Storm Forecast
Valid: Fri 13 Jul 2007 06:00 to Sat 14 Jul 2007 06:00 UTC
Issued: Thu 12 Jul 2007 16:42
Forecaster: TUSCHY

SYNOPSIS

Finally a significant change of the European weather pattern is underway. A well defined upper-level trough just west of Europe digs further towards the south, enhancing intense WAA over parts of western Europe.
Another upper-level trough over eastern Europe shifts towards the east, while a broad high pressure area over the Mediterranen constantly gains influence, while expanding towards the north / northeast.

DISCUSSION

.... Central Russian Uplands up to an area S / SW of the White Sea and SE Finland...

Conditions for an outbreak of severe thunderstorms over a broad area will come together with all modes of severe weather being likely, including large hail, severe wind gusts and tornadoes !

The area of interest has to be split up because of the widespread nature of this event and nearly simultaneous thunderstorm initiation.

...Central Russian Uplands...

The focus for thunderstorm development will be a slowly eastward shifting and wavy cold front. There are indications that a weak surface depression well towards the S / SE of Moscow can evolve as an intense upper-level streak enters the area of interest from the south.
A way more diffuse situation for thunderstorm initiation arises when investigating the upper-levels. Numerous disturbances and linked UVV fields rotate around the main trough although it's not exactly clear how strong the convective feedback (problem) of the global models is which results in a pretty washy idea how strong each disturbance will finally be.

Latest thinking is that yesterday's storms along the cold front will go on and re-strengthen during the morning hours, as instability builds up in front of this boundary. Yesterday's surface synoptic data were sparse but showed temperatures around 30°C and dewpoints of 10-15°C. GFS developed a broad area of more than 11g/kg mixing ratios and in combination with at or above 7K/km lapse rates at mid-levels more than 1000 J/kg MLCAPE and 2000 J/kg SBCAPE can be released.
30m/s DLS and enhanced LL shear ( although depending on the final strength of the LL depression ) pose a severe wind gust and large hail risk. A few tornadoes are possible if a more backed wind field / up to 17m/s LL shear verify.

...SW / S of the White Sea and parts of Finland ...

Again...the main focus for thunderstorm initiation will be the eastward moving cold front. Latest model runs consistently showed the evolution of a more robust LL depression along the wavy frontal boundary south of the White Sea and this should support a significant increase in LL shear with up to 20m/s just south / east of the White Sea area. Right now it looks like most parts of the warm sector will be capped so best shear will be well displaced from the strongest thunderstorm activity.
DLS of 20-25m/s and LL shear up to 10m/s will be present along the front with an attendant severe wind gust risk. Impressive SRH values at BL- 3km and nice mid-level lapse rates create a favorable environment for large hail with very large hail being possible.
Environment will be conducive for each more discrete thunderstorm to become tornadic.

All this said we expect a few squallines to cross the level 1 / 2 area from the S / SW . Warmer mid-levels along the eastern part of the level areas would also favor more discrete supercell development with the aforementioned risks.

...Central part of Sweden and Norway...

A pool of cold mid-level air over the area of interest, strong signals of augmented 0-3km CAPE release, a westward moving convergence zone and weak DLS will all support an enhanced risk for a few funnel / short-lived tornado reports during the day-time hours.

... Ireland and United Kingdom...

An intense LL depression is forecast to develop SW of Ireland, moving rapidly towards the NE. Attendant warm front will cross UK from the south and should reach Scotland during the night hours. A broad shield of stratiform rain will keep temperatures in the lower tens (western / central UK ) and no surface based convection expected during the day .
Chances for a few thunderstorms should increase after 00Z as cold front and dry slot enter SW UK / SE Ireland. DLS of 25-30m/s will be present and each convective segment could support an enhanced downward transport of stronger winds. No level area was issued because confidence is growing that no more than an isolated thunderstorm will occur.

... NE-France, Belgium, the Netherlands and extreme western Germany...

Aforementioned LL depression over UK supports not only an intense warm-up process over the area of interest but also a significant increase of surface dewpoints. Yesterday afternoon,dewpoints of 15-19°C were common over NW France while spreading eastwards.
Mid-levels will stay warm, but strong diabatic heating ( ~ 25°C ) and dewpoints in the upper tens should help to release some low-end instability. EL values also indicate that deep convection will be indeed a distinct possibility and DLS of 15m/s and up to 200 J/kg SRH3 values will be present, too.
Belgium should be the focus for a few severe thunderstorms with large hail as the main threat, if trends in latest model output won't change significantly.
Even an isolated tornado can't be ruled out as LL shear constantly increases during the late afternoon / early evening hours and LCLs stay low.