Storm Forecast
Valid: Sat 27 Feb 2010 06:00 to Sun 28 Feb 2010 06:00 UTC
Issued: Sat 27 Feb 2010 06:03
Forecaster: VAN DER VELDE
A level 1 was issued for parts of Portugal, Spain and France for tornadoes and convectively enhanced severe wind gusts. The threat of damaging winds of non-convective nature, however, will be much larger.
A level 1 was issued for SW Turkey for a chance of local excessive convective precipitation.
SYNOPSIS
Dangerous storm-force winds and gusts of more than 25 m/s, locally 35 m/s are previsioned Saturday and early Sunday for Portugal, western and northern Spain, and western France as an intense depression "Xynthia" (FU Berlin) traverses the region. Currently calculated center pressures range between 965-968 hPa in GFS, GME, UKMO and ECMWF models. There are still some differences in timing between different models and in the steepness of pressure gradients. GFS seems to be faster and milder, respectively.
At Sunday 00Z GFS18Z places the low over Bretagne which is 3 hours earlier than in the other models. Same for Saturday 12Z where GFS has the low closer to the coast of Portugal. HiRLAM (AEMET) shows the slowest system motion.
DISCUSSION
...Portugal, W/N Spain, W France...
At the cold front GFS depicts some indicators of slight instability, mainly below 3 km. Lapse rates will be near neutral and a strongly forced convective line may result. Within a few hundred km of the occlusion point the angle of line and winds will be the most perpendicular and favorable for a squall line, although this was more pronounced in runs of two days ago. Nevertheless, any convection may mix winds down from a deeper layer, and over NW Iberian Peninsula mean 1-3 km AGL speeds are predicted to be more than 80 kts.
Kinematics are also very favorable for tornadogenesis with SREH over 350 mē/sē where instability is predicted, as well as 20-30 m/s shear in the lowest 1000 m.
Reason that only level 1 was issued is the confidence level that deep convection indeed will develop.
...SW Turkey...
Some CAPE is present and relatively slow storm motion and onshore moist air advection could yield isolated flash floods from thunderstorms. This slight threat spreads eastwards.
