Severe Thunderstorms Expected over Czechia, E. Germany and Poland



  • Valid: Sat 15 Jun 2019 06:00 to Sun 16 Jun 2019 06:00 UTC
    Issued: Thu 13 Jun 2019 22:11
    Forecaster: TUSCHY

    A level 2 was issued across the Czech Republic, E-Germany and parts of Poland mainly for large to very large hail, severe wind gusts and excessive rain. The tornado risk is maximized from NE-Germany into N-/CNTRL Poland.

    A level 1 surrounds the level 2 with similar hazards but lower probabilities.

    A level 1 was issued for S-Poland to Bulgaria and to the Dinaric Mountains mainly for large to isolated very large hail, severe wind gusts and excessive rain.

    A level 1 was issued for SE-France, NW-Italy and parts of Switzerland mainly for large hail, severe wind gusts and excessive rain.

    A level 1 was issued for CNTRL-Italy mainly for large hail, an isolated severe wind gust event and locally excessive rain.

    photo:ESTOFEX;link:http://www.estofex.org/cgi-bin/polygon/showforecast.cgi?text=yes&fcstfile=2019061606_201906132211_2_extendedforecast.xml&fbclid=IwAR2SDvHsC2P_eZXF6aofw_e8fQ4wB1CKUvAYxYYEWfIbI5EtF7hfQPQvbJs;desc:Extended forecast valid Sat 15 Jun 2019 06:00 - Sun 16 Jun 2019 06:00 UTC.;

    SYNOPSIS

    Yet another day with a deep cyclonic vortex over far NW Europe and broad ridging from N-Algeria/Tunisia to E-Europe. Of interest is a gradual weakening trend of the ridge (especially along its western periphery) with lowering thickness while strongest synoptic-scale subsidence resides over far SE-Poland. Low heights over Turkey reflect a quasi-stationary and weak upper trough while a progressive NW-erly flow regime still impacts far NE-Europe.

    A cold front surge over the Baltic States stalls in form of an extensive and wavy synoptic-scale cold/warm front, whose exact position remains uncertain that far out. A plausible setup would be a strengthening warm front extending from a structuring and leisurely northward drifting depression over E/NE-Germany further east towards N-Poland into S-Belarus/N-Ukraine before changing back to a cold front further east. A broad warm sector with unstable conditions evolves to its south.

    DISCUSSION

    Parts of Germany, Czech-Republic to Poland

    First of all, major uncertainties exist over NE-Germany into far NW-Poland, with which storm mode and coverage we emerge from the previous night. Either discrete supercells or a large thunderstorm clusters are possible. This area probably experiences substantial modifications in the following outlook. Same for N-CNTRL Germany.

    With the approach of a strong short-wave from SW-Europe, mid-level flow backs more to the south, allowing an EML to spread far north and finally over the western part of the extensive warm sector from E-Germany, the Czech Republic into Poland. BL moisture - pushed far south/east due to the previous cold surge - is in full swing back to the W/NW and undercuts the EML, resulting once again in impressive MLCAPE values in the 1500-3000 J/kg range (ICON more aggressive than IFS). Peak values occur most likely along confluent low-tropospheric zones and the synoptic-scale warm front (e.g. far N-Czech Republic, far E-Germany and CNTRL-Poland).

    Strongest shear resides to the north of the warm front, but 10-15 m/s DLS is still forecast to impact most of the warm sector. Interacting with the mentioned thermodynamics, initiating or ongoing thunderstorms over NE-Germany pose a risk of large to destructive hail and severe wind gusts before noon. Slow storm motion and tendency for rapid clustering with short hodographs also increase the flash flood risk rapidly. A strong tornado risk may evolve in the 06-12Z time frame over NE Germany (ongoing from the previous night) with any discrete and surface based thunderstorm riding along the warm front. This is also valid for far N-CNTRL Germany.

