Storm Forecast

Storm Forecast
Valid: Fri 24 Mar 2017 06:00 to Sat 25 Mar 2017 06:00 UTC
Issued: Thu 23 Mar 2017 17:19
Forecaster: TUSCHY

A level 2 was issued for NE Spain, extreme SW France and the Balearic Islands mainly for excessive rain. A tornado threat exists, probably maximized over the Balearic Islands.

A level 1 surrounds that level 2 mainly for excessive rain, an isolated tornado, large hail and severe wind gust risk.

SYNOPSIS

A pronounced cyclonic vortex is situated over SW Europe and serves as main player for organized thunderstorms during the forecast. The mid-level air mass within that vortex remains cold with readings well below -30C at 500 hPa. Numerous impulses circle the main vortex:
One exits W-France during the start and crosses the Bay of Biscay to the west, a second one affects NW-Morocco beyond sunset while the third and strongest one lifts north from N-Morocco/NW Algeria and affects the W-Mediterranean. The latter impulse is supported by a temporal coupled upper-level jet configuration which inserts ample upper divergence. This configuration gradually breaks apart during the night with one maximum of upper divergence moving to NE Spain and another one drifting to Corsica/Sardinia.
Over N/NE Europe, a wave train of progressive upper-level troughs results in unsettled and windy conditions over a broad area. Shallow postfrontal offshore convection affects parts of CNTRL Norway. Downward mixing causes gusty winds with the strongest activity.

Within the boundary layer, numerous frontal boundaries are analyzed over Europe but only one serves as focus for DMC activity.
A pronounced N-S aligned baroclinic zone expands from S-France to N-CNTRL Algeria. As this boundary interacts with the northeast/northward moving and strengthening impulse aloft, falling surface pressure is forecast along that baroclinic zone. The result will be a surface low pressure channel, which runs from NE Spain to N-CNTRL Algeria. Embedded in this low pressure channel is a wavy front. Further improving conditions aloft with respect to the upper jet configuration result in a developing surface low next to the Balearic Islands within this channel of low pressure. This surface low is forecast to move to the NW, towards NE Spain, during the forecast while strengthening. This development results in a sharpening warm front to the NW of the low and a cold front to the SE. In general, the main model suite (GFS/EZ) has a good handling in timing/strength of that vortex.

DISCUSSION

... Portugal and W-Spain ...

Placed beneath the broad and deep cyclonic vortex, cold mid-levels and weak shear affect the area of interest during the day. A rather shallow and symmetric low/mid-tropospheric cold-core low drags along the W coast of Portugal to the south. Ahead of that low, modest BL moisture advection occurs and spreads N/NE towards Portugal/W-Spain. Forecast soundings show very weak shear at all levels and small/thin low/mid-level CAPE profiles. In general, expect pulsating convection with gusty winds and graupel. However, mountainous convection over W-Spain may tap into still dry BL air mass. Diurnal heating enhances boundary layer T-Td spread with deepening and drying subcloud layers. We won't exclude a sporadic and short-lived downburst event, where wind speeds could approach severe limits. The overall risk is too localized and marginal for a level area. Convection rapidly decays around sunset.

An isolated waterspout risk exists along the SW coast of Portugal in the vicinity of the southward moving vortex. This risk will be maximized, as the vortex moves ashore. The overall hazard is too marginal for a level area however.

... S-France ...

In response to the evolving depression/strengthening mid-level impulse S of the Balearic Islands, a warm front over S-France slides westward during the day. Augmented LL mixing ratios along that front, a constantly strengthening low-/mid-tropospheric wind field and steepening mid-level lapse rates due to ongoing differential temperature advection result in a favorable setup for leisurely westward moving thunderstorm clusters especially over S-CNTRL and SW France. Forecast soundings show mainly elevated convection with that activity, but maximized moisture pooling within the zone of persistent confluent flow along the front may support near surface based convection. Intense LL shear would favor an isolated tornado or severe wind gust risk mainly along the coast. Otherwise, excessive rain will be the main hazard. We kept this event with a broad level 1 due to the progressive nature of the front but upgraded SW France, where ingredients for training convection persist well into the evening hours. Excessive rainfall is also possible along the Massif Central, where orographic enhancement could push rainfall amounts above 100 l/qm/24h. The activity diminishes from E to W before another round of heavy precipitation approaches S-France during the end of the forecast from the S.

... Balearic Islands to NE Spain ...

Ingredients show a substantial risk for excessive rain in the area of interest. As discussed above, the surface low gradually intensifies next to the Balearic Islands. Ahead of the wavy front, a low-level frontal jet strengthens and deepens up to 600 hPa with a SE/NW orientation. Forecast soundings in this belt of moist air show a deep marine layer with BL mixing ratios around 9 g/kg beneath steepening mid-level lapse rates, which result in 500-1000 J/kg MLCAPE. The reasoning for the level distribution is the following:

a) The level 2 for the Balearic Islands was issued due to enhanced confidence in isolated supercells/well organized multicells during initiation time (noon/afternoon). DLS of 20 m/s and strengthening LL shear indicate a risk of tornadic thunderstorms before growing upscale into clusters. The exact path of the depression and onset of CI add some uncertainty, but we want to highlight the overall favorable conditions for severe thunderstorms. Beside the tornado risk, large hail and severe wind gusts are possible next to excessive rain.

b) The level 2 between NE Spain and Sardinia was added as the setup is favorable for training/back-building convection along the gradually northward pushing front. The mesoscale most likely dictates, where/when training V-shaped clusters evolve. We thought about downgrading NE Spain, as upstream convection could reduce the inflow into any developing or ongoing cluster. However, we can't rule out a cluster developing just along the coast of NE Spain, bringing the most dangerous part (with respect to flash flood producing rain) of such a cluster ashore. Also, orographic enhancement probably supports rainfall amounts of more than 100 l/qm, enhancing the flash flood risk. The thunderstorm risk ends from S to NE betimes. An isolated tornado event is possible along the coast.

Further east, upscale growing convection probably develops into a large cluster / MCS, which moves to the N/NE. A temporal severe wind gust threat could evolve with that activity beside the more elevated nature of convection. Confidence in how far east this activity evolves is low, but we decided to include Corsica/Sardinia into a level 1 due to the overall favorable atmospheric conditions. An isolated tornado threat and excessive rain will be the main hazard.

During the end of the forecast, isolated thunderstorms may erupt over and W of the Balearic Islands as mid-levels cool to -25C and below. Gusty winds, graupel and an isolated waterspout event is possible.

... Carpathian Mountains ...

BL moisture from previous days seems to be worked over as a general downward trend in ML mixing ratios is forecast by numerical models. Combined with weakening mid-level lapse rates, the main focus for isolated CI will be a southbound moving cold front, which crosses the area during the daytime hours. 500 J/kg MUCAPE and 15-20 m/s DLS should add a temporal risk for organized updrafts with hail and gusty winds the main but sub-severe hazard. The activity diminishes after sunset although isolated thunderstorms may continue along the front well into the second part of the night due to ongoing weak MUCAPE and frontal lift. Nothing severe is anticipated.

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