vendredi 10 octobre 2014

European Storm Forecast Experiment

Storm Forecast
Storm Forecast
Valid: Fri 10 Oct 2014 06:00 to Sat 11 Oct 2014 06:00 UTC
Issued: Thu 09 Oct 2014 14:26
Forecaster: TUSCHY
A level 2 was issued for parts of S-France mainly for excessive rain. Another level 2 was issued for far NW Italy mainly for excessive rain.

A level 1 surrounds the level 2 mainly for an isolated tornado, large hail and severe wind gust risk. Excessive rain is also forecast.

A level 1 was issued for parts of NW Italy mainly for excessive rain and large hail.

A level 1 was issued for NE Spain mainly for an isolated large hail event.

A level 1 was issued for SW Spain mainly for excessive rain and an isolated waterspout event.


A big trough covers all of NW Europe, while downstream ridge remains in place from Italy to the Ukraine. A weakening upper low becomes trapped over Turkey as geopotential heights increase to its north. Unsettled conditions persist.


... S-France to NW Italy ...

The Massif Central to the Gulf of Lion to the Ligurian Sea will be the main region for today's severe risk.

Brisk mid/upper layer SW-erly flow extends from the S-Bay of Biscay to the Baltic Sea. Deep layer flow exhibits a rather large angle to the Pyrenées, so lee effects over extreme SW France cause a broad area with less low-tropospheric pressure. This enhances an already ongoing onshore flow along the coasts of S-France. The shape of the orography helps to speed low-tropospheric winds up towards the Rhone Valley. Trajectories emerge from the NW Mediterranean, where TPWs range between 25-35 mm, which is not exceptionally high. Atop, persistent 300 hPa divergence affects S-France and combined with numerous passing short-waves, repeated thunderstorm development is anticipated. Roughly 1 kJ/kg MLCAPE exists along the coast.

Low-tropospheric winds dictate when and if storms train, but latest guidance maintains 20-30 kt winds (850 hPa) offshore and stronger winds onshore due to orographically channeling. Magnitude oscillates with passing short-waves but in general keeps its strength during the forecast. However we miss a confined LLJ core, which could act as focus for onset of training. That's probably why QPF maxima show a mixture of persistent upslope flow along the S-Massif Central or some temporal training along the immediate coast. Anyhow, excessive and flash flood producing rain is likely and a level 2 was added.
Tail-end storms and in general any more discrete storm along the coast has enough shear to work with to add a supercell risk to that forecast (20 m/s DLS and more than 200 m^2/s^2 SRH-1). An isolated tornado, strong to severe wind gusts and large hail accompany those storms. In fact an isolated tornado risk can occur well inland and despite uncertainty in CAPE magnitude, the level 1 was expanded north.

A similar scenario evolves further east over the Ligurian Sea. Weaker LL winds and less forcing limit the risk of an organized and long-lived training cluster, but repeatedly onshore moving and temporarily training convection probably brings heavy to locally excessive rain to that area. Despite uncertainties and restrained QPF maxima, impressive downstream moisture and persistent onshore flow should be enough for localized excessive rainfall amounts and hence another level 2 was issued.
The level 1 was was expanded far inland as 15-20 m/s DLS and 800 J/kg MLCAPE may support a few thunderstorms with large hail.

Especially during the night, thickness lowers a bit over the W-Mediterranean, but no real lift mechanism can be detected to break the remaining cap. Can't rule out an isolated storm over the islands or even off the Atlas mountains, but we think that ongoing capping issues should keep the liftime short. With abundant CAPE to work with, any storm could become severe due to large hail. Confidence in such a scenario remains too marginal for expanding the lightning areas far south. However, Corsica was included into the level 1.

... Spain ...

A wavy front bisects Spain from SW to NE and pools better BL moisture. This somewhat offsets meager mid-layer lapse rates, but diurnal mixing should keep final MLCAPE around 500 J/kg over NE Spain, around 800 J/kg over SW Spain and between 200-500 J/kg in-between. 20 m/s DLS over NE Spain may cause a few storms to become better organized with an isolated large hail threat. Less shear but high low-tropospheric moisture and repeatedly crossing storms bring an heavy rainfall risk to SW Spain. Both regions were highlighted by level 1 areas. SW Spain may also see an isolated waterspout risk with better LLCAPE.

Between both level 1 areas, clustering and slow moving storms may pose an heavy rainfall risk, but meager CAPE precludes organized storms and hence a level 1.

... Rest of the lightning areas ...

No organized DMC activity is forecast with lack of shear and/or CAPE. An isolated funnel/short-lived tornado event can't be ruled out over SW Finland, where diurnal heating supports a temporal increase of LLCAPE build-up but this risk remains too conditional for a level 1. The same for Ireland and Great Britain, where any mesoscale convergence zone could act as focus for a few funnel reports...mainly along the westward facing coasts, before onshore mixing begins.

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