Storm Forecast
Valid: Fri 14 Jul 2017 06:00 to Sat 15 Jul 2017 06:00 UTC
Issued: Thu 13 Jul 2017 18:41
Forecaster: TUSCHY

A level 1 was issued for NW-Germany mainly for an isolated large hail event.

A level 1 was issued for SE-Germany, the S-Czech Republic, parts of Austria, NE-Italy, parts of Slovenia and W-Croatia mainly for isolated large hail, severe wind gusts and a low-end tornado risk.

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

A level 1 was issued for parts of E-Spain mainly for isolated large hail and a severe downburst event. A very isolated very large hail event is possible.

A level 1 was issued for W-Russia and the CNTRL-Ukraine mainly for isolated large hail, excessive rain and severe wind gusts. A low-end tornado risk exists.

SYNOPSIS

A deep (steering) vortex is situated over N-Norway/Sweden with a gradual motion to the north. Another impulse enters the southwestern periphery of that vortex and evolves into a second vortex over the Baltic Sea and adjacent regions. The configuration of both vortices results in an upper-level trough, which encompasses an area from N- Norway/Sweden to Hungary and the far W-Ukraine.

A brisk W-/NW-erly flow regime continues over W-/CNTRL Europe between that trough and a large high pressure area over SW-Europe.

Regarding fronts, no frontal intrusion is expected over the Mediterranean, where hot and mostly humid conditions persist. Elsewhere, a wavy front over E-/SE-Europe separates a warm and moist air mass to its east from a cool and drier one to its west. Over CNTRL-Europe, an occlusion exits the North Sea to the SE during the start of the forecast and affects Benelux, Germany and the Czech-Republic during the forecast.

DISCUSSION

... NE-Spain ...

A conditional risk of a few severe thunderstorms exists although CI will be quite uncertain.

A pool of very rich/deep LL moisture is present over the far W-Mediterranean (between the Balearic Islands and E-Spain). N-to NW-erly LL-/mid-level flow over S-France sparks both a Mistral event and a anticylonic LL flow regime S of the Pyrenees (e.g. NW of the Balearic Islands). (Speed) convergence along the W-fringe of the Mistral pushes the rich BL moisture constantly west with an increase in depth. In addition, a strong thermal low over Spain supports a diurnal driven onshore flow regime along the E-coast of Spain. This pushes surface dewpoints in excess of 20 °C ashore.

Impressive 850 hPa temperature of 25-30 °C beneath rather cool mid-levels of -10°C result in very steep mid-level lapse rates of 8-9.5 K/km. The overlap of this EML with the aforementioned BL moisture result in a confined region with capped 1-2 kJ/kg MLCAPE. The only chance to see isolated CI will be either due to upslope flow along the eastward facing slopes or during the passage of weak short-waves, which may spark an isolated mountain storm. The question will be, if this activity is able to develop robust updrafts against entrainment of very dry surrounding air and if any storm can build into the capped but very unstable air mass over E-Spain. Any storm, which manages to survive that environment will see favorable conditions for organization with 0-6 km shear in excess of 20 m/s. Large or isolated very large hail will be the main hazard although a localized severe downburst event in the mountains is well possible, given the very dry and deep subcloud layer with LCLs in excess of 2.5 km AGL. The risk of isolated activity continues all night long.

... Austria, NE-Italy, Slovenia, parts of Germany and the Czech Republic ...

Already during the start of this forecast, a SE-ward moving diffluent upper-level trough interacts with an unstable air mass over N-Italy and a cluster of thunderstorms is expected to continue from the previous night. This cluster should push east with isolated severe over NE-Italy and Slovenia (300-600 J/kg MLCAPE and 20 m/s DLS). Isolated large hail and severe wind gusts will be the main hazard.

Later-on during the day, diabatic heating results in an increase of CAPE ahead of this cluster. The best BL moisture should stay along and just south of the Alps. The cluster's anvil debris may limit insolation over SE-Austria, so we think the overall severe risk remains marginal and confined to extreme SE-Austria and N-Slovenia. In addition, a more humid air mass over the N-Adriatic Sea may also result in some re-strengthening of that cluster with an isolated severe risk (severe wind gusts and isolated large hail). This activity may affect Croatia around noon with a rapid weakening trend forecast.

Behind that cluster however, rich BL moisture (due to the preceding rainfall / evapotranspiration) and insolation will push MLCAPE in the 400-800 J/kg range over SE-Austria. DLS in excess of 20 m/s may support a small corridor and confined time-frame for a few organized multicells/isolated supercells. The main risk will be isolated large hail and severe wind gusts. Despite weak LL shear, rich LL moisture and deviating storm motions may increase SRH on a local scale and we can't exclude an isolated tornado event. The risk diminishes after sunset.

Further north, over SE-Germany into NE-Austria, the same problem exists with convection from the previous night, succeeding anvil debris and limited insolation. During the afternoon hours, enough diabatic heating is anticipated for the build-up of several hundreds J/kg of weakly capped SBCAPE. Forecast soundings show the possibility that updrafts may struggle to take profit of the 0-6 km shear (20-25 m/s) due to limited vertical growth. However the chance exists for a few deeper updrafts in regions with localized better BL moisture. In additon, 0-3 km shear is also enhanced due to moderate 700 hPa flow. A few organized multicells/isolated supercells are forecast with an isolated large hail and severe wind gust risk. A low-end tornado risk exists similar to SE-Austria. Convection weakens after sunset.

Further north, over E-Germany and the Czech Republic, less shear but similar MLCAPE support a few strong to isolated severe thunderstorms. Isolated large hail will be the main hazard.

Over NW-Germany, another impulse and the SE-ward moving occlusion intract with a moderately unstable air mass with SBCAPE of 300-600 J/kg. A forced line of convection is forecast to affect NW-Germany during the daytime hours with graupel and gusty winds. An isolated large hail event can't be ruled out with 15 m/s DLS but the risk dminishes rapidly around sunset.

During the night, an ongoing risk of elevated thunderstorms exists over CNTRL-/E-Germany into the Czech Republic as the impulse dives SE with MUCAPE ahead. A solid line of convection may evolve over the Czech Republic into NE-Austria beyond 00 UTC in the left exit of a strong mid-level jet. Graupel/isolated large hail may accompany strongest activity.

... CNTRL-Italy ...

Roughly 1 kJ/kg MLCAPE and 10-15 m/s DLS are forecast over CNTRL-Italy. Orography itself but also a passing short-wave should support scattered thunderstorms with large hail, severe wind gusts and excessive rain. Rapid clustering is forecast betimes before a weakening trend starts during the night.

... NW-Russia and the CNTRL-Ukraine ...

Along/east of the wavy front, 600-1000 J/kg MLCAPE and 10-15 m/s DLS overlap. With some diabatic heating and the front itself, scattered to widespread CI is anticipated. Pulsating convection and a few multicells cause an excessive rainfall, large hail and isolated severe wind gust risk. Somewhat stronger LL shear next to the front may also support a localized tornado event as LCLs remain rather low (below 800 m). Clustering occurs betimes with a transition to an isolated excessive rainfall risk, before the convection weakens during the night.

Heavy rain may occur north of the steering vortex over far N-Norway as the warm/moist air mass wraps around the center. Beside a low-end risk of elevated thunderstorms, the main event should be stratiform in nature. Hence we only expanded the lightning areas into the region. Localized flash flooding is still possible.

Over CNTRL-Ukraine a back-building cluster with an augmented excessive rainfall risk is forecast although most of the activity occurs to the east of our forecast area. Hence only a small level-1 was issued. The risk diminishes beyond 18 UTC.