Valid: Sun 29 Oct 2017 06:00 to Mon 30 Oct 2017 06:00 UTC
Issued: Sun 29 Oct 2017 00:12
Threat levels were issued for large parts of central Europe mainly for severe convective wind gusts and to a lesser extent for tornadoes.
Amplified long waves govern the circulation in Europe. A 500 hPa trough over eastern Europe and a deep surface low with its center over Estonia are opposed by a 500 hPa ridge and an extensive surface high over western Europe and the near Atlantic.
In-between, a very strong NW-erly flow curves from Scandinavia across central Europe to the Aegean Sea. Embedded in this mid-level jet streak, a powerful short-wave trough ejects from Denmark (Sun 00 UTC) to the Black Sea (Mon 00 UTC) and reinforces the eastern European long-wave trough. Strong synoptic lift ahead of this short wave and in the jet's left exit also stimulates the formation of a secondary surface cyclone over the Skagerrak Sea Saturday night, which is expected to deepen below 970 hPa while it moves across Poland into Belarus on Sunday. This new cyclone will bring a remarkable autumn storm to central Europe, probably including a round of severe convection along its cold front.
... central Europe (cold front passage) ...
A tongue of moist air from the North Sea is advected eastward ahead of the cold front (2m dewpoints around 10°C across N Germany and around 12°C in the Netherlands at Sun 00 UTC, in line with the forecast models). It is overrun by steepening lapse rates, as the approaching short-wave trough lifts and vertically stretches the air. This interaction will likely create neutral to marginally unstable profiles up to 4-5 km height by early Sunday morning, and the formerly stratiform rain shield along the cold front is expected to break up into convective elements in E Germany and W Poland.
Vertical wind shear is more than supportive for storm organization below the mid-level jet streak: 0-3 km bulk shear increases from 10 m/s in central Poland to 30 m/s near the Alps, where 0-3 km storm-relative helicity also peaks above 400 m^2/s^2. The best overlap of all "ingredients" for organized convection is anticipated in the 06 to 14 UTC time frame, as the most active part of the cold front crosses east-central Europe. Linear convection with wind gusts very probably above 25 m/s and possibly, at least locally, above 33 m/s is expected to develop. Fine-meshed models agree on such a scenario.
The cold front will probably be located along a line Wroclaw (Poland) - Prague (Czech Republic) - Nuremberg (Germany) at Sun 06 UTC and will quickly move SE-ward. In the first few hours of this forecast period, the most active line segments with the highest risk of lightning and severe to localized extreme wind gusts will likely follow the Polish / Czech border. By mid-morning, the eastern line segments will gradually weaken and the most active parts should shift across the E and S Czech Republic into E Austria, W Slovakia, and finally Hungary, following the path of the strongest synoptic forcing.
The area to the east of the Alps becomes a particular focus. As the westerly flow increases ahead of the cold front, diurnal heating and westerly downslope winds (which may easily become severe themselves, but are not included into the ESTOFEX threat level scheme) may push the temperatures temporarily close to 20°C in the late morning. The deeper and drier boundary layer further increases the risk of severe downbursts as the convective line arrives. In addition, a pocket of westerly 850 hPa winds up to 40 m/s (!) overspreads this area exactly in the time frame when the cold front is expected to arrive - e.g., around 10 UTC in Vienna and Bratislava, 11 UTC in Budapest and 13 UTC at the Hungarian / Serbian / Croatian border triangle.
All in all, the setup appears menacing enough to issue a confined level 3 area. Prior to the cold front's arrival, the formation of a "dryline" boundary between poorly mixed, more humid air to the east and warmer, drier Foehn subsidence to the west should be monitored! If it appears, it would mark a preferred path for the axis of the most extreme wind gusts.
Convection will gradually weaken in Croatia, Serbia and W Romania in the late afternoon, as both the strong wind field and the synoptic lift start to disintegrate.
