shear profiles along the coasts of Corsica and Sardegna add an isolated tornado threat. The level 2 was issued for excessive rain, as repeated onshore moving thunderstorms and the orography probably cause very high rainfall amounts.

The ongoing orographic flood event along the S Alps was added to our level scheme due to abundant MUCAPE impacting this area (increasing the risk of numerous thunderstorms, which are embedded in the stratiform rain, causing extreme hourly rainfall rates). Hence the level 2 was expanded also all the way to the S-Alps. Change-over to mountain snow with intense snowfall rates also affects the final runoff but this scenario is not included in our level decision.

A low probability lightning area was added for the S North Sea but the thunderstorm risk diminishes beyond 12Z.


During the overnight hours (until 06Z) ...

... the cold front shifts east, it crosses most of the Adriatic Sea and it impacts the CNTRL / E Ionian Sea until 06Z. The most likely scenario seems to be a southward building V-shaped MCS to enter both sea areas with excessive rain being the dominant risk (maximized along the coasts of the SE Adriatic Sea). Especially for S Italy an augmented overnight tornado risk exists with stronger CAPE signals and ongoing extreme shear (plus better chances for discrete convection). Damaging wind gusts and isolated large hail can occur with more discrete activity.

Non-severe WAA convection spreads far NE and probably all the way to S Poland. The lightning areas were expanded accordingly.
We expanded the level 2/level 1 areas far east towards Slovenia and W Hungary as an active cold front passage is forecast during the night. CAPE decreases gradually inland, but extreme shear and dynamics assist in an ongoing nocturnal damaging wind and tornado threat! Given amount of shear, even a signficant tornado event is well possible over extreme SE Austria to Slovenia and extreme NW Croatia.

Ahead of another low-tropospheric impulse over the W/CNTRL Mediterranean, backing winds keep the training/excessive rainfall risk alive along the N coast of the Adriatic Sea until 06Z.

Further west (between Sardegna and CNTRL Italy) strong to severe wind gusts are possible with scattered thunderstorms ahead of the NE-ward moving LL impulse and beneath the lifting upper trough. SBCAPE in the 300-600 J/kg range and 20-25 m/s 850 hPa flow in a well mixed postfrontal air mass keep that wind risk augmented until 06Z. An isolated tornado event along the coasts is still possible.

A low probability lightning area was added for parts of W Germany beneath the left exit region of a powerful mid-level jet (beyond 00Z).

... SW Portugal ...

A cold front crosses Portugal during the night from NW to SE. Moderate prefrontal moisture advection and weak mid-level lapse rates keep the instability and the general thunderstorm risk on the marginal side. Heavy rain is possible given the origin of the subtropical air mass, but the progressive nature and low CAPE keep this event below the level 1 threshold.

Non-severe offshore convection occurs W/NW of Portugal.