Storm Forecast
Valid: Fri 20 Mar 2015 06:00 to Sat 21 Mar 2015 06:00 UTC
Issued: Thu 19 Mar 2015 23:17
Forecaster: TUSCHY

A level 1 was issued for S/E Spain mainly for excessive rain, large hail and an isolated tornado event.

SYNOPSIS

Looking at the 500 hPa loop makes one dizzy as various upper lows affect many parts of Europe. A vertically stacked vortex over the Iberian Peninsula drifts a bit to the east with no major fluctuations in strength. A smaller upper cold-core low over W-France gets caught by the Spanish vortex and continues to move to the SW over the Bay of Biscay, over extreme NW Spain and thereafter towards the open Atlantic. Regarding the mid-levels, a cold-core low over SE Europe features the coldest air with 500 hPa readings around -35 °C. It shifts a bit to the SE towards Turkey with rather cold air filtering in from the NW.
Very cold air invades NE Europe with enhanced convection over offshore areas due to the augmented heat flux. No DMC activity occurs during the forecast.

DISCUSSION

... W-France ...

Spotty and short-lived thunderstorms may occur beneath the upper low, where steep lapse rates over meager BL moisture cause slim CAPE build-up around peak heating. Graupel and gusty winds will be the main hazard and the activity decays after sunset (despite one or two offshore storms during the night, which is too marginal for a lightning area).

... Spain ...

Forcing remains diffuse as LL pressure field features an elongated channel of low pressure and numerous minima. Aloft, no distinct jet core affects the area with the subtropical jet running far south (Morocco and Algeria). Cold low/mid tropospheric air overspreads the far W-Mediterranean and results in quite robust MLCAPE of 400-800 J/kg and local peaks aoa 1 kJ/kg. Not much lift is needed to surpass the LCL, so we already expect numerous ongoing clusters of showers and thunderstorms during the start of the forecast. The bulk of activity may be displaced towards S/SE Spain as the dry N-African air needs some time for mixing. Weak steering flow and enhanced BL moisture along the coasts indicate a good chance for slow moving thunderstorms with excessive rain. Stronger storms over S/SE Spain may also produce an isolated large hail event, whereas the overall wind risk remains low(despite a localized downburst event). The risk extends far inland and local flash flooding or an isolated large hail event are well possible in addition to a few funnel/an isolated tornado report. During the night, the electrified activity diminishes, but slow moving convection remains capable of producing localized heavy rainfall events.

During the night, the focus for enhanced thunderstorm activity shifts more towards the Balearic Islands, where a warm front sharpens. This front is characterized by a strong moisture/CAPE gradient with a developing near front-parallel 35-40 kt BL flow from SE to WNW. In addition an upper trough over that region becomes better defined with enhanced UL divergence. A rather big cluster of showers and thunderstorms is forecast to evolve over the Balearic Islands with an excessive rainfall and large hail risk as forecast soundings show up to 1 kJ/kg CAPE for elevated thunderstorm activity. The tornado threat depends on the degree of 850 hPa capping, which seems to be quite strong until 06Z. Heavy rain spreads NW and affects NE Spain during the night. Coastal areas may also see emebdded thunderstorms, which could locally enhance the rainfall threat. Local area models show isolated probabilities for 50-80 l/qm in 12 h, but confidence in the final placement remains too low to warrant an upgrade. Strongest QPF signals for NE Spain also start beyond our forecast range. Therefore a broad level 1 area was issued.