Severe storms in Europe: A level 3 is issued for parts of Croatia and Italy



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    Valid: Sat 22 Jun 2019 06:00 to Sun 23 Jun 2019 06:00 UTC
    Issued: Sat 22 Jun 2019 00:10
    Forecaster: PISTOTNIK

    • A level 3 is issued for parts of Croatia mainly for large hail and to a lesser degree for severe convective wind gusts.
    • A level 3 is issued for a small area in NE Italy mainly for large hail and to a lesser degree for tornadoes and severe convective wind gusts.
    • A level 1 and level 2 are issued for N Italy, SE Austria, Slovenia, Hungary, parts of Croatia, Bosnia-Herzegovina, Serbia, Romania and Bulgaria mainly for large hail, excessive convective precipitation and to a lesser degree for severe convective wind gusts.
    • A level 1 and level 2 are issued for the rest of Austria, Germany and the S Czech Republic mainly for excessive convective precipitation.
    • A level 1 and level 2 are issued for Slovakia, SE Poland and the SW Ukraine, as well as for the W half of Turkey for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.
    • A level 1 is issued for parts of Belarus and NW Russia mainly for excessive convective precipitation and to a lesser degree for large hail, severe convective wind gusts and tornadoes.
    • A level 1 is issued for NE Turkey, Georgia and the Russian Caucasus region mainly for large hail and severe convective wind gusts.

    Synopsis

    Steering cyclones are placed over Finland and over the Atlantic near 50N 15W. A meandering W to SW flow at 500 hPa and a diffuse frontal boundary near the surface stretch from the Bay of Biscay to NW Russia at their southern flank.

    South of this frontal boundary, very warm to hot, often moist and unstably stratified air covers the majority of the continent. Under mostly weak geopotential/pressure and temperature gradients, a cut-off low moves from the Alpine region to the N Balkans and another one is stationary over the Black Sea, whereas mid-level ridging and a surface anticyclone dominate from France to Germany and Poland.

    Discussion

    NW Russia, Belarus, NW Ukraine

    The cold front of the Finnish cyclone slowly moves eastward. CAPE on the order of 300 to 1000 J/kg was sampled by the Friday 12 UTC radiosondes and higher values were indicated towards the SW by 2m dewpoints around 20C near the Polish-Belorussian border, though the precicted CAPE values up to 2000 J/kg appeared unlikely.

    A similar CAPE magnitude - 300-1000 J/kg over Russia, up to 1500 J/kg over Belarus and the W Ukraine - is expected on Saturday, overspread by 0-3 km shear between 10 and 15 m/s under mostly unidirectional wind profiles. Isolated storms may be active already in the morning.

    Coverage will increase in the afternoon. CAPE mostly based on rich low-level moisture and wind profiles favorable for side- and backbuilding multicells point to a main risk of excessive rain. If storms manage to stay discrete or in case of favorable inflow/outflow interaction, transient supercells with additional risks of large hail, severe wind gusts or a tornadoes are not ruled out, in particular near a dryline that borders the CAPE reservoir to hot, dry air from the Russian prairies further SE.

    Convection will gradually turn elavated and weaken after sunset.

    Alpine region, N Italy, N Balkans, Hungary

    Particularly rich low-level moisture has gathered on the warm side of the frontal boundary and is overspread by the fringes of a Saharan elevated mixed layer. Widespread, daily convective overturning limits CAPE over the Alps to some hundred J/kg, whereas the airmass south of the Alps remained largely capped until Friday evening, yielding CAPE on the order of 1500 J/kg per 12z Rivolto, Zagreb and Belgrade soundings based on 2m dewpoints around 18C, and regionally up to 2500 J/kg where 2m dewpoints exceed 20C.

    On Saturday, a confined mid-level jet curves around the cut-off low over the Alpine region. In its left exit, strong lift and increasing mid-level flow overspread this potent airmass from the west. By afternoon, 0-3 km shear up to 15 m/s and 0-6 km shear up to 20 m/s should become available. Interaction with the cut-off low also creates a subtle wave within the diffuse frontal boundary with temporary warm air advection and veering wind profiles.

