Friday, October 12, 2018

Hurricane Leslie: will it affect Canary Islands ? Updated

Hurricane Leslie yesterday evening with the Canary Islands and the Moroccan coastline on the right
The tropical storm system named 'Leslie'  has been meandering in the Atlantic for 19 (!) days now without having made up its mind where to finally dissipate. Most of that time it was, and still is, a powerful large hurricane with a current central pressure of 969 hPa and maximum winds exceeding 150 km/h. It is now situated about 730 kms SSW of the Azores and rapidly moving with increasing forward speed towards the island of Madeira, where the first ever tropical storm warning has been issued. Some models suggest that it may finally dissipate near or over the Canary Islands as a remnant low from Sunday, Oct. 14th 2018. There may be some stronger winds up to gale force and the heavy swell generated by Leslie may affect the Canary Islands as well as some showers and the odd thunderstorm, but it seems unlikely that it will be a weather system of great significance, if any, in these islands. According to several models it should turn more to the north towards the Iberian peninsula and not affect the Canaries at all, but other models are in disagreement. Here's an extract from a bulletin of the National Hurricane Centre in Miami explaining the situation and the various track forecasts:
''...The initial motion is now 065/24.  The guidance is in good agreement that this general motion should continue for 24 h or so. After that, there is a major model divergence.  The GFS, ECMWF, and the various consensus models now show a continued east-northeast motion until landfall in Portugal or Spain.  On the other hand, the UKMET, UK ensemble mean, and Canadian models show a turn toward the south and then back to the west (- which would bring it to the Canaries - Ed.) Since the previous forecast showed a southward and westward turn, the new forecast track will also follow this scenario.  However, it is shifted well to the east of the previous track due to the forecasts of the other models. Needless to say, the latter portion of the track forecast is low confidence.
Leslie should gradually weaken as it approaches Madeira Island during the next 24-36 h. After that, the intensity forecast is dependent on what track the cyclone takes.  If it follows the GFS/ECMWF scenario, the system would likely make landfall on the Iberian peninsula and quickly dissipate.  If it follows the UKMET scenario, a combination of shear, cool water, and dry air entrainment should cause weakening.  Since the track forecast is closer to the UKMET scenario, the intensity forecast follows the previous forecast in having Leslie decay to a post-tropical low by 72 h.  However, this is also a low confidence forecast due to the uncertainties in the track forecast. ...''
In 2005 when hurricane Delta powered up again after having almost disintegrated west  of the Canaries and gradually regained tropical characteristics, it was briefly a subtropical storm on November 22, then moved erratically for a few days before moving towards the Canary Islands. It became a powerful extratropical storm just before it passed to the north of the archipelago bringing severe disruption, widespread damage, and claiming seven lives there. However, please note that this was a different and very unusual scenario and most unlikely to be repeated by Leslie, and current indications are that it will not bring any disruptions.
The Spanish and the Canarian authorities have not issued any alerts or warnings for Leslie, but are monitoring the situation under a pre-alert for the western Canaries. However, a warning for strong winds and heavy seas has been issued for the high seas area called 'Madeira' which includes the high seas just north of the Canaries. Some cruise ships which were scheduled to cross the Atlantic westward are now sheltering in the Canaries until the system has dissipated.
UPDATE edited 10:30 pm, Oct.12th 2018:
The latest model runs all predict that Leslie will pass the Canary Islands much further to the north (see latest graphics below to compare with the earlier one above) and instead of having an impact here will instead affect the Iberian peninsula. Below a quote from the latest bulletin of the NHC in Florida:
''...Leslie is moving quickly east-northeastward on the southern side of a powerful longwave
trough centered over the north Atlantic. The track forecast reasoning has changed since the last advisory, with the GFS, ECMWF, and now the 06Z run of the UKMET all showing a more eastward motion of Leslie toward the Iberian Peninsula as the cyclone is picked u by the aforementioned trough. The new NHC track forecast has been adjusted significantly to the north and east, especially beyond 24 hours...
...Key Messages:
1. Leslie is expected to bring near hurricane-force winds on Saturday to portions of Portugal as a powerful post-tropical cyclone. Tropical-storm-force winds are also likely to affect portions of western Spain.
2. Leslie is expected to produce rainfall amounts of 25 to 75 mm (1 to 3 inches) with isolated amounts as high as 100 mm (4 inches) across portions of Portugal and Spain, which could cause flash flooding. ...''

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