Storm Forecast
Valid: Wed 30 Sep 2009 06:00 to Thu 01 Oct 2009 06:00 UTC
Issued: Wed 30 Sep 2009 04:38
Forecaster: PUCIK

A level 1 was issued for Eastern Spain mainly for excessive rainfall.

SYNOPSIS

A mid-level cyclonic vortex filled with arctic air resides over most of Northern Europe with strong west-northwesterly flow at its southern fringes. The trough is expected to amplify over Eastern Europe by Wednesday morning with another disturbance developing over SW coasts of Norway, in the left exit region of 40 m/s jet-streak at 500 hPa. To the south, synoptically quiescent conditions will prevail with a cut-off low over Spain gradually filling. Unstable airmass over the western Mediterranean will result in abundant showers and thunderstorms, mostly over Spain during the Wednesday. Later on, thunderstorm will likely occur over the Balearic Sea and the activity should spread also towards Sardinia and Corsica and Algiers.

DISCUSSION

... Eastern Spain and surrounding coastal areas...


During the previous two forecast periods, thunderstorms resulting in excessive rainfalls have hit the region, resulting in flooding. As for Wednesday, situation is expected to ease off, as the cut-off low will weaken and slowly decay with no strong lift associated with it. The easterly surface winds should weaken and shift a bit northwards. Over the Mediterranean Sea, significant destabilization is forecast, with MLCAPE values exceeding 2000 J/kg. With high water vapour content in the airmass, weak tropospheric flow and storm initiating due to the coastal uplift, parts of Spain might still see excessive rainfall with thunderstorm activity persisting from Tuesday night. Towards the evening, storms will very probably cease to reform over the region and the period of severe weather will draw to the end. Level 1 is introduced for the region with the highest threat of strong convective rainfall during the Wednesday morning and noon hours. As of Wednesday 04 UTC, MCS has formed, parallel to the mean flow, suggesting its persistence over certain areas. The whole system will slowly shift northwards of its current location (

To the southeast of Spain a belt of moderate DLS values ( 15 m/s) and a blob of high SREH ( over 200 J/kg in 0-3 km layer) is simulated by models in noon hours, indicating an environment conducive to well organised thunderstorms. Lacking lift and at the same time enhnanced CINH values should prevent thunderstorms forming and no severe threat is anticipated.

... Rest of Spain and Portugal...

Diurnally driven thunderstorms will form over the region in a weak flow regime. Due to the low shear values ( under 10 m/s in 0-3 km layer) , only weakly organized, multicell clusters or single cells are expected. As thunderstorm movement will be very slow and simulated LCLs quite low ( mostly under 1500 m), a very isolated event of excessive rainfall is not ruled out. The coverage should be too low, however, to warrant a Level 1.