Storm Forecast
Valid: Tue 17 Jul 2007 06:00 to Wed 18 Jul 2007 06:00 UTC
Issued: Tue 17 Jul 2007 10:31
Forecaster: VAN DER VELDE

SYNOPSIS

A thermal trough extending southeastward over southern Scandinavia from a low pressure area over Norway and British Isles is the main feature of today. The associated cold front overrun by strong differential vorticity advection at the mid/upper level shortwave trough moves into the unstable warm sector causing numerous thunderstorms over western Poland into Sweden, occluded into Norway, while a convergence zone near the warm front over the Baltic states into northern Ukraine provides a secondary focus for convection, occluding later during the period.
The cold front has become parallel to the flow with an axis of moisture remaining north of the Alps into eastern-central France. Strong southwestern flow occurs through a region from France into southern Scandinavia.

DISCUSSION

...SE Sweden...

Strong dynamic forcing associated with the shortwave trough is likely to sustain a line of convection at the cold front during afternoon into the evening. Theta-e plots suggest the front is more established at mid levels, causing potential instability to be present at/after its passage. Moderate to strong deep layer shear (20 m/s) and moderate low level shear (> 8 m/s) aid in storm organisation and may develop rotating updrafts with chances of large hail and an isolated tornado (LCLs low) but the primary threat given the linear forcing is a squall line with potentially fairly widespread severe gusts. The line should cross the Baltic Sea while weakening, and reach into Estonia and southwestern Finland.

...W Poland...

While GFS develops barely any convective precipitation here, 00Z soundings of Lindenberg and Greifswald as well as GFS's own signal of a deeply uncapped boundary layer with more than 1000 J/kg MLCAPE and deep convergence should be sufficient for a couple of potent storms, likely long-lived multicell clusters (moderate deep layer shear and somewhat enhanced SREH) with a good chance of large to very large hail and severe gusts, aided by the dry boundary layer.

...Lithuania, Latvia, Estonia...

While a source of forcing is barely available (the warm front, if anything), 00Z Kaunas sounding has indicated a very unstable setup with possibly 2000 J/kg MLCAPE during the day. It seems likely from GFS that the weak capping must result in a few storms, confirmed by some precipiitation signals. At the warm front SREH is enhanced to over 100 m2/s2, and shear increases further towards the evening when the cold front encroaches onto the warm sector. The same kind of storms as in Poland may be expected, i.e. strong multicell and perhaps a supercell storm, with chances of large hail and severe gusts.

...E France into S Germany...

In this area there is 25-30 m/s deep layer shear and locally more than 200 m2/s2 SREH, but marginal instability. Isolated storms may form, that can develop as supercells with a chance of large hail and perhaps a severe gust.

...Ireland...

Significant low-level instability has been indicated by GFS within the flat low pressure area, with the 00Z Valentia profile being virtually unstable all the way from the ground up, with very weak winds near the surface. This is very favourable for land/waterspouts. To a lesser extent this is also true for the UK, but better near-surface convergence is forecast for Ireland, to provide vorticity to spin up.