Storm Forecast
Valid: Fri 17 Jul 2015 06:00 to Sat 18 Jul 2015 06:00 UTC
Issued: Thu 16 Jul 2015 23:04
Forecaster: TUSCHY

A level 2 was issued for E-Germany and SW Poland mainly for large to very large hail, severe wind gusts and excessive rain. An isolated tornado event is possible.

A level 2 was issued for SW France mainly for large to very large hail, severe wind gusts and excessive rain. An isolated tornado event is possible.

Level 1 areas surround both level 2 for similar risks but with less coverage.

A level 1 covers parts of CNTRL France and Spain mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Austria and N-Italy mainly for large hail, excessive rain and severe wind gusts.


SYNOPSIS

A very complex forecast evolves with lots of dephasing in respect of ingredients and an abnormally developing synoptic-scale vortex just west of Ireland.
All models agree in track and strength of that vortex, which features a tight pressure gradient in the lowest 2 km AGL. Phase diagrams show a distinct warm-core signature as influx of modified subtropical air continues towards its center. This is an unusual strong vortex for that time of year e.g. with core pressure approaching 990 hPa in some model runs. As often seen with such features, a steady motion exists, increasing overall confidence in the forecast of numerical models, which indicate peak strength NW of Ireland during the day. Increasing strength of that vortex towards lower levels also means strong gradient flow in the lower to mid troposphere (up to 5 km AGL), which spreads northeast and affects N-France, NW-Germany, the North Sea and UK during the forecast.

Convective-wise, a short-wave has to be mentioned, which crosses the Netherlands around sunrise and Denmark during the late afternoon/evening while weakening. This wave crests the tip of a ridge and turns more to the E/SE and hence towards Poland during the day. Intresting to see ongoing thickness increase as ridge builds NE but models keep the wave intact.

Positive tilted trough over the Baltic Sea turns SE as ridge to its west builds north/northeast. Rising surface pressure beneath that trough indicate a constant weakening trend but accompanying cold mid-levels atop seasonable moist boundary layer cause adequate MLCAPE build-up for scattered thunderstorms.

The Mediterranean resides beneath a firm ridge with a hot air mass in place.

Regarding fronts, the driving force for progressive boundaries is the vortex next to Ireland, which sends an occlusion over the North Sea to the E during the day. To the south, a complex frontal concept evolves with a warm front stretching from NE Germany to the SE around noon. Further to the SW, a cold front approaches W-Germany but turns more or less quasi-stationary especially towards France. GFS continues to indicate a prefrontal convergence zone, which could be also analyzed as another weak cold front or even as a dryline as drier air temporarily filters in from the W.

The final look will be for the ingredients. EML plume from the Iberian Peninsula and SW France advects E/NE, which was sampled nicely in yesterdays' soundings. Impressive mid-level lapse rates of 8 K/km are forecast over Germany. Overlap of seasonable moist BL and aforementioned lapse rates offers widespread 800-1500 J/kg MLCAPE, with ECMWF remaining on the more conservative side. Localized more robust values are possible. Even better BL moisture exists over N-France towards Benelux, but meager lapse rates limit CAPE.
Shear relaxes somewhat over Germany during the daytime hours but increases substantially from France thereafter as compact low/mid-level jet approaches.


DISCUSSION

... Germany, Denmark and Poland ...

It is likely that a cluster of showers/thunderstorms already exists over the S-North Sea/Denmark to NW Germany during the start of the forecast as the short-wave passes by. Steep lapse rates overspread the area and 600-1000 J/kg MUCAPE exist. DLS of 20 m/s favors organized elevated convection with a large hail risk. Dry sub-700 hPa layer also indicates a chance for enhanced downdraft momentum, which could induce a localized severe wind gust risk despite a shallow stable boundary layer. In case of pockets of surface based CAPE over W/NW-Germany, as proposed by GFS, an isolated tornado event is possible due to strong LL directional and speed shear.

This activity spreads E/NE around forenoon and has to be monitored closely for re-intensification/becoming more surface based. Substantial LL shear/SRH/CAPE overlap exists over NW Germany and in case of ongoing thunderstorm activity, an enhanced tornado/severe wind gust risk exists next to large hail.
Most models however favor re-intensification from far N-Germany to the Ore Mountains around noon. Despite increasing separation of strongest shear/best CAPE, a confined corridor with adequate LL shear / DLS and helicity exists from far N-Germany to SW-Poland. Numerical guidance in general remains very reluctant with CI. The reason for that could be a dogged inversion at 800-900 hPa, but eastbound spreading negative Q-vector divergence and enhanced mesoscale convergence at lower levels increases confidence in at least isolated CI from Schleswig Holstein to NE Germany to SW Poland. Impressive forecast soundings exist over far E- Germany with more than 2000 J/kg MLCAPE, very steep lapse rates and 15-20 m/s DLS. Any storm, which breaks the cap will grow deep and most likely acquires rotation with a large to very large hail threat. Severe downbursts and excessive rain are forecast, too and lowest 2 km AGL seem supportive for the development of a cold pool driven system in case of localized more active thunderstorm development. This activity spreads E/SE and therefore also affects SW Poland and the far N-Czech Republic during the evening hours. Despite the presence of a hot and well mixed air mass, moisture pooling and a diurnal decrease of LCLs during the evening hours may also add an isolated tornado risk, especially near the coast of NE Germany and over far E-Germany/SW Poland. Another focus for an augmented tornado risk exists over N-Lower Saxony and Schleswig-Holstein, where cooler BL is present. There, forecast hodographs feature looping lower parts although weak winds keep SRH-1 on the lower side.

