Storm Forecast
Valid: Fri 30 Apr 2010 06:00 to Sat 01 May 2010 06:00 UTC
Issued: Thu 29 Apr 2010 18:20
Forecaster: TUSCHY
A level 1 was issued for parts of Spain and extreme south France mainly for large hail.
A level 1 was issued for extreme S-France mainly for isolated, excessive rainfall.
A level 1 was issued for N-Morocco and NW Algeria mainly for large hail - even significant - and severe wind gusts. To a lesser extent also for excessive rainfall.
SYNOPSIS
The model consensus is good, showing a constant flattening of broad ridging over central Europe, peaking at 576gpdm over the west central Mediterranean. Further deamplification and a more zonal alignement is forecast during the following 24h, as strong lead wave, centered over N-Germany at the start, gradually rounds the ridge eastwards while weakening. The trough over the Bay of Biscay re-strengthens as another impulse moves in from the west with some amplification and an overall slow eastward progress. Another upper trough retrogrades a bit to the westsouthwest over the central Mediterranean, but it looks like no vortex drips off that configuration.
Ensemble forecasts also reflect that pattern well with some discrepancies in respect of how strong the trough over the Bay of Biscay will finally be with increasingly higher confidence further east (central Europe) in respect of a developing SW-erly flow at mid-levels.
As upper level support vanishes, a quasi-stationary surface front sets up from the Pyrenees to the Alps and merges with a cold front (up to Poland), which also slows down due to the increasingly meager angle between the background flow and the front itself. No further front is forecast in conjunction with convective interests over the rest of Europe.
DISCUSSION
... Pyrenees and surrounding regions, parts of Spain...
After the departure of the strong lead wave (over N-Germany at 06Z), the area resides in a weakly forced environment until well into the night hours, before a stronger vorticity maximum approaches from the west. However, this constellation is prime for weak short waves, ejecting out of the re-forming upper trough to the west, spreading eastnortheastwards during the forecast. GFS/COAMPS for example support that scenario with gradually improving high-level jet configurations over that region, which resides beneath a consolidating coupled jet configuration between a departing streak over France and a northeastward/evolving one over the Balearic Islands.
A weak thermal ridge affects the area during peak time heating and overall tilt of this trough is not supportive for more robust destabilization. However, good BL moisture is present, especially north of the Pyrenees, where stalling boundary nestles to the Pyrenees. Coupled to somewhat steeper mid-level lapse rates as EML to the southeast expands somewhat to the north, up to roughly 500J/kg SBCAPE south and up to 800 J/kg north of the Pyrenees will be realized.
No confined jet core affects the area, although more or less unidirectional profilers in forecast soundings reveal 15-20m/s speed shear. A few thunderstorms may attain some organization with a large hail and strong wind gust risk. The main story however will be rapid upscale growth into a cluster of storms over/north of the Pyrenees. Models disagree in respect of QPF maxima, but GFS indicates some good rain for extreme S-France for the past few runs, and the overall synoptic set up would assist a similar scenario. Despite rapid BL stabilization beneath northeastward expanding cluster, strong wind gusts, marginal hail and locally excessive rainfall are all possible due to slow storm motion. Thunderstorm coverage will decrease after sunset, as CAPE finally vanishes.
Over central Spain, convection also peaks during the afternoon/early evening hours with strong multicells possible. Large hail and strong wind gusts remain the main hazard.
... S/SE-France, SW Switzerland ...
Stalling boundary remains the focus for a few thunderstorms, but weakly capped conditions/persistent moisture advection from the south hint on an ongoing shower/isolated thunderstorm risk throughout the daytime hours, changing back to non-thundery after sunset. Any thunderstorm remains short-lived and weak in nature with no appreciable severe risk. The main story will be locally heavy rain amounts due to persistent onshore flow from the south towards the stalling boundary but also over SW Switzerland (orographic lift). No real focus for anything more widespread can be seen in current data, so no level 1 was introduced.
... NE Germany, Poland and the Czech Republic ...
Gradually weakening short wave and an eastward sliding cold front will be the focus for some convection. LL shear over NE-Germany and N-Poland is augmented with minimal SBCAPE and better instability/weak shear over the Czech Republic later on. Hence, no severe risk is forecast with that activity.
... N-Morocco...
A loaded gun-type environment gradually evolves during the day. However one ingredient is missing: lift. Hence, initiation has to await better forcing around sunset and onwards for at least isolated thunderstorm development. A northeastward spreading EML and good BL moisture along the coast result in modest MUCAPE of at or above 1000 J/kg. Kinematics improve throughout the day with DLS of 15-20m/s and some directional shear, so multicells and an isolated supercell event are well possible. Large hail events, even a significant one,are forecast next to strong to severe wind gusts before storms grow upscale into one or more clusters, which move eastwards. We will monitor further model outputs and also have to think about upgrading, if the significant hail risk becomes more widespread than currently anticipated.
Another feature of interest could be a developing surface depression to the south,enhancing the LL shear, so the tornado risk also has to be evaluated, if later model runs confirm this trend.
