Forecast Update
Valid: Fri 24 Aug 2007 20:00 to Sat 25 Aug 2007 06:00 UTC
Issued: Fri 24 Aug 2007 20:16
Forecaster: TUSCHY

SYNOPSIS

Please refer to the outlook, issued at Thu 23 Aug 2007 21:07 !


DISCUSSION

...E-Spain, the Balearic Islands...

An update / upgrade becomes necessary to reflect increasing confidence in the development of severe thunderstorms.

Latest ( 18Z ) IR / WV images show a cluster of storms just north of the African coast with at least two embedded intense storms ( strong overshooting tops and " enhanced V " structure at latest Met9 image ). At least 2 SSW-NNE aligned streets of cumuli can be seen towards the west of this cluster with a few showers / storms just southwest of the Balearic Islands.

Latest synop data of and around the Balearic Islands show persistent 15 - 20kt NE-erly winds and dewpoints in the upper tens. 12Z sounding ascent of Palma de Mallorca indicates nice shear and veering. The thermodynamic dispatch of the PBL is not too representative as diabatic warming was suppressed by a broad Cu/Sc field at 12Z and some warming occured since then.

GFS still produces moderate to high instability values ( MLCAPE of 1kJ/kg ) and modified soundings indeed show the chance for some deep convection although 12Z soundings exhibit quite warm mid-/low levels.
A few cogitations about the thunderstorm scenario during the next few hours:

Although models like GFS show a constant warming of the 800hPa layer ( above 20°C ) and hence a stout cap, storms managed to develop north of Africa. DAAG 12Z sounding ist representative and shows the very steep lapse rates above this layer and hence some robust instability release. This cluster should continue towards the NNE but should stay just east of the Balearic Islands. Severe thunderstorm threat ( mainly severe wind gusts ) should persist through the night hours although shear weakens somewhat.

Bad data coverage of this area precludes to go into more details but current thinking is that outflow boundary of this cluster will work its way towards the west during the next few hours. This boundary and impressive mixing ratios will be present just east of Spain and as upper-level energy rotates towards the north, scattered storms are expected to initiate in the level-2 area.
The same over eastern Spain, where LL moisture increased significantly during the past few hours and scattered thunderstorm development looks increasingly likely.

Although the main uncertainty will be the degree of the instability ( especially inland ), expect a broad area of at least low to moderate instability release with higher values just offshore.
Each storm will rapidly gain strength and organize. DLS will be about 25-35m/s and LL shear about 15m/s with enhanced SRH values. This was also supported by latest wind profiler data of Lannemezan, where some veering at lower levels was present.

Expect an enhanced tornado threat mainly offshore and along the coastal areas of eastern Spain, but even further inland, a tornado report can't be excluded ( although storms should be more elevated ). Each storm will pose a risk of severe to damaging wind gusts and an isolated large hail risk further inland.
Strong easterly inflow in a deep layer and high PWAT values point to the possibility of flash flooding, especially if a cluster of storms develops.

The threat will persist during the night hours and should slowly expand towards the north ( possibly affecting the coastal areas of S-France during the morning hours mainly with heavy rain).

Otherwise some modifications were done to the outloo map.