Storm Forecast
Valid: Sun 15 Nov 2015 06:00 to Mon 16 Nov 2015 06:00 UTC
Issued: Sat 14 Nov 2015 18:50
Forecaster: TASZAREK

A level 1 was issued for British Isles mainly for the severe wind gusts.

SYNOPSIS and DISCUSSION

High pressure with warm and dry mid levels covers most of the Medditerean region where convection is inhibited. Large horizontal temperature contrast between northern Europe with cold air mass and southern Europe with warm air mass creates strong thermal wind (jet stream). It extends from British Isles, through Germany, Hungary and Ukraine providing dynamic airflow with large directional and speed vertical wind shear. A wave that will disturb air flow within this feature is forecast by NWP models in the evening hours and will influance British Isles and North Sea.

Warm air advection in the warm sector of the approaching trough will provide moist low levels (mixing ratio ~ 10 g/kg, dew points ~ 12C) that ahead of the approaching cold front will destabilize atmosphere and result in a thin CAPE up to 100-200 J/kg. Although a convection will occur in a very dynamic air flow, a shallow free convective layer will be able to benefit only from MLS that in NWP models is forecast to exceed 25 m/s. The same value is assigned to values of non-convective wind gusts that will be likely due to horizontal pressure gradient. However, strong and persisted QG-lift along the cold front suggest that storms may organize into compact bowing linear features capable of producing scattered severe wind gusts stronger than 25 m/s. A large level 1 area covers those regions where at least isolated gusts are expected with passing thundery showers.

Due to steep vertical temperature lapse rates in the inflowing cold air mass (7.5 C/km), some thunderstorms will be also possible over mild sea surface in offshore and coastal areas where instability may increase to 200-400 J/kg. Along some part of the W Norwegian coast, the additional factor will be orographic lift that will support convective development.

It needs to be mentioned, though, that the background wind field is strong enough to enable severe wind gusts even outside of deep convection (and hence outside the scope of ESTOFEX) over Benelux, Germany and Czech Republic. The highest values will also be confined to coastal areas which are adapted to such wind speeds.