Storm Forecast
Valid: Sun 15 Nov 2009 06:00 to Mon 16 Nov 2009 06:00 UTC
Issued: Sat 14 Nov 2009 19:52
Forecaster: TUSCHY

A level 1 was issued for Portugal and extreme NW-Spain mainly for excessive rainfall amounts.

SYNOPSIS

Huge, cyclonic complex over the NE-Atlantic continues to spread southwards. A bouncing SW-erly jet affects western Europe during the forecast with strengthening ridging downstream over the central Mediterranean. More winter-like conditions persist over E/N-Europe.

DISCUSSION

... Portugal and extreme NW-Spain ...

Along the NW coast of the USA, a similar pattern with a direct connection to the tropical airmass is well known as the "Pineapple Express" with its origin in the Hawaiian area, resulting in intense rainfall along the NW coast. A similar pattern is in full progress now west of Portugal with a far southward digging trough over the Azroes and the advection of a tropical airmass on direct way towards the Iberian Peninsula.

Latest IR/WV data has an extensive baroclinic leaf, running from the Azores all the way towards the Bay of Biscay. Numerous convective clusters developed south of this baroclinic zone and visualized the very unstable character of this airmass. The main upper trough, still way out over the Atlantic, gradually approaches from the west, pushing a wavy surface front towards the SE. Along this boundary, a broad and slowly strengthening surface depression moves NE-wards and causes a north (morning hours/noon) and southward (night hours) oscillation of the boundary over Portugal. Very weak lapse rates at all levels affect the region due to moist ascent throughout the troposphere. A gradually weakening 20-30m/s LLJ and input of high PWAT airmass cause heavy rainfall over the level 1 area with the acticity slowly shifting southwards during the day while weakening. Thunderstorms upstream occurred, so despite marginal CAPE release, isolated, embedded thunderstorms are forecast. Heavy rain risk diminishes during the night hours.

... E-Atlantic, the English Channel and the S-North Sea ...

Linked to mid-level impulses, swaths of enhanced convection affect the English Channel and the S-North Sea during the daytime hours and an area SW of UK during the night hours. Due to the fact, that cold mid-levels are needed to build-up some CAPE, showers/thunderstorms evolve in a weakly sheared environment beneath the center of those vertically stacked disturbances. Strong gusts will be the main risk. Heavy rain could affect S-Ireland during the very end of the forecast period, but no thunderstorm activity is expected and hence this event is no subject to our risk scheme.

A broad warm sector overspreads most parts of France during the night. Some marginal CAPE is forecast by GFS just south of the warm front, but this is an outlier in the model pool. Weak forcing intersects the warm front during the morning hours, but right now, there are no well-founded reasons to expect thunderstorms with this activity.