Storm Forecast

Storm Forecast
Valid: Sun 17 Nov 2013 06:00 to Mon 18 Nov 2013 06:00 UTC
Issued: Sun 17 Nov 2013 03:26
Forecaster: VAN DER VELDE

A level 1 was issued for northeastern Catalonia/southern France mainly for excessive convective precipitation and tornado chance.

A level 1 was issued for the southern Mediterranean Sea mainly for large hail.

A level 1 was issued for central Norway mainly for severe convective wind gusts.


Low pressure areas dwell over the western and eastern Mediterranean, a west-east ridge is present between the Atlantic and central Europe, and large low pressure areas move across the northern Atlantic and Russia.
The main focus is the western Mediterranean low centered just east of Catalonia. It advects unstable moist air to the France-Spain border area where it is lifted over the Pyrenees and lower ranges. Convective lines may slowly move across the shoreline, causing locally excessive precipitation sums. Some WRF place the activity south of the Pyrenees for the entire period. Strong vorticity may spin up into waterspouts near the core of the low and its convergence arms, although the north side of the low appears rather windy.
The southern Mediterranean lies under the left exit of the African jet and a cold front moves in. Warm air advection creates high storm-relative helicity values > 250 mē/sē sufficient for supercells, capable producing large hail. But, model-predicted CAPE values are quite weak, below 200 J/kg, only increasing just before 06Z Monday.
Interesting is also the central Norway area. A disturbance comes ashore around 18Z with slight CAPE, a strong deep convergence signal in GFS, and 1-3 km average winds over 30 m/s. Over land the friction creates very large SREH and 0-1 km shear. Depending how well convection becomes organized, storms with severe gusts could develop (but if >32 m/s it needs a higher threat level).

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