Storm Forecast
Valid: Mon 01 Jul 2013 09:00 to Tue 02 Jul 2013 06:00 UTC
Issued: Mon 01 Jul 2013 09:56
Forecaster: PISTOTNIK

A level 1 was issued for Estonia and E Latvia for severe wind gusts.

SYNOPSIS

A change in the large-scale weather pattern is underway: The disintegrating European long-wave trough and the deep warm air advection regime over far Eastern Europe finally give way to a more zonal flow which enters the scene from the Northern Atlantic. The leading feature of this change is a pronounced short-wave trough which moves across the Baltics into Southern Finland, while it gets an increasing negative tilt as it turns into the remnants of the Southerly flow over Northeastern Europe. A second short-wave trough moves from the British Isles into Denmark and Northern Germany. However, its amplitude and the strength of the attendant frontal system are dampened by broad warm air advection which follows up from the West. Quiescent conditions in a warm to hot air mass previal across the Mediterranean region.
With this change, the focus for deep convection starts to leave Eastern Europe and to disperse across the rest of the continent again. Various regions will see moderate thunderstorm activity on Monday with a low-end severe weather threat.

DISCUSSION

... Baltics ...

Strong QG lift overspreads the region ahead of the leading short-wave trough. Fairly steep low- to mid-level lapse rates and a few hours of insolation will result in a narrow band with a deep convective boundary layer and neutral to slightly unstable profiles aloft. Forecast models agree on the appearance of a belt with weak CAPE in an environment of very strong deep-layer shear (25-35 m/s).
The quick separation of low-level forcing, which moves eastward with an occlusion, and upper-level forcing, which moves northward with the short-wave trough, keeps the time frame with a favorable overlap of ingredients quite limited in space and time and speaks against the development of a longer convective line. The most likely scenario is a handful of strongly forced multicells or short bowing lines with a risk of severe wind gusts, which can develop in Estonia and Eastern Latvia in the noon to afternoon hours and may reach the coast of Southeastern Finland by evening before they start to weaken again.
Limiting factor is the low amount of boundary layer moisture, which will keep CAPE very limited and may award the convection a more winter-like character with low cloud tops and weak lightning activity. However, since even non-electrified convection may produce (marginally) severe wind gusts, a low-end level 1 is issued.
A second round of isolated to scattered postfrontal convection will develop under the base of the trough over Western Lithuania and Latvia later in the afternoon. However, by then vertical wind shear will drop too low to promote an enhanced severe weather risk any more.

... Finland via Russia into E Ukraine ...

Along a shallow surface trough associated with an old thermal ridge, rich boundary layer moisture (dew points in the upper tens) and some hours of insolation will allow the buildup of low to moderate CAPE again. Scattered afternoon thunderstorms are forecast, though activity will be less widespread than in the previous days due to cold air advection in the lower troposphere. Vertical wind shear will be too weak to support a noteworthy severe weather risk.
The convergence zone sharpens somewhat in a belt across Central Finland, where a marginal risk of excessive precipitation could evolve with a storm motion that is almost parallel to the boundary. However, highest CAPE is displaced to the Northeast and latest satellite imagery shows extensive cloudiness over the region of interest, hence it is doubtful how much convection will be involved with this precipitation. For the same reason, it is also considered unlikely that strong convergene and/or the partial overlap between CAPE and enhanced low-level shear (>10 m/s) which appears in some forecast models can be fully converted into briefly rotating updrafts, though the brief spin-up of a weak tornado cannot be ruled out in the very moist boundary layer. All in all, both threats are considered too low for a level 1.

... France, Belgium, Luxemburg, Germany ...

Steepening lapse rates overspread the region with the onset of warm air advection from the West, and low-level moisture accumulation increases along the trailing cold front which slowly looses its characteristics. Forecast models (in particular) show CAPE buildup in the order of 400 to 800 J/kg towards evening, which has often shown to be overly optimistic in similar situations in the past. Scattered afternoon and thunderstorms are forecast mainly along the frontal boundary, but since vertical wind shear does not increase before nightfall and warm air advection plus limited insolation keep the time frame for surface-based storms short, severe weather is not particularly likely.
The increasing lift provided by the approaching short-wave trough will likely keep scattered and elevated convection going on into the night hours. ECMWF shows a particularly sharp trough axis and consequently simulates upscale growth of overnight convection into an MCS, which shall travel across the Northern half of Germany and into Western Poland by Tuesdays morning. While a background of sufficient large-scale lift support and increasing deep-layer shear (15-20 m/s) makes this scenario well possible, limited CAPE release and the elevated nature of this feature should still make severe weather unlikely. A marginally severe rain event cannot be ruled out, though.

... Alps, Pyrenees, Atlas mountains ...

Warm air with steep lapse rates and plentiful insolation allows the regional buildup of moderate CAPE where rich low-level moisture from humid forelands or sea surfaces can be ingested into upvalley and upslope circulations. Isolated to scattered afternoon thunderstorms are forecast. Large hail and marginally severe downbursts are possible with the strongest storms, but dry environmental air and a lack of more organized lift decrease the confidence in strong and sustained updraft development. Storms will likely stick to orographic features and will mostly decay again around sunset.