Storm Forecast
Valid: Fri 12 Sep 2014 06:00 to Sat 13 Sep 2014 06:00 UTC
Issued: Thu 11 Sep 2014 16:59
Forecaster: TUSCHY
A level 1 was issued for parts of Portugal and NW Spain mainly for an isolated large hail event and excessive rain.
A level 1 was issued for the N-Adriatic Sea and surroundings mainly for excessive rain and an isolated large hail and tornado event.
A level 1 was issued for parts of the Czech Republic and SW Poland mainly for an isolated tornado threat.
SYNOPSIS
Blocking pattern (resembling an omgea structure) persists over most of Europe. A cold-core low over the W-Alps drifts south and over the N-Adriatic Sea until the end of the day. An extensive low pressure area is situated over the E-Atlantic and steers an increasingly moist air mass towards W-Spain and Portugal. In between, ridging keeps the weather calm regarding organized DMC. Over Scandinavia, calm weather persists beneath a broad high pressure area.
DISCUSSION
... Portugal and NW Spain ...
Subtropical air advects onshore but weak mid-layer lapse rates keep MLCAPE in the 500-800 J/kg range. Until sunset no distinct lift mechanism is seen in model fields but the left exit of a subtropical jet streak and weak upper divergence support a few thunderstorms over N-Portugal and extreme NW Spain. DLS oscillates around 15 m/s, so a few large hail events and heavy rain will be the main risk.
During the night, the mid/upper flow turns more cyclonic ahead of a strengthening disturbance, which approaches the W-Iberian Peninsula during the end of the forecast. The flow becomes more diffluent and weakens, so DLS drops to 10-15 m/s. However, N-ward spreading EML atop the marine air mass pushes onshore CAPE to 1 kJ/kg and offshore CAPE to values in excess of 1.5 kJ/kg. A few storms continue to fire and large hail/heavy rain remain the main hazard.
... N-Adriatic Sea, Slovenia, Croatia, W-Hungary, the Czech Republic, SW Poland and parts of Italy ...
In the range of the cold-core low, persistent isentropic lift / WAA bring unsettled and rainy conditions to a large part of CNTRL Europe. W-Croatia/Slovenia will see highest rainfall amounts as thunderstorms move ashore during the day. Orographically enhanced rainfall and embedded thunderstorms could push rainfall sums to 100 mm/24h at a few places. A weak cyclogenesis within the lower troposphere and the depression's placement over CNTRL Italy limit the risk of organized/training convection in those areas and we kept a level 1 for this event. Stronger storms along the coasts could bring isolated large hail due to 500-800 J/kg MLCAPE and 15-20 m/s DLS. An isolated spout-type event along the coasts is also possible.
The level 1 was expanded to N-Italy, where augmented LLCAPE and low LCLs indicate an chance for a few spout-type tornado events. A slight warm-up at 850 hPa should lower the risk from W to E over N-Italy beyond 15 Z.
A disturbance ejects out of the base of that cold-core low during the overnight hours and travels rapidly to the N. EZ and GFS both show a swath of enhanced QPF from E-Croatia to E-Austria as stratiform rain with embedded thunderstorms occurs. Anticipated rainfall amounts remain below the level 1 threshold.
Finally, the Czech Republic and SW-Poland see an upswing of thunderstorm probabilities during the afternoon and evening hours as low-tropospheric WAA strengthens. Cap is weak along the westward moving warm front and a window of opportunity for a few organized thunderstorms exists between 09-18 UTC. The best environment evolves along the warm front (in the level 1 area), where SRH-1 around 150 m^2/s^2, LCLs of less than 800 m, 15 m/s 3 km bulk shear, 500-1000 J/kg MLCAPE and 150 J/kg LLCAPE overlap. An isolated tornado and severe wind gust risk exists. Storms cluster betimes and move to the NW with an heavy rainfall risk.
... Rest of the lightning areas ...
Weak shear/meager CAPE preclude organized DMC.
