Storm Forecast

Storm Forecast
Valid: Mon 08 Jul 2013 06:00 to Tue 09 Jul 2013 06:00 UTC
Issued: Sun 07 Jul 2013 21:54
Forecaster: TUSCHY

A level 1 was issued for the Pyrenees mainly for isolated large hail and severe downbursts.

SYNOPSIS

An opulent surface high pressure area is situated over Ireland/UK and it exerts influence on most parts of NW/N-Europe. The only noticeable disturbance crosses far W-Russia which also weakens/opens up betimes while moving to the SE. In vorticity maps, there are some weak indications of numerous short waves circling the ridge and moving down along its eastern fringe, crossing the CAPE-plume along/south of the Alps. Despite the fact that this sounds reasonable synoptic-wise and is in line with weak mid-level height falls over S/C-Europe, it is hard to say if those signals come from the orography, from model QPF maxima or indeed have a pure meteorological background. Right now we won't create our forecast and especially the initiation reasoning due to a certain short wave but keep the forecast more general ... e.g.: there will be some kind of synoptic support for CI but this parameter remains diffuse both in space and time.
An extensive cyclonic vortex remains in place over the C-Mediterranean and assists in enhanced initiation probabilities for this area.

At the surface there are no signs of any healthy looking frontal boundary although numerous more or less pronounced convergence zones either from degenerating frontal boundaries by the past frontal intrusions or from what was left over by yesterday'sconvection can be analyzed. Beside of the orography, those convergence zones will also play a role for CI.

DISCUSSION

... Lightning areas ...

In general mid-level lapse rates remain weak from France to Hungary but seasonably BL moisture beneath assists in moderate MLCAPE build-up on the order of 500 to 1000 J/kg on the broader scale. However, local moisture pooling/deepening of the BL moisture assists in areas with diminished diurnal mixing and mean surface dewpoints in excess of 15 C are forecast on a regional scale. This explains why some models show spots with MLCAPE in excess of 1500 J/kg. Shear remains very weak mainly from Austria to the east, where a few strong pulsating thunderstorms area forecast, which might grow upscale into a few loosley organized and cold pool driven thunderstorm clusters. Nothing more than marginal hail, strong to isolated severe wind gusts and heavy rain (slow storm motion!) is forecast.
Kinematics don't change a lot when going from Switzerland to the the west, e.g. France. However, 1-8 km shear increases somewhat, which might enhance upper ventilation a bit. Also, the CAPE field looks quite impressive especially over S-France. However the main meteorological parameters to look at will be both the very dry mid-level layer but also the well mixed and high-LCL environment over SW-France. Storms might be able to develop healthy downdrafts with a risk of a few severe downburst events. The main reason for not going with a level 1 is the constant displacement of the QPF forecasts from the best environment for downbursts. However thunderstorms from far W-Austria, Switzerland into S-France will be able to produce locally heavy rainfall amounts, isolated large hail and strong to isolated severe downburst events ... the latter risk maximized over S-France.

DLS improves somewhat over NE Spain and a marginal level 1 was added for the Pyrenees, where large hail and severe downbursts become the main risk. Not sure if storms will evolve away from the Pyrenees due to strong capping and no noticeable forcing mechanism seen in the mpas. Therefore we kept the level 1 along the Pyrenees. An isolated thunderstorm event can't be ruled out over most of C/N-Spain and N-Portugal as intense diabatic heating erodes the cap and mid-level heights decrease somewhat, but the air mass remains utterly dry, so we're not surprised about mixed QPF outputs from models (GFS once again being very optimistic regarding initiation due to better BL moisture/less diurnal mixing). We added a 15-% lightning area where confidence in evening/overnight CI is somewhat enhanced. If an updraft manages to survive entrainment, there might be a very isolated large hail and severe downburst risk, but this hazard remains too uncertain to go with a level 1.

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