Storm Forecast
Valid: Fri 03 Mar 2017 06:00 to Sat 04 Mar 2017 06:00 UTC
Issued: Thu 02 Mar 2017 14:43
Forecaster: TUSCHY

A level 1 was issued for W-France mainly for a few large hail and an isolated severe wind gust event.

A level 1 was issued for the NW Mediterranean mainly for a few severe wind gusts and an isolated tornado event. A very isolated large hail event is possible.

A level 1 was issued for an isolated tornado and severe wind gust event over SW Portugal.

SYNOPSIS

Two prominent progressive longwave troughs affect parts of Europe. The first one over E-CNTRL Europe is broad and encircled by rather weak mid / upper jets. It ingests a dry and stable air mass. Hence despite steep mid-level lapse rates, no substantial CAPE build-up is expected for thunderstorm activity.
The second one approaches the Iberian Peninsula and the Bay of Biscay from the west. This sharp trough continues to amplify while an intense 45 m/s mid-level speed max dives south towards its base. This is also well reflected in PV fields, which highlight a more negative tilt of the trough axis during the end of the forecast period. In between, mid-level ridging affects an area from the CNTRL Mediterranean to CNTRL Europe.

Looking down to the lower troposphere, an extensive cyclonic vortex is placed over the far N-Bay of Biscay with falling surface pressure forecast over the far W-Mediterranean, as the sharp upper trough approaches. High pressure dominates SE Europe.

DISCUSSION

... W-France, the Iberian Peninsula and the W-Mediterranean ...

An extensive warm sector spreads out over Spain and W-Europe between a northbound moving warm front over the English Channel and Benelux and a leisurely eastbound moving cold front over the Bay of Biscay, which also becomes more wavy in response to surface cyclogenesis over extreme SW France during the night. Strengthening deep-layer flow, which points near normal to the Pyrenees results in a descending/warming column of mid-/low-tropospheric air north of the Pyrenees. This supports a constant steepening of the mid-level lapse rates. A plume of well mixed air spreads north towards W France during the day. In addition, an EML plume advects N/NE from Morocco and Algeria and overspreads E Spain and the far W-Mediterranean during the forecast. A limiting ingredient will be the quality of the marine layer over the W-Mediterranean which features low BL mixing ratios. Persistent onshore advection with brisk S/SE-erly surface flow is forecast over NE Spain and SW/S France, which enhances the BL moisture during the day. MLCAPE of 200-600 J/kg is forecast in those regions especially with temporal diabatic heating. CAPE maximum will be during the afternoon hours over W-France and during the night along the coastal areas of the NW-Mediterranean.

0-6 km bulk shear in excess of 25 m/s and 15-20 m/s 0-3 km shear both support organized multicells/isolated supercells. LCLs of 1-2 km over W-France indicate rather high-based convection, so a few large hail events will be the main hazard. The hail risk may be maximized in rotating updrafts, which are possible with 0-3km SRH in excess of 150-200 m^2/s^2. Gusts should stay sub-severe in most cases although an isolated severe downburst event can't be ruled out due to potential downdraft enhancement by evaporative cooling in the well mixed subcloud layer. Rapid clustering of storms should switch the aforementioned severe risk more to a localized heavy rainfall hazard. The severe risk diminishes after sunset.

Betimes, more elevated convection spreads N/NE over N-France, as weakening MUCAPE plume advects north. An isolated large hail event is still possible. Localized nighttime thunderstorm activity is possible all the way to Benelux, far NW Germany and the S-North Sea but very slim MUCAPE values should keep activity very isolated and sub-severe.

Active convection beneath the base of the upper trough over Portugal and W/NW Spain should result in graupel and gusty winds. The cold low/mid-level column of air combined with a few strong updrafts (up to 400 J/kg SBCAPE) could even result in an isolated hail event. The risk diminishes rapidly around sunset with loss of daytime heating. A short-wave approaches SW-Portugal during the end of the forecast with ongoing model discrepancies regarding wave amplitude and attendant WAA. Current idea is that at least low-end MUCAPE materializes in the level 1 area, where a very isolated tornado and severe wind gust event can't be ruled out. Despite forecast soundings offering more stable inland soundings, neutral stratification along the coast and just inland could support downdrafts, which could bring 25-30 m/s wind gusts down to the surface. This level 1 remains very uncertain and depends on the final track, amplitude and timing of the wave (with GFS favored right now).

Nocturnal convection along the coasts of NE Spain and S France will see lower LCLs with strong LL speed and directional shear, so an isolated tornado event is possible. In addition, LLCAPE increases from W to E which supports an isolated tornado risk. Local moisture pooling along the coast could result in elongated CAPE profiles, with deeper updraft growth. This would support the risk of temporarily rotating updrafts with an isolated large hail and severe wind gust risk. Betimes, this activity should grow upscale into an extensive cluster, which rides east along the S-coast of France from W to E.

We expanded the level 1 far to the SE over the W-Mediterranean to account for some thunderstorms along the eastbound racing cold front with 200-500 J/kg MUCAPE. A 25 m/s speed max at 700 hPa comes with the cold front, so severe wind gusts are possible (e.g. 20-25 m/s 0-3km shear over the Balearic Islands during the cold front passage). Combined with intense LL shear, an isolated tornado event can't be ruled out, although LLCAPE will be meager. The SE fringe of that level 1 remains unclear as cap strength diverges in latest model data and some eastward expansion is possible.