Storm Forecast
Valid: Fri 14 Mar 2014 06:00 to Sat 15 Mar 2014 06:00 UTC
Issued: Thu 13 Mar 2014 18:52
Forecaster: TUSCHY

A level 1 was issued for severe wind gusts for the N-North Sea and parts of W-Norway.

A level 1 was issued for NE Algeria and N-Tunisia mainly for excessive rain.

SYNOPSIS and DISCUSSION

The shift of the longbreathed high pressure area from central Europe to the northeast Atlantic culminates in a change of the steering pattern over N/CNTRL Europe. A mid-layer wave, centered beneath the left exit of an intense 90 m/s jet at 300 hPa, experiences persistent upper divergence. Combined with a tight temperature gradient along the NW/SE aligned frontal zone, a rapid deepening of the vortex is forecast (e.g. a surface pressure drop of 20-25 hPa/ 24h, dependent on the model and model run). Attendant cold front crosses the North Sea and approaches Denmark/N-Germany during the second part of the overnight hours.

A tongue of strongly modified subtropical air along/ahead of the cold front, intense forcing, placement in the favorable exit region of a 50 m/s mid-layer streak and a SE-ward pushing dry slot should cause a forced line of convection along the cold front. The highest risk for an organized line with deeper updrafts exists from Scotland to the E/SE. 850 hPa winds in excess of 30 m/s and no serious friction in the boundary layer (offshore) likely cause a swath of severe wind gusts. We thought about adding a level 2, but warm EL temperature forecasts and SSTs well below 10 °C increase uncertainty how strong instability build-up/updrafts will be.
Over Norway, decreasing prefrontal moisture, onset of 30-35 m/s gradient flow in response to the wrap-around occlusion and rough orography make it a) difficult to distinguish between severe wind gust due to the tight gradient or convectively enhanced downdrafts and b) unlikely that a solid line of convection will spread far inland. Severe wind gusts remain likely and a rapidly fracturing convective line adds to some enhanced gust potential. Therefore the level 1 was expanded well inland.
For the S-North Sea, Denmark, NE/NE Germany and NW Poland, the SE-ward surging front moves beneath the mid-layer jet and therefore in more stable conditions and away from mid/upper divergence. A cold-frontal rainband affects those areas and with 850 hPa winds in excess of 25 m/s, severe wind gusts will be possible all the way to central Poland until 06Z. No deep moist convection is forecast to accompany the band so we kept those regions out of a level 1. History however proved that isolated lightning strikes can occur well inland and in the middle of the night (mainly where orographic lift comes into play).

Over NE Algeria and N-Tunisia, latest synop data confirm rather high BL moisture with surface dewpoints in excess of 10 °C. A combination of gradually cooling mid-levels (500 hPa less than -20 °C) and some diabatic heating result in widespread 300-600 J/kg MLCAPE. No shear is forecast, so pulsating storms with rapid upscale growth/clustering are expected. Heavy rain and sleet will be the main hazard. A rainfall level 1 was issued due to storm motions of less than 10 kt and effective PWs in excess of 15 mm. Flash flooding is well possible. Besides, an isolated funnel event is also possible due to a quite unstable 0-3 km layer.

Forecast soundings from south and central Finland show neutral to slightly unstable profiles in a strongly sheared environment. In fact, shear seems to bee too strong for sustained updrafts with only 50-100 J/kg CAPE forecast. Still, showers and spotty short-lived thunderstorms may enhance downward momentum with severe wind gusts at the surface. A lightning area and an equivalent level 1 (for that kind of shear)were considered, but confidence in initiation remained too low. Still, rather steep LL lapse rates and 20-25 m/s near BL flow indicate a strong to severe wind gust risk with any stronger/more persistent updraft.

For the rest of Europe, quiescent conditions regarding deep moist convection persist.