Storm Forecast
Valid: Mon 08 Apr 2013 06:00 to Tue 09 Apr 2013 06:00 UTC
Issued: Sun 07 Apr 2013 19:16
Forecaster: TUSCHY

SYNOPSIS

Despite long-range forecast data gradually showing an improving polar vortex structure and a persistent shift of the coldest air mass from Scandinavia to the west, this change won't have any serious issues on today's outlook. However, a somewhat more active thunderstorm period could gradually evolve over W/C-Europe during the end of the week with transition period in full progress.

Today, no major surprise is found on the maps with a far south displaced frontal zone running from Spain to N-Africa although a gradualy northward shift can be already seen as the next mid-level impulse joins a bit more to the north.
A broad upper trough over SE Europe shifts east while filling wheras the bitterly cold cold-core vortex over N-Europe keeps its strength during its leisurely eastward shift.

The higher dewpoints remain confined to the Mediterranean although some BL moisture improvement is expected over France with the forecast pattern shift towards a more zonal flow regime.

DISCUSSION

... France ...

Latest synop data show surface dewpoints around 0°C over most of France with 5-7 °C offshore over the Bay of Biscay and N-Spain (7th, 19Z). GFS is the most bullish model regarding BL moisture recovery with EZ being a bit more reluctant. We see no reasons why arguing with GFS although this thin layer of best moisture will be prone to diurnal mixing. Hence, for now we kept EZ as the most likley outcome for most of France excluding the immediate proximity of a surface front, where moisture pooling may push dewpoints up to the GFS forecast.

A filling upper wave crosses N/C-France during the daytime hours from west to east, while a NW-SE aligned surface pressure trough sets-up. An occlusion accompanies that surface pressure trough with a gradual shift to the north during the day, crossing all of France until the evening hours. Postfrontal mixed maritime air mass (better BL moisture) and modest mid-level lapse rates due to the eastward travelling mid-level wave result in a broad area with low-end CAPE build-up. Highest probabilities for a few short-lived storms arise along the N-ward moving occlusion, where BL moisture pooling occurs but also over E-C France, where LL convergence markedly increases during the late afternoon/evening hours. Despite 10-15 m/s DLS, the main activity will be a pulsating one with strongest storms producing marginal hail/sleet and gusty winds. A very isolated more pronounced updraft is still possible, given EL temperatures below -30°C, so a very isolated large hail risk can't be ruled out mainly over N-C France. With nocturnal BL stabilization rapidly starting after sunset, the thunderstorm activity rapidly vanishes during the early night hours and onwards. A sporadic storm or two are also possible south of the lightning area, but we kept the lightning area to the region with the best overlap of parameters.

Far SE France and far NW Italy were also added to a different lightning area, as a few short-lived thunderstorms are possible during the passage of a weak mid-level wave (12-18Z). Nothing severe is expected with that activity.

A stronger upper wave crosses the N-Bay of Biscay during the night and approaches NW France until 3 to 6Z. Favorable placement in the left exit region of an healthy 45 m/s mid-level jet and a rapid cool-down at mid-levels next to the influx of maritime Atlantic air may prepare another possibility for isolated thunderstorm development over W-NW France during the second part of the night. With 850 hPa winds increasing to 20 m/s, strong wind gusts will be the main hazard with that activity beside marginal hail/sleet.

... W-Turkey ...

Despite the upper trough departing to the east, a weak surface depression just offshore of SW-Turkey slows the soutbound moving cold front down, so enough BL moisture exists during the daytime hours for isolated to scattered thunderstorm activity...probably until the onset of the night. Weakly capped 200-500 J/kg MLCAPE and weak shear assist in a few pulsating storms with marginal hail and gusty winds. The activity rapidly winds down after sunset, excluding far SW Turkey, where ongoing showers/sporadic thunderstorms may last well into the overnight hours.

... NE-Poland to Estonia ...

A weakly defined cold front crosses those regions from NW to SE during the forecast period. Weak prefrontal moisture advection increases mixed-layer mixing ratios up to 4g/kg, which is enough for some low-end CAPE build-up, given 500 hPa temperatures well below -30 °C. Isolated and short-lived thunderstorms with marginal hail are forecast. 15-30 m/s DLS exists (increasing from north to south), but convection remains low-topped due to scarce moisture support, so cloud bearing shear will be significantly lower. The activity will rapidly diminish after sunset.