Valid: Sat 19 Jan 2013 06:00 to Sun 20 Jan 2013 06:00 UTC
Issued: Fri 18 Jan 2013 22:35
A level 1 was issued for Southern Portugal, parts of Spain, for Gibraltar and northernmost Morocco for severe wind gusts.
The dominant feature is a rapid cyclogenesis which has started over the Atlantic Ocean and enters Northwestern Spain at the beginning of the forecast period (Sat 06 UTC) at peak intensity. Central pressure is forecast to deepen from ~1000 hPa to ~975 hPa within an 18-hour time frame from Fri 12 UTC to Sat 06 UTC. Needless to say that the accompanying dynamics and the forcing for upward vertical motion are very strong in the range of the low center and along its cold front / occlusion. This is well indicated by an accompanying sharp upper-level trough, a dry intrusion and a tropopause anomaly, all of which are very pronounced according to latest satellite and NWP data at the time of writing (19 UTC). During the forecast period, the cyclone continues to move eastward across Spain, while it gradually slows down and starts to weaken. The short-wave trough cuts off into a separate upper-level low.
Further downstream, another mature low gradually fills up over Romania, and a short-wave trough in its periphery moves eastward from Greece towards Turkey and the Black Sea.
Meanwhile, Easterly winds with the advection of continental cold air create wintery and stable weather conditions across continental Europe, the British Isles and Scandinavia.
The appearance of a highly dynamic feature like this cyclone from the poorly sampled Atlantic region always poses a big challenge to forecasters. This time, the model pool can largely agree on the maximum intensity of the rapidly developing low and a track across the Northern half of Spain, hence a certain confidence seems permissible that this feature will keep to the schedule. Likewise, scattered to widespread severe wind gusts can reliably be expected in the southwestern quadrant of the cyclone's track. However, major uncertainties concern the question about an involvement of convection, and hence a reflection in the threat level scheme of this outlook.
The cold front is forecast to hit the Portuguese west coast around 03 UTC on Saturday, probably with an accompanying convective line and rather widespread severe wind gusts (please refer to the previous day's outlook), and to enter Spain at the beginning of the current forecast period (Sat 06 UTC). The frontal system will rapidly occlude as soon as it impacts the continent. This means that the low-end CAPE of the subtropical maritime air in the warm sector (probably in the 100-200 J/kg range) will soon become elevated and exhausted. Nonetheless, the strong forced ascent along the cold front and beneath the short-wave trough is thought sufficient to keep temperature profiles neutral across the lower troposphere. It is therefore well possible that the convective line can race a few hundred kilometers eastward over the Spanish mainland, before the unfavorable topography and the de-coupling from the strongest synoptic forcing will lead to a substantial weakening. Besides, deep-layer shear remains strong (up to 40 m/s) but the low-level wind field rapidly slackens below 25 m/s at 850 hPa, pointing to a decrease of the severe gust potential as the front proceeds over Southeastern Spain. All in all, severe wind gust threat seems to be robust enough to warrant a level 1 for an area from Southern Spain to the Morocco coast.
Isolated lightning could occur all the way North to the low pressure center, which will also be characterized by neutral profiles and strong rising motions. However, the much weaker background wind field will preclude any severe wind gusts there.
Another focus of interest is the range of the back-bent occlusion, which will cross the Northern half of Portugal between 06 and 12 UTC. 850 hPa winds in excess of 35 m/s and the probable development of a "sting jet" point to a threat of widespread damaging wind gusts. However, it needs to be emphasized that this robust severe weather threat is not related to convection and hence not reflected in our threat level scheme.
Contemporaneously, post-frontal convection will likely affect the southern half of Portugal, adjacent Spain and Gibraltar in a slightly unstable air mass (CAPE around 200 J/kg). Hodographs are straight and deep-layer shear is strong enough (>30 m/s) to organize the maritime convection into short bowing lines early in the forecast period. The according severe wind gust threat is reflected by a level 1 area as well. This threat is cut short after 12 UTC, when the wind field quickly slackens beneath the trough axis and strong subsidence overspreads the region.
...Balearic Islands, Western Mediterranean...
Ahead of the low pressure system, warm air advection overspreads the Western Mediterranean region, before the occlusion moves into this area in the afternoon and evening. GFS shows patches of instability, but it will probably be capped for a while. Hence, it is doubtful if the synoptic- and mesoscale lift ahead of the upper-level trough and along the occlusion will be sufficient to initiate convection. Around the Balearic Islands, a few hundred J/kg of CAPE and strong deep-layer shear (>30 m/s) nicely overlap in the GFS forecast. A level 1 was drawn for a chance of severe wind gusts in the case that the occlusion can reorganize a convective line.
Further south and east, the capping gets stronger and the synoptic lift support does not arrive before nightfall. This makes thunderstorms very questionable in the broad warm air advection regime, even though GFS shows the development of patchy CAPE as far East as the Tyrrhenian and Ionian Sea overnight.
Scattered and mainly non-severe thunderstorms are expected in the range of the short-wave trough over the Ionian Sea and the Turkish West coast in the morning to afternoon hours. An isolated waterspout or heavy rainfall event is not completely ruled out. Activity becomes elevated and weakens while moving onshore.