Storm Forecast
Valid: Fri 18 Aug 2006 06:00 to Sat 19 Aug 2006 06:00 UTC
Issued: Thu 17 Aug 2006 19:22
Forecaster: TUSCHY

SYNOPSIS

Intense upper-level longwave trough, placed over the eastern Atlantic / extreme western Europe, will continue to affect most parts of Europe via few disturbances, embedded in the SW - erly flow.
Downstream of this system, intense heat will continue to build NE-ward, and hence very hot but dry weather can be expected over most parts of the central and eastern Mediterranean.

DISCUSSION

... Western Mediterranean...

The constellation of a slightly positive tilted upper-level trough over western Europe in conjunction with intense WAA on its downstream side causes an unusal ( for mid August ) intense and splitted upper-level SW-erly flow over the western Mediterranean and parts of central Europe.... Models agree in placement and strength of both features very well.
The southern streak // well developed in all levels // should continue its slow eastward drift due to the upper-level trough, lifting out torwards the NNE, although very slow.
A frontal boundary, not very active, will be aligned nearly parallel to the background flow across the western Mediterranean, which should cause a very faint eastward drift during the forecast period.
Despite the slow motion, baroclinity along this front will increase during the afternoon and early evening hours mainly over Sardinia and Corsica, when a splash of very hot ( 25-30°C at 850hPa ) desert airmass will be advected northeastward over Tunisia and the southern Tyrrhenian Sea....Main concern for TSTM development evolves along the conjunction point of frontal boundary and mountainous topography ( like Corsica )....The question will be, how far towards the north the warming at low-levels will occur ( and hence the strengthening cap ).....GFS also indicates a slight cool down of the mid-levels, even affecting Sardinia during the night hours, but confidence in storm development in such an environment that far towards the south will be too low for expanding the risk area.
Current thinking is, that a few storms will develop from the Balearic Islands NE-ward and also over Corsica.
Kinematic parameters will be impressive with 35-40m/s DLS , but expect only a few storms to develop.... Main risk will be a severe - damaging wind gust risk.

There are indications that the slight cool down in the mid-levels could also support TSTM development over N-Italy during the night hours..... Instability release should be on the lower side, but shear will be very intense with 30-35m/s and any developing storm should pose mainly a significant severe wind gust / isolated larger hail risk.
Currently, there is no evidence that an organized line of storms will form, but coverage of TSTMs will be monitored.

...Portugal and most parts of Spain...

WV loop already showed a convectively inactive, but well developed upper-level disturbance over the eastern Atlantic, moving towards the south.... This feature is forecast to cross the area of interest from W towards the ENE during the day hours, while weakening....
Convective activity should increase, while moving onshore and main instability release will occur mainly in the highlighted areas....
Well separated instability - shear should preclude any significant TSTM organisation, but there will be a marginal hail risk due to steepening lapse rates.

...Central and eastern France, parts of Switzerland , western Germany , the Netherlands and Belgium....

Such as the Mediterranean, slowly eastward moving frontal boundary will be the main focus for TSTM development.... A series of upper-level disturbances will cross the area of interest ... Right now, there will be two regions with enhanced severe weather potential.

One will be over SE/E France / W/NW Switzerland, where kinematic parameters will be favorable for a few organized storms..... 25-30m/s DLS but weakened mid-level lapse rates favor a few severe wind gust reports....
Low LCLs and increasing LL shear ( enhanced SRH 0-1km ) due to a strengthening LL-jet over SE France should also pose a risk for possible tornadic storm activity.

The second one will be over the Netherlands, Belgium and extreme NW Germany.... Although DLS will be not quite intense compared to the other regions, kinematic parameters will be fine enough for an enhanced severe TSTM threat.... Main concern will be a broad sector of augmented LL shear , 150-200 m**2/s**2 SRH 0-1km values and low LCLs.
Strong-severe wind gusts and a few tornado reports will be possible.

GFS is one of the few models, indicating another round of TSTM development over central France during the night hours... Shear will be strong enough for storm organisation and area will be monitored for a possible organized line of storms, but confidence in this scenario is low right now.

... United Kingdom and Scotland...

The center of the upper-level trough will cross the area of interest..... Release of low-end to moderate instability as a consequence of steepened lapse rates should help for scattered TSTM development..... Mesoscale boundaries and an area over Scotland ( next to a N-ward departing front ) could see locally enhanced LL shear and don't want to exclude a short tornado report, but over-all severe weather threat should be limited.

.... S-Sweden, N-Poland,Belarus and Lithuania...

A slowly northward shifting occluded front will be the focus for isolated TSTM development.... Most of the convective signals are calculated on the cold side of the boundary and hence well separated from the best shear.
A small region fo superposing LL shear and instability will exist over NE Poland and Belarus and storms, rooting into the boundary layer, could produce one or two tornadoes and a few severe wind gust reports.