Valid: Wed 24 May 2017 06:00 to Thu 25 May 2017 06:00 UTC
Issued: Wed 24 May 2017 06:32
A level 1 was issued for N Africa and Iberian coastlines mainly for large hail and severe wind gusts.
A level 1 was issued for N / NW Italy, Slovenia and NW Croatia mainly for large hail and severe wind gusts.
At mid to upper troposphere, a ridge stretches from N Africa to France and UK. Strong flow (jet-stream) surrounds this synoptic-scale feature with an embedded short-wave trough, that will quickly translate from Denmark towards N Italy and NW Balkans during the forecast period. Most of the thunderstorm activity will be observed in the area of lower geopotentials and cooler mid-tropospheric temperature over much of E Europe, ahead of the slowly advancing cold front in rather weak pressure gradient field and thus also quite weak vertical wind shear. Further activity is expected also along the coastlines of Iberia and N Africa.
... Iberia and N Africa coastlines ...
Models suggest a narrow zone of overlap of a plume of very steep mid-tropospheric lapse rates with a moist airmass along the coasts. Isolated to scattered initiation will likely be provided by a sea-breeze circulation in interaction with the local terrain as several hundreds to around 1000 J/kg of MLCAPE form with daytime heating. Rather weak vertical wind shear is foreast for Iberia, while around 15 m/s of DLS is simulated over N Africa. Overall severe threat will be higher over N Africa, where well-organised multicells or even brief supercells are anticipated, but pulse storms over coastlines of Iberia may also be capable of large hail and severe wind gusts thanks to steep lapse rates.
... N / NE Italy, Slovenia, NW Croatia ...
A rather complicated scenario will unfold over the region as the northerly flow strengthens during the day, increasing DLS to values around 25 m/s over N and NE Italy towards the evening and night hours. 00 UTC sounding Rivolto sounding shows steep lapse rates up to 600 hPa, likely generated by the downslope winds in the lee of the Alps. With abundant day-time heating, this sounding could yield substantial CAPE. However, satellite imagery reveals thick cirriform cloud cover, likely hindering heating as of now. Furthermore, it is not likely this is going to change much during the day and convective initiation is highly uncertain atm. Several models simulate or hint on CI especially with the passage of short-wave trough embedded in the jet-streak towards the evening and night hours. Therefore, will introduce a Lvl 1 for the possibility of supercells capable of large hail and severe wind gusts in the second half of the forecast period.
... SE to E Europe ...
A large area exists with moderate degree of lower tropospheric humidity and dewpoints around 14 deg C. Several hundreds to around 1000 J/kg of MLCAPE are forecast at the height of the surface heating. Convective initiation will be tied to the advancing cold front, local, often orographically, induced circulations or outflow boundaries left over by former convection. Vertical wind shear will remain rather weak, with mostly disorganised convection over most of the warm sector. Shear will be stronger just ahead of the advancing cold front, perhaps allowing for some well organised multicells. It is difficult to pinpoint any area with enhanced threat as significant cloud cover lingers over much of the area and thus will refrain from issuing any Lvl 1. However, stronger cells may be capable of locally higher precipitation amounts and marginally large hail.