    Despite mentioned uncertainties about the storm mode, models generally agree in the early activity (before noon) to lift north with the warm front, leaving a capped warm sector behind. At least partially sunshine is then forecast. However, uncertainties continue with potentially laid out outflow boundaries or potential trailing convection in response to 30kn SE-erly winds at 850 hPa. In case of adequate air mass recovery or even an uncontaminated warm sector, another round of severe thunderstorms with large to very large hail, severe wind gusts and excessive rain is possible over NE Germany beyond noon.
    With all those uncertainties we did not yet upgrade the western fringe of the warm sector to a level 2 which certainly has to be considered in the following SWODY-1. This is also valid for far N-CNTRL Germany.

    Further south over the Czech Republic into N-Austria the air mass should stay capped until noon, despite weak cyclonic flow aloft. Beyond noon, CI is forecast along the Alps and the Bavarian Mountains with activity growing upscale into NE-ward pushing clusters, which enter an increasingly more unstable air mass over the Czech Republic. Uncertainties about BL moisture in models keep CAPE spread substantially with IFS the more reluctant one (roughly 1500 J/kg). Large hail, severe wind gusts and heavy rain are the main hazards and we upgraded parts of the Czech Republic due to the risk of significant hail and swaths of severe wind gusts in case of a maturing MCS event.

    Over Poland, most models indicate a capped warm sector until the evening. However, we would not be surprised to see the German activity to enter NW Poland before noon, continuing all day long to the SE along the warm front/ CAPE axis. Persistent 25-35 kn SE-erly winds at 850 hPa would support the idea of a SE-ward building cluster of thunderstorms into a strong to extreme CAPE reservoir. This would occur mainly cold-pool driven as not much support is available on the synoptic-scale.

    There is a chance that this cluster grows into a huge MCS/potential MCC over CNTRL-Poland during the evening and night. SRH-3 in excess of 200 m^2/s^2 and 15 m/s DLS favor a supercell risk with tail-end or more discrete thunderstorms, which can produce destructive hail, severe wind gusts and excessive rain. The latter risk increases during the night with the maturing thunderstorm cluster. In addition, a risk of strong tornadoes exists over N-Poland along and S of the warm front with favourable speed/directional shear.

    We issued very broad level areas, for now, to account for all those uncertainties and potential internal storm dynamics like cold pool driven MCS events. We also expanded the level 1 far north of the warm front to account for convectively enhanced heavy rain and isolated large hail over far S-Sweden with lots of MUCAPE and 20 m/s DLS.

    S-Poland to Bulgaria and along the Dinaric Mountains

    An extensive warm sector beneath the flat ridge features moist LLs beneath mid-level lapse rates in the 7-8K/km range. Capped 1500-3000 J/kg MLCAPE is forecast with forecast soundings showing frightening weak CIN during peak heating. Local convergence zones and mountain/valley circulations could all spark orographic thunderstorms, which certainly would grow into intense thunderstorms despite DLS of less than 10 m/s. Large/very large hail and excessive rain with local severe downburst events are possible and a broad level 1 was added.

    SE-France to far NW-Italy into Switzerland

    A slowly approaching synoptic-scale cold front from the W assists in downstream moisture recovery and MLCAPE in the 500-1000 J/kg range with higher peaks along with the coastal areas. DLS of 20 m/s with straight hodographs show a favourable environment for isolated splitting supercells and organized multi cells with large hail and severe wind gusts, which also affect far NW Italy. Betimes the activity grows upscale into a cluster, which impacts Switzerland but diminishing CAPE lowers the severe risk (maybe isolated flash flooding which does not yet justify a level 1).

    CNTRL-Italy

    Mountainous convection is forecast on an isolated scale during the afternoon, especially along the western fringes due to prevailing westerlies. MLCAPE of 1500-3000 J/kg and 20 kn DLS favour organized multi cells with large/very large hail, severe downbursts and excessive rain. Limited thunderstorm coverage precludes an upgrade for now.

    Turkey

    Weak shear and moderate CAPE keep any severe risk on an isolated scale. Local flash flooding due to slow storm motions is possible.


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