Storm Forecast
Valid: Sat 11 Apr 2015 06:00 to Sun 12 Apr 2015 06:00 UTC
Issued: Fri 10 Apr 2015 22:42
Forecaster: TUSCHY

SYNOPSIS

The predominant ridge over S-CNTRL Europe weakens while thickness gradually increases further west (far E-Atlantic and SW Europe). This transition is used by numerous weak mid/upper waves, which drift east and cross France and Germany during the forecast. Over S-Portugal/Spain, a cold-core cut-off low moves slowly east but loses forward speed betimes and eventually becomes stationary over SE Spain.
Progressive trough/ridge pattern continues over N-Europe.

DISCUSSION

...S-Spain ...

Yesterday's weak onshore flow pushed BL moisture dewpoints in the low to mid tens. This layer is very thin, seen in latest sounding data and prone to diurnal mixing. Despite somewhat drier air atop, air mass remains rather moist until roughly 3 km. With colder temperatures at mid-levels, a broad area with 300-600 J/kg SBCAPE should evolve during the noon/afternoon hours. Shear at all levels remains weak despite somewhat stronger winds below the tropopause, but don't expect updrafts to grow that deep. Hence, disorganized thunderstorm activity with graupel and gusty winds is expected. Activity diminishes after sunset as the BL becomes more stable.

... NE/E Germany and the Alps...

All parameters for somewhat enhanced convection come together, but their stamping remains marginal at best. Uncertainties increase due to complex background forcing. A NE-ward moving IPV maximum weakens betimes and becomes elongated in a N-S manner while the main trough/forcing approaches the area of interest from the west during the afternoon hours. The first IPV maximum pushes a convergence zone to the NE (ill defined) while the second and strongest forcing will bring a cold front rapidly to the east. Models show no coincident signals, if the convergence zone or the cold front itself will result in CI.

Latest thoughts are that the prefrontal convergence zone remains inactive due to its early arrival and a thick shield of cirrus clouds which limits diabatic heating. Background (synoptic-scale) subsidence adds to the overall pessimistic picture.

Hence, CI probably occurs over N-Germany during the early afternoon hours, as convective temperatures will be surpassed in a weakly capped air mass. Enhanced upper divergence beneath the left exit of an approaching 45 m/s mid-level jet will support that idea. Numerical models offer various solutions of weak prefrontal convergence zones, which should induce adequate LL lift for CI. The cold front itself, which affects NW Germany during the evening hours and E-Germany around midnight, could also be accompanied by a few thunderstorms, although less BL moisture and loss of daytime heating should keep this risk marginal at best.
Weak shear precludes organized updrafts and graupel/gusty winds will be the main hazard. Beyond sunset, thunderstorm probabilities decrease rapidly. Local swaths of higher lightning density are possible, but missig foci for clustering activity preclude the 50-% lightning area for now.

A few thunderstorms are also possible over the Alps, where regionally enhanced BL moisture and modest lapse rates result in pockets with 200-400 J/kg SBCAPE. GFS seems to be a bit too bullish with moisture advection and hence we kept this risk in a low probability lightning area. Nothing severe is expected. The same for the Czech Republic.

A very isolated thunderstorm is also possible over parts of Poland, but low coverage precludes a lightning area for now.