Storm Forecast
Valid: Fri 03 Feb 2017 06:00 to Sat 04 Feb 2017 06:00 UTC
Issued: Thu 02 Feb 2017 19:12
Forecaster: TUSCHY

A level 1 was issued for parts of N-Portugal, NW-Spain and W/SW-France mainly for severe or damaging wind gusts and an isolated tornado risk.

A level 1 was issued for parts of SE France, N-Italy and the N-Adriatic Sea including surrounding regions mainly for an isolated severe wind gust and tornado risk. One or two large hail events are possible.

SYNOPSIS and DISCUSSION

An extensive trough is placed over the NE Atlantic with downstream ridge anchored over N-Europe. A belt of strong westerlies spreads east over the Iberian Peninsula to the CNTRL-Mediterranean. Numerous short-waves circle the Atlantic trough and affect W-Europe with variable intensities. A confined cold-core low over Belarus drifts SE towards the W-Ukraine.

The following foci for more organized convection exist:

a)
During the start of the forecast, a strong depression over the Bay of Biscay moves E to NE and crosses UK during the later afternoon/evening hours. An occluded frontal system spreads east and affects W-France during the daytime hours.

Between 09Z-15Z the attention turns to the arriving front (NW France). Despite some indications of dry slot/front overlap, current guidance remains rather restrained regarding thunderstorm probabilities. The main concern with respect to the wind risk exists with the potent cold conveyor jet, which won't be covered by us (a straight-line and mostly non-convective wind event). Slim CAPE (due to dry above moist layer stratification) and favorable shear compete with strong subsidence (well visible in Q-vector fields), so main convection should stay shallow in nature. 25-35 m/s near BL winds in a well mixed air mass assist in severe wind gusts with any convective element, which can produce a more sustained downdraft. A level 1 was issued along the coast and a bit more inland as CAPE begins to vanish.

Further south, best chance for thunderstorms is expected over SW France, where a mid-level thermal trough overspreads a marine BL air mass. SBCAPE of up to 600 J/kg, up to 150 m^2/s^2 SRH-1 and 8-12 m/s LL shear overlap and support a few better organized updrafts especially during peak heating (12Z-16Z). Strongest cells may contain isolated large hail, severe wind gusts and they could also produce an isolated tornado event. Once again, subsidence is forecast, but weaker compared to NW France. Hence, we expect more active showers/thunderstorms spreading east before decaying more inland as they outrun CAPE.

b)
A second cyclogenesis is forecast over the S-Bay of Biscay beyond 18 UTC. This development is fostered by impressive kinematics and dynamics. Placed in the left exit of a powerful 50 m/s mid-level jet, strong deepening rate is anticipated over the S-Bay of Biscay. Models show a developing shallow warm-core structure with an intense LLJ in the warm sector. 50 m/s ! near BL winds are probably a mixture of intense gradient winds, some funneling between the depression and the Cantabrian Mountains and a developing cold conveyor or even a potential sting jet. Extremely damaging surface gusts in excess of 40-45 m/s are forecast between 21Z-06Z over the far S-Bay of Biscay, approaching far SW France during the end of the forecast. Convection in this region of extreme winds should stay shallow - if any convection manages to develop at all (probably dry slotted). We expanded the level 1 further south (including parts of NW Spain and extreme N-Portugal), as cold front may see enough inland CAPE (on the low-end side), for convective elements with enhanced downward transport. Severe wind gusts will be the main risk with that activity. An upgrade may be needed for this part if the dry slot spreads further south than currently expected.

Of interest will be a region just to the south of the depression's center, where the dry slot overspreads a most BL air mass in combination with intense lift (probably E-CNTRL Bay of Biscay). Thunderstorms may evolve in that region and could affect W-CNTRL/NW France during the end of the forecast. Again, severe wind gusts and an isolated tornado will be the main hazard. This level was merged with the level 1 from a).

c)
Numerous more or less pronounced mid-level waves affect far S-France and N-Italy during the forecast. Those waves are also visible in the surface pressure field and temporarily/regionally enhanced thunderstorm chances exist in the level 1 regions. Strong shear and MLCAPE of 400-800 J/kg support organized convection with severe wind gusts, isolated large hail and an isolated tornado risk.
For S-/SW France, the time-frame between 15Z-21Z seems conducive for an isolated longer-lived storm event or two, although the shape of the trough and subsequent moisture return is still handled differently in model fields. 21Z-03Z seems to be conducive for isolated thunderstorms over N-CNTRL Italy, where better moisture return may support healthy inland CAPE (up to 500 J/kg). This activity then spreads east over the N-Adriatic Sea, affecting parts of Croatia during the end of the forecast. Next to the aforementioned risk with more isolated cells, small bowing systems may also support confined swaths with more severe wind gusts.