Extended Forecast
Valid: Sun 14 Jun 2009 06:00 to Mon 15 Jun 2009 06:00 UTC
Issued: Fri 12 Jun 2009 19:50
Forecaster: TUSCHY
A level 1 was issued for E/NE-France, Luxembourg, Belgium, SE-Netherlands and most parts of Germany mainly for large hail and severe wind gusts.
A level 1 was issued for Portugal and extreme NW-Spain mainly for large hail and severe wind gusts.
A level 1 was issued for W-Russia mainly for large hail and severe wind gusts.
SYNOPSIS
Downstream of an approaching NE-Atlantic trough, WAA overspreads western/central Europe with hot and increasingly unstable air. Another upper trough over E/SE-Europe stays put with ridging in between those features bringing hot and stable conditions to the central Mediterranean.
DISCUSSION
... Central/E-France, Switzerland and SW-Germany ...
The pattern change over the past few days culminates in this configuration with flat ridging over the central/western Mediterranean and a slacking depression just west of Ireland. A brisk SW-erly flow pattern will establish over the Iberian Peninsula and France with embedded short waves rotating to the NE/ENE. This configuration also allows a far northward advance of the Spanish plume, although fanning out over France. In fact, 2-4km lapse rates barely topping 7K/km is not that impressive, but ought to be more than adequate, given the moist/warm boundary layer.
The potential flaw of GFS06Z (12th June) with surface dewpoints up to 25°C over central France gave way to more realistic dewpoint forecasts, which are also in line with EZMWF. The evening surface data of the 12th already had dewpoints in the lower to mid tens along the W-coast of France but also over extreme S-France and a combination of persistent advection and evapotranspiration during the upcoming 36h ought to yield dewpoints at least in the mid to upper tens over France, probably maximized along any convergence zone.
NOGAPS and GFS agree in the approach of a vorticity lobe from the SW during the later afternoon/evening hours, although initiation will likely occur independently of that forcing as atmosphere will be weakly capped. Scattered thunderstorms are forecast over central/eastern France from the early afternoon hours onwards, maybe even moving into NW Switzerland/SW-Germany although it is not yet clear how far east initiation will occur.
Given MLCAPE probably around 1000J/kg and DLS between 10-20m/s, large hail is forecast with isolated large hail events within the realms of possibility. However, weak directional shear and modest lapse rates only justify a level-1 for now. Severe wind gusts are possible, too. An upgrade may be issued if boundary layer quality can be assessed with new data.
.. N-France, Luxembourg, Belgium and W/CNTRL and N-Germany ...
The main story for the forecast is the interaction of a rapidly eastward moving short wave over the Netherlands and N-Germany with a SW-NE aligned surface boundary, running from N-France to NW-Germany and then straight to the east to NE-Germany. Initiation along this boundary starts early due to weak capping probably already during the late morning hours and increases in coverage thereafter, also aided by the eastward moving short wave. Kinematics are strong and enough for rapid storm organisation, given DLS around 30m/s and 15m/s at lowest 3km. We're not yet sure if dewpoints in the mid-tens are able to reach N-Germany although GFS and EZMWF agree well. The current thinking is that severe wind gusts are the main risk, especially if storms ride along the E-W aligned boundary over N/NE-Germany as this whole pattern points to the possibility of bow echoes.
... Spain and Portugal ....
As the main upper trough draws near from the west during the evening/night hours, thunderstorms eventually increase in coverage and intensity. Maximized activity will likely occur over Portugal, where a better thermodynamic environment is present. However, isolated thunderstorms and probably elevated in nature are also possible over Spain during the night hours. DLS is strong enough for large hail and a level-1 was issued for those places, where initiation looks most likely. Spain has to be monitored closely as enough MCUAPE/speed and directional shear overlap for hail producing storms.
... S-Finland eastwards ...
A strong autmn-like depression is situated over N-Europe and a tongue of warm/moist air is drawn far to the west, potentially as far west as central Sweden. A rapidly northward moving cold front will be the focus for scattered thunderstorms and strong LL shear/enhanced 0-3km CAPE release indicate a good chance for tornadoes/severe wind gusts. However, the exact strength and path of this depression is crucial for highlighting the region with highest tornado chances. Consistent with decreasing instability betimes, the thunderstorm risk also vanishes over central Finland around sunset. Further to the east, better boundary moisture keeps atmosphere more unstable with ongoing thunderstorms well into the night. An isolated large hail/strong wind gusts is possible with that activity but most of it occurs too far to the east (beyond our forecast area). A level-1 for parts of Finland may be needed if the current strength and path of the depression indeed verifies.
