Storm Forecast
Valid: Sun 26 Jun 2011 06:00 to Mon 27 Jun 2011 06:00 UTC
Issued: Sat 25 Jun 2011 16:46
Forecaster: TUSCHY

A level 1 was issued for parts of Portugal and NW-Spain mainly for large hail (significant events possible) and strong to severe wind gusts.

A level 1 was issued for parts of UK mainly for strong to severe wind gusts, isolated large hail and an isolated tornado risk.

SYNOPSIS

The structure of a large-scale trough at roughly 20° W remains a bit difficult to analyze with numerous short-waves exiting/entering that feature during the forecast. Overall, a slow movement to the east is forecast and the trough axis will cross the 20th degree of longitude during the end of the day. Further amplification and an overall neutral tilt allow low-end impact to the European thunderstorm forecast (despite UK). However, ahead of this trough, a disturbance, centered over the Canary Islands, is expected to become absorbed by the approaching trough. This disturbance lifts to the north just west of Portugal although models diagree in the final placement of this feature.
Large-scale ridge downstream affects W/CNTRL Europe with hot and dry conditions. A deep cyclonic vortex becomes quasi-stationary over the Black Sea and results in unsettled conditions over far SE-Europe.

DISCUSSION

... Portugal and NW-Spain ...

Main problem is the missing/weak forcing, which is expected for those areas. In fact, all models agree in the dissolving of the disturbance of the Canary Islands just west of Portugal. Nevertheless, weak cyclonically curved flow affects the region during the night hours and may allow a few thunderstorms to develop. Also, rough terrain may also permit a few thunderstorms to overcome the strong cap during the evening and early night hours.

Deep, near dry-adiabatic profiles (e.g.850 hPa - near 500 hPa) are forecast as EML spreads north/northeastwards. We don't expect that weak onshore advection of a maritime Atlantic air mass may erode cap enough for any surface-based initiation, but 1000-1500 J/kg MUCAPE may likely evolve with large mid-level CAPE profiles.

15-20 m/s 0-6 km bulk shear and veering within ongoing WAA regime in that CAPE environment support a favorable set-up for severe to damaging hail. EZ and GFS both show initiation around 18Z with EZ decreasing the activity after 21Z, whereas GFS keeps further thunderstorm initiation alive well after midnight (stronger forcing). NOGAPS on the other hand sees the highlighted area capped and dry. For now, we can't exclude at least an isolated thunderstorm and hence we went ahead and issued a level 1. Nowcast of the "Canary Island disturbance" will play a major role regarding final thunderstorm coverage. Any discrete and longer-lived thunderstorm has the potential to produce large to very large hail (> 5cm). Also, well mixed and deep subcloud layers promote a risk for severe downburst events with high-based convection. Thunderstorms move off the coast and may affect parts of the Bay of Biscay during the night.

... Parts of central UK ...

Model discrepancies remain significant regarding QPF with EZ the drier outlier and GFS/WRF the more bullish ones regarding initiation. Persistent moisture advection from the south and the tip of the northeastward spreading EML create an environment with modest MLCAPE (500 - 800 J/kg) and a weak cap. Also mid-level forcing will cross the highlighted area from the southwest during the evening hours onwards, so at least isolated thunderstorms may evolve. Thunderstorms would be within a favorably sheared environment (20 m/s DLS, 15-20 m/s 0-3 km shear, modest LL speed/directional shear), so isolated large hail, strong to severe wind gusts and an isolated tornado event may occur with strongest thunderstorms. In fact, would not be surprised to see an enhanced tornado risk with locally more than 50 J/kg 0-3 km SBCAPE and large-scale veering of the 850-500 hPa wind field next to very low LCLs. Isolated thunderstorms move rapidly off to the E/NE with dry conditions thereafter.
EZ breaks out some weak precipitation signals over far SW-UK during the morning hours of the 27th, but this remains an outlier for now, so do not highlight that in this outlook.

... Further areas of interest ...

The gradually intensifying depression over the Black Sea may push thunderstorm clusters from E-Ukraine to the west during the end of the forecast with heavy rainfall. However, there are not yet any indications for a concentrated excessive rainfall event, matching our level criteria.

Also, GFS has some weak convective precipitation signals over south-central France during the past few runs. However, to break the cap with 850hPa temperature at or above 20°C, we indeed need those GFS magical surface dewpoint numbers of 20 °C and above (roughly 10K bias compared to ECWMF during peak time heating). We will leave this area without a lightning area for now.