Extended Forecast
Valid: Sun 30 Aug 2015 06:00 to Mon 31 Aug 2015 06:00 UTC
Issued: Fri 28 Aug 2015 19:54
Forecaster: TUSCHY

A level 1 was issued for parts of Spain, NE Portugal and W-France mainly for large hail and severe wind gusts.

A level 1 was issued for N-France, Belgium, the Netherlands, N-Germany and the SW Baltic Sea (including surrounding areas) mainly for severe wind gusts, large hail and an isolated tornado threat.

SYNOPSIS

Large-scale European synoptic pattern features a positive tilted longwave trough over Scotland and Cape Finisterre. No significant change of its geometry is forecast during the eastward shift. Numerous vorticity maxima circle that trough with ongoing inhomogeneous timing/placement in model pool.

Pronounced high pressure with a deep warm-core remains in place over SE Europe. Its western fringe features somewhat cooler mid-levels, which ensures enhanced probabilities for DMC along the frontal zone.

Elsewhere, no organized DMC is forecast in Europe due to separation of CAPE/shear fields.

DISCUSSION

... Spain to France to N-Germany ...

DAAG 12Z (28th) samples EML with deeply mixed mid-level layer. EML plume spreads north during the following 24-48h with GFS forecasting 7-8 K/km over Spain and 6-7 K/km further north.

The Iberian Peninsula experiences prolonged period of onshore flow of rich maritime (W-Mediterranean) moisture with 0-1km mixing ratios in excess of 17 g/kg. Further north (e.g. N-France, Benelux and N-Germany), main moisture source will be a stalled front (CNTR-France to S-Germany to S-Poland on Saturday morning), which shifts north as a warm front during the day of Saturday and becomes quasi-stationary again on Sunday. It acquires a SW-NE orientation and runs from Brittany to N-Germany. Current model guidance has a good handling of moisture along that front, so confidence in moisture forecast on Sunday is quite high. Peak values occur along wavy frontal zone with embedded weak waves.

DLS beneath frontal zone is quite strong with 20-25 m/s and similar values in the 0-3 km layer. Main question will be how strong NE-ward traveling waves will be, which could induce regionally enhanced overlap of shear/CAPE. Same for LL shear, which offers peak values in the immediate proximity of the wavy front.

Forcing remains the most uncertain parameter that far out. In general, thickness tends to increase a bit during the day before decreasing again from SW to NE during the evening and night. Latest Q-vector forecasts show strongest impuls exiting the base of the main trough during the morning hours to the NE and affecting the N-Sea beyond sunset. This feature seems to be displaced to the north of the front. Further east, numerous weaker waves cross the front during the day and insert numerous regions with temporarily and regionally enhanced thunderstorm probabilities. In addition, diurnally enhanced thermally direct circulation of roughly coast-parallel front may add further foci for CI. Placement of that front differs in models with some keeping it just offshore and others more over Benelux and NW Germany.

Shear/CAPE magnitude with shear vectors aligned parallel to front support rapid upscale growth of thunderstorms with small bowing line segment of organized convection along the front. Swaths of severe wind gusts, large hail and an isolated tornado risk would be the main hazard. Convection, which evolves further east of the front experiences stronger cap/weaker forcing, which would increase the risk of more discrete convection. Large hail and severe wind gusts would be the main hazard. Progressive nature of convection should keep the risk of training convection on the low-end side.

In general, confidence in organized DMC is highest along the front and hence the level 1 was issued for that region. Further broadening of the level 1 area may be needed if confidence in CI further east increases with later model runs. Cap strength has to be monitored closely.

Convection over Spain and W-France seems to be mainly diurnal driven during the day with enhanced dynamic CI during the evening and night as the main trough approaches from the W. Forecast soundings show 10-15 m/s DLS and steep lapse rates, so large hail and a few severe downbursts are possible.
SE Spain offers impressive CAPE values, but strong cap at 800 hPa and weak synoptic-scale lift should keep CI probabilities low.