Storm Forecast
Valid: Fri 18 Jan 2008 06:00 to Sat 19 Jan 2008 06:00 UTC
Issued: Thu 17 Jan 2008 21:42
Forecaster: TUSCHY

SYNOPSIS

During the past few days, the nordhemispheric streamline pattern became very wavy with 4-5 well structured long-wave troughs. For us the most interesting one is a wave, which entered the North Atlantic during the past few days and moved slowly towards the east. Its positive tilt resulted in a wide area of WAA on its downstream side and hence a gradual increase in geopotential heights over SW Europe started and is still underway.
At the same time, geopotential heights stay quite low east / southeast of Greenland and this constellation now results in a very intense height gradient over NW Europe.
During the following 24 hours, the upper-level streak strengthens constantly to more than 95m/s just to the north of Ireland and conditions are favorable for a rapid cyclogenesis somewhere over Scotland and eastwards.

Otherwise the weather over Europe stays stable and only the central Mediterranean gets a few thunderstorms under a broad upper-level trough, but conditons for thunderstorm organisation are marginal at best.

DISCUSSION

....An area south of Crete...

As mentioned above, atmosphere is quite unstable under the pool of cold mid-levels and moderate instability release in the order of 200-500 J/kg SBCAPE can be expected. Lapse rates at low-levels steepen during the forecast period and as background flow stays weak, an isolated waterspout report can't be excluded.
Set-up looks too conditional for issuing a level-1 area.

...Scotland, northern North Sea and parts of Norway...

The past few model runs of GFS brought the central pressure of the rapid cyclogenesis down to 962-968hPa with previous runs having values of 950-965hPa. In fact everything points to an intense depression and despite the uncertainties in the final strength between different models and from run-to-run, the model pool agrees quite well in the track of this feature, crossing Scotland between 12Z-18Z and reaching western Norway around 00Z.
Comparing the analysis of 18Z, 17th Jan. GFS has a good handling of the area of interest west of Ireland in both the location as well as intensity and a rapidly eastward expanding and cooling cloud shield of the approaching and developing depression can be already seen west of Ireland. With this in mind we can start to highlight areas of main concern as overall scenario seems to be on track and in general agreement with models.

...12Z-18Z Scotland...

The first round of convection should start as early as 11-12Z over Scotland, as intense WAA goes on. Despite a rapid warming of the lower 3km, airmass at 500hPa stays cool with 500hPa temperatures around -20°C, which is not that cold, but in combination with warm 0°C at 700hPa, lapse rates steepen to more than 7km / K. Combined with intense forcing, a few elevated thunderstorms can't be ruled out although coverage and duration of those storms will be limited as instability release will be scanty and UVV field rapidly leaves Scotland to the east/northeast. SRH-3km values bust the scale with more than 1000 J/kg but high wet-bulb zero levels and expected weak updraft strength should suppress any severe thunderstorm risk.

A whole other story between 14Z and 18Z as the rapidly deepening depression crosses Scotland just to the north. The main problem right now is to find a compromise between the proposals of the different models as strength and position result in a quite different temperature advection at all levels.
We went with the most consistent trend, indicating the best chance for instability release over Scotland, decreasing rapidly to the south where atmospheric stratification is more stable.
Theta_e ridge is quite impressive for this time of year as mixing ratio reaches values of near 6g/kg over Scotland and dewpoints, soaring to 9-11°C. Not much cooling needed for some instability release.
Forcing is impressive as cold front and upper-level trough axis approach from the WNW and we expect an active cold front to cross Scotland during this time-frame with embedded showers and thunderstorms.
Kinematic parameters hint on an enhanced risk for rotating storms ( 0-1km shear of 25m/s, 0-1km helicity of more than 300J/kg and winds at 850hPa in excess of 40m/s ).
Parcel layer depth of more than 2km and signals of strong convergence at lowest levels in such a sheared environment were enough to release an high-end level-1 for the coastal areas. Not much needed to upgrade this area, but we first want to see the exact evolution of this depression before issuing higher probabilities.
Beside the damaging wind gust risk, a few tornadoes are likely, too.
A level-1 was also issued for most parts of Scotland as widespread severe-damaging wind gusts in this slightly unstable airmass become an increasing threat. We do not think that convective activity will be that extensive that a level-2 can be justified, but this area has to be monitiored, too.

...18Z-06Z northern North Sea, parts of Norway, Denmark and S-Sweden...

The intense depression continues to deepen, while racing eastwards. GFS continues to show a very interesting scenario with a still well established theta-E ridge all the way up to SW Norway but a rapid flattening of the thermal ridge, as temperature distribution becomes more zonal. This results in a rapid cool down at all levels while BL moisture is still quite humid and hence the whole S/SE quadrant of this depression becomes quite unstable.
The main interest right now is an area Orkney Islands eastwards, where up to 500kJ/m^2 weakly capped ICAPE are forecast. This area is highly sheared with winds at 850hPa between 30-40m/s , LL shear of 20m/s and helicity values of 150J/kg. The main focus for thunderstorm acitvity will be along the eastward racing cold front, reaching western Norway around 21Z.
Offshore, helicity values are not too impressive and right now we expect mainly a severe-damaging wind gust risk with those storms.

We expanded the level-1 area well to the east, as atmosphere becomes slightly unstable over a broad area, including western Norway, Denmark and SW Sweden after 00Z.
LL shear is still favorable for a few tornadoes and in addition, intense 35-40m/s streak at 850hPa continues to spread eastwards, crossing southern Norway between 00Z and 06Z. Severe to damaging wind gusts are a distinct possibility with each not necessarily electrified convective activity.

Overall configuration and the slightly unstable but strongly sheared sector point to a potential damaging wind event over SW-Norway and degree of convection decides if a broad level-2 becomes necessary later on.
For now we will stick with an unsual large level-1 area for a winter-time depression.