Storm Forecast
Valid: Mon 15 Apr 2013 06:00 to Tue 16 Apr 2013 06:00 UTC
Issued: Sun 14 Apr 2013 20:19
Forecaster: TUSCHY

SYNOPSIS

A marginally toppled omega-like pattern persists over Europe with two major troughs flanking weak ridging over C-Europe. The trough over the E-Atlantic sends numerous short-waves towards NW Europe which dictate any thunderstorm risk over C-Europe, whereas a pronounced cold core low over far SE Europe causes widespread shower and thunderstorm activity.

At the surface, a cold front extends from the Iberian Peninsula to NW France to UK at 18 Z (at the 14th), which moves slowly east during the forecast. The eastward shift will be modulated by crossing short waves and regionally enhanced deep convection, which likely results in a wavy appearance of the cold front during the end of the forecast, which then runs from Finland to Germany to S/C France.

A gradually structuring LL depression just south of Turkey sends a warm front north, which is placed over SW/N Turkey during the complete forecast. A cold-core low moving atop the moist warm sector results in a favorable set-up for active shower/thunderstorm development.

For the rest of Europe, no frontal boundary is seen, which influences thunderstorm development during the forecast.

DISCUSSION

... Romania, Bulgaria and Turkey ...

Ongoing NE-erly flow over Romania and Bulgaria causes advection of a modified and moderately moistened BL air mass to spread ashore while evolving deep tropospheric vortex south of Turkey draws coldest mid-level air south and just atop that moist air mass over Romania and Bulgaria. Q-vector fields look not supportive for any pronounced forcing during the forecast, so the main focus will be either diurnal driven initiation in a weakly capped air mass or initiation along numerous NW-SE aligned and SW-ward moving convergence zones. MLCAPE is quite low with 100-300 J/kg, so updrafts likely struggle to reach the charging zone. Nevertheless there are a few spots, which may see a few more healthy updrafts and hence a 15-% lightning area was added. We expanded that lightning area far east to include a few afternoon storms over E-Romania and Bulgaria due to deepening parcel layer depth and some LL convergence with mesoscale convergence zones. The thunderstorms remain sub-severe with sleet/marginal hail the main risk.

A way more supportive set-up for widespread CI exists over parts of Turkey, where rough orography, the aforementioned warm front, attendant warm sector and cooling mid-levels create weakly capped 200-500 J/kg MLCAPE during the daytime hours. Numerous pulsating thunderstorms likely grow upscale into a messy cluster of showers and thunderstorms. Marginal hail and gusty winds will be the primary risk during the initiation stage, transforming more into a gusty wind (outflow driven) and locally heavy rainfall risk. Thunderstorms along the S coast of Turkey may produce an isolated waterspout event with better LL CAPE present. After sunset, the main activity shifts east/southeast and exits our forecast area.

... Germany ...

Incoming front from the west looks not healthy at all with latest synop data showing a wind shift and some rain showers over UK and remote sensing data indicating some high-level clouds with that front (18Z at the 14th).
The approach of that front remains inactive and unspectacular until around noon, when a weak vorticity lobe grazes the front while crossing the Netherlands from SW to NE. Weak PVA ahead of this short-wave should increase mid-level clouds in the postfrontal sector, which is supported by latest model data. Hence, temporarily increasing mid-level vorticity/shear and differential diabatic heating may act frontogenetical with a slight increase of the frontogenetical circulation also seen in increasing LL convergence and deepening parcel layer depth/moisture pooling along the front. Forecast soundings in this area, although showing some faint decrease of the mid-level temperature, still offer a warm nose at mid-levels, resulting in weak mid-level lapse rates. Therefore the main CAPE-build-up likely occurs in the 850-600 hPa layer with EL temperatures showing spotty -30 °C in this area as well. Combining all that points to an isolated thunderstorm risk between 12-18Z over NW Germany with the mesoscale playing a major role regarding initiation.

Placed beneath the right entrace of the upper level jet and with 15-20 m/s DLS forecast (decreasing from N to S), any longer lived thunderstorm will be able to temporarily organize. Marginal hail (isolated large) and gusty winds (LCLs aoa 1km ahead of the front) accompany that activity with storms spreading ENE until sunset. High resolution models constantly show at least 1 or 2 storms which could be longer-lived and strong to severe before growing upscale into a NE-ward moving cluster of showers/thunderstorms. If clustering indeed occurs, 15 m/s 0-3 km shear and high LCLs may support a swath of strong to isolated severe wind gusts. Again, the mesoscale dictates where those storms finally evolve which is not yet predictable that far out but the most likely area currently seems to be W-Germany. Despite the slim chance for a few storms to temporarily acquire severe characteristics, no level 1 was added...but this region was highlighted by a 50-% lightning area to reflect the clustering potential.

During the overnight hours, a stronger vorticity maximum crosses N/C Germany from SW to NE. This likely results in another period of thunderstorm development over C and W-Germany which spreads NE. Enough MUCAPE is expected due to a modest cooling trend at mid-levels in connection to the incoming forcing. Once again, marginal isolated hail and gusty winds will be the main hazard. Of note is the pretty aggressive QPF of EZ during the night over far N-Switzerland and S-Germany, where a few embedded thunderstorms are forecast within more widespread shower activity. Hence the lightning area was expanded far S as this solution looks not unrealistic (given CI over the Vosges/Black Forest as the forcing approaches during the evening hours).

... Spain and France ...

In absence of any mid-level forcing, only orography and regionally enhanced convergence along the NE-SW aligned cold front support sporadic thunderstorm development during the forecast. However, the coverage of that activity does not support any lightning area despite C/S-Spain, where orography and strong diabatic heating may support a few afternoon thunderstorms. This activity remains sub-severe although inverted-V profiles may point to a few strong wind gusts with otherwise weak thunderstorms.