• Administrator

    Valid: Fri 12 Jun 2020 06:00 to Sat 13 Jun 2020 06:00 UTC
    Issued: Fri 12 Jun 2020 01:45
    Forecaster: PISTOTNIK

    • A level 1 and level 2 area issued for a large belt from SW Russia, the Ukraine, N Romania, Hungary, Slovakia, the Czech Republic, E Germany, Poland, Lithuania and Belarus mainly for large hail, excessive convective precipitation, severe convective wind gusts and to a lesser degree for tornadoes.

    • A level 2 is issued for south-central France mainly for excessive convective precipitation and to a lesser degree for large hail, severe convective wind gusts and tornadoes.

    • A level 1 was issued for S and E France mainly for excessive convective precipitation, large hail and to a kesser degree for severe convective wind gusts.

    • A level 1 was issued for SE Romania, Bulgaria, N and central Turkey mainly for excessive convective precipitation and large hail.

    • Level 1 was issued for NE Algeria, NW Tunisia and S Turkey for large hail and severe convective wind gusts.

    Synopsis

    A blocking pattern is in place. A large anticyclone over Scandinavia is opposed by an active cyclone over the Bay of Biscay and a filling mid-level low from the S Balkans to Turkey.

    Very warm air from Russia is advected westward into E and central Europe at the northern flank of the cut-off lows.

    Discussion

    Northern Black Sea region, Poland and Germany

    Similar to Thursday, the highest severe weather potential arises in the belt of maximized warm air advection, where low-level moisture accumulates to robust CAPE and synoptic lift works on the erosion of the (initially pronounced) capping inversion.

    With expected dewpoints in the upper tens, CAPE between 500 to 1500 J/kg should be available again after some hours of daytime heating.

    Vertical wind shear is rather weak with values around or below 10 m/s even across deep layers, but warm air advection creates veering low-level hodographs with moderately enhanced 0-3 km storm-relative helicity between 50 and 150 m^2/s^2.

    All in all, this environment seems prime for some well-organized storms with a strong tendency to grow upscale and produce all kinds of severe weather, which was already confirmed by numerous severe weather events on Thursday (including hailstones up to 7 cm in size).

    To the NE of this boundary, low-level moisture is expected to be shallow and vulnerable to thermal mixing processes, which should create dry and deeply mixed vertical profiles and reduce CAPE to a few hundred J/kg as soon as daytime heating takes over.

    Hence this boundary will attain the characteristics of a dryline and will not only be the prime site of convective initiation, but also the northeastern margin of the area at the highest severe weather risk.

    photo: Windy.com; licence: cc

    Under a pronounced cap and limited synoptic lift (aside from the ongoing warm air advection), convective initiation will happen rather late, possibly not before early to mid-afternoon, and may even completely fail across some areas. However, once it happens, it will likely set off a cascade of secondary initiations at outflow boundaries.

    Best chances for the "convective nucleus" to form exist over and around Moldova, from where storms would likely move into N Romania and the SW Ukraine and possibly enter Hungary, Slovakia and Poland if they survive into the night.

    In general, storms can contain embedded supercells and will quickly grow upscale into large clusters. The main risk at early stages is large to very large hail, followed by cold-pool-driven severe wind gusts and very heavy precipitation later on.

    One or two brief tornadoes are not ruled out in case of favourably colliding gust fronts or outflow boundaries.
    Signals for convective initiation are less convincing further eastward along the sea breeze front in the Ukraine and SW Russia and further northwestward towards central Europe.

    Scattered storms can form over mountains in Romania, Slovakia, the Czech Republic and SW Poland, but may struggle against the cap once they detach from them.

    A low-probability scenario, shown only by WRF, is a surviving overnight MCS over Hungary whose outflows could improve the chances for new storms on Friday also over flat terrain and further westward. However, at the time of writing (Thu 23 UTC), there are no indications for this outlier scenario to materialise.

    In E and N Germany, stratus clouds will probably break up too late to allow surface-based storms already in the afternoon.

    Chances for scattered storms to form or to move in from the Czech Republic and Poland increase in the evening and overnight, but they will likely be elevated and their severe weather risk is limited.

    Screenshot 2020-06-12 at 12.03.59.png

    Poland, Kaliningrad, Lithuania, Belarus, Ukraine and Russia

    Ahead of the western cut-off low, a few hundred J/kg CAPE in moist and moderately warm air overlap with strong vertical wind shear (15-20 m/s across the 0-3 km layer). Synoptic lift from a short-wave trough and frontal lift of a slowly advancing cold front across France will likely create scattered storms in the course of the day.

    The highest severe weather risk evolves over south-central France, where constantly backbuilding storms originating from the Massif Central could create more than 100 mm rain within a few hours and produce a very dangerous flash flood situation (probably maximalized in the creeks discharging northwards).

    With higher CAPE up to 1000 J/kg and enhanced 0-1 km shear under a southerly low-level jet, tail-end storms can contain mesocyclones with a risk of large hail, severe wind gusts or tornadoes.

    Further north and east, the lower CAPE magnitude and the almost front-parallel storm motion limit the severe weather risk and shift it betimes to heavy rainfall. Convection will gradually become elevated and weaken in the evening and overnight while it spreads into BeNeLux, Germany and Switzerland.

    Balkans and most of Turkey

    Beneath the eastern cut-off low, scattered daytime-driven storms form in an environment of low to moderate CAPE and weak vertical wind shear. The main risks are heavy rain and localized marginally large hail.

    Algeria, Tunisia and Turkey

    Daytime heating of moist onshore and upslope circulations create some hundred J/kg CAPE, which overlap with enhanced vertical wind shear beneath the subtropical jet (up to 15 m/s in S Turkey, up to 25 m/s in Algeria and Tunisia).

    Afternoon storms will stay isolated to scattered, but pose a main risk of large hail (especially with good access to coastal moisture) and severe downbursts (especially in the drier, more deeply mixed air further inland).


  • bonjour ,

    Pourquoi laisser des informations qui sont Anciennes ....?

  • Administrator

    @bernard17 Hello, this is article and we do not delete old articles.

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