Storm Forecast
Valid: Sun 15 Jun 2014 06:00 to Mon 16 Jun 2014 06:00 UTC
Issued: Sat 14 Jun 2014 21:27
Forecaster: GATZEN
A level 1 was issued for north-eastern Spain mainly for large hail.
A level 1 was issued for the south-western Mediterranean mainly for large hail and severe wind gusts.
A level 1 was issued for the central Mediterranean mainly for excessive precipitation and large hail.
A level 1 was issued for the Aegean region including Macedonia and Bulgaria mainly for large hail, excessive precipitation, and to a lesser extend for severe wind gusts.
A level 1 was issued for central Turkey mainly for severe wind gusts and large hail.
SYNOPSIS
A long-wave trough has established over Europe, with its centre over southern Scandinavia and western Russia. Northerly winds still advect cool and dry air into most regions, whereas the overlap of rich moisture and steep lapse rates is pushed further south into the central and southern Mediterranean. Cold mid-level air is present near the base of the trough over western Russia and parts of eastern Europe, and steep lapse rates will form in response to diurnal heating. Another area of steeper lapse rates will be a cut-off low that forms over France and the west Mediterranean.
DISCUSSION
North-eastern Spain
The mid-level trough centre will be located over north-eastern Spain on Sunday. Cold air advection will be in place and dry air masses are expected to spread eastward over northern Spain. During the day, diurnal heating will help to sustain steep lapse rates to the south of the Pyrenees. Additionally, some moisture return is expected due to the sea breeze near the Mediterranean Sea. A brief overlap of these factors may lead to CAPE of a few 100s J/kg in the noon and afternoon.
Although this instability will be slightly capped, initiation is quite likely in the cool mid-level air mass given low-level convergence and upslope flow. Storms may then organize given 15 m/s 0-3 km vertical wind shear due to easterly surface winds of the sea breeze and northerly winds aloft. Some large hail is not ruled out as a consequence. The set up is quite marginal and a low-end level 1 is issued.
Storms will rapidly decay in the evening when the sea breeze ends, leading to dry air advection from the west.
South-western Mediterranean
Cold air advection is underway over the west Mediterranean ahead of the approaching cut-off low. On Sunday, a strong mid-level jet streak evolves to the south of the Balearic Islands due to an increasingly sharp frontal temperature gradient. In the noon hours, weak low-level warm air advection is expected along the front due to slightly southward turning winds at 850 hPa.
Given robust moisture evolving near the frontal boundary as well as steep lapse rates, CAPE is forecast and thunderstorms may form. These storms will have a rather good potential to organize given strong deep layer vertical wind shear due to easterly surface winds below the strong mid-level jet streak. Latest GFS indicates up to 30 m/s deep layer vertical wind shear. Therefore, supercells and bow echoes may develop capable of producing large hail and severe wind gusts.
The main limiting factor may be missing lift to break the cap. Latest models do not indicate strong low-level convergence. However, current thinking is that local modifications of the low-level wind field will allow for initiation, as well as outflow boundaries and sea-breeze convergence zones. Storms will start to spread south in the wake of the jet streak, and upslope flow may allow for storms over northern Algeria as well. Large hail seems to be the main threat with these storms as vertical wind shear will still be rather strong.
Central Mediterranean
At the eastern flank of the Iberian cut-off low, weak flow is expected over the central Mediterranean. Rich low-level moisture is expected to remain over most places, although dry air advection gradually increases from the east. An overlap with remaining steep lapse rates is likely over most places, and CAPE near 1000 J/kg is reasonable. Storms are forecast along sea-breeze or outflow boundary convergence lines through-out the period. Given the weak vertical wind shear, storm organization is limited. Clusters of slow-moving storms may pose a threat of excessive precipitation, whereas pulse storms can also produce some large hail. Given the high coverage of storms, a low-end level 1 is issued.
East Mediterranean to Turkey and Bulgaria
Ahead of the cut-off low over the west Mediterranean, a southerly flow will evolve over the eastern portions including the Aegean Sea. An embedded short-wave trough will travel north-east into the Aegean Sea on Sunday, providing cooling mid-levels due to QG forcing.
The affected low-level air mass will quite unstable given a broad overlap of rich low-level moisture and steep lapse rates. Storms will start over the mountains of Greece, Macedonia, Bulgaria, and Turkey and spread into lower terrain later on. Limited storm organization due to weak vertical wind shear is forecast, but some stronger storms are not ruled out with large hail the main threat. The high coverage of storms also posed a threat of excessive precipitation and isolated severe wind gusts.
The threat of severe wind gusts will increase over Turkey, where inverted-v profiles indicate a deep boundary-layer, supportive for dry microbursts. On the other hand, the more isolated nature of the storms over Turkey will limit the threat of excessive precipitation. Storms will weaken after sunset, but a few storms may go on until the morning.
