Storm Forecast
Valid: Fri 19 Oct 2018 06:00 to Sat 20 Oct 2018 06:00 UTC
Issued: Fri 19 Oct 2018 08:42
Forecaster: PISTOTNIK
Level 1 and level 2 areas are issued for NE Spain and the Balearic Islands as well as for Sicily mainly for flash floods and to a lesser degree for (non-supercellular) tornadoes over water.
Level 1 areas are issued for S Spain and Morocco as well as for parts of the Ionian Sea for large hail, flash floods, (supercellular) tornadoes and to a lesser degree severe convective wind gusts.
SYNOPSIS
A meandering zonal flow is present north of 55N. Two embedded short-wave troughs and attendant frontal systems of surface cylones proceed from Finland into NW Russia and from the N Atlantic to the Norwegian and North Sea, respectively. Deep convection is not foreseen with any of these features.
To the south, a large surface anticyclone with its center over England creates quiescent synoptic conditions for the majority of Europe. At mid-levels, a stationary cut-off low is placed near the Strait of Gibraltar. Another, smaller one with no surface signal slips from Slovakia to Serbia.
DISCUSSION
... E and NE Spain ...
Decent low-level moisture (2m dewpoints often around 18C) creates CAPE on the order of some hundred J/kg. Main concern for convective initiation is a pronounced, persistent convergence zone which has formed at the forward flank of the cut-off low from the Spanish E coast (near Valencia) to the Tunisian N coast. 10-15 m/s easterly winds are in place ahead of it and pump plentiful moisture towards the Spanish coast. Intense backbuilding thunderstorms started to form on Thursday afternoon along this convergence zone. They will likely persist during much of the current forecast period. The convergence zone moves northward only very slowly. Coastal areas of E (and later NE) Spain will be exposed to heavy rainfall for several hours. Peak accumulations may easily reach 200-300 mm in this time. Dangerous, possibly life-threatening flash floods are likely! This risk remains very high until the evening and only slowly subsides while shifting north into Catalonia overnight.
A similar but weaker risk also covers the Balearic Islands, in case the tail of the backbuilding storms becomes stationary there. In addition, a few non-supercellular tornadoes may spin up at the convergence zone. Weak vertical wind shear keeps the hail and wind risk low.
... Morocco, NW Algeria, S Spain ...
A mid-level jet streak curves around the cut-off low and improves the conditions for organized storms. The intensifying SW-erly flow will advect an elevated mixed layer (EML) offshore, creating CAPE around or above 1000 J/kg under increasing 0-3 km shear (storm relative-helicity) with impressive values up to 25 m/s (400 m^2/s^2) between Morocco and Spain by the end of the forecast period.
Synoptic lifts makes it likely that some elevated convection will emerge from Altocumulus clouds. If it manages to break the capping inversion beneath the EML and root down to the warm and moist maritime boundary layer, very well organized storms may result. The "worst case scenario" would be upscale growth into an MCS with a possibility of embedded supercells. In this case, large to very large hail, heavy rain and a few tornadoes become likely. Since this scenario may unfold only after Sat 06 UTC (or not at all), only a level 1 is issued for the current forecast period.
... Sicily and around ...
Conditions in terms of CAPE and vertical wind shear are similar to the western Mediterranean region - i.e., mostly a few hundred J/kg under weak vertical wind shear, with increasing values of both quantities only to the very south (mainly outside our forecast domain). However, synoptic subsidence under a strengthening mid-level ridge makes convective initiation and sustenance less likely than on Thursday.
The remnants of an intense MCS may still affect parts of Sicily in the morning (especially its east coast). A pronounced flash flood risk extends for some hours into the current forecast period. If the tail end of this MCS continues to survive over the Ionian Sea, or if its outflow boundaries trigger new convection there, better organized storms with a conditional risk of all kinds of severe weather are not ruled out.
Otherwise, deep convection turns increasingly unlikely, though low cloud bases and converging surface winds may promote a few waterspouts, even in case convection is not deep enough for lightning any more. All mentioned risks continue to decrease until Saturday morning.
... Turkey ...
Warm and moderately moist air features a few hundred J/kg of CAPE under weak vertical wind shear. Scattered afternoon storms are expected mainly over orographic features. A capping inversion and a lack of synoptic lift support will probably suppress convective initiation in coastal and offshore areas. Severe weather is unlikely.
