Storm Forecast
Valid: Wed 27 Jun 2012 06:00 to Thu 28 Jun 2012 06:00 UTC
Issued: Tue 26 Jun 2012 23:13
Forecaster: TUSCHY

A level 1 was issued for N-Spain and parts of France mainly for an isolated large hail and severe downburst events. Beneath storm clusters, locally heavy rain will be another hazard. Also an isolated very large hail event can't be ruled out over W/NW France

A level 1 was issued for parts of UK and Ireland mainly for heavy rainfall amounts, isolated large hail and strong wind gusts.

SYNOPSIS

A long-wave trough is still situated over NE/E-Europe with another trough approaching western Europe during the forecast. Weak ridging in-between covers parts of the Mediterranean area.

DISCUSSION

... Parts of Spain, Bay of Biscay, parts of France, UK and Ireland ...

An extensive warm sector evolves over the area of interest ahead of an approaching trough over the E-Atlantic. A pronounced EML mixes to the N/NE during the day and overspreads a BL with increasing moisture. For this forecast, a medium between GFS and EZ was used regarding surface dewpoint forecast as GFS seems to be too bullish and EZ too reluctant. With dewpoints in the upper tens already present, we would not be surprised to see a few spots in the lower twenties mainly over W/NW France. With steepening lapse rates atop, capped MLCAPE exceeds 1kJ/kg during the afternoon hours onwards over NW France with lower values to the east, UK/Ireland and towards N-Spain.

During the daytime hours, a strengthening ridge and intensifying WAA regime probably preclude CI for most parts of the forecast area. The main area of interest will be Ireland and UK, situated along the tip of the ridge, where weaker cap and better forcing probably assist in scattered CI. DLS remains modest with 10-15 m/s forecast, so pulsating storms/isolated organized multicells are possible. This activity weakens during the evening hours.

During the daytime hours, a few thunderstorms also evolve over N-Spain, as a weak PV streamer moves in from the W (beside weak forcing as a mid/upper wave grazes the area of interest). During the evening hours onwards, a few small storm clusters may evolve with a movement to the NE (e.g. offshore over the SE Bay of Biscay and also towards far SW-France). Large hail and an isolated severe wind gust event is possible during the daytime hours (discrete storm mode) with a trend to hevy rain, marginal hail and strong wind gusts during the clustering stage.

The focus then shifts to the west ahead of a weakening short wave, which crosses the Bay of Biscay during the evening hours onwards. The interaction of modest forcing and the aforementioned CAPE plume results in numerous areas, where CI is likely, mainly from NW/W-France to SW-UK/Ireland. A coupled jet configuration will be in place, so current thinking is that numerous large storm clusters evolve with a gradual movement to the E/NE. Shear in all levels increases by 5-10 m/s, so well organized multicells/isolated supercells are likely. The main activity will be elevated with ongoing deep/intense WAA, but we can't exclude a few near surface based storms mainly over NW France, where highest BL moisture is forecast. Conditions seem supportive for a large MCS to affect Ireland and UK during the night. Heavy to isolated excessive rainfall amounts will be the main hazard next to isolated large hail and strong wind gusts. Storms further south (NW/W-France) remain more discrete, so large hail (one or two very large hail events) and a few severe wind gust events are well possible.

Right now, there is no sign that the overnight MCS event may produce any swath of enhanced severe wind gusts over UK/Ireland due to modest LL/mid-level flow and late arrival of strongest forcing. Hence a broad level 1 will cover the risk for now.

... The rest of the European lightning areas ...

Either shear or instability parameters will be too low for an augmented severe risk.

Over E-France and parts of Germany, DLS will be strong (up to 20 m/s), but CAPE will still be on the lower end side (with EML plume still displaced far to the west). If indeed a stronger updraft manages to evolve, an isolated large hail event may be possible. Otherwise strong wind gusts and marginal hail will be the main hazard before storms decay after sunset. There is a moderate chance for a few small line segments evolving within the 50-% thunderstorm area due to enlarged and straight hodographs with weak looping present in the lowest 3 km over Saxony (Germany). Hence, an isolated well organized multicell event can't be ruled out in the 50-% thunderstorms area with isolated large hail/strong wind gusts and a very low-end tornado risk.

The alpine region of Switzerland, parts of Austria and parts of N-Italy will see some CAPE (500-1000 J/kg MLCAPE) but weak shear (DLS of 10-15 m/s over Switzerland, decreasing to less than 10 m/s over Italy). As local topography and modest shear may still be able to assist in a few better organized storms, an isolated large hail and heavy rainfall event is forecast (but too marginal for a level 1). Further south, over C-Italy, a few strong pulsating storms may produce isolated large hail and a severe downburst event or two before weakening after sunset.

500-1000 J/kg MLCAPE but very weak shear overlap over NE Europe, where scattered pulsating storms are forecast. Given locally strong CAPE fields, we can't exclude an isolated large hail event, but the main risk will be heavy rain with slow moving storms. This activity lasts well into the night.

Inland moving cold front sparks numerous thunderstorms over NW-Turkey, especially during the interaction of the cold front circulation and the rough topography (mainly between 9Z-15Z). Further inland, thunderstorm activity rapidly decays. 20 m/s DLS is adequate for a few well organized multicells with isolated large hail and strong wind gusts.