Storm Forecast
Valid: Fri 15 Jun 2007 06:00 to Sat 16 Jun 2007 06:00 UTC
Issued: Thu 14 Jun 2007 22:47
Forecaster: TUSCHY

SYNOPSIS

A nice SW-erly flow regime is present over Europe.
An upper-level trough over NW Europe stays nearly stationary during the forecast period with a conjunction of the polar / subtropical jet over SW Europe.
Over Sweden and Finland a way more progressive depression is forecast to move quickly towards the NE with a significant cool down on its downstream side.
S and SE Europe continues to see a diurnal driven thunderstorm activity in a weak pressure gradient environment.

DISCUSSION

....Extreme western Russia...

At 18Z, 14th June, a strong 995hPa depression is centered over southern Sweden, moving towards the NE .
A well defined cold front takes shape and is characterized by a sharp dewpoint drop. Replication regarding placing and model forecast show a nice match and confidence is high that a strong cold front will continue to race towards the east / northeast.

Strengthening moisture advection ahead of this front will help to support a surge of nice dewpoints ( 10-15°C ) well towards the north, while strong diabatic heating will help to bring surface temperatures above 25°C over the area of interest.
Moderately steepened mid-level lapse rates and the aforementioned boundary layer will result in up to 500 J/kg MLCAPE release with decreasing values towards the north.

Kinematic and dynamics are impressive with this system. Strong PVA and an intense UVV field will cross the area from the SW . DLS of 35m/s ( up to 27m/s at 850hPa ) will be present and expected well mixed boundary layer / steep LL lapse rates support an downward impulse transport of higher levels (
30m/s at 700hPa and 35-40m/s at 500hPa ).

A line of storms should develop / go on during the morning hours east of a line Estonia - Belarus and a constant increase in convection can be expected. Widespread severe - damaging wind gusts will be possible, especially if embedded storms bow out / develop supercell characteristica.
An isolated tornado can't be ruled out, but the tornado threat should be maximized in the prefrontal airmass, where LL shear will be enhanced.

... N-Italy, SE France, parts of Austria, parts of Germany, the Czech Republic and parts of Poland....

An upper-level trough will stay over UK and attendant trough axis will rapidly swing towards the NE ( E-France at 12Z, central / eastern Germany at 06Z,16th June ).
At lower levels, a wavy cold front shifts eastward, whereas propagation speed depends on numerous features, like topography and weak surface depressions.

The structure of the trough ( framed by strong upper-level streaks ) results in intense UL divergence values over SE France during the early morning hours. This should help to re-activate thunderstorm activity along the slowly eastward moving cold front and a cluster of storms should evolve during the early morning hours.
DLS of 20-25m/s will be strong enough for storm organisation with a marginal hail / gusty wind risk.
This complex should slowly shift towards the NE and a weakening trend can be expected because of limited insolation / bad timing and slim instability values.

Further towards the south, storms will shift towards the east in a constantly better environment with at least moderate instability release. DLS of 15-20m/s and enhanced SRH values favor an isolated large hail/ tornado / severe wind gust risk with the stronger cells, especially if discrete storms can evolve over N-central and northeast Italy later in the day.
The cluster of storms should start to weaken east of the level-1 area as it encounters an area with lower instability values and when upper-level support rapidly weakens.

A more robust severe weather threat looks like to evolve over SE / E and NE Germany during the early afternoon hours, as strong forcing at upper-levels and the eastward moving frontal boundary all favor scattered to widespread thunderstorm initiation.
Although strong to severe storms can evolve all-over the line, two areas seem to have a better environment for storm organisation:

SE Germany, where nice mid-level lapse rates, moderate instability release and 15m/s DLS will be present. Meso-GFS charts have clear signals of a possibly organized line of storms, which will cross SE Germany from the west during the afternoon hours. Marginal hail and severe wind gusts will be possible with stronger storms.
SRH3 of 250J/kg indicates a possibly enhanced large hail risk, especially if a few discrete thunderstorms can evolve ahead of the eastward moving line.

The other area will be NE Germany. At lower levels, a channel of low surface pressure / a possibly closed surface low will evolve during the afternoon hours somewhere over E-Germany. Instability will be better compared to S-Germany and strong diurnal heating should bring surface temperatures to 25-30°C over NE Germany and just above 20°C over N-Germany.
Scattered storms will evolve along the slowly eastward moving cold front and DLS of 15m/s will support strong to severe thunderstorms.
T-Td spread should be lower over N-Germany, where an isolated tornado could be a distinct possibility.
Further towards the east , a better mixed BL and steeper mid-level lapse rates support large hail reports and strong downbursts.

During the evening hours, storms will also fire up over N / NW Poland, where LL shear will be augmented and low LCLs indicate the risk of an isolated tornado.
LL shear could get even stronger than currently anticipated if a closed surface system evolves somewhere over E-Germany / the Czech Republic and this could boost the tornado possibility over NE Germany and NW/N Poland....mainly during the later afternoon / evening hours !

Two points, which should be mentioned:

Typical convective feedback problems of the global models (e.g. with yesterday's convection ) could cause significant shifts in the final position of the cold front and strongest thunderstorm activity. In addition, convective debris could also impede insolation over the highlighted areas. An update will be written,if significant changes will occur and parts of the broad level-1 area may need an upgrade later-on.

A pretty weak steering flow, impressive UL divergence ( a coupled jet configuration around the base of the upper-level trough ) and a high energetic arimass all suggest the chance for torrential rainfall, especially if storms start to cluster. This is not reflected in our criteria !

....S-UK....

A pronounced convergence zone will be present and moderate instability release looks reasonable, given the cool down at mid-/upper-levels.
Shear will stay weak, but low LCLs,an eastward spreading field of enhanced mid-level vorticity and nice 0-3CAPE values will be present and conditions look fine for a few short-lived tornadoes to spin up.
No level area will be issued due to the overall weak shear environment.

...Northern France....

Scattered storms will evolve in a weakly unstable environment with weak DLS values. A combination of low LCLs and a broad area of enhanced SRH values for 1km will be present and a short tornado can't be ruled out mainly over NW and N France. Low coverage and weak shear / low instability preclude any higher probabilities.