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 Slight Risk (Level1) in France and some other places in EU

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Andre-ESCT

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Join date : 2008-09-20
Age : 45
Location : NDS/Germany

PostSubject: Slight Risk (Level1) in France and some other places in EU   Wed Feb 04, 2009 9:23 am



Storm Forecast
Valid: Wed 04 Feb 2009 06:00 to Thu 05 Feb 2009 06:00 UTC
Issued: Wed 04 Feb 2009 01:05

Forecaster: PUCIK

SYNOPSIS

A widespread trough has settled over Europe, its axis stretching across E Atlantic. Several impulses, including closed circulations will rotate around it. These mid-level impulses will become a foci for thunderstorm development. At the surface a large and deep low pressure system stalls under the trough with several separate centers. The low is expected to slightly deepen and new closed centers will develop by the western coasts of Norway and Iberia. The Russian part of the continent is dominated by a high, slowly retreating to SE.

DISCUSSION

... Western coast of France and Portugal...


Approach of the deepening low will result in the considerable strenghtening of the low level flow, with windspeeds at 850 level reaching 20-25 m/s. The ageostrophic component of the flow, resulting from the friction will yield high values of low level shear ( between 10 - 15 m/s) and at the same time SREH for 0-3 km layer might reach 250 J/kg. DLS should increase to more than 20 m/s just ahead of the approaching impulse. Abundant synoptic scale lift and an advection of moister air ahead of the surface low will result in a marginal instability with few hundred J/kg of MLCAPE. Due to the high values of shear and helicity, we expect well-organised storms with few rotating updrafts. The only real threat should be isolated tornadoes, thanks to the high LLS and SREH, especially in the coastal areas with the best instability / shear overlap. Few marginally severe wind gusts might also occur ( thanks to the strong perpendicular flow to the boundary) and a low end Level 1 is warranted. Threat should be confined mostly to the period 18 - 21Z in case of France, 09Z - 12Z in case of Portugal and will quickly diminish afterwards.

... Morocco to S Spain...

A similar situation described in the above paragraph is expected also in this area. However, GFS in more runs insists on the instability being advected well inland in case of southern Spain. This will provide better overlap of enhanced low level shear / SREH and of surface-based instability. Moreover, low LCLs and strong convergence of low level flow around the coastal areas should also contribute to the environment conducive to few weak ( F0 - F1) tornadoes. Southern Spain will also probably experience a heavy rainfall, with storms re-developing and entering the area for few consequent hours ( as suggested by GFS convergence signals, persistent synoptic-scale lift and simulated precipitation amounts) mostly between Thursday 00Z and 09Z. The above mentioned threats will continue into the next forecast period.

... E coast of the Adriatic / Ionian sea...

A marginal Level 1 is issued for the area as a chance of a few tornadoes will persist from the previous forecast period. The threat is expected to diminish completely by noon with the mid level impulse leaving the area. The overlap of instability and strong low level shear will be confined in this case strictly to the coastal areas.

... N Aegean...

As a mid-level impulse will arrive, destabilization is expected to occur but MLCAPEs will probably stay below 800 J/kg. Nevertheless strong wind profiles are anticipated - southwesterly flow at 500 hPa level will exceed 25 m/s and at the same time, 850 hPa level winds will also reach 25 m/s, blowing from the south. Therefore, wind shear will be confined mostly to the low levels, yielding LLS of more than 15 m/s. DLS should stay in order of 20 - 25 m/s. Moreover, veering of the winds with height will result in a belt of enhanced SREH values, which will exceed 250 J/kg on the front side of the approaching trough. Only weak / isolated precipitation signals are simulated by GFS, so storms might be more of an isolated / supercellular nature. Tornadoes should be the primary threat but marginally severe hail is not ruled in supercells, as well as few severe wind gusts.


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Slight Risk (Level1) in France and some other places in EU
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