Storm Forecast
Valid: Fri 09 Nov 2007 06:00 to Sat 10 Nov 2007 06:00 UTC
Issued: Thu 08 Nov 2007 20:47
Forecaster: TUSCHY

SYNOPSIS

An active period shapes up for parts of Europe.
Downstream of a broad and compact upper-level trough over northeast North-America and eastern Canada, WAA over the northern Atlantic is on the increase, culminating in a strengthening high pressure area just west / southwest of Ireland. Geopotential heights are forecast to increase in this area during the next 24-36 hours, supporting an augmented geopotential height gradient over NW Europe.
This in turn results in a rapidly strengthening upper-/ and mid-level wind field north / east of Scotland. GFS develops a jet core of about 160kt at 300hPa and 90-100kt at 500hPa. Regarding the reliability.... Sounding reports of Iceland represent the model environment quite well. UW-CIMSS also indicates a rapidly organizing and strengthening jet northeast of Scotland and latest QuikSCAT scan had up to 70kt uncontaminated wind fields just north of Scotland ( although most of the data looks like to be inaccurate ). Surface wind gust reports were about 20-25m/s over Scotland and well more than 30m/s offshore during the past few hours ( 15-17Z, 8th Nov.).
So there are indeed enough indications that a compact and intense windstorm area is forming east of Scotland ( refering to 17Z, 8th Nov. ).
The upper-level jet is forecast to race towards the southeast, overspreading Germany from the NW while weakening gradually during the end of the forecast period.
Like we saw it during the past few days, such an intense jet on the upstream side of an upper-level trough supports an increase in the trough's amplitude ( in this case all the way to Greece ).
Cold and stable conditions prevail over western Russia while weather stays warm and stable over most parts of Spain and Portugal.

As a side-note:
There are no indications that a polar low is embedded in the broad postfrontal CAA regime but an area west of Norway and the North Sea has to be monitored for any development.
The low pressure area southwest of the Azores has no further indications of better convective activity next to the center and further development is not expected during the forecast period.
An intense Mistral event will go on during the forecast period, suppressing any convective activity over the western Mediterranean.


DISCUSSION

... Belgium, the Netherlands, Germany ...

Like during the CAA-events, which occured for the past few days, there is a concern regarding the thermodynamics. CAA is compact and occurs nearly simultaneous at all levels, but the long distance over the still warm North Sea helped to warm the low-levels and hence temperatures at 850hPa are not expected to fall below -5°C. Diurnal warming will be limited and surface temperatures over Germany are forecast to stay below or around 5-6°C . This in combination with the 850hPa temperatures results in weak LL lapse rates.
Cooling at mid-levels is way more significant and lapse rates steepen to 8°C/km.
Current thinking is that thermodynamic stratification still supports deep convection which can be seen in the expected EL temperatures, too.
Having a glance on the latest IR / WV images reveals an healthy postfrontal airmass with widespread convective activity and confidence is growing that we will see numerous thunderstorms over a broad area and we therefore decided to highlight quite a large area.
It looks like we have to deal with three different regions/mechanisms of enhanced thunderstorm chances:

a)
The first region is the coastal area of NW Germany and the Netherlands. SSTs are still well above 10°C and a well mixed BL is forecast to spread inland. LL lapse rates are enhanced and in combination with steep mid-level lapse rates, deep convection is possible. The main risk will be severe wind gusts as winds at 850hPa of 25-30m/s could be easily mixed down.
Another risk will be a tornado threat as surface friction takes its toll on the LL wind field and enhances shear and storm relative helicity (0-1km shear of 10-15m/s and SRH1 of up to 200 J/kg). This threat extends well inland and so does the level-1 .

b)
The next focus for a few thunderstorms will be areas, where topography enhances updraft strength ( which could offset the aforementioned more negative aspect of weak LL CAPE and supporting more robust instability values at the mid-levels due to very steep lapse rates ).

c)
Right now the main concern arises along a rapidly southward shifting convergence zone ( it looks like a back-bent occlusion ), affecting extreme northern Germany at 06Z and central and eastern Germany between 9 and 15Z. This would yield the possibility of a line of thunderstorms with an enhanced risk for severe to damaging wind gusts and tornadoes, as shear and helicity values are considerably enhanced. Global models like GFS dismiss this scenario and we can't ignore potential foehn effects downstream of Scandinavia and slight warming at all levels so we did not issue any level area but probabilities have to be enhanced significantly if storms indeed form.
Marginal hail could accompany stronger storms.

The main severe weather risk in all areas will be a severe wind gust threat not necessarily bound to electrified convection. Please refer to the latest statements of your local weather office!


..Parts of Poland, the Czech Republic and northern Austria...

The focus for a few thunderstorms will be along the eastward moving convergence zone ( discussed in c) ) and will affect those area between 16 and 00Z. Instability values are low, but deep convection is likely due to very cold temperatures especially at mid-levels. As winds at 850hPa stay at 25m/s and shear at lower-levels is enhanced we can't exclude an isolated tornado and a considerably enhanced severe wind gust risk! A level-1 could be issued if later sounding informations and model data reveal a better environment for an higher thunderstorm coverage.
Marginal hail could accompany stronger storms.

The main reason why we did not issue a broad level area for the aforementioned regions (due to the severe wind gust threat) was that we want to make a clear distinction between gusts, accompanied by thunderstorms and the more straight-line and very intense wind field.

...Aegean and Ionian Sea...

Surface pressure is forecast to gradually fall over a broad area and a cooling atmosphere should support low-end to moderate instability release mainly offshore. The wind field and hence shear won't be outstanding strong but LL shear along the coast could locally support an isolated tornado risk mainly over the southern Adriatic / northern Ioanin Sea, where 0-3km CAPE values show a rapid increase during the evening and night hours.

...Greece, Albania, Macedonia, and NW and western Turkey...

A developing surface depression west of the Black Sea results in a constantly moistening boundary layer over the area of interest and in combination with gradually cooling mid-levels, moderate instability release looks increasingly likely. LL shear is augmented ( 10-15m/s and up to 200J/kg SRH1) and a few tornadic cells could evolve out of this set-up. The main risk will be over Greece and NW Turkey. The latter one also has an enhanced severe wind gust risk as winds at 850hPa accelerate to 25m/s.

...Central Baltic Sea...

LL lapse rates under the depression are not impressive but should be steep enough for some instability release and as shear stays weak, waterspout could develop along mesoscale boundaries, under stronger convective updrafts and along the coast. Overall set-up is not supportive for issuing a level-1.

...West Norway...

Although history showed us that many CAA events do not result in any electrified convection at this time of year, parameters all hint on a risk for at least an isolated thunderstorm event. Temperatures at all levels cool down significantly and deep convection can be expected ( also supported by very cold EL temperatures ). Thunderstorms could produce an isolated tornado although marginal hail and gusty winds should be the main risk.