Denmark in contrast will be affected by the cluster of elevated storms until noon and diabatic heating could be limited. Nevertheless, re-development of a few strong to severe thunderstorms is expected during the afternoon hours. Strong shear and modest CAPE support an isolated severe risk mainly over S-Denmark. All kind of severe is possible, although on a very isolated scale. Hence the level 1 was expanded all the way to far S-Denmark.

Further south over S-Germany, air mass should be capped with only weak forcing noticed in numerical models. Still, early morning IPV max from the North Sea also extends far south and crosses S-Germany during the day. Hence, expect at least isolated CI over mountainous areas with an isolated severe risk. Would not be surprised to see models downplaying initiation a bit too much (e.g. forenoon convection over the Black Mountains spreading east and afternoon convection from the Alps spreading NE), but don't want to argue against all models. A considerable large hail and severe downburst risk exists with any sustained/deep updraft.

During the night, a wavy and quasi-stationary boundary bisects Germany. The wind field intensifies from W to E and the overall pattern supports the idea of bowing segments, crossing CNTRL Germany from SW to NE during the night. 20 m/s 0-3 km front-parallel shear and about 150 m^2/s^2 SRH-1 are forecast, which would support a severe wind gust and isolated tornado threat. However, uncertainties exist, if this activity remains surface based. Forecast soundings keep this activity elevated, which would lessen the severe risk. 500-1000 J/kg MUCAPE and 25 m/s DLS however support an isolated large hail threat, so a broad level 1 was issued.
Further to the north (N-Germany), leisurely eastbound moving cold front also sparks a few thunderstorms. Despite meager CAPE, strong shear at all levels could produce better organized shallow convection with a strong to severe wind gust threat.

Poland also experiences numerous overnight storms, mainly elevated in nature. Strong DLS and modest MUCAPE results in a large hail risk. Therefore the level 1 was expanded far east.

... Spain and France ...

The main problem during the forecast is to identify a distinct lift mechanism. Quasi-stationary front becomes the focus for scattered DMC all day long with a peak activity during the evening and overnight hours. Daytime activity may be hampered by capping conditions until late in the afternoon.
The focus for isolated CI is placed mainly along the Pyrenées during the late afternoon hours and later on also over S/CNTRL France. 1000-2000 J/kg MLCAPE and 10-15 m/s DLS assist in organized multicells with large hail, excessive rain and strong to severe wind gusts. Very large hail is possible N of the Pyrenées, where up to 2500 J/kg MLCAPE seems realistic, given amount of BL moisture and very steep mid-level lapse rates.

Otherwise during the night, the setup resembles the overnight one over Germany, although confidence is higher that surface based convection exists, given higher BL moisture. Hence we would not be surprised to see numerous smaller-scale bow echoes to race to the ENE with a considerable strong to severe wind gust threat next to large hail. However with lack of forcing and no perfect match of CAPE/shear fields, confidence in a confined level 2 area is not yet present.

We expanded the level 1 over Spain far SW as overnight convection could produce a few large hail reports (20 m/s DLS and 400-800 J/kg MUCAPE).

... Alps ...

Scattered diurnal driven CI is forecast over far SW Austria. The environment features strong CAPE in excess of 1000-1500 J/kg and 10-15 m/s DLS (with 15 m/s 0-3 km shear just to the south of the Alps). This assists in the development of strong to severe multicells, which pose a large to isolated very large hail, excessive rain and downburst risk. Betimes, a temporarily cold pool driven line of DMC should shift towards NE Italy and Slovenia with an ongoing large hail and enhanced severe wind gust threat, before weakening during the evening hours. This was reflected by a level 1 area.

... NE Europe ...

400-800 J/kg MLCAPE and weak shear support diurnal driven scattered to widespread CI of weakly organized convection. Isolated large hail is possible over Latvia and Estonia where a tight DLS gradient exists and a few better organized multicells could evolve. Further north, slow moving and clustring storms pose an localized excessive rainfall risk. The activity diminishes beyond sunset. Expected low coverage of severe reports should not justify a broad level 1.