    While everything points to an outbreak of severe thunderstorms, details are still difficult to assess. In the northern part of the concerned area, both CAPE and vertical wind shear will be limited, but very moist air and plentiful synoptic and orographic lift create a typical environment for excessive precipitation and flash floods.

    This risk is maximized in the Italian Alps, S and E Austria and W Hungary, where most models agree on precipitation peaks exceeding 100 mm. A wedge of CAPE will likely also wrap around the cut-off low and be squeezed out in a strongly convergent wind field over the southern parts of the Czech Republic and Bavaria.

    Towards the south, the main risk shifts to discrete and well-organized storms. Possible initiation sites include the tail ends or outflow boundaries of the storm clusters over the Italian and Austrian Alps, or individual storms forming over the northern parts of the Appennines and Dinaric mountains and moving onto adjacent forelands.

    These storms can easily turn into strong multicells or supercells with a main risk of large to very large hail.

    The further south, the more CAPE is available (possibly well in excess of 2000 J/kg in inland parts of Croatia and Bosnia-Herzegovina), but the less favorable the balance between the capping inversion and the synoptic lift will be. As usual in these volatile setups, the margin between a "null event" and a memorable severe weather outbreak is therefore quite narrow, and the southernmost updrafts that still manage to persistently break the cap will likely develop into the most extreme storms.

    In case convection grows upscale into an MCS later on, additional risks of cold-pool driven severe wind gusts and excessive rain would evolve and such a feature could travel as far east as E Hungary, N Serbia or even SW Romania overnight.

    This scenario is rather questionable, but still reflected by an eastward expansion of the lightning and risk level areas across much of the Pannonian plains.

    Two small areas are upgraded to a level 3 after awaiting the Friday 12z and 18z runs:

    (1) Inland Croatia: all convection-resolving models agree on strong supercell signals, hence confidence in some extreme hail events is high enough.

    (2) NE Italy between the Appennines and the lower Po river: Onshore and uslope flow of very moist air from the Adria might create more than 2000 J/kg CAPE under strongly veering low-level wind profiles. Whereas a premature reduction of CAPE either by dry downslope winds or by morning convection pose limiting factors further west, an undisturbed sea breeze regime should be maintained and not disturbed until the afternoon closer to the Adriatic Sea.

    By then, a second vorticity maximum on a more southerly track will erode the cap and convective initiation at the sea breeze front or outflow boundaries appears to be only a question of time. Supercells with all kinds of severe weather are very likely, including a possibility of extremely large hail and one or two tornadoes.

    rest of E-central and SE Europe, W half of Turkey

    In an environment of moderate CAPE (some hundred to around 1000 J/kg) and almost zero vertical wind shear, at least scattered daytime-driven storms are expected over orographic features. The main risks are large hail in initiating stages and flash floods plus isolated downbursts later on.

    A particularly high storm coverage is expected over the northern Carpathian arc from Slovakia and the SW Ukraine to N Romania. By evening, several large storm clusters with an ongoing severe weather risk may propagate some distances northward into the forelands over the Ukraine before they finally decay overnight.

    NE Turkey, Georgia, Russian Caucasus region

    SW-erly mid-level flow ahead of the cut-off low over the Black Sea advects an elevated mixed layer from the Turkish Plateau over Georgia and from the Caucasus mountains over adjacent forelands (though the latter belt is less than 100 km wide). CAPE will likely reach 1000 J/kg or more, though it will be strongly capped.

    Warm air advection and subtle vorticity maxima will create some lift. Scattered elevated altocumulus convection is expected, and some surface-based storms will likely form over high mountains in the course of the day.

    It is unclear if lift will suffice to make some storms tap into the rich lowland moisture and break the cap there. If they do, robust CAPE and 0-6 km shear up to 15 m/s can promote storm organization with a risk of large hail and severe downbursts.

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