Storm Forecast
Valid: Sat 17 Jan 2015 06:00 to Sun 18 Jan 2015 06:00 UTC
Issued: Fri 16 Jan 2015 15:30
Forecaster: PISTOTNIK

A level 1 was issued for NE Italy and coastal areas of Slovenia and Croatia for excessive convective precipitation.

A level 1 was issued for parts of Portugal for severe convective wind gusts and excessive convective precipitation.

SYNOPSIS

A highly amplified mid-level trough stretches from Norway to Morocco at the beginning of the forecast period. Its tip will cut off over northern Algeria on Saturday while the remnant translates eastward. Further downstream, a deep southwesterly flow stretches from the western and central Mediterranean across central Europe all the way into northwestern Russia.
Near the surface, a large and mature cyclone starts to fill up over northern Scandinavia. Its long and wavy cold front stretches from the Baltic States across Poland and the Alpine region towards Algeria and makes slow southeastward progress. Very mild air is present ahead of it and cool maritime air behind it.

DISCUSSION

... Italy, Adriatic Sea and W Balkans ...

A 15 m/s southerly low-level jet pumps unseasonably rich Adriatic moisture (surface dewpoints 10-12°C) towards the southern Alps and the Dinaric mountains ahead of the cold front. A superposition of orographic lift and moderate large-scale ascent ahead of the trough will likely suffice to create and instantly release limited CAPE in the almost saturated air mass. Backbuilding convection may cause heavy rainfall for a couple of hours. A wave development will temporarily decelerate the cold front in the wake of the Alps before it starts to work its way southward along the Croatian coast. .
Peak rainfalls in numerical forecast models are not too extraordinary (around 60mm/6h) and a rather limited coverage of embedded storms is expected, but the overall setup seems to justify a low-end level 1. The risk is highest in northeastern Italy and adjacent Slovenia in the 06 to 15 UTC time frame, before it shifts southeastward, following the intersection of the cold front and the upslope flow regime along the Dinaric mountains.
A similar situation is present along the west coast of Italy, but with less moisture and weaker upslope flow, and therefore without a risk level. Under regionally enhanced 0-3 km vertical wind shear around 15 m/s, one or two better organized storms are not ruled out over the Tyrrhenian or Adriatic Sea, but the lack of more robust CAPE makes severe weather unlikely.

... Portugal ...

A short wave at 500 hPa ejects southeastward at the back side of the main trough and induces the birth of a surface cyclone over the Atlantic Ocean, which will reach the Iberian Peninsula in the second half of the forecast period in its mature stage. Moderate to strong synoptic lift overspreads the cold front, which will make a landfall after 21 UTC. Especially WRF indicates the possibility of a convective line with a pronounced wind shift. 850 hPa winds around 25 m/s may suffice to create isolated severe wind gusts at the surface. A few flash floods are possible as well.
The final degree of these risks will depend on the track of the surface cyclone, and how far northward and inland a prefrontal tongue of warm and particularly moist subtropical air can be advected. Despite a limited confidence of electrified convection and a tendency of latest model runs to shift the best area for organized convection more to the southwest (where it could stay offshore), another low-end level 1 is issued.

... North Sea, British Isles, Celtic Sea, Bay of Biscay ...

Scattered to widespread showers and weakly electrified storms will form beneath the base of the trough, which is filled with very cold mid-level air (-34 to -38°C at 500 hPa). Activity will be most active over the Bay of Biscay early in the forecast period, in coastal areas of Ireland and over the Celtic Sea in the evening and overnight, and over the North Sea all day long. Severe weather is not expected.