Storm Forecast
Valid: Mon 08 Oct 2018 06:00 to Tue 09 Oct 2018 06:00 UTC
Issued: Sun 07 Oct 2018 09:22
Forecaster: TUSCHY

A level 1 was issued across parts of the W-Mediterranean mainly for excessive rain. In addition a waterspout risk exists east of E-CNTRL Spain.


SYNOPSIS

No change in today's forecast with respect to the zonal flow regime over N-Europe. Progressive low-amplitude ridges and waves cross Scandinavia from W to E with standing waves along the S-Scandes in response to an intense cross-flow regime and a stable thermal stratification up to roughly 600 hPa. Predominant very dry low-tropospheric air starts to moisten from the SW as modified Atlantic air arrives over S-Norway and Sweden. No thunderstorm risk exists as slim mid-level lapse rates and mostly hostile BL moisture preclude any CAPE build-up.
During the night, a deep plume of tropical moisture approaches far NW Europe with MIMIC-TPW analyzing values in the 40-50 mm range. In fact this evolving atmospheric river is fueled by the long-lived tropical cyclone LESLIE. This high moisture content and embedded short-waves (sliding along that atmospheric river) will create very wet conditions for Scotland/N-Ireland with the start of that event during this forecast period. No thunderstorm activity is forecast and hence this heavy rainfall event won't be reflected in our level scheme.

Therefore the main player regarding thunderstorm probabilities remains a strong cut-off over the Iberian Peninsula, which sags south along the E coast of Spain. Of interest is a temporal intensification of that vortex down to the lower troposphere during the night with a tightening meridionally aligned temperature gradient over the W-Mediterranean. In response to that, low/mid tropospheric winds increase and advect a moist marine air mass towards S France.


DISCUSSION

...W-Mediterranean...

Two foci for severe exist. One is centered along the E-CNTRL coast of Spain beneath the upper low. An enhanced vertical temperature gradient results in scattered thunderstorms, which reveal slow storm motions. Heavy rain is the dominant threat. With slightly positive SST anomalies and SSTs in excess of 24-25 C, broader LL CAPE profiles are forecast and a waterspout risk exists - probably maximized along any mesoscale convergence zone.

The second area of interest will be the region east of the cyclonic vortex, which runs from S-France to the south. Initially dry offshore winds over S France turn to a SE-erly direction is response to the overall backing of the wind field upstream of that vortex. A confluent LL wind regime evolves beyond 00 Z and a broad area of thunderstorms is forecast in an environment with 400-800 J/kg MLCAPE and weak DLS. The responding winds in 850 hPa support elongated LL hodographs, supportive for training convection in addition to a broad influx of unstable and moist air from the SE. The late arrivial of most favorable ingredients for heavy rain and mainly offshore activity keep this event in a level 1 until 06Z. Also, upstream building convection cuts off the influx of most unstable air to thunderstorms next to the French coast which should also temper the overall rainfall risk along the coast until 06 Z.

...CNTRL Mediterranean...

Weak shear and instability cause slow moving pulsating convection with local instances of heavy rain.