Valid: Mon 01 Jan 2018 06:00 to Tue 02 Jan 2018 06:00 UTC
Issued: Mon 01 Jan 2018 02:47
Forecaster: VAN DER VELDE
A level 3 was issued across southwestern France mainly for extremely severe convective wind gusts and tornado probabilities.
A level 2 was issued across western and central France, as well as the north coast of Spain, mainly for severe convective wind gusts and tornado probabilities.
A level 1 was issued across western Mediterranean and Tyrrhenian Sea mainly for severe convective wind gusts.
A large low pressure system with the core over western Norway remains in place over northwestern Europe. An activating satellite low pressure system tracks from the Atlantic south of Ireland through the English Channel towards Belgium and on to eastern Germany. Strong non-convective winds are forecast across northern France and southern Germany. However, the passage of its cold front from Bay of Biscay into southwestern France meets all criteria for severe convective weather. Cold upper air advection combined with 6-7 g/kg BL mixing ratio over the Celtic Sea has caused steep lapse rates and a few hundred J/kg CAPE, while models draw a sharp cold front with strong low-level convergence and differential theta-e advection, supported by a strong PV anomaly.
The cold front will likely trigger a line echo wave pattern (LEWP) with bowing line segments responsible for wind gusts of 30-35 m/s as is present in the 1-3 km layer. 0-1 km shear vectors over 15 m/s (locally even 20 m/s) magnitude will be in place across N Spain and most of France, supporting tornadogenesis as well as the bow echoes. Convection is likely to survive even inland as strong moisture advection, dynamic destabilization factors, lapse rates and orographic lifting are carried as far as central France.
As the front moves on, its structure may be torn apart between Pyrenees and Alps after 15Z, by the cut-off of moisture supply, but convection will likely recover before Corsica and Sardinia around 21Z, while the flow weakens.
Valid: Fri 08 Jun 2018 06:00 to Sat 09 Jun 2018 06:00 UTC
Issued: Thu 07 Jun 2018 23:26
A level 3 is issued for Slovenia and N Croatia mainly for large hail and severe convective wind gusts.
A level 1 and level 2 are issued for N and E Italy, SE Austria, SW Hungary and Croatia mainly for large hail, severe convective wind gusts and excessive convective precipitation.
A level 2 is issued for the Adriatic Sea mainly for tornadoes and to a lesser degree large hail and severe convective wind gusts.
A level 1 and level 2 are issued for Bosnia-Herzegovina, Montenegro, parts of Serbia and Albania mainly for large hail and to a lesser degree severe convective wind gusts and excessive convective precipitation.
Level 1 and level 2 areas are issued for S Germany, N Austria, Slovakia, S-ernmost Poland, NE Hungary, Romania and the SW Ukraine as well as for NE Turkey, parts of Georgia, Armenia and the Russian Caucasus region mainly for large hail and excessive convective precipitation, and to a lesser degree severe convective wind gusts.
A level 1 and level 2 are issued from NW Germany into Switzerland mainly for excessive convective precipitation and to a lesser degree large hail.
Another deepening polar trough and a surface cold front translate from Finland into W Russia. Otherwise, mostly weak pressure gradients remain in place across much of Europe. Cut-off lows at 500 hPa are located NW of Scotland, NW of Iberia and over central Italy.
Warm and moist air covers much of the continent south of England, S Norway and the Ukraine. It often allows moderate CAPE in reponse to daytime heating, mostly under weak vertical wind shear and weak dynamics. However, a confined window for an outbreak of organized storms opens up ahead of the Italian cut-off low, where particularly hot air wedges from SE Europe towards Austria and increasingly overlaps with enhanced vertical wind shear.
... Italian E coast, Adriatic Sea, Croatia, Slovenia, SE Austria, SW Hungary, Bosnia-Herzegovina, Serbia, Montenegro, Albania ...
