Storm Forecast
Valid: Thu 21 Jan 2021 06:00 to Fri 22 Jan 2021 06:00 UTC
Issued: Wed 20 Jan 2021 23:10
Forecaster: GATZEN

A level 1 was issued across northern Portugal for tornadoes, wind gusts and locally extreme rainfall.

SYNOPSIS

A strong, negatively tilted trough migrates across the North Sea towards Scandinavia. This is associated with intense cyclogenesis over the North Sea at the begin of the period. A deep tropopause fold moves over the North Sea, causing steepening lapse rates at low levels and some CAPE can form. However, the equilibrium level temperature is high below the fold, and thunderstorms are not expected.

To the south, further troughs affect the Bay of Biscay during the afternoon and the Iberian Peninsula late in the period. These troughs can be associated with deep moist convection, especially due to better low-level moisture and deeper steep lapse rates.

Downstream, ridging spreads across Europe, and large scale subsidence limits the development of strong low-level convergence. Best potential is over the west and central Mediterranean due to weak embedded mid-level troughs. However, latest models do not indicate that sufficient CAPE will develop.

DISCUSSION

Northern Portual

In the evening, an intense short-wave trough progresses across the Atlantic, with a strong mid-level jet streak in its wake. QG forcing and a well-developed tropopause fold affect the air mass that becomes unstable with CAPE in the order of some 100 J/kg. Close to the shore, low-level vertical wind shear gets strong with 15+ m/s 0-1 km bulk shear. This will be more than sufficient for rotating low-level updrafts. These can produce tornadoes and severe wind gusts. Main limiting factor is the small time window as stroms will rapidly move towards the mountains where CAPE is low and storms will weaken. Together with the wind threat, some heavy rain can also occur when storms move into the highter terrain and profit from upslope flow.