Storm Forecast
Valid: Thu 17 Sep 2009 06:00 to Fri 18 Sep 2009 06:00 UTC
Issued: Wed 16 Sep 2009 19:26
Forecaster: GATZEN

A level 2 was issued for western Albany and western Greece mainly for excessive rain.

A level 2 was issued for eastern Spain and the Balearic region mainly for excessive rain.

A level 1 was issued for the west Mediterranean and the east Mediterranean mainly for excessive rain.

SYNOPSIS

The cut-off low centred over the Iberian Peninsula will remain during the forecast period. Curving around its base, a strong south-westerly mid-level jet is expected to move into the west Mediterranean Sea during the period. This is associated with warm air advection and an upper ridge that builds over the central Mediterranean region in the wake of an amplifying short-wave trough spreading into the Aegean. While high pressure dominates north-western and central Europe, an intense trough moves across Scandinavia.

DISCUSSION

West Mediterranean region

The cut-off over Iberia evolves a dipolar structure with an intense southern centre rotating across southern Spain and later into western Spain. The southward extension of the cold mid-level air mass enables a strong mid-level jet that is expected to spread north-eastward across the west Mediterranean Sea. At lower levels, warm air advection becomes likely, while the boundary-layer moisture increases during the day. With a low-level mixing ratio expected around 10 g/kg, latest soundings clearly indicate that instability will build in the afternoon and evening hours.

While the forcing will be limited over most places to the east where the central Mediterranean ridge will be present, quite strong forcing is likely along the eastern and north-eastern coasts of Spain, where upslope easterly winds will evolve ahead of the approaching surface low. The chance of thunderstorms is quite high during the period over this region. Given the high precipitable water content and expected convective activity for all of the period over the same region, excessive rain is forecast. Thunderstorms will likely be organized given strong deep layer vertical wind shear, and large hail is not ruled out completely. Late in the period, the strong mid-level jet streak will likely lead to another intensification of the convection, and one or two MCS will likely develop and spread eastward into the west Mediterranean. Along the leading gust front, severe wind gusts are not ruled out. Along the north-eastern coasts of Spain, the increasing LLJ and low-level vertical wind shear may be favourable for tornadoes, but decreasing low-level buoyancy will likely limit the potential. The main threat will be excessive rain.

Southern Adriatic to southern Aegean

Downstream of an amplifying ridge over the central Mediterranean, a mid-level short-wave trough migrates across the east Mediterranean embedded in the strong mid-level westerly flow. At lower levels, rich moisture over the central Mediterranean will spread eastward ahead of low surface pressure over the Adriatic. Given some QG forcing in the range of the short-wave trough and quite weak capping of the very moist boundary-layer, thunderstorms are likely especially along the western coasts of Greece where upslope flow will provide additional lift. Although the vertical wind shear will gradually decrease in the range of the mid-level trough, convection may likely evolve into mesoscale convective systems that are expected to produce excessive rain as they will likely affect portions of the Adriatic for a couple of hours. Embedded supercells are also forecast as strong low-level winds will create 15 m/s vertical wind shear in the lowest 3 km, and large hail or tornadoes may form locally. Storms will likely spread eastward in to the Aegean in the evening and night hours. While rather stable conditions are expected over the northern portions due to less low-level moisture, rather good low-level buoyancy may form over the southern Aegean. Given strong vertical wind shear and forcing, one or two MCS are forecast that may also affect Crete late in the period. Severe wind gusts and local tornadoes are not ruled out, while excessive rain is expected to be the main threat.