Storm Forecast
Valid: Sat 27 Jan 2018 06:00 to Sun 28 Jan 2018 06:00 UTC
Issued: Fri 26 Jan 2018 20:21
Forecaster: TUSCHY

A level 1 was issued across parts of E-Spain mainly for excessive rain and an isolated tornado threat.

A level 1 was issued across SW-Sweden mainly for an isolated severe wind gust threat.

SYNOPSIS

A cold-core low over the extreme SW-Mediterranean chokes off of the westerlies and retrogrades to the WSW. A short wave circles the eastern and during the night also the northeastern fringe of that vortex, adding additional large-scale forcing over NW-Algeria with favorable conditions for low-/mid-tropospheric vortex intensification along the coast. This is shown in numerous models, which show a constantly intensifying LL vortex just off the coast of NW-Algeria, rolling to the W/SW. The main uncertainty will be how long that vortex remains offshore above slightly warmer coastal waters. Those models, which keep the vortex offshore for a longer time, show pretty robust LL vortex intensification with an augmented ageostrophic acceleration of the LL wind field. Right now, model spread and missing indications in phase diagrams keep confidence in any significant warm-core development on the lower-end side. In addition, at least GFS seems to have a decent problem with the convective feedback, dragging a persistent QPF maximum next to the center towards far NW-Algeria. Despite those questions, this feature should be monitored closely until landfall regarding intensification changes.

Further north, deep-layer flow regime turns zonal with a potent jet racing towards S-Scandinavia. A rapidly intensifying vortex beneath the left exit of a powerful 65 m/s mid-level jet near the Faroe Islands decouples from the frontal zone, but remains a powerful cyclone as it gradually approaches SW-Norway during the night.

A ridge from the far NE-Atlantic builds east on the south side of the jet and spreads towards CNTRL-Europe during the night. Over NE-/E-Europe, a cold and dry air mass precludes DMC activity.

Regarding fronts, an east/southeastbound racing occlusion will be the main focus for thunderstorm activity, which affects S-Norway and S-Sweden during the night.

DISCUSSION

... E-/SE-Spain and N-Algeria ...

As the short-wave emerges over the far SW-Mediterranean after sunset, upper support for vortex intensification overspreads that area and results in an intensifying LL depression, paralleling the coast of NW-Algeria. No serious DMC activity is anticipated as EML remains displaced to the E.

Further north, responding ageostropic low-/ mid-tropospheric flow creates a strengthening W-E aligned convergence zone east of SE-Spain. Northward fanning EML overspreads that region and results in a broad area with enhanced MLCAPE in the 400-800 J/kg range. The LL flow equals or exceeds the speed of the mid-level flow and with persistent onshore moisture transport, a cluster of numerous showers and thunderstorms probably evolves in the level 1 area. Weak DLS limits the risk of an organized MCS event, but slightly enhanced probabilities for temporal training in addition to orographic lift could result in rather high rainfall amounts over SE-Spain. Disorganized nature of that cluster and offshore convection which probably disrupts the inflow betimes limit the general severe risk (e.g. excessive rain). Still local excessive rainfall amounts will be possible ... especially in regions with orographically enhanced upslope flow. An onshore and coastal level 1 was issued for that risk and for an isolated tornado event near the coast, where LL speed/directional shear increase in response to increasing friction.

... SW-Sweden ...

An intense jet drives a slowly deepening mid-level wave over S-Sweden east during the night. The main focus for thunderstorm probabilities exists in the left exit of a 55 m/s mid-level jet. This wave is well coupled with an eastbound moving occlusion, which reaches SW-Sweden around midnight from the W. Beyond midnight however the wave continues to race east and runs ahead of the surface front. Hence, subsidence is forecast to catch up with the front. In addition, enhanced onshore trajectories over NW-Europe limit the BL moisture content of the prefrontal air mass. Therefore the current idea is that intersection of dry slot and occlusion may produce an increasingly convective backside of the front, but no organized line is currently forecast. In addition, with subsidence gaining magnitude betimes, the question arises if this temporal flare up of convection already decreases around 06 UTC. With 20 m/s LL shear in place, a few severe wind gusts are possible with any stronger convective cell. A small level area was added where confidence in enhanced convectively induced severe wind gusts.