Storm Forecast

Storm Forecast
Valid: Mon 17 Feb 2014 06:00 to Tue 18 Feb 2014 06:00 UTC
Issued: Sun 16 Feb 2014 22:03
Forecaster: PISTOTNIK

A level 1 was issued for NE Italy, parts of Slovenia and Croatia for severe wind gusts, large hail and tornadoes.


The rupture of the formerly dominant Western European trough leaves a complex weather pattern. Its remaining Northern part accelerates Eastward into the Baltics, Belarus and Russia. The cut-off low over Morocco gets absorbed by a new, amplyfying Atlantic trough. In-between, weak ridging moves from Western into Central Europe.
Near the surface, aged and filling cyclones are located over Northern Scandinavia and near Ireland, and temperate Atlantic air has invaded most of the continent. A still active and pronounced frontal boundary runs from Romania via Northern Italy to Southern Spain. Extremely warm air is present to its South with 850 hPa temperature anomalies up to 13K over the Balkans. This frontal boundary is also overspread by a mid-level jet and is the main area of interest for convective developments on Monday.


... NE Italy, N Balkans ...

The air mass on the warm side of the boundary is characterized by steep mid-level lapse rates of 7-8 K/km which spread as far Northeast as Romania. Rich low-level moisture is pumped towards the boundary with Scirocco/Jugo winds over the Adriatic Sea. Likely a few hundred J/kg of CAPE will develop, though they will be strongly capped towards the South.
A small but pronounced vorticity maximum and tropopause fold race Eastward from Northern Italy (06 UTC) to Romania (18 UTC) before they disintegrate. Their rising motions will erode the cap and trigger a wave development along the frontal boundary. In its warm sector, excellent wind profiles for organized convection will be available with 0-6 km shear of 30-40 m/s, 0-3 km storm-relative helicity up to 400 m^2/s^2 and 0-1 km shear regionally well in excess of 10 m/s.
An area of rain and partly embedded thunderstorms is expected to cross Northeastern Italy and the Northern Adriatic region in the 06 to 12 UTC time frame with the passage of this frontal wave. Especially more discrete tail-end storms may turn into multi- or supercells, and a level 1 was issued for a chance of severe wind gusts, 2-3 cm sized hail and one or two tornadoes. Further North, the elevated remnants of embedded storms will be squeezed against the Southern Alps, and another round of heavy precipitation will tread a path from West to East. Its quick translation should keep the excessive rain risk below our level 1 criteria, but further flooding and mudslides are possible with the contribution of saturated soils and melting snow.

Things get somewhat tricky further inland over the Balkans, where the day will start with cool Northeasterly surface winds and a stable boundary layer. This will cause the returning warm air advection difficulties to work its way down to the surface. Nonetheless, limited area models like BOLAM and WRF show that patches of CAPE could develop in case of sufficient sunshine and enough lift support by the vorticity maximum aloft. Again, wind profiles would be excellent for organized storms and any storm that manages to root down to the surface on the cool side of the frontal boundary could even become tornadic.
Limiting factors are the limited diurnal heating at this early time of the year and the poor timing of the dynamic forcing: it seems like the strongest vorticity lobe will exit the reagion already until 12 UTC, though the flow stays still unsteady in its wake and themodels disagree on details. Current thinking is that convective initiation will be rather isolated and that the storms, if any, will struggle to stay surface-based as soon as they cross the frontal boundary. In case of "success" the wind, hail and tornado risk would continue Eastward all the way into Romania, but this scenario is too doubtful for an expansion of the level 1 area.

... Western Mediterranean ...

Another jet streak curves around the cut-off low over Morocco. Large-scale lift continuously increases over the Western Mediterranean Sea, where the jet's left exit and the next swell of warm air advection will nicely overlap. A rapid CAPE buildup is forecast overnight, when a cyclogeneis off the Algerian coast will contract the moist boundary layer, which is topped (and capped) by an elevated mixed layer from the Sahara.
A strongly sheared and helical flow will be present for most of the forecast period, but will likely relax by the time that the cap can be eroded. Isolated to scattered but probably non-severe thunderstorms are expected to flare up late in the forecast period.

... Ireland and offshore areas of W Europe ...

Scattered thundery showers will form in the well-mixed Atlantic air behind an occlusion that moves onshore in Ireland, Western Spain, Protugal and Morocco. An isolated water- or landspout is not ruled out in Ireland, where the cyclonic background vorticity is maximized.

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