Storm Forecast

Storm Forecast
Valid: Tue 05 Aug 2014 06:00 to Wed 06 Aug 2014 06:00 UTC
Issued: Mon 04 Aug 2014 19:24
Forecaster: PISTOTNIK

A level 1 was issued for the Russian Kola peninsula and large parts of Scandinavia for large hail, severe wind gusts, excessive precipitation and to a lesser extent for tornadoes.

Level 1 and level 2 areas were issued from the W Baltics and E Poland to the Balkan states, Greece and the S Alpine region mainly for excessive precipitation and to a lesser extent for severe wind gusts and large hail.

A level 1 was issued for the Adriatic Sea mainly for waterspouts.

A level 1 was issued for parts of Italy and for NE Spain mainly for large hail and to a lesser extent for severe wind gusts.

SYNOPSIS

A blocking high pressure system slowly moves from Finland into Northwestern Russia. It is surrounded by a wreath of lower 500 hPa geopotential from the Black Sea across the Alps to the North Sea with generally very weak gradients.
Along its Southern flank, the subtropical jet stream runs from the Bay of Biscay across Northern Spain into Southern Italy before it fans out over the Eastern Mediterranean. The less pronounced polar jet curves northward from the British Isles to Northern Scandinavia.
Late in the forecast period, another weak short-wave trough and an associated non-convective frontal system from the Atlantic will spread into England and Western France.

DISCUSSION

... Scandinavia, Russian Kola peninsula ...

An elevated mixed layer is advected from Bulgaria along the Western flank of the blocking high all the way northward into Lapland. On Monday, the hot air connected to the surface in some inland areas in Southern Sweden and resulted in temperatures up to 35C, though associated with a pronounced drying as the low-level moisture was mixed out. The more interesting environment for thunderstorms was provided when the capping inversion and the rich moisture beneath it could be maintained, especially along the Swedish coast where station observations showed temperatures mostly between 23 and 28C and impressive dewpoints around 20C. This combination resulted in more than 1500 J/kg of CAPE according to the Monday 12 UTC Sundsvall sounding, a quite rare event at this high geographical latitude.
Similar CAPE values are expected again on Tuesday in the pronounced warm air advection regime, limited to the North and West by a boundary which acts as a warm front over Lapland and a slowly advancing cold front over Norway. Convective initiation is expected as soon as cross-frontal circulations, upslope winds near the mountains or sea breezes erode and break the cap, repeatedly aided by large-scale lift from warm air advection and passing weak vorticity lobes. Scattered thunderstorms will form mostly from noon onward and move almost parallel to the boundary to the North, while constantly grazing an environment of vertical wind shear around 15 m/s near the cold front. Much of this shear is concentrated in the lowest 3 km and associated with slightly enhanced storm-relative helicity (100-150 m^2/s^2), which is sufficient for an organization into multicells and possibly a few supercells. Large hail, severe wind gusts and excessive precipitation can be expected, the latter especially in case a large cluster forms towards evening. The strongest shear will be present in close vicinity to the frontal boundary, but may be associated with infiltrating cooler air and therefore not be fully consumable by the storms. Nonetheless, models simulate a broad belt with enhanced 0-1 km wind shear in excess of 10 m/s, hence not even an isolated tornado is not ruled out in case storms can still stay surface-based close to the boundary (primarily depending on the amount of insolation).
Activity may go on well into the night while it continues to move northward into Lapland and then northeast- to eastward over the Kola Peninsula with a slowly decreasing severe weather risk.

... E, SE and Central Europe ...

A fairly large region will again see scattered to widespread, mostly daytime-driven, slow moving thunderstorms in an environment of CAPE up to 1000 J/kg, precipitable water between 30 and 40 mm and almost no shear.
A good part of the activity will be concentrated along the tail of the same stalled cold front over Western Lithuania, Kaliningrad and Eastern Poland, where converging winds will continue to concentrate the already rich low-level moisture, likely resulting in dewpoints close to 20C. A front-parallel storm motion from South to North will further enhance the flash flood risk. It was decided to say with a high-end level 1 due to uncertainties about the exact position of the cold front, which may be modulated by a possible wave development that moves from Northern Poland onto the Baltic Sea in the morning.
Further away from the cold front, and in general further South and East, the moisture availability becomes more difficult to asses. Unless persistent convergence zones manage to form, strong daytime heating may regionally mix out much of the low-level moisture and result in hotter and drier conditions with limited CAPE (refer to the Monday 12 UTC Kaliningrad, Legionowo, and Bukarest soundings). In that case, storm coverage and the heavy rain risk would be lower, but pulse storms have a higher chance to bring 2-3 cm sized hail and isolated severe downbursts.

Well behind this ill-defined cold front, weak to moderate positive vorticity advection re-activates the still moist air mass in a belt from Germany towards Italy and the Balkans during the day.
Areas of greatest concern are Bosnia and its surroundings, where forecast models agree on persistent convergence of very moist air (dewpoints around 20C) from both sides along the Dinaric Mountains. Widespread storms are likely to form till the afternoon and can grow into several large clusters with erratic motions in the evening and overnight. Flash floods are the primary risk and might become a major problem, as most limited area models agree on regional precipitation maxima in excess of 100 mm overnight. Localized large hail and cold-pool driven severe wind gusts are additional hazards. Activity will further spread into Albania, FYROM and Northern Greece in the evening and overnight, as well as backwards onto the Adriatic Sea where a number of waterspouts may result along convergence zones later at night.
To the North of the Alps, slightly cooler and drier air will spread from France eastward on Tuesday. Limited insolation due to rather thick morning cloudiness and some areas of rain could limit the convective potential. However, as soon as the sun comes out, at least scattered thundery showers are expected to form again. With reduced precipitable water around 25 mm and probably not more than 500 J/kg of CAPE, the heavy rain risk seems to be lower than on the previous days. However, isolated events are still not ruled out due to the slow storm motion. Other severe weather is even less likely.

... Italy, NE Spain ...

Near the Southern fringe of the highlighted areas, storms may benefit from moderate to high CAPE (possibly in excess of 1000 J/kg) and enhanced deep-layer shear beneath the subtropical jet - if they manage to break the strong cap.
The lift support of a travelling vorticity anomaly should result in at least scattered afternoon storms over the Italian mainland. In Catalonia, deep-layer shear is even stronger (more than 20 m/s) but convective initiation is more doubtful. If storms manage to detach from the mountains and tap into the very moist sea breeze, they can organize very well.
Large hail and severe downbursts are the main risks. In Italy, isolated flash flooding could result as well in case of a higher storm coverage.

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