Storm Forecast
Valid: Fri 15 Aug 2008 06:00 to Sat 16 Aug 2008 06:00 UTC
Issued: Thu 14 Aug 2008 21:16
Forecaster: TUSCHY/GROENEMEIJER

* * * A major severe-weather outbreak is forecast for parts of the central Mediterranean, the northwestern Balkans and parts of central Europe. Severe thunderstorms with tornadoes, damaging wind gusts and significant hail can be expected. * * *

SYNOPSIS

A very dynamic upper-level trough, centered over France during the morning hours, is on its way to the east, approaching the Alpine region during the late afternoon/evening hours as a negative tilted and still strengthening upper trough. This feature dictates the weather over most parts of Europe as intense WAA downstream of this system spreads northeastwards, while much cooler and more stable air infiltrates NW Europe. Between both airmasses, a quasi-stationary and undulating frontal boundary has established during the past days. This boundary changes into a cold front over central Italy, Austria, the Czech Republic, and SW Poland, and into a northward advancing warm front over central and northern Poland. Very hot conditions continue over the central Mediterranean, eastern and southeastern Europe.

DISCUSSION

Since the Extended Forecast for this day has been issued, global models have converged regarding the overall development. Exception is the UKMO, which still keeps the developing surface depression south of Poland, while others now agree on a northward shift of a surface depression into Poland.

...NE Italy, Slovenia, parts of Croatia, western Bosnia and Herzegovina...

The aforementioned mid/upper-trough should be located across central Italy during midday and the rest of the area during the late afternoon/early evening hours. A strong mid-level wind field downstream of the trough overspreads the area (e.g. 30-40 m/s at 500 hPa) creating 0-6 km bulk shear around 30 m/s. As a surface cyclone forms and deepens across the Po Valley and northern Adriatic Sea, and the low-level wind field reacts to that, strong veering winds with height are expected to develop. High storm-relative helicity with values over 200 m^2/s^2 0-3 km storm-relative helicity should result. In addition to those already impressive values, low-level shear increases over NE Italy/W-Slovenia and W-Croatia during the midday hours with 0-1 km storm-relative helicity values at or above 200 m^2/s^2.

Forcing should be provided by strong cyclonic vorticity advection as the negative tilted trough approaches the area.

The boundary layer airmass was already very moist yesterday with dewpoints in the upper tens/ lower twenties and this airmass will be present during the day. A well evolved EML advects to the northeast as shown by the sounding of Dar-El-Beidaand central Italian soundings. Widespread 1000-2000 J/kg MLCAPE with higher MUCAPE values are forecast.

Thunderstorms are likely ongoing just south of the Alps during the early morning hours as strong moisture advection persists. This activity, composed of multicells/isolated supercells, will keep going during the day, while storms develop southwards in an increasingly more unstable airmass as they approach the Adriatic Sea. Each more or less discrete storm/mature supercell poses a risk of hail >5 cm and severe wind gusts. The tornado threat, already present during the morning hours, increases over Slovenia and parts of Croatia during the afternoon and especially evening as low level shear increases significantly. Moreover LCL heights will lower after sunset. A few strong tornadoes are possible. The threat continues well into the night. In addition, there is a risk for waterspouts across the northern Adriatic Sea during the night hours, as a pronounced thermal through crosses the area and the background wind field decreases.

...Central and NW Italy...

Over central Italy, thunderstorms are also expected to develop as the upper trough axis approaches. This may occur as early as the late morning or early afternoon. The environment is favorable for very large hail possibly above 5 cm. Damaging wind gusts are also possible for a deep well-mixed subcloud layer will be present (LCLs still well above 1 km). During the afternoon, the cold front at the surface advances SE-wards and LCL heights become lower. Abundant low-level instability release, rich boundary-layer moisture and strong deep-layer shear overlap and a few tornadoes may occur. A strong tornado cannot be ruled out given the degree of shear and instability. As the thunderstorm activity shifts SE-wards, they encounter a more capped environment, and will likely decrease in coverage coverage. However, intense forcing could push thunderstorm development further to the south than global models indicate.

More thunderstorms could well develop across NW-Italy during the afternoon, as the trough axis crosses those regions. Instability will be available and an isolated severe event including some hail is not ruled out.

