Storm Forecast
Valid: Sat 16 Mar 2013 06:00 to Sun 17 Mar 2013 06:00 UTC
Issued: Fri 15 Mar 2013 21:50
Forecaster: TUSCHY

A level 1 was issued for SW UK mainly for an isolated large hail and tornado risk.

SYNOPSIS

A deep channel of low geopotential heights remains anchored over Europe. A cold air mass is in place over most of Europe with bitterly cold conditions ongoing over E/SE Europe. A pronounced vortex enters the scene from the west and becomes quasi-stationary over Ireland, UK and Scotland. A slightly warmer and increasingly more humid air mass starts to affect parts of Portugal and Spain and another pronounced air mass boundary sets-up over N-Portugal/Spain.

DISCUSSION

... Aegean Sea and W/SW Turkey ...

With the cold mid-level air mass still present atop the Aegean Sea, expect another day with active shower and thunderstorm development before a southbound moving cold front cuts off that risk from north to south during the overnight hours. Until then, scattered DMC is forecast with marginal hail and gusty winds. Once again, the waterspout risk will be somewhat elevated especially along the coasts, but decreasing LL convergence signals during the afternoon hours should bring the overall risk down during the day. During the night, only isolated thunderstorms are forecast, which exit the forecast area to the E.

... France and parts of the UK ...

Seasonably moist post frontal cold sector air mass beneath modest mid-level lapse rates may result in a few short-lived thunderstorm events. The main foci for CI will be those regions, where postfrontal air mass (with cellular convection) advects onshore during peak daytime heating (e.g. NW France and SW UK). Those spots will see some SBCAPE build-up. DLS remains weak over most areas and therefore only gusty winds and marginal hail/sleet is forecast.

An exception will be SW-UK, where a low-end severe risk may materialize. The placement of a near vertically stacked low just to the south of Ireland during the afternoon hours assists in good onshore moisture advection beneath an ongoing diffluent mid/upper-level flow regime. A lot depends on how strong the lower part of that vortex will be regarding the magnitude of backing of the LL wind field. Still, with 15 m/s DLS and enough time for diabatic heating forecast, a few strong thunderstorms may evolve during a short time-frame between noon and sunset. Forecast soundings show good speed shear with improving directional shear next to 300-500 J/kg SBCAPE, so storms will be capable of producing isolated large hail. Some SRH-1, low LCLs and LL CAPE in excess of 100 J/kg may also result in a very isolated tornado event. The risk rapidly diminishes after sunset.

During the overnight hours, the eastward moving cold front interacts with a prefrontal moisture surge from the W-Mediterranean (advected moisture becomes more elevated atop the stable nocturnal BL). At the same time, mid-levels cool down a bit which only increases mid-level lapse rates atop the surface cold front, so I would not be surprised to see the initiation of a few elevated thunderstorms along the eastward moving cold front. However, this front becomes masked betimes with washed out temperature gradients and a near parallel alignement to the background flow. Hence, only a sporadic thunderstorm risk may occur.

... W of Portugal and NW Portugal ...

A low probability lightning area was issued for the expected convective activity although history proved that convection in subtropical air mass hardly manages to produce lightning activity mainly due to weak lapse rates. Same here, but proximity to a W-E aligned frontal boundary just to the north keeps mid-level temperatures a bit down during the forecast, which is reflected by a slight increase of mid-level lapse rates in latest model data. Beside that, heavy rain likely occurs along the intersection point of the leisurely southward moving front and the tongue of subtropical air (e.g. N-Portugal). No level area will be issued due to a low-end confidence in electrified activity over N-Portugal.