Storm Forecast

Storm Forecast
Valid: Sat 12 Jul 2014 06:00 to Sun 13 Jul 2014 06:00 UTC
Issued: Fri 11 Jul 2014 23:07
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for Belarus and the Ukraine for severe wind gusts, large hail, excessive precipitation and to a lesser extent for tornadoes.

A level 1 was issued for NE Poland, the Russian exclave of Kaliningrad and Lithuania mainly for excessive precipitation.

A level 1 was issued for N Italy mainly for excessive precipitation and for the N Adriatic Sea mainly for waterpouts.

A level 1 was issued for N Turkey for large hail and severe wind gusts.

A level 1 was issued for a small area in N Norway and N Finland for large hail and to a lesser extent for severe wind gusts.

SYNOPSIS

A multi-core upper-level low still covers most of Europe. Three of its individual centers are situated over Denmark, the Gulf of Bothnia and the Westernmost Ukraine, respectively. A fourth one moves from Southern France into Central Italy. The whole complex connects with another extensive upper-level low near Iceland on Saturday, temporarily resulting in a broad trough with a strong negative tilt. Unsettled and cool to moderately warm conditions prevail in the entire range of this trough.
Near the surface, the dominant cyclone slowly regresses northwestward from the Western Ukraine across Northeastern Poland towards the Baltic Sea. Ahead of it, a tongue of very warm air is advected into the Ukraine, Belarus and the Baltic states.
High pressure brings mostly sunny and warmer-than-usual conditions into Iberia and into Scandinavia.

DISCUSSION

... Ukraine, Belarus, Lithuania, NE Poland, Russian exclave of Kaliningrad ...

Steep lapse rates from the Southeast overspread very moist low levels with dewpoints around 17C in the warm sector of the cyclone. This will likely result in CAPE on the order of 1000 to 2000 J/kg on Saturday. Like usual under warm air advection regimes, the bulk of the thunderstorm activity will initiate rather late from the afternoon onwards. However, travelling vorticity maxima will then further facilitate convective initiation, and widespread storms are expected to form.
Moderate deep-layer shear (around 15 m/s) and enhanced 0-3 km storm-relative helicity (100-200 m^2/s^2, locally up to 300 m^2/s^2) are sufficient for an organization into multi- and supercells which can bring the full spectrum of severe weather. The main risk changes from large hail in the storms' initial stages to severe wind gusts and excessive precipitation as soon as they merge into large clusters. A few tornadoes are possible especially towards the evening, when a beginning decoupling of the near-surface layers further enhances low-level wind shear, and in general in case of a favourable collision of outflow boundaries.
A level 2 was issued for those parts of the Ukraine and Belarus where forecast models show consistent signals of large storm clusters which seem capable of producing longer swaths of severe wind gusts. In the large surrounding level 1 area, similar risks are expected but with a lower coverage. Towards the West (Poland, Kaliningrad, Lithuania), heavy precipitation will be the main hazard with the westward-moving storms while they gradually become elevated and start to weaken.

... Northern Turkey ...

Forecast models agree on at least moderate (>800 J/kg) but strongly capped CAPE over the Black Sea. Northeasterly sea breezes beneath moderate Southwesterly flow aloft create deep-layer shear on the order of 20 m/s. If a few storms manage to break the cap along the sea breeze front (aided by a short-wave trough aloft that crosses the region from West to East), they can turn well-organized with a risk of large hail and isolated severe downbursts.

... Scandinavia ...

Like on Friday, moderate CAPE will form in response to diurnal heating and moisture accumulation in the vicinity of a long convergence zone which aligns with the Norwegian Alps and extends into Lapland and Karelia. Scattered afternoon storms are expected, most of them non-severe due to a lack of vertical wind shear.
However, near the Northern rim of the unstable area, again a few better organized storms are possible as deep-layer shear rises to 15 m/s. Large hail and isolated severe wind gusts may occur.

... other thunderstorm areas ...

The slowly decaying upper-level low leaves a messy situation across much of the continent. Areas of augmented low-level moisture and daytime heating will result in the buildup of low to moderate CAPE. Scattered diurnally-driven storms are expected. With very weak vertical wind shear, lower CAPE than on the previous days and no pronounced convergence zone any more, most of this activity should be non-severe. Isolated instances of heavy downpours or marginally severe hail or wind events are not ruled out, but coverage should remain too low for a level 1.
The only exception is the Northern Italian region, where increasing vorticity advection ahead of the French upper-level low will likely increase the coverage of storms and make them go on well into the night. Large, ill-organized clusters with a primary risk of heavy precipitation are expected. The center of the activity will move onto the Northern Adriatic Sea overnight and may spawn a few waterspouts.

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