Storm Forecast
Valid: Sat 06 Oct 2018 06:00 to Sun 07 Oct 2018 06:00 UTC
Issued: Sat 06 Oct 2018 04:46
Forecaster: GATZEN
A level 1 was issued across the Tyrrhenian and Adriatic Sea, southern and central Italy, and western Balkans mainly for extreme rainfall and to a lesser extent waterpouts.
A level 1 was issued across south-eastern France and north-eastern Spain and the north-west Mediterranean mainly for severe wind gusts and to a lesser extend extreme rain and large hail.
DISCUSSION
SYNOPSIS
The Mediterranean cut-off slowly lifts and weakens during the period. Main convective activity is expected along a convergence zone at the eastern flank of the cut-off, although lapse rates weaken with time. To the north-west, a sharp trough crosses the British Isles, the North Sea, and Scandinavia. Its southern part cuts off over France.
DISCUSSION
Adriatic and southern Italy region
Still rich low-level moisture is present across the Ionian Sea. A convergence line remains east of the lifting cut-off low where thunderstorms are likely to go on. Lapse rates will decrease during the day, resulting in skinny CAPE profiles with high PW values. Given weak vertical wind shear, slow moving storms are expected along the convergence zone that spreads slowly east, affecting the western Balkans. Here, upslope flow will support initiation of storms. Excessive rain and local flooding is forecast, with the main focus highlighted by level 2. Additionally, water spouts are not ruled out, in particular within the southern portions of the convergence line.
Tyrrhenian Sea
Although the cut-off low moves north-east, some CAPE is still forecast given rich low-level moisture in the range of a weakening low-level vortex. Skinny CAPE and almost no vertical wind shear will allow for slow-moving, heavy rain producing storms, capable of local flash floods where land is affected. Waterspouts are not ruled out, but weak low-level buoyancy may limit the potential.
Northern Spain and southern France into north-west Mediterranean
As the intense trough approaches and cuts off, a well-developed vort-max spreads over France. Its southern periphery crosses the Pyrenees from west to east in the afternoon and evening. Ahead of the trough, an elevated mixed layer advects over the Pyrenees, overlapping with rich low-level moisture. While CAPE build-up is questionable north of the Pyrenees due to a rather early approach of the cold front during the noon, substantial CAPE is likely over north-eastern Spain. Current thinking is that a few storms form ahead of the cold front over south-eastern France, rapidly merging to the cold-frontal rain band. In the wake and south of the Pyrenees, some more storms are likely. Given strong vertical wind shear, especially in the lowest 3 km, storms are expected to grow upscale, forming an MCS over the north-west Mediterranean at night.
Main threat will be some severe wind gusts over southern France especially in the later stage of the developing MCS. Farther south and east, isolated cells may organize into multicells and supercells given about 20 m/s deep layer vertical wind shear. Large hail, severe wind gusts, and locally flash floods are not ruled out. The severe potential continues overnight when the system spread across the Mediterranean.
France into western Germany
Warm air advection increases over eastern France and western Germany ahead of the forming cut-off low. Due to diurnal heating and QG lift, lapse rates increase during the day. Steep lapse rates will overlap with rich low-level moisture ahead of a cold front. Some weak CAPE of about 600 J/kg may form. Initiaton is not ruled out during the afternoon and evening. Storms that form will profit from about 15 m/s deep layer vertical wind shear and 10 m/s 0-3 km vertical wind shear. Although hodographs are straight and multicells will be the dominant convective mode, locally severe storms may form, capable of severe wind gusts, some marginally severe hail, and local flash floods. Overall threat is too low to warrant a level 1.
