Storm Forecast
Valid: Fri 08 May 2020 06:00 to Sat 09 May 2020 06:00 UTC
Issued: Thu 07 May 2020 23:58
Forecaster: PISTOTNIK

A level 1 is issued for N Portugal, N Spain and far-SW France for large hail, excessive convective precipitation and severe convective wind gusts.

A level 1 is issued for the Bay of Antalya for excessive convective precipitation and non-mesosclonic tornadoes (waterspouts).

SYNOPSIS

The main frontal zone is displaced far northward and runs from Iceland across Scandinavia to NW Russia. To the south, temperature and pressure gradients are weak and the forecast maps look typically spring-like. Two cut-off lows move leisurely eastward over the Ukraine and the eastern Mediterranean Sea. Another one approaches Portugal from the west. In-between, the area south of the British Isles and Poland is covered by a broad ridge and sees a return of warm air with SW-erly flow.

DISCUSSSION

... N Portugal, NW half of Spain, France ...

A diffuse warm front lifts north ahead of the approaching cut-off low. An elevated mixed layer (EML) spreads from the Moroccan Atlas and the Spanish Plateau towards the Bay of Biscay and France in the warm sector, and CAPE forms in response to daytime heating and low-level moisture trapped beneath the capping inversion. The SW-erly flow provides moderate vertical wind shear (0-3 km: 10-15 m/s) under veering wind profiles especially near the warm front (0-3 km storm-relative helicity between 100 and 200 m^2/s^2).
Aided by synoptic lift, one or two clusters of elevated, embedded convection will likely drift NE-ward from N Spain to W France already in the morning and continue or re-form periodically over central and NE France during the entire forecast period. A higher storm coverage is expected in the evening and overnight, when isentropic lift by warm air advection increases. The probability of large hail and excessive convective precipitation rises as soon as storms become surface-based, but even in the afternoon, it is questionable if this will happen due to CAPE limited to a few hundred J/kg and a rather strong capping inversion. Hence these regions fall just below a level 1. The leading activity of elevated convection can reach S Germany by Saturday morning.
Strong insolation in the warm sector allows up to 1000 J/kg CAPE over the northern parts of Portugal and Spain as well as the French Pyrenees forelands. By early afternoon, the cap should break over the mountains, and scattered to widespread thunderstorms are expected. Multicells will be the dominant mode. A high-end level 1 is issued for large hail, excessive precipitation and isolated severe downbursts. Storms quickly become elevated and lose their severe weather potential when the move over coastal areas and the Bay of Biscay, and generally weaken after sunset. It is unclear if they manage to move into SW France in the evening, where CAPE and shear are similar but the capping is stronger and synoptic lift weaker.

... SE half of Spain, N Morocco and N Algeria ...

Sea breezes penetrate far inland at this time of the year, feeding some moisture beneath the EML. An overlap of CAPE (a few hundred, possibly locally up to 1000 J/kg) and enhanced vertical wind shear forms in the sea breeze regime which would be favorable for well-organized storms. However, it is highly unlikely that daytime heating and the lift of upslope flows will suffice to break the enormous cap.
Forecast models do, however, agree on increasing elevated convection (probably from Altocumulus levels) in the evening and overnight, when persistent warm air advection and several vorticity maxima create synoptic lift. These storms will very likely stay non-severe.

... interior SE Turkey ...

A cold front slowly moves SE-ward across the Turkish highlands. On its warm side, some patches of low-end CAPE may form under very strong vertical wind shear (15-25 m/s) but critically depend on sufficient daytime heating and pockets of enhanced moisture. Some embedded storms are possible near the front line. If prefrontal storms manage to form in the afternoon and propagate off the long and straight hodographs, they could organize into supercells with a risk of large hail and severe wind gusts. However, it appears more likely that forming updrafts will rather be sheared apart than benefit from the strong kinematics. Severe weather is therefore quite conditional and only a low-probability thunder area is issued for now.

... Turkish south coast around the Bay of Antalya ...

With onshore flow and near the center of the cut-off low, moisture-laden air with around 500 J/kg CAPE is exposed to fairly strong lift. Storms will extend into the 06-12 UTC time period of this forecast. Flash floods are possible, as well as (non-mesoscylonic) waterspouts along the nocturnal land breeze front or other zones of strong convergence. Convection will quickly weaken afterwards.