Storm Forecast
Valid: Sat 13 Jul 2013 06:00 to Sun 14 Jul 2013 06:00 UTC
Issued: Sat 13 Jul 2013 08:32
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for parts of Portugal, Spain and Algeria for large hail and severe wind gusts.

A level 1 and level 2 were issued for N and Central Italy for large hail, severe wind gusts, tornadoes and excessive precipitation.

A level 1 was issued for W Slovenia, W Croatia and W Bosnia-Hercegovina for large hail and to a lesser extent for severe wind gusts.

A level 1 was issued for SW Ukraine for severe wind gusts.

A level 1 was issued for parts of Belarus for large hail and severe wind gusts.

SYNOPSIS

The Eastern European cut-off low decelerates and gradually fills up over Belarus and the Ukraine. It is connected to another shallow upper-level low over Northern Portugal by a weak zonal upper-level trough. This pattern results in a continuancy of rather low 500 hPa temperatures across the Southern half of the continent.
The subtropical jet stream runs straight across the Southern Mediterranean and the Maghreb states, where the cap beneath the Saharan elevated mixed layer is too strong to allow convection. The polar jet is displaced to the far North, where a distinct speed maximum enters Scotland and Norway behind a positively tilted short-wave trough.
Most of the convection on Saturday will occur in the broad region in-between, where regionally very high instability, mesoscale augmentation of vertical wind shear and various foci for dynamic forcing set the stage for an active thunderstorm day despite a rather inconspicious large-scale weather pattern.

DISCUSSION

... N Portugal, Spain ...

The daily thermal low will reinforce on Saturday in response to diurnal heating, and rich low-level moisture will advect inland again with onshore and upslope flows along the Spanish North coast. The surface and sounding observations of the last few days have revealed that the circulation of this heat low extends upward to as high as 700 hPa with 5 to 10 m/s Northeasterly winds. This means that the moisture will be deep enough to reduce its exposure to daytime mixing processes. After a series of days with afternoon and evening thunderstorms, local moisture ¨recycling¨ can play an additional favourable role. Satellite imagery of the last couple of days suggested strong instability release with well-organized storms as soon as this moist boundary layer was heated enough to break the cap and connect with the very steep mid-level lapse rates aloft. Not much doubt is left that the CAPE maxima between 2000 and 3000 J/kg in the forecast model will materialize again in a narrow, West-East aligned belt over Northern Spain on Saturday, though this instability will be strongly capped along the coast.
Deep-layer shear is in the 10 to 15 m/s range on a large scale and may again be augmented to 20 m/s in case of a well-established sea breeze and upslope flow regime. The novelty in Saturday´s setup is a vorticity lobe that swings northeastward and overspreads the region with large-scale lift, which will weaken the cap and facilitate convective initiation over Northern Spain. Scattered to widespread thunderstorms will form in the afternoon and evening hours, and a quick organization into strong multicells and a couple of supercells is forecast. A level 2 was issued for the region with the expected highest storm coverage for a chance of large to very large hail and severe downbursts. Storms may grow upscale into one or two MCSes and can persist well into the night, while the severe weather threat slowly diminishes with cooling surface layers. Northern Portugal was included into a level 1, since various models show signals that the storm clusters start to move Southwestward overnight on the back side of the upper-level low´s center.

The areas further inland in Central Spain will again see very dry and deeply mixed profiles. Isolated thunderstorms may form in the afternoon and evening hours, first over orographic features and later along an organized outflow front which may move Southward from the activity over Northern Spain, similar to Friday (see HRV satellite imagery). Severe (dry) downbursts and large hail are possible with stronger storms. However, strong entrainment will keep the thunderstorm coverage low.

Higher instability may build up again along the coast of Eastern Spain and in the Ebro valley, where moist Easterly upslope flow establishes. In an environment of 1000 to 2000 J/kg CAPE and a local augmentation of deep-layer shear to 20 m/s (along with some low-level veering), a similar scenario like in Northern Spain can be expected with any storm that forms: a quick organization into a strong multicell or even supercell with a significant threat of large to very large hail and severe wind gusts. However, the lift support of the vorticity maximum is weaker and arrives later. This means that the strong cap may suppress most of the convection, and confidence in more than isolated and struggling storms is not high enough for another level 2 area over Eastern Spain.

... Western Med, Sardegna, Corsica ...

