Storm Forecast
Valid: Sat 14 Nov 2009 06:00 to Sun 15 Nov 2009 06:00 UTC
Issued: Sat 14 Nov 2009 00:06
Forecaster: TUSCHY
A level 2 was issued for parts of the English Channel and the coastal areas of S-UK mainly for severe wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for W/NW/NW-France mainly for severe wind gusts and tornadoes.
A level 1 was issued for most parts of the UK, surrounding the level 2 mainly for severe wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for N-Portugal and extreme NW-Spain mainly for excessive rainfall.
SYNOPSIS
Extensive cyclonic vortex over far NW-Europe steers numerous disturbances around its fringes, which affect western and N-Europe during the forecast. Ridging over the Mediterranean suppresses convection, whereas cooler/stable conditions persist over E-Europe.
DISCUSSION
... S-UK and the English Channel ...
The main story of the day will be the passage of the upper trough axis with the attendant intense depression/pressure gradient over S-UK/the English Channel.
The wind forecast is straightforward as 35-40m/s at 850hPa will mix down easily towards the surface, so damaging and potential life-threatening wind gusts affect the southern part of the UK until 18 UTC, exiting the area thereafter towards the east. Some weakening of the wind field is forecast over far SE-UK, but kinematics are still adequate for isolated extreme wind gust events.
The tornado risk is already more uncertain but most likely maximized along the coastal areas of S/SE-UK, where some onshore moisture causes marginal CAPE. Strong tornadoes will be possible, if more persistent updrafts can evolve. However this brings us to the main uncertainty: convection.
The region will be placed beneath the weakening sting jet, where intrusion of dry, high-level air and constantly evaporatively cooled airmass inhibits deep convection. This is also seen in warm EL temperature forecasts and slim convective precipitation signals. However, dry slotted areas could also see some insolation and not much BL modification is needed along the coast for deeper convective updrafts. A strong vorticity love also crosses S-UK during the daytime hours, so despite slim signals, at least supportive conditions for deep convection can't be ruled out along the coast and SE-UK.
Probably the most likely scenario will be a rapidly E/NE-ward racing forced line of convection over S-UK. Conditions onshore become worse, so the strongest activity remains confined along the south coast (probably enhanced by diurnal heating) and the level 2 was expanded inland, where the tornado risk is enhanced next to the damaging wind gust threat. As convection is forecast to play at least a partial role in this severe wind event, a level 2 became necessary. The overall risk diminishes from west to east during the day. The level-2 was expanded well towards the west, as latest data indicates a slow-down of the eastward progression of the strongest winds.
Further north, west-central UK, an isolated tornado event is also forecast, given some marginal CAPE and strong shear. A level 1 may cover this risk due to the overall weaker shear.
... NW/N France ...
A prolonged period of deep convection is forecast to affect W/NW and N-France during the day and even affects parts of W-Belgium during the evening hours. A broad swath of enhanced directional LL shear and 100-500 J/kg MLCAPE (decreasing towards the ENE) overlap, so tornadoes and strong to severe wind gusts are likely in the broad level-1. Isolated large hail also increases during the afternoon hours, as strongest updrafts with low WBZ values occur. The risk diminishes during the evening and night hours, as CAPE decreases. Thunderstorm probabilities over the English Channel remain augmented during the night hours with an enduring severe wind gust risk.
... Parts of Portugal and extreme NW-Spain ...
A favorable pattern for intense rainfall amounts set-up during the end of the forecast over the area of interest (00 UTC onwards). Numerous waves, embedded in the intense baroclininc leaf, cross the area from the SW with a constantly falling pressure at the surface. A 50m/s mid-level jet and a persistent 25-30m/s LLJ overspread the area and assist in the advection of modified tropical airmass towards the coast. Again, conditions support embedded convection due to CSI release, so we decided to include low-end thunderstorm probabilities. The heavy rain event keeps going beyond 06 UTC.
... W/SW Switzerland ...
Ahead of the eastward trailing cold front, a strong LLJ supports adequate moisture advection well towards the north. A combination of cold mid-levels, the approaching front during peak time heating and a strong vorticity lobe from the SW indicate a temporarily flare up of convection in the highlighted area (afternoon hours). It is still unclear, how much CAPE will become available, but any stronger updraft grows into a strongly sheared environment (35-40m/s DLS), so an isolated, organized thunderstorm is possible. The main risk will be large hail and strong to severe wind gusts. Confidence in an augmented thunderstorm coverage is not yet high enough for a level 1 but an upgrade may be performed later-on, if new data confirm current trends.
