Mesoscale Discussion
Valid: Thu 15 Sep 2016 12:00 to Thu 15 Sep 2016 22:00 UTC
Issued: Thu 15 Sep 2016 12:00
Forecaster: TUSCHY

Mesoscale discussion for the SE-Bay of Biscay:

A vigorous depression over the S-CNTRL Bay of Biscay continues to drift to the ESE. Buoy 62001 reported southerly winds of 32kt at 00 UTC, decreasing to 4 kt from the SE at 05 UTC, recently backed to E with 37 kt and with the latest report to NE with decreasing wind speed. SST was just around 20 °C with a pressure of 996 hPa. Latest satellite data also confirm and increasing trend of enhanced/longer-lived convection along the eastern and northern part of the vortex.

On the current depression's path, the vortex is about to enter SSTs of more than 22 °C which is a positive anomaly of 2K from the 1961-90 period. Compared to yesterday, deep layer shear already decreased to 25-30 kt and shear is about to weaken further during the following 24h by 10-15 kt. Hence, the vortex now moves in an increasingly more favorable thermodynamic and kinematic environment.
Current increase of DMC especially within the eastern fringe may be the result of aforementioned improving conditions. In addition, the latest SSM/IS scan indicates an increasingly moist troposphere ahead with PWATs increasing to 25-30 mm. At 10:14 UTC, near boundary layer winds in excess of 35 kt were observed per satellite mainly along the southern fringe of the vortex or just offshore of the N-coast of Spain. This may be the result of the deepening vortex and an attendant strengthening pressure gradient, but also due to some channeling effects between the vortex and the Picos de Europa (N-coast of Spain).

If the current trend of increasing convection next to the center of the vortex continues (which is well possible given aforementioned environmental setup), a shallow warm core structure may evolve, which is fostered by numerous model data and cyclone phase diagrams. A constant SE-ward motion - as proposed by data - would indicate a landfall over extreme SW France or east of Bilbao (Spain) between 18-00 UTC. Severe wind gusts would be the main risk but enhanced LL shear with low LCLs may also support an isolated tornado threat during the evening and overnight hours. Heavy rain will be possible.