Storm Forecast
Valid: Fri 29 Jun 2012 06:00 to Sat 30 Jun 2012 06:00 UTC
Issued: Thu 28 Jun 2012 22:47
Forecaster: TUSCHY

A level 2 was issued for W/NW Germany mainly for large hail (a few very large hail events possible), strong to severe wind gusts, heavy to excessive rainfall amounts and an isolated tornado event.

A level 1 surrounds the level 2 mainly for isolated large hail, strong to severe wind gusts and heavy rainfall amounts. An isolated tornado event is possible.

SYNOPSIS

A strong trough just west of Europe advects a very warm and moist air mass far to the NE, which affects most of C-Europe during the forecast. Downstream of this feature, ridging covers most of S/E-Europe and keeps deep moist convection more isolated.

DISCUSSION

... Germany ...

The stage is set for another round of organized DMC events over most of Germany. However there remain numerous 'flies in the soup' with the evolving event.

Main uncertainty will be the behavior of overnight's convection. Discrete stom clusters were ongoing over E-France and parts of C-Germany (at 21Z), which will probably survive nocturnal BL stabilization (becoming more elevated). Attendant anvil cirrus contamination, left-over stratiform rain-shields with cold pools and leftover outflow boundaries all influence the start of the forecast period.

As of 06Z, we therefore expect ongoing thunderstorm activity (isolated in nature), probably leveling off a bit with mostly sub-severe thunderstorm activity. However, thunderstorm coverage and intensity increases already around forenoon mainly along the wavy and quasi-stationary synoptic front over W/NW-Germany but also within the weakly capped warm sector further east. Compared to yesterday, cap at 800-950 hPa weakens by a few degrees with unseasonably moist air mass beneath, so not much extra forcing needed for CI. Weak background (large-scale) subsidence may keep activity more isolated until noon.

Around noon onwards, a mid-level vorticity lobe grazes Benelux/NW-Germany during the afternoon and evening hours, providing some extra lift for N-Germany. Mid/upper streamline pattern reveals good high-level diffluence over N-Germany, so confidence in scattered initiation is good and becomes more conditional further south.

Warm sector features surface dewpoints in the lower twenties with an unseasonably depth of that moisture seen in forecast soundings. EML plume fans out during the forecast with weakening lapse rates (despite S/SE Germany, where another alpine EML plume advects to the NE). This may keep final CAPE magnitude in the 1-1.5 kJ/kg MLCAPE range, although a few higher values are well possible.

Shear improves during the day from west to east with 20 m/s DLS present, lessening to 10-15 m/s over E-Germany. Despite the grazing vort lobe, no real curvature in the SW-erly flow regime can be detected, so SRH remains low to modest.

With that background knowledge in mind, we expect initiation of well organized mutlicells/a few supercells from noon onwards mainly within the level 2 area with more isolated CI further to the south. Dominant storm mode is a bit uncertain with long and straight hodographs forecast, which may be supportive for splitting storms. Weak forcing may assist in discrete storm structures, which will be capable of producing large to very large hail and a few strong to severe downburst events. Betimes, storms tend to cluster (with most favorable environment for a large storm cluster/MCS probably over N/NE-Germany during the evening hours onwards) and swaths of strong to severe wind gusts may evolve. Despite aforementioned weaker directional shear component, LCLs remain around 1-1.5 km with a lowering trend during the evening hours and enhancement of the LL flow on the mesoscale (e.g. along an outflow boundary) may be supportive for an isolated tornado event. Heavy to excessive rainfall amounts are also likely beneath storm clusters. One focus for potential serious flash flooding (on a local scale) may arise over C-Germany during the afternoon hours, as diffluent wind field and rich lower tropospheric moisture (e.g. 0-1 km average mixing ratios above 15 g/kg) overlap. However, the majority of the high resolution models show very limited initiation over C/E-Germany until sunset, with main bulk of activity along the wavy synoptic front further to the west. Nevertheless, we exapnded the level 1 far to the east to account for some nighttime convection with heavy to excessive rain the main risk (e.g. E-Germany).

Thunderstorm coverage intensity and coverage slowly decreases during the night, but we won't rule out a few severe storms area-wide with isolated large hail and heavy rainfall amounts.

The level 2 was issued for those areas, where deterministic and probabilistic forecasts of high resolution model data reflect most probable initiation. However, displacement between model QPF and reality can be significant, so don't concentrate on the final lines of the level 2 area too much (in addition to all the aforementioned uncertainties).

Thought about adding a small level 1 for S-Bavaria, as SW-erly flow strengthens a bit, which may assist in a few storms to move off the Alps. However I see minimal support by any model. In case an isolated storm is able to become longer-lived, large to very large hail will be the main hazard.

... Parts of S-Denmark, parts of Sweden and the S-Baltic Sea ...

The weak short wave from Benelux/NW Germany continues to travel to the NE and also affects the highlighted areas mainly during the afternoon hours onwards. Despite already ongoing isolated 'severe' activity during the daytime hours, the main event will probably be an overnight MCS, as a mature storm cluster from N-Germany moves to the NE. Modest forcing and northward advecting CAPE plume probably keep this cluster going all night long, although the activity may becomes elevated betimes (especially as it moves atop the Baltic Sea). Especially over S-Denmark and S-Sweden, surface based activity will still be able to produce large hail, strong to severe wind gusts, heavy rainfall and an isolated tornado event. Betimes, the main hazard shifts to heavy rain and strong to severe wind gusts.

... E-France, Switzerland and Austria ...

CI further south becomes more conditional with stronger cap, nearly non-existent forcing and somewhat lower BL moisture content. Hence, initiation will probably be bound to the topography or local convergence zones. 1000 J/kg MLCAPE and LCLs in excess of 2 km (with good DCAPE) point to a few severe thunderstorms with large hail, severe downbursts and locally heavy rainfall. For now, we just expanded the level 1 from Germany to E-France, where CI is most likely. For the rest, thunderstorm areas were included without a level 1 due to the very isolated thunderstorm coverage.