Storm Forecast
Valid: Fri 03 Jan 2014 06:00 to Sat 04 Jan 2014 06:00 UTC
Issued: Fri 03 Jan 2014 00:02
Forecaster: TUSCHY

A level 1 was issued for Ireland, UK, NW-France, parts of the Netherlands and Denmark mainly for severe to damaging wind gusts and an isolated tornado event.

A level 1 was issued for NW-Portugal, NW Spain and SW Spain mainly for severe wind gusts, heavy rain and an isolated tornado event.

SYNOPSIS and DISCUSSION

Fueled by an outbreak of very cold arctic air from E-Canada, an extensive low pressure area grows into a 970 hPa vortex, which covers most of the N-Atlantic. Embedded in that vortex, numerous smaller-scale vortices or mid-tropospheric waves exist. One of those vortices is placed between Ireland and Scotland with a surface pressure between 955 and 960 hPa. This feature drifts to the north/northeast during the forecast with a gradual weakening trend anticipated. Thereafter, numerous more or less pronounced mid-tropospheric waves cross Ireland and UK. Finally a pocket of very cold air at 500 hPa enters the scene from the W during the night and results in a rapidly amplifying trough west of Ireland. Those features will be discussed below:

Ireland and Scotland, ongoing at 06 Z until 18 Z:

Despite missing convection, a short hint at that wind event seems reasonable. Phase diagrams and diverse model data show one of the most remarkable warm-core events for quite some while with the sub-960 hPa depression N of Ireland moving to the NE ... even showing a deep warm-core structure. A well structured occlusion, connected to a subtropical air mass, bends into that vortex and to its south. This scenario provokes a compact damaging wind field beneath a 850 hPa jet core with wind speeds of 35 to 45 m/s (peak strength during the start of the forecast). Despite gradual weakening a swath of damaging winds likely affects Ireland and Scotland during the daytime hours from SW to NE. Despite the magnitude of that wind event, missing convection precludes the issuance of any level areas.

S-C Ireland, UK all the way to Denmark:

Behind an eastward sliding occlusion (the same, which provokes the scenario above), deep CAA will be underway with 500 hPa temperatures falling to -30 °C and less. SSTs west of Ireland exceed 11 °C with a marginal decrease towards Ireland and UK. Sufficient vertical lapse rates are forecast for active and deep post-frontal marine convection. Strong and still predominantly unidirectioanl shear accompanies that convection, so downward mixing of 25 m/s winds at 850 hPa cause severe winds to be the main hazard ... next to sleet. However, at least two embedded mid-layer impulses during that forecast increase concerns about more organized convection:

The first wave affects S-UK/NW France (09 to 15Z) with a rapid SW-NE motion. Surface pressure fields confirm a weak stamp at low-levels with a temporal increase of BL mixing ratios. Coupled to cooling mid-levels, 200-500 J/kg SBCAPE seems likely over S-UK and NW France. Faint backing ahead of that wave also increases LL directional shear with latest GFS output confirming SRH-1 in excess of 200 m^/s^2. Modest forcing, favorable placement beneath the left exit of a powerful 500 hPa jet, a weakly capped air mass, a 70kt storm motion vector aligned near parallel to a LL wind shift/convergence zone and aforementioned CAPE increase confidence in numerous fast moving showers/isolated thunderstorms with some bowing (severe wind gust) and tornado potential. A compact line of deeper convection could also cause a swath of severe wind gusts. This wind gust risk extends also offshore (east of SE-UK) and may affect the Netherlands and Denmark between noon and the evening hours. Behind that convection, a temporal decrease of convection is forecast over S-UK. However, GFS/WRF show a tongue of slightly unstable air to reside over the Netherlands into Denmark during the evening and overnight hours. A few thunderstorms are possible and favorable directional and speed shear once again point to severe wind gusts and an isolated tornado threat. Hence the level 1 area was expanded well inland over the Netherlands and Denmark.

Around noon, an even more pronounced wave approaches Ireland from the west and affects UK during the late afternoon and evening hours. This one keeps directional shear enhanced although decreasing BL moisture and late timing should insert a weakening trend of onshore CAPE during the late afternoon and evening hours. With 850 hPa background flow increasing to 30 m/s it will be hard to distinguish between gusts from the gradient wind flow and convectively induced gusts. The risk however exists for severe to damaging wind gusts .. especially next to showers/isolated thunderstorms. An isolated funnel/tornado event can't be ruled out as well.

Finally a third wave with very cold mid-layer air (500 hPa below -35°C) pushes east towards Ireland. Rapid amplification of that wave into a large upper trough induces a slowdown and keeps that trough west of Ireland. Nevertheless, it taps into rather moist marine air west of Portugal, Spain and over the Bay of Biscay. Differential WAA increases mid-level lapse rates atop onshore streaming moist marine air over the Iberian Peninsula and France. Isolated to scattered elevated and non-severe thunderstorms are forecast. The westward facing coasts of N-Portugal and Spain however could see surface based activity with strong shear. Severe wind gusts, an isolated tornado event, heavy rain and marginal hail are possible with that activity. France was excluded due to the late arrivial of the trough (probably beyond 06Z).
A similar risk is forecast for SW Spain between 00-06 Z. A tongue of subtropical air / high moisture points towards SW Spain with strong shear and moderate MLCAPE. Heavy rain, severe wind gusts and an isolated tornado event are possible.

For the rest of Europe no thunderstorm activity is anticipated (despite isolated storms south of Turkey beneath cooling mid-levels).