Steep lapse rates, originally created over the Sahara desert and reinforced over the Appennines and the Dinaric mountains, spread over the area of interest. With 2m dewpoints of 16-20C over the Pannonian plains and up to 23C over the Adriatic Sea, around 2000 J/kg of CAPE are predicted to build on Friday, locally even more. Warm air advection will create a pronounced cap, but lift ahead of the cut-off low will start to increase from the SW as the day proceeds. At the same time, vertical wind shear rises to 15 m/s (20 m/s) across the lowest 3 (6) km under slightly veering profiles.
Scattered convection will likely still be left over from the previous night over N and E Italy or will form already before noon, aided by increasing synoptic lift and a mess of outflow boundaries and other disturbances of the low-level wind field. Competition and destructive interference may lower the severe weather risk somewhat, but at least scattered large hail, severe wind and heavy rain events are expected with this activity. Tail-end storms towards the southeast may organize best into supercells.
It is worth mentioning that the majority of the model pool, including convection-permitting models like WRF and MOLOCH, predict scattered activity across the northern and central Adriatic Sea throughout the forecast period. Scirocco winds would advect a very moist boundary layer with enhanced 0-1 km shear into these storms, which would very likely become supercellular. If such a high storm coverage over the Adriatic Sea indeed verifies, numerous tornadoes could form, most of them well off the coastlines, though.
In the evening and overnight, clustering storms will increasingly move onshore or form anew in coastal Croatia, Montenegro and Albania with a gradually decreasing but still enhanced risk of all kinds of severe weather.
Further north and east of the Adriatic Sea, it is most difficult to assess where and when the cap will be broken. Convective initiation will stay isolated at best until mid-afternoon. Possible sites for solitary early "cap breakers" are the southern Alps (if upvalley flow is strong enough) or outflow boundaries, laid out by several storm clusters from the previous night that are moving northward in E Austria and W Hungary near the nose of the warm air advection regime at the time of writing (Thu 22 UTC). Despite an expected struggle against the cap, these early storms, if they form, may already organize well and bring large hail.
In the course of the afternoon, the probability of convective initiation rapidly increases over the Dinaric mountains in W Slovenia and NW Croatia. While vertical mixing will keep both CAPE and wind shear lower in these initiating regions, storms will quickly intensify and organize as soon as they tap into the abundant CAPE reservoir and better shear further northeast. Convection-resolving models agree on quick upscale growth into a large MCS with a most likely track across much of Slovenia in the evening, including the bordering regions of Croatia and Austria. Supercells will likely bring large to very large hail in initiating stages or generally at the southern flank of such a system. In addition, a tornado is not ruled out, though this risk is mitigated by not too low cloud bases and rather inconspcuous low-level shear. Along the MCS's main body, severe straight-line winds will betimes become the main risk. They may become quite widespread and include some extreme events (>32 m/s).
The mature and finally slowly weakening MCS could persist well into the night or even into the next morning while it continues its track northeastward into E Austria, Hungary and possibly even Slovakia. It will likely bring a prolonged period of heavy rain, and flash floods become another dangerous hazard in the course of the night, especially in S and E Austria where soils are often already saturated. In addition, severe wind gusts or large hail are still possible at the leading edge or under more discrete updrafts, respectively.
Similarly abundant CAPE and enhanced vertical wind shear is expected over the eastern parts of Croatia and Bosnia-Herzegovina as well as in Serbia, but the capping inversion will be stronger towards the south (indicated by the Thu 12 UTC Trapani, Pratica di Mare and Brindisi soundings) and synoptic lift arrives later and stays weaker. It is uncertain whether the cap can be broken at all. However, every storm that forms over the Dinaric mountains and descends into the Pannonian plain can easily become supercellular with a risk of large to very large hail. Severe downbursts and flash floods become additional hazards in case convection manages to grow upscale. In that case, it may also persist into the evening and night. | Gewijzigd: 8 juni, 19:05 uur, door Lako