Storm Forecast
Valid: Tue 14 Apr 2009 06:00 to Wed 15 Apr 2009 06:00 UTC
Issued: Tue 14 Apr 2009 06:37
Forecaster: VAN DER VELDE
SYNOPSIS
Two large low pressure areas, one over the Atlantic Ocean and the other over the southern Balkan, dominate the map. In between these main lows, over central Europe, is a large area of weak gradients and slight low pressure in which vertical profiles are locally marginally unstable.
A cold front has become nearly stationary at the west coast of France at 00Z.
The main thundery action of the day should occur when the upper trough interacts with instability buildup over France. The GFS model predicts several shortwave troughs embedded within the highly cyclonal flow, while the left exit region of the jetstream should arrive around 21Z near the western Pyrenees.
However, the small amount of moisture to work with yields only a modest amount of CAPE in the model. CAPE is thought to reach higher values over northern France, but the forcing will be very limited for most of that area.
Models do agree about precipitation, though.
DISCUSSION
...western France, northern Spain...
00Z soundings indicate that still quite some moistening and lifting is needed before profiles are conditionally unstable. Values of 6 g/kg average mixing ratio in the lowest kilometer in Santander appear correctly in GFS. A few hundred J/kg CAPE should be possible around the Basque area. Predicted values reach higher over central and eastern France in mesoscale models, but absence of favorable shear and lift suggests limited chance of severe weather there.
Over Spain and western France, 6km deep layer shear will be higher than 20 m/s while 3 km SREH is 150-300 mē/sē near the SW France coast. This environment is favorable for multi- and supercellular storms producing large hail. Especially as better dynamics arrive during the evening, the chance of initiation of an MCS is good. It could produce severe gusts locally if it lines up perpendicular to the shear vector (i.e. NW-SE), but in general a linear system appears to line up along the shear vector today. A secondary severe threat area appears warranted for northwestern France, where shear is moderate as well and deep convergence is forecast. Large hail will be the main threat.
Note that low-level shear increases after 18Z to values >10 m/s locally, and is somewhat favorable for a tornado, but in the relatively moisture-starved, marginally unstable situation with cooling BL this development seems unlikely.
GFS 18Z predicts a rather aggressive jet and deep layer shear >30 m/s over Spain during the night with the left exit region still near the Pyrenees, and uncapped parcels are still displayed at that hour by the model. Given also SREH increasing, and more than 12 m/s shear component normal to low-level theta-e gradients, another thunderstorm system, with severe gusts, could develop on the south side of the Pyrenees. However, given so poor instability and being the first such setup of this year, there is not much confidence in this scenario at this moment.
...central Turkey...
By GFS guidance, presence of instability, combined with convergence zones and around 250 mē/sē SREH, 30 m/s DLS and more than 10 m/s LLS, suggest that severe storms can occur. While the environment is rather dry, storms that succeed can be strong and fast moving with large hail and severe downburst winds.
