Storm Forecast
Valid: Mon 15 Oct 2018 06:00 to Tue 16 Oct 2018 06:00 UTC
Issued: Mon 15 Oct 2018 05:40
Forecaster: GATZEN

A level 1 was issued for the western and central Mediterranean mainly for excessive precipitation and to a lesser extent for tornadoes.

A level 1 was issued for Portugal mainly for tornadoes and excessive precipitation.

A level 1 was issued for eastern Algeria and Tunisia mainly for large hail and severe wind gusts and excessive precipitation.


SYNOPSIS

Remnants of the former tropical storms LESLIE and MICHAEL will affect Iberia, the west Mediterranean, and southern France on Monday. Weak lapse rates will limit potential of convective storms. However, embedded thunderstorms are likely in particular over the Mediterranean Sea. To the east, a short-wave trough affects the central Mediterranean, another is located across the Aegean Sea. Warm air advection and low-level convergence will be present over the central portions of the Mediterranean. Cold air advection will be dominant over Greece and parts of the Aegean. Over Germany, a cut-off low will develop within the broad ridge from the British Isles to eastern Europe. Western Scandinavia is crossed by an intense short-wave trough embedded in the polar jet.

DISCUSSION

Western and central Mediterranean

Main convective potential is expected across the central and western Mediterranean. Main severe potential is due to excessive rain along the frontal boundary ahead of ex-LESLIE that moves towards Sardinia and Corsica and southern France. High precipitable water and skinny CAPE profiles especially in the northern portions in addition to a storm motion parallel to the frontal boundary, strong low-level shear supporting back-building and a long fetch across the Mediterranean Sea indicate a potential of heavy rain. Main limiting factor is the weak lapse rate that reduces CAPE and results in warm equilibrium levels. However, embedded storms are expected embedded in stratiform rain and pose a threat of flash flooding. The threat will slowly progress eastwards across the Mediterranean, whereas is remains almost stationary over southern France, though slowly decreasing. A level 1 was issued due to the partly stratiform nature of precipitation. The overall threat of flash floods can be widespread, though. Next to excessive rain, tornadoes are not ruled out given strong low-level vertical wind shear that overlaps with low-level buoyancy in particular near the coasts.

Portugal

At the flank of the short-wave trough associated with ex-MICHAEL, a strong jet streak diggs south over Portugal. Onshore transport of rich moisture and increasing lapse rates at low levels beneath an upper level front will allow for weak CAPE at low levels. Warm equilibrium levels will limit thunderstorm potential. However, it is not ruled out that a few tornadoes can develop with showers along a frontal boundary moving south in the morning and noon. Additional, excessive precipitation is expected, partly due to deep moist convection embedded in the stratiform rain band.

Atlas mountains

Low-level moisture has been advected far into Algeria and Tunisia. On Monday, a well-developed short-wave trough crosses the region from the west. CAPE in the order of 500 J/kg is forecast due to steep lapse rates advected northward ahead of the short-wave trough. Forecast soundings indicate rather high CIN, but initiation seems to be likely over the mountains. With 20 m/s deep layer vertical wind shear, supercells can develop, capable of producing large hail and severe wind gusts. Locally, excessive precipitation is not ruled out. Late in the period, the trough affects the south Mediterranean east of Tunisia. Increasing CAPE and low-level convergence will support thunderstorms in a strongly sheared environment. Supercells and multicells are expected, and locally, large hail and tornadoes are forecast.

Southern Italy and Sicily

Along a convergence line, storms are expected in an area with high precipitable water. Skinny CAPE profiles and slow-moving storms pose a risk of local flash flooding, enhanced due to upslope flow along the south-eastern flanks of mountains.