Storm Forecast

Storm Forecast
Valid: Mon 09 Jul 2007 06:00 to Tue 10 Jul 2007 06:00 UTC
Issued: Sun 08 Jul 2007 19:48
Forecaster: TUSCHY


Streamline charts show a disturbed weather pattern over most parts of Europe with two cyclonic swirls centered just east of Great Britain and extreme western Russia.
Downstream of this systems, very warm / hot airmass spreads towards the N / NE while cooler air gets infiltrated on their upstream side.
Numerous boundaries over the Alpine region and western Russia will be the focus for an active convective period.


.... N - Italy ...

Numerous strong to possibly severe thunderstorms will affect the area of interest during the forecast period with the main threat being strong to severe wind gusts and large hail.

As usual in such cases when cold fronts cross the Alpine region and being aligned nearly parallel to the background flow only slow progression of today's cold front towards the S / SE can be expected.
Models like GFS, GEM and UKMO agree with the overall scenario, as cold front should start to disengage from the Alpes ( late afternoon / early evening hours ) and will slowly shift towards the SE.
Surface temperatures again soar to 30C and the surface wind field should response to this strong diabatic heating and a nice southerly jet should support the influx of a moist airmass well towards the north.

As thunderstorms will go on / evolve already during the morning hours over the western Alpine region ( the "Valle D'Aosta" and the " Lombardia" ), a pool of cool air is expected to spread towards the south, creating a constantly strengthening convergence zone over N-Italy, which will be the main player regarding the thunderstorm activity. Latest thinking is that this zone will shift southeastward very slowly or even be stationary just south of the mountainous region until the afternoon hours, when the cold front will finally overtake this zone, racing rapidly towards the SE during the evening hours.
In addition to strong forcing at lower levels, GFS has an impressive UVV max. crossing the area from the west during the evening hours and position of the mid-level jet also favors strong lift, so a cluster of storms looks likely during the evening hours, crossing NE Italy from the W / NW.

DLS of at or above 30m/s and lapse rates of up to 7K/km will be present at mid-levels, pointing to the enhanced severe wind gust threat, especially if storms line up / bow out.
200 J/kg SRH for the lowest 3km will be available and in combination with a well mixed BL, hail matching our severe weather criterion looks likey, especially if discrete storms evolve ahead of this line.
In addition, LL shear increases during the evening hours and as LCLs lower, the tornado threat rise, too.

Due to the prolonged period of thunderstorm activity and the moist airmass ahead of this cold front, flash flooding will be a distinct possibility.

... E-Austria, parts of Hungary and Slovenia, Slovakia, parts of the Czech Republic and Poland...

The overall evolution of the main upper-level system is pretty diffuse as numerous short waves travel rapidly out of the base towards the NE and eastward shifting upper-level trough axis arrives pretty late.
In addition, convective debris of yesterday's convection and aforementioned Alpine convection could help to limit the diabatic heating so main negative fact will be limited instability release.

Latest thinking is that during the early morning hours, storms will move out of the central Alpine region towards the east in an environment with at least marginal instability release over E / SE Austria.
DLS will increase during the day and should help to enhance the severe wind gust risk, especially if a more organized line of storm can evolve.
Highest risk for severe storms can be found over N-Slovenia, SE and E Austria, western Hungary and western Slovakia, where strongest instability and shear overlap and beside severe wind gusts, an isolated large hail report can't be excluded ( although mid-levels stay pretty warm ).

During the evening hours, another round of thunderstorms over S-Austria can be expected as the complex of storms over NE-Italy ( please refer to the discussion above ) shifts towards the east. Intense DLS will favor a isolated severe wind gust report.

Storms will develop northeastward, affecting the eastern Czech Republic and Poland during the afternoon hours. Again...thermodynamic profilers don't support any significant instability release, but strengthening DLS should support a line of storms with an enhanced strong to severe wind gust threat.
An update may become necessary if better instability release can be established !

...SE parts of United Kingdom , most parts of France, Belgium and the Netherlands...

A broad pool of cold mid- / upper-levels is forecast to cover the area of interest. GFS indicates nice moisture advection ahead of a LL depression over Ireland and surface temperatures will be between 15 - 19C. This would yield a stout amount of 0-3km instability release in combination with low LCLs. Local convergence zones could be the focus for a few funnel reports / short-lived tornadoes.
Forecasting such small scale features like convergence zones contains enough uncertainties for not issuing any level.

The same for central / NE / E France, Belgium, the Netherlands and NW Germany. Moisture streamline advection charts show a few convergence zones, shifting rapidly towards the E / SE. Again....a few funnels / short lived tornadoes are possible as favorable instability dispersion and low LCLs will be present.

.... Extreme western Russia ....

A warm and moist airmass flows northward, downstream of a LL depression. LL shear of 10m/s and DLS of 15m/s in the warm sector will be present and a mixture of multicells / supercells will be possible with an isolated large hail / severe wind gust risk.
LCLs are pretty high but an isolated tornado report is not out of the question, especially if dewpoints are higher than currently expected.

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