Mesoscale Discussion

Mesoscale Discussion
Valid: Thu 18 Oct 2018 16:00 to Fri 19 Oct 2018 06:00 UTC
Issued: Thu 18 Oct 2018 14:08
Forecaster: TUSCHY

This Mesoscale Discussion was issued to update the synoptic conditions which result in a potential dangerous excessive rainfall event durig the overnight hours along the E coast of Spain. The already issued level 2 is still valid!

The morning ASCAT scan revealed a tight LL vortex over NE Algeria which is in line with the latest model guidance. This vortex responds to an approaching stronger impulse over S Spain and the resulting interaction of both vortices (Fujiwhara effect) results in a NW-erly motion towards SE Spain during the evening and night (valid for the mid-level vortex whereas the LL depression probably reforms offshore of E Spain). The pressure gradient between rising pressure over France and this vortex increases and results in a respectable low-tropospheric wind field with 45-50 kn easterlies at 850 hPa. The same is true for the boundary layer, where a tightening zonally aligned convergence zone evolves to the north of the LL vortex, which is also accompanied by brisk E/NE erly LL winds (maximized just offshore due to channeling between the vortex and the coast). Enhanced onshore moisture advection of a warm/unstable air mass (400-800 J/kg SBCAPE) is therefore still forecast.

Placed beneath the NE quadrant of the already mentioned strong cyclonic vortex over far S Spain and enhanced by a short-wave, which circles its E quadrant, a prolonged period with intense upper divergence sets up over E-CNTRL Spain. Hence vigorous convection along a sharp convergence zone with effective moisture advection is forecast and results in long-lasting and back-building/training convection. Latest MIMIC-TPW data indicates a broad area with 40 mm PWAT over the W Mediterranean and higher values along any convergence zone (atop SSTs in the 22-24 C range), so the general environment continues to support excessive rainfall and potential life-threatening flash flooding in the highlighted area. Rainfall amounts well in excess of 200 l/qm beneath training convection are likely on a regional scale with a broad swath of more than 100 l/qm in most of the MD area until 06Z.

Initial multiple bands probably merge into one or two dominant convective bands during the night, which is the time, when efficient rainfall production and extreme rainfall rates can occur. The rainfall risk also extends inland due to orographic support. The onset of training convection is currently forecast during the evening hours with the most dominant bands of convection developing during the night.

We expanded the MD offshore and to the south to account for latest nowcast trends, which show a building convective band between Ibiza and Valencia, whereas the convection SW of Barcelona seems to remain progressive due to the stronger background flow (but still capable of producing excessive rain when moving ashore during the next couple of hours). Looped hodographs and low LCLs also support a tornado risk along the coast and along the convergence zone although meager LL CAPE may lower that risk a bit.

A gradual motion of the convergence zone to the north is anticipated until 06Z as the short-wave passes by and with the departing/onshore moving LL vortex, ingredients slowly detoriate for organized convection. However this will probably not be the case before 06Z and hence the excessive rainfall threat (albeit weakening a bit) continues.

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