Storm Forecast
Valid: Sat 27 Oct 2018 06:00 to Sun 28 Oct 2018 06:00 UTC
Issued: Fri 26 Oct 2018 13:07
Forecaster: TUSCHY

A level 2 was issued across the Balearic Islands mainly for excessive rain.

A level 2 was issued across extreme SE France and far NW Italy mainly for excessive rain.

Both level 2 areas are surrounded by a level 1 mainly for excessive rain and isolated tornadoes/severe wind gusts.


SYNOPSIS

Cyclonic Rossby wave breaking over the Baffin Bay is accompanied by impressive cyclogenesis and downstream ridge amplification SE of Greenland and over Iceland. Attendant WCB is based in the deep tropics, where active convection strengthens the building ridge in addition by diabatic heat support. Resulting broadening/northward amplifying PVU shield highlights the intense/deep WAA regime with a 2-3 sigma deviation of the mid-level geopotential heights along the core of the intense ridge (SW of Iceland). Supergeostrophic winds at high-levels drive a highly abnormal 90 m/s 300 hPa jet south towards Portugal. Downstream of this ridge a digging positive tilted trough impacts the W/CNTRL Mediterranean during the end of the forecast with a northward building ridge over SE Europe.
The geometry and placement of the upper pattern supports a rapidly downward building vortex over the W Mediterranean along an intensifying baroclinic zone. This vortex and its wind field affect most of SW and S Europe and is the start of an high-impact weather event for many areas along the Mediterranean.
The evolving cold/warm fronts over the W Mediterranean serve as main focus for DMC activity.

The rest of Europe is avoid of instability either due to lack of BL moisture or meager mid-level lapse rates.

DISCUSSION

... W-/CNTRL-Mediterranean ...

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

... conditions gradually improve for scattered to widespread CI between SE Spain and NW Italy. The structuring baroclinic zone will be the focus for enhanced CI as backing low-tropospheric winds (due to falling pressure E of Spain and over the Balearic Islands) point with an enlarging angle towards the baroclinic zone and therefore support enhanced isentropic lift. In response to the deepening vortex the downstream wind profile becomes increasingly WAA swayed with deeply veered vertical wind signatures. LL winds also back due to the strengthening low-tropospheric ageostrophic flow response. Hence forecast hodographs show supportive profiles for organized convection along that structuring warm front. Missing input of N African air keeps mid-level lapse rates and instability on the low-end side (400-800 J/kg MLCAPE) but still enough for healthy updrafts with an attendant isolated tornado and severe wind gust threat, stretching from the Balearic Islands towards NW Italy.
Latest data points to an evolving LL depression next to the Balearic Islands, which would increase LL winds and training potential, although track and intensity of the depression will be crucial for that threat (and still features some uncertainties). Overall slow moving convection between E Spain and the Balearic Island poses an excessive rainfall risk with deep moist profiles, skinny CAPE profiles and uncontaminated influx of warm/moist (unstable) air from the S. Again, structuring vortex and delayed response of the near BL wind field may keep the training risk on the isolated side until 18Z.

East of the baroclinic zone (e.g. Corsica, Sardegna and to the south/east) the building ridge and rather dry mid-level air plus some capping issues with mild sub-700 hPa temperatures keep CI isolated with local CI along the SW facing coasts due to frictional convergence. Nothing severe is forecast until the evening.

S-Spain may see a temporal severe risk during the afternoon/evening as onshore moisture advection ahead of the trough axis beneath cool mid-levels creates a confined tongue of 300-600 J/kg SBCAPE. Shear is adequate for isolated large hail and severe wind gusts but the risk vanishes after sunset due to BL stabilization and as the cold front finally pushes offshore.

Isolated to scattered CAA convection is forecat over parts of the Bay of Biscay (non-severe) and a similar non-severe activity is forecast in the WAA regime east of the Alps (with a far NE ward fanning MUCAPE plume). Hence the lightning areas were issued in those areas. Could see isolated rain issues over Slovakia, where isentropic lift and orography could result in local heavy rain events, but the general risk seems to stay too marginal for any level issuance.


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

... the low-tropospheric winds respond to the deepening vortex over a broad area with intensifying S-erly winds between Spain and Greece. The severe risk over the Gulf of Genoa ramps up quickly as backing low-tropospheric winds create an elongated N-S aligned fetch of warm/moist air (MLCAPE in the 500-1000 J/kg range) pointing towards NW Italy/far SE France. The mentioned LL vortex moves from the Balearic Islands towards SE France during the night. During its passage LL winds increase and create a more conducive LL wind profile for training convection along the coast of NW Italy (although no real LL wind speed maximum can be detected, enhanced frictionally induced convergence along the coasts could offset that missing parameter). The area around and S of Corsica/Sardegna remains avoid of convection until far into the night, so no inflow contamination is expected and excessive rain becomes a distinct threat fo the coastal areas. During the night the threat for regionally training convection enlarges towards SW facing coasts of CNTRL Italy and the E coast of the Adriatic Sea but limited time (until 06Z) does not support a level 2 (although this convection continues in SWODY 2 with excessive rain!). Slightly curved hodographs and low LCLs also indicate a tornao risk along the coasts.

Further inland over N Italy, the wind field changes to low shear at low levels and stronger shear at mid/upper levels. Although no cold air damming scenario is forecast over N Italy, nocturnal cooling should serve as adequate "cool air dome" for persistent isentropic lift, only enhanced along the S Alps. Excessive rain with embedded electrified convection is possible far inland (with MUCPAE plume approaching S-Austria during the night) and maximized along the S-Alps. This excessive rainfall event gradually turns into an heavy snowfall event along the mountain range, with embedded convection and intense precipitation rates bringing snow levels far down. Please refer to the local national weather offices for this threat.

The severe risk (isolated tornado and excessive rain) continues over the Balearic Islands and E of E-Spain during the night in response to a new LL depression, which is forecast to form beyond 00Z. Drier air filters in from the S during the end of the forecast and gradually ends the thunderstorm risk (with still ongoing DMC between the Balearic Islands and E Spain).
Due to the long time of potential severe and numerous rounds of excessive rain, we upgraded this area to a level 2. Mesoscale once again dictates, where the most severe rain risk resides.