Storm Forecast
Valid: Fri 17 Jan 2014 06:00 to Sat 18 Jan 2014 06:00 UTC
Issued: Thu 16 Jan 2014 21:51
Forecaster: TUSCHY

A level 1 was issued for Portugal mainly for large hail, heavy rain, strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for SW Spain mainly for heavy rain and an isolated tornado event.

A level 1 area was issued for NW Italy (Gulf of Genoa) mainly for heavy rain.

SYNOPSIS

A deep longwave trough over W-Europe becomes established with a steady motion to the east. Anomalous cold temperatures in the mid-tropospheric layer advect far south and approach N-Morocco during the day. Downstream ridge keeps most of the Mediterranean and adjacent areas stable (regarding DMC) and warm, whereas a strong high pressure area with chilly temperatures over Norway, Sweden and Finland builds south a bit. A pronounced and quasi-stationary baroclinic zone remains in place, which extends from S-Norway all the way to Belarus.

DISCUSSION

... Portugal ...

Ongoing advection of a warm and moist air mass towards Portugal and Spain beneath constantly cooling mid-levels creates moderate onshore MLCAPE. Up to 800 J/kg are forecast with peak values over S-Portugal (least onshore mixing and best BL moisture). CAPE decreases to the north and east with quite sharp CAPE gradients towards the east/Spain.
The approaching trough axis and an onshore moving (already occluded) surface front create a prolonged period for active thunderstorm development....both offshore and onshore. No pronounced jet streak crosses the area of interest, but the forecast wind field still creates adequate shear for long-lived thunderstorms (0-6 km bulk shear of 15-20 m/s). A few forecast soundings from S-Portugal indicate the chance for a mixture of organized multicells and isolated supercells with large hail and strong to severe wind gusts. Also, an overlap of moderate LLCAPE and locally enhanced LL shear indicate an isolated tornado/waterspout risk along the SW/S coast of Portugal. A few cold-air funnels/short-lived tornado events however can also occur further north, over N-Portugal. During the late afternoon hours, environmental conditions become increasingly supportive for upscale growth of ongoing convection into numerous clusters of showers/thunderstorms...not necessarily well organized. Heavy rain will be possible with strongest clusters.

The level 1 was expanded towards SW-Spain due to an augmented risk for repeatedly onshore moving showers/thunderstorms and temporal back-building (as 20 m/s LLJ becomes aligned near parallel to onshore pointing moisture tongue). LL shear, although not particular strong, still indicates a chance for isolated tornado events.

... S-UK ...

With faint onshore SBCAPE, low LCLs and weak/moderate LL shear, isolated thunderstorms are forecast with a low-end funnel/short-lived tornado risk. Sleet and gusty winds remain the main hazard. The risk for isolated storms diminishes during the afternoon hours from S to N as 15 mm PWAT tongue and warming mid-levels over the English Channel approach far S-UK with heavy (but stratiform) rain. We don't want to rule out an isolated embedded storm as forcing passes by.

... N-CNTRL Mediterranean ...

The overall set-up seems favorable for a mixed mode of repeatedly onshore moving showers/thunderstorms and more organized bands of training activity along the N/NE coast of the Gulf of Genoa and the NE-Adriatic Sea. A deep plume of rich moisture, strong shear and numerous impulses, grazing the areas just to the north, also point to that risk. The only missing ingredient will be meager LL moisture/weak mid-level lapse rates and therefore low-end CAPE. Strongest activity occurs until noon, with a temporal lull thereafter and another increase during the night. 50-100 mm/24 h are possible with higher amounts due to upslope flow. With faint SBCAPE signals and a low-end thunderstorm risk, lightning/level 1 areas were added for the Gulf of Genoa. The NE-Adriatic Sea looks too stable and hence no level area was issued for now.