A small but pronounced short-wave trough travels eastward. Forecast soundings show almost saturated mid-levels with moist-isentropic lapse rates, resulting in extensive mid- and upper-level cloudiness. The trough´s lift will likely be converted into elevated convection with scattered, locally thundery showers over the Western Mediterranean Sea and Sardegna. It is unlikely that this convection will root down into the strongly capped boundary layer and benefit from the rich low-level moisture and good shear conditions.
Further North, Corsica will see scattered, surface-based afternoon storms again under weaker vertical wind shear. They might bring a localized heavy rainfall event, but due to their brief nature and limited coverage no level 1 is issued.

... Italy ...

A voriticity maximum is approaching from the Northwest and will spread Northern and Central Italy and the Balkans till the evening. Very rich low-level moisture with surface dew points up to 22C is still in place in the wake of the Alps and along the shorelines. 00 UTC soundings from Milano and San Pietro Capofiume already showed plenty of CAPE, and with daytime heating and further steepening lapse rates ahead of the trough CAPE around 2000 J/kg can be expected. Moderate Northwesterly flow aloft results in deep-layer shear around 15 m/s, in the course of the day probably even enhanced to 20 m/s with low-level onshore flow along the Italian East coast.
In the second half of Friday night, a long-lived and right moving storm (probably a supercell) already formed over Lombardia and travelled southward into Northern Emilia-Romagna. Currently it seems to be weakening, but further storms are expected to initiate throughout the day and even persist into the night. Highest coverage is expected over the Southern Alps and the Appennin mountains in the afternoon hours, but at least scattered storms are forecast to make it into the lowlands as well. They may bring the full spectrum of severe weather with large hail, severe wind gusts, excessive rain and even one or two tornadoes. A level 2 was drawn for the region with Easterly onshore winds and best low-level moisture, where instability as well as shear and low-level veering (0-3 km SRH between 100 and 150 m²/s²) seem to be maximized. Otherwise, it was decided to stay with a high-end level 1.

... W Balkans ...

Lift ahead of the trough and daytime heating will aid in the (re-)buildup of low to moderate CAPE in an environment of 15 to 20 m/s deep-layer shear. Scattered afternoon and evening storms are forecast especially along the Dinaric mountains, which may organize into multicells. Isolated large hail and (to a lesser degree) severe wind gusts are possible. Further South, only isolated storms will form and no threat level seems to be necessary.

... E Europe ...

In the range of the old cut-off low, convective activity will be maximized in two zones:
(1) In a belt along the decaying occlusion from Central Ukraine across Belarus into Lithuania, again CAPE in the order of 500 to 1200 J/kg will likely be realized, and scattered to widespread thunderstorms can be expected. Towards the rear edge of this belt, deep-layer shear increases to 15 m/s, and a short-wave trough swings Northward into Belarus. In this area, better storm organization into multicells with a threat of large hail and severe wind gusts can be expected. Due to a storm motion almost parallel to the boundary, localized excessive rain becomes another, subordinate threat.
(2) Cool air convection with scattered, rather low-topped thunderstorms will follow over Eastern Poland and the Western Ukraine. CAPE is limited to a few hundred J/kg and most of the activity will be placed beneath weak vertical wind shear near the center of the low, hence an isolated landspout seems to be the only risk. However, further South, deep-layer shear increases to 20 m/s and another potent short-wave trough travels from Poland into the Southwestern Ukraine. In this region, a handful of strongly forced multicells with a chance of isolated severe wind gusts can be expected, and a low-end level 1 is issued. These storms will move eastward and will later be quenched by the subsequent back-bent occlusion.

... Norway, Sweden ...

A cold front from the West will move into an area where low CAPE is expected to build up in response to diurnal heating and vorticity advection aloft. Isolated to scattered, low-topped thunderstorms are possible in the afternoon and evening hours. Deep-layer shear would be sufficient for multicell development (15-20 m/s), and the models even show signals resembling a convective line with a cold pool formation which would enhance the severe wind threat. However, latest surface observations indicate a rather cool and moist boundary layer, i.e. even after some daytime heating, not the best environment for such a scenario is anticipated. Besides, the strongest QG lift will slump northeastward over Norway and will only graze the unstable area. Hence it was decided to go without a level 1.

... British Isles ...

Isolated to scattered afternoon thunderstorms are likely under anticyclonic influence. Both instability and shear will be too weak for an augmented severe weather threat.