Storm Forecast
Valid: Fri 09 Mar 2018 06:00 to Sat 10 Mar 2018 06:00 UTC
Issued: Thu 08 Mar 2018 15:22
Forecaster: PISTOTNIK

A level 1 is issued for Portugal and W Spain mainly for excessive convective precipitation and to a lesser extent for tornadoes and severe convective wind gusts (the latter mostly to the N).

SYNOPSIS

Spring-time cyclonic conditions prevail over Europe. A low-pressure system moves from Denmark towards Belarus while it fills up. Another one, deepening, approaches the Bay of Biscay from the west. A broad, moderate zonal flow is present at their southern flank. Warm air advection increases over western, central and southern Europe ahead of the new Atlantic cyclone, and the flow backs to the southwest. The last remnants of continental polar air continue to recede into Scandinavia and northeastern Europe.
Despite the dynamic setup, the largely separated "ingredients" - steep lapse rates on the cyclonic side and rich low-level moisture on the anticyclonic side of the main frontal zone - will keep this forecast period rather quiet in terms of deep convection.

DISCUSSION

... Portugal, W Spain ...

Despite rather poor lapse rates, plentiful low-level moisture allows a few hundred J/kg of CAPE in the warm sector of the Atlantic cyclone. The deep southwesterly flow features enhanced vertical wind shear and storm-relative helicity (15-20 m/s and 100-300 m^2/s^2, respectively, across the lowest 3 km).
Strong onshore and upslope flow will cause heavy precipitation, possibly with peak values above 100 mm in the present forecast period and with embedded convection. From 15 UTC onwards, a short-wave trough swings in from the southwest and creates additional lift. It becomes increasingly likely that thunderstorms will become involved then. Kinematic conditions are good enough to allow organized storms, though their embedded nature may be somewhat hindering. While flooding is the main risk, any supercell that forms may produce a tornado in this environment of low cloud bases and strong low-level shear.
After 00 UTC, the cold front will enter Portugal and Galicia from the west. A convective line may form and may produce severe wind gusts (in conjunction with tight pressure gradients).

... W and central Poland ...

Synoptic lift in the left exit of a mid-level jet streak and daytime heating may create a temporary window for ~100 J/kg of CAPE under strongly sheared, veering wind profiles at the southern flank of the filling cyclone. Shower activity may organize into multicells or a comma-like feature with a risk of a few severe wind gusts or an isolated tornado. It is questionable whether this convection will grow deep enough to produce lightning, and hence no level 1 is issued.