Storm Forecast
Valid: Tue 04 Jun 2013 06:00 to Wed 05 Jun 2013 06:00 UTC
Issued: Mon 03 Jun 2013 22:48
Forecaster: TUSCHY

A level 1 was issued for W-Finland mainly for isolated large hail.

A level 1 was issued for NW Turkey mainly for large hail.

SYNOPSIS

An extensive low pressure area extends from France all the way to Russia with continued weak heigh gradients present over most parts of Europe. With the polar and subtropical jet displaced north/south of our forecast area no significant change in the geometry of the mid/upper level height field is forecast during the forecast period.

DISCUSSION

... C/E Europe ...

Weak height gradients remain in place, inducing a weakly sheared environment. Seasonably BL moisture beneath weak to modest mid-level lapse rates results in widespread 400-800 J/kg MLCAPE. Regional moisture pooling (e.g. along a decaying frontal boundary from C/E Poland into Belarus) may increase MLCAPE to 1000 J/kg in some places. DLS of 5 to 10 m/s assists in rapid upscale growth/clustering and numerous loosley organized storm clusters with erratic storm motions are forecast. Heavy rain will be the main risk with isolated large hail and strong wind gusts well possible mainly during the initiation stage. As seen 24h ago, a few clusters might produce local swaths of strong wind gusts due to temporary cold pool acceleration, but this risk remains too uncertain and too marginal for a level 1.
Yesterday's soundings already sampled the moist and unstable 0-3 km layer over C/E Europe. Once again, low LCLs and some LL CAPE are forecast and therefore an isolated tornado event is possible, especially along better organized convergence zones or storm interactions on the mesoscale. This also includes the offshore/coastal areas of the Aegean and N-part of the Ionian Sea. Thunderstorms gradually decay during the first part of the night, although any subtle mid-level wave or the orography itself can assist in developing convection all night long (especially over the Ukraine).

A level 1 was added for NW Turkey as DLS increases to 15-20 m/s and hence multicells/isolated supercells are forecast with large hail the main risk. The thermodynamic environment however remains marginal with 500 J/kg MLCAPE and hence only an isolated severe risk is currently anticipated.

... W- Finland ...

A deepening trough over Norway and Sweden chokes off the belt of westerlies and drops to the south as an healthy looking cold-core low with 500 hPa readings below -30°C. The model/ensemble spread of this process decreased and confidence in strength and location of this feature is reasonably high. At the surface, a wavy front extends from W-Finland to Estonia with some westward motion as a warm front expected from W-Finland towards E-Sweden. Further south, the front remains more or less quasi-stationary over Estonia. Strongest frontogenesis is forecast over Estonia but otherwise, forcing remains a bit diffuse through the forecast. The right entrance region of a strengthening mid-level jet along the eastern fringe of the upper low may induce some mid-/upper level support next to increasing cyclonic curvature of the flow.

A tight shear gradient is forecast, but at least some overlap of 15 m/s with existing 500-800 J/lkg MLCAPE tongue east of the boundary exists. Isolated CI is forecast with a few better organized multicells during their mature stages. A few large hail and strong wind gust events are possible. In fact, a few models show a temporal overlap of 1000 J/kg MLCAPE and 15 m/s DLS with thick CAPE fields in the hail growth layer, so a few hail events of 2 cm are likely with any developing storm. Also, forecast soundings/hodographs show strong directional shear along the western part of the westbound moving front with enlarged lower parts of a few forecast hodographs and SRH-1 of 100-200 m^2/s^2. An isolated tornado event along that boundary can't be ruled out although time frame for surface based storms is limited. We therefore expanded a level 1 further west. Limited thunderstorm coverage however makes this event a borderline level 1.

Thunderstorms gradually decay after sunset due to cooling lower levels and westward moving forcing.