Storm Forecast
Valid: Fri 27 Dec 2013 06:00 to Sat 28 Dec 2013 06:00 UTC
Issued: Thu 26 Dec 2013 21:42
Forecaster: SCHLENCZEK

A LVL2 was issued for S Norway mainly for severe wind gusts and to a lesser extent for tornadoes.

A LVL1 was issued for parts of Denmark, Sweden and the coastal regions of W Europe mainly for severe wind gusts.

A LVL1 was issued for UK and Ireland mainly for severe wind gusts.

A LVL1 was issued for S Greece for excessive convective precipitation.

SYNOPSIS

There are basically two regions in Europe where deep moist convection can be expected on Friday:

An upper trough over the central Meditarranean will move slowly to the east. Ahead of this trough, some enhanced deep layer shear might aid storms to organize into some slightly severe multicellular systems with severe wind gusts and large hail as primary threat. In the vicinity of the upper cold core, vertical shear is very weak but this may lead to excessive convective rainfall.

The big deal, however, is a developing cyclone which moves across the E Atlantic towards the British Isles. Along the cold front of this cyclone and near the post-frontal trough axis, some instability is found in an environment of very intense winds at low to mid levels. Such configurations are prone to produce widespread severe wind gusts which are at least partly convective. It is also not ruled out that an isolated tornado may form along the cold front.

DISCUSSION

...S Norway, S Sweden, Denmark, NW Germany, Netherlands, Belgium, NW France to NW Iberia...

The track of the developing cyclone is forecast with a good accuracy and therefore the regions with the greatest threat of severe weather which is at least partly associated with convection are obvious. All past runs of GFS on Dec 26 show the greatest overlap of strong front-normal winds and instability around the south coast of Norway on Friday noon. It is likely that storms along the cold front will form a narrow line which will enhance the already given threat of severe winds. Some gusts are likely to exceed 33 m/s as the 850 hPa background flow is in order of 35 m/s, probably enhanced by convective downdraughts. In such a situation, there is also a slight risk of tornadoes as favourable kinematic conditions with very high SRH3 and SRH1 are expected. The area with the greatest threat of wind gusts > 33 m/s accompanied by deep moist convection is covered by a LVL2 area.

The other regions mentioned above do also experience some low-end instability combined with strong vertical shear and a quite intense background flow. Nevertheless, the front-normal wind components are considerably weak in the other regions which will reduce the chance of a linear MCS and therefore the risk of severe wind gusts. The whole region along the cold front will likely get enhanced SRH and strong LL shear, so a tornado is not ruled out either.

...UK and Ireland...

During the morning and afternoon hours, the cyclone moves across the northern parts of the British Isles and some low-end instability in the wake of the cold front may lead to showers and thunderstorms which are likely to produce severe wind gusts in the given kinematic environment. As mentioned before, isolated tornadoes are not so uncommon for such a situation.

...C Mediterranean, Adriatic and Ionian / Aegean Sea...

A weakening upper trough moves slowly eastwards. The regions ahead of the trough will get some hours of strong onshore advection of moist and unstable air which may lead to excessive convective rainfall in some places. Apart from the heavy rain threat, isolated severe wind gusts are not ruled out. The surrounding regions will likely get showers and thunderstorms but the risk of severe weather is considerably marginal.