Forecast Update
Valid: Fri 18 Jan 2013 18:00 to Sat 19 Jan 2013 06:00 UTC
Issued: Fri 18 Jan 2013 17:32
Forecaster: TUSCHY

A level 1 was issued for Portugal and extreme W-Spain mainly for severe wind gusts and an isolated tornado risk. There exists the possibility of a damaging wind event during the end of the forecast period over parts of Portugal.

A level 1 area was issued for the E-Aegean Sea mainly for heavy rain.

SYNOPSIS and DISCUSSION

This outlook was issued mainly to evaluate the wind/tornado risk over Portugal with newest data. For the rest of the outlook, only cosmetic changes were performed as the outlook seems to be on track (e.g. downgrading the level 1 over the NW Aegean Sea).

... Portugal and W/NW Spain, 00Z onwards ...

This event encompasses 2 rounds, first the cold front passage at roughly 03Z and the damaging wind event with attendant back-bent occlusion.

WV imagery and latest ASCAT data confirm the interaction of a strong vort max with a pronounced hyperbaroclinic zone at roughly 45°N/ 40°W. Since then, a rapidly intensifying and expanding wind field was noted in available remote-sensing data. Sampled wind speeds over the NE Atlantic agree nicely with latest model output, showing a 60 m/s mid-level jet just about to approach the depression. Placed beneath its exit region, the rapid deepening phase of the depression seems to be a likely outcome. Inflow trajectories into this depression emerge from the tropics, so beside an increasing dynamic component, latent heat release becomes another factor to assist this rapid deepening phase. Main model guidance hints at a 20 hPa pressure drop with some members exceeding 30 hPa before landfall over NW-Portugal just after midnight.

Phase diagrams in the past few model runs showed some fluctuation regarding the final strength of the depression, but there was and still is a persistent sign of a warm-seclusion, affecting Portugal during the end of the forecast. Landfall may disturb explosive deepening somewhat, but we think that a well structured back-bent occlusion will be in full progress as this depression comes ashore.

a) cold front between 00Z onwards:
There still exists some uncertainties regarding the strength and structure of this cold front. Despite a pool of dry air currently entering the depression's center, we think that this depression will already reach its occlusion stage before landfall. Hence, the dry slot will already wrap around the center and may be displaced from the eastward surging cold front (in addition to ongoing mixing with the ambient more humid air mass). Nevertheless, the magnitude of the PVA/lift will be impressive and a forced line of convection accompanies the cold front, probably LEWP-ish. Prefrontal moisture advection (surface dewpoints in the mid-tens) and neutral lapse rates atop may provide some low-end CAPE support. This cold front will be embedded in a 25-30 m/s near BL flow, so widespread severe wind gusts can be anticipated during the cold front passage. An isolated tornado event along that line is possible, given intense speed and moderate directional shear.

b) back-bent occlusion 03Z onwards:
Confidence increased that a concentrated swath of damaging wind gusts may accompany that back-bent occlusion during the end of the forecast with gusts along the W-coast likely exceeding 35 m/s. No DMC is anticipated with this feature, so this magnitude of wind will not be reflected in our warning scheme. For warning information, please refer to your national weather service, e.g. ipma.