Storm Forecast
Valid: Fri 29 Mar 2013 06:00 to Sat 30 Mar 2013 06:00 UTC
Issued: Fri 29 Mar 2013 00:09
Forecaster: TUSCHY

A level 1 was issued for parts of Portugal and Spain mainly for an isolated large hail, a strong to severe wind gust event and an isolated tornado.

SYNOPSIS

There are not many changes to talk about for most parts of Europe. The frontal zone remains located far south over the Mediterranean and numerous disturbances move along that frontal zone to the east. Cold and dry air to the north of the boundary quashes DMC chances, so the focus once again resides over SW Europe. There, a plume of modified tropical air with TPWs in excess of 30 mm gradually overspreads Portugal and Spain from west to east as a brisk zonal flow becomes established. Numerous weak disturbances in this flow regime will result in an active period for convection.

DISCUSSION

... Portugal and Spain ...

As the plume of moisture and warm air spreads east, temperature gradients over the Iberian Peninsula get worse with gradient winds at all levels show a marked increase during the forecast period. DLS will increase to 25-30 m/s with LLS up to 15 m/s, so kinematics seem perfect for organized convection. However, especially the thermodynamics raise concerns about the potential updraft strength of those showers/thunderstorms. Forecast soundings show thin and elongated CAPE profiles with warm mid-levels (as one can expect in a modified tropical air mass), so mid-level lapse rates remain weak throughout the forecast period. Another issue will be the rough orography, which may result in widespread convection within the weakly capped air mass, limiting diabatic heating. Also, long-lived DMC seems unlikely when marginal CAPE and the rough topography overlap. Hence a 15-% lightning area was issued although confidence increased during the past few runs that there probably will be spots with higher lightning activity, given patchy EL temperature distributions below -20 °C. However, orography and mesoscale moisture distribution will dictate that coverage. The main initiation will await the "strongest" forcing (although pretty modest in strength) around noon. Between 12-18Z, the most active convective period is forecast with a decrease thereafter over Portugal and Spain, but ongoing activity over the Bay of Biscay and the adjacent coastal areas.

Despite marginal CAPE, a level 1 was issued , as any more persistent updraft will grow in a favorably sheared environment, so we can't rule out a few better organized multicells or even an isolated supercell. The hail risk will be damped by the warm temperatures, but in case we see a rotating storm or two, an isolated large hail event is possible. Isolated severe wind gusts will be another concern as 850 hPa winds increase to 15-20 m/s, but once again, the air mass with widespread convection (limiting diabatic heating) won't offer a mixed BL, so strong downdrafts will be needed to bring those winds down to the surface. Finally strong LL shear (SRH-1 up to 150 m^2/s^2) with LCLs at or below 600 m point to an isolated tornado risk although limited buoyancy in the lowest 3 km limits the risk. With aforementioned hight PWAT values and the potential upscale growth into numerous weakly organized shower/thunderstorm clusters, heavy rain accompanies that activity but there are no signals for any level-1 rainfall event. After sunset, thunderstorms gradually decay although an isolated and likely elevated event within the WAA regime is still possible mainly over C/E Spain, S France and the far W-Mediterranean until 06Z.

During the overnight hours probably the most dense lightning activity will evolve over the Bay of Biscay with DMC beneath the cold base of the eastward moving mid-level wave. Cold EL temperatures and SBCAPE in excess of 500 J/kg gave us enough confidence to issue a 50-% lightning area. Storms remain sub-severe and gradually approach W-France during the end of the forecast.

... C-Mediterranean and Greece ...

A few sub-severe and short-lived thunderstorms are expected at numerous places during the forecast period. Regionally enhanced moisture (e.g. along westward facing coasts) support that activity.