...SE and E Austria, W and central Hungary, NE Croatia, Czech and Slovak Republics...

Uncertainties about the developing surface depression and its effects on the final strength of the shear complicate the forecast for this area considerably. In the past 24 hours, a few models simulated warm air to be advected as far west as eastern Austria. In that case, E/SE Austria could see a strong instability build-up during the morning/midday hours just as well as further east. Instability in excess of 1000 J/kg, deep-layer shear of 25-30 m/s and low-level shear in excess of 10 m/s overlap ahead of the frontal boundary, which moves to the SE during the afternoon/evening hours as a cold front.

Multiple rounds of thunderstorms are possible. During the late morning/afternoon hours a lead vorticity impulse crosses the area from the SW, which may cause a first round of storms. These will be capable of producing large hail. Hail <5cm can occur with thunderstorms over extreme S/SE/E Austria, W Hungary, and possibly move into SW Slovakia. In addition, low-level shear and 0-1 km storm-relative helicity increase during the day and combined with the impressive low-level CAPE, tornadoes are possible and even strong tornadoes can occur. The rough topography over Austria could have a bimodal effect, both in decreasing storm organisation or locally increased shear, which cannot be forecast, but east of the mountainous area, a risk for long-lived supercells will exist.

More thunderstorms will probably evolve during the evening hours and overnight as the intense upper trough draws near the area of interest. Strong upper-level divergence, very strong 0-2 km deep convergence, an intensifying surface depression moving northward across western Hungary create an environment favorable for severe weather. Most notably, very high 0-3 km storm-relative helicity will, per GFS, develop on the surface low's northwestern flank (up to 600 m2/s2) across western Hungary and eastern Austria. Hence, a threat of a few tornadoes, including strong tornadoes will exist. A few large hail/severe wind gusts are also possible. Otherwise, the threat should rapidly switch over to a widespread flash flood risk as the complex of storms moves only slowly NNE-wards. Embedded and possibly elevated thunderstorms north of the surface front will locally increase rainfall rates.

Thunderstorms may persist into the night across S-Austria, as another cluster of storms spreads NE-wards, but as instability release is low, this will mainly pose a heavy rainfall threat again for those areas. In general, numerical models have major problems to forecast the exact track of numerous MCSs that far out (e.g. convective feedback problem).

...Poland...

The aforementioned wavy frontal boundary will get modified by the developing surface depression over Poland and is expected to move northward across Poland as a warm front. Cold air slowly advances from the W during the evening/night hours. The warm sector, which covers most parts of Poland, is characterized by a warm boundary-layer with dewpoints near 20°C in vicinity of the frontal zones. Shear along the warm front and next to the depression's center should increase during the day with locally up to 20 m/s low-level shear and 0-1 km storm-relative helicity values of more than 300 m^2/s^2. LCL heights should be low near and just north of the warm front. Each thunderstorm that develops near this boundary has a high chance to acquire strong low-level rotation and produce strong tornadoes. During the late afternoon hours, the low-level wind field increases even further with readings of more than 25 m/s at 850 hPa. This not only increases the risk of rotating storms, but enhances the risk for damaging wind gusts as well. Numerical models suggest that storms may cluster across central and/or northern Poland and subsequently move northeastward into Lithuania and further northeastward. If such a cluster develops, it could well develop into a bow echo with widespread damaging winds. In comparison with the Extended Forecast issued earlier, we have upgraded parts of the area to a level 3 because a regional tornado outbreak appears very well possible in addition to damaging wind gusts.

Both shear and instability values are also impressive over SW Poland, but directional shear should be much weaker. This should help to abate the tornado risk, but storms moving off the hodograph could still produce tornadoes. The main risk will be a severe-damaging wind gust and isolated large hail threat as shear at the lowest 3km increases to 25m/s.


...Belgium and the Netherlands...

Placed under the broad upper trough axis, cold mid-/upper levels will support deep convection during the midday/afternoon hours. Shear is very weak, but there will be some CAPE release in the lowest 3 km and hence a few funnel/tornado/waterspout events can occur. A marginal level 1 was therefore introduced. Thunderstorm activity and coverage should decrease after sunset.

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