Storm Forecast
Valid: Sat 12 Aug 2017 09:00 to Sun 13 Aug 2017 06:00 UTC
Issued: Sat 12 Aug 2017 09:15
Forecaster: TUSCHY

A level 3 was issued for parts of Latvia, Estonia and surroundings mainly for tornadoes (strong ones possible), severe to damaging wind gusts and very large hail.

A level 2 surrounds that level 3 with similar hazards but lower probabilities. In addition, excessive rain is forecast.

A level 2 was issued for parts of E-CNTRL and SE-Europe mainly for large to very large hail, excessive rain and severe wind gusts. A few tornado events are possible.

A level 1 surrounds all level areas with a similar risk but with lower probabilities.

A level 1 was issued for N-Algeria mainly for large hail and excessive rain.

SYNOPSIS

A deep longwave trough is analyzed from Scandinavia all the way down to the CNTRL-Mediterranean. It is forecast to weaken during the following hours and despite its effort to choke its southern part off the westerlies, no evidence of a real cut-off process exists yet. Still, during the end of the day, this trough is mainly composed of one strong cold-core vortex over the Adriatic Sea and a broad northern part, where numerous progessive short-waves emanate from the main trough and affect the southern parts of Scandinavia. In between, over CNTRL-Europe, the structure of that trough weakens constantly during the day.
Downstream ridging results in muggy and hot conditions over SE-Europe, as moist and warm air advects north.
Upstream, a zonal flow pattern with numerous embedded mid-level waves becomes established over NW-Europe. Hot and stable conditions persist over SW-Europe.

Latest surface analysis indicates two high pressure areas, one affecting SW-Europe and another one being analyzed over parts of Belarus, N-Ukraine into W-Russia. Weak pressure gradients exist in between, but a low pressure channel with numerous minima also accompanies the aforementioned longwave trough. One LL depression can be found over Hungary with another one over the S-Baltic Sea. The latter one drags a warm front to the NE, stretching a broad warm sector over the Baltic States, where the main severe risk for today evolves. Attendant cold front runs south and merges into a wavy and quasi-stationary frontal boundary, which extends all the way to the CNTRL-Mediterranean. It separates a warm, moist and unstable air mass to its east fromt a cooler and more stable one to its west and therefore also serves as focus for thunderstorm activity during the day.

DISCUSSION

... Lithuania, Latvia, Estonia, parts of Belarus and SE-Finland ...

A broad warm sector is evident in latest data with surface dewpoints in the upper tens to lower twenties. A persistent south/southeasterly flow brought the moist air mass into the warm sector and 00Z soundings in this advective flow regime also show an unseasonably deep layer with high moisture content (e.g. 33041 Gomel) next to widespread 13 g/km mixing ratio or more, averaged over the lowest 1000 m. This moisture affects all of the Baltic States and extends into Belarus with gradually decreasing values further to the south.

Aloft, a pool with steep mid-level lapse rates advected over E-Europe north during the night. It now affects the warm sector with upstream soundings showing a well mixed mid-level air mass (e.g. 33317 Shepetivka). As a result numerical guidance agrees well in the development of a warm sector with strong to extreme CAPE values. MLCAPE will be in the 1500 to 3000 J/kg range with peak values probably confined to the area with strong BL convergence and maximized BL moisture (e.g. E-Latvia and far N-Belarus). MUCAPE runs even higher.

Although a bit displaced from the frontal zone, DLS is more than adequate to sustain long-lived updrafts and a W-E gradient of isoshear exists with 20-25 m/s 0-6 km shear to the west and 10 m/s to the east of the warm sector.

During the daytime hours, forcing will be the main problem as most of the warm sector still resides beneath an anticyclonically curved mid-level flow regime. In addition, a cap may hold up for some more time although missing sounding data exacerbates the assessment of the final cap strength. With full sunshine and departing, thinning clouds only confined to Estonia right now (08 UTC), cap will certainly weaken during the following hours. During the afternoon hours, isolated CI is possible over W-Lithuania and W-Latvia, where a confluent flow regime is analyzed along the slowly eastward shifting cold front. Any developing storm will already become severe with large hail and severe wind gusts a risk. Elevated convection is likely over Estonia with an isolated large hail risk. A very isolated storm could also evolve further into the warm sector (e.g. further east) along mesoscale boundaries. Again, all hazards will be possible with such an event.

During the afternoon and especially during the evening hours, scattered to widespread CI occurs, as a strong short-wave approaches from the W and the area resides beneath the right entrance of a 30 m/s mid-level jet streak. Expect a mixture of new CI or further intensification of already ongoing convection.

The combination of a strong forcing overspreading the eastward sliding cold front, 20 m/s 0-6 km and 0-3 km shear and near front parallel aligned shear vectors with the mentioned CAPE indicate a good chance for a long-lived bow echo or potential derecho to affect the highlighted area during the evening hours into the night. Widespread severe wind gusts with numerous extreme events are well possible. Embedded supercells may also pose a large hail risk and the resulting big cluster also poses an excessive rainfall risk.

Further east towards N-Belarus and E- Latvia, weaker synoptic scale forcing and placement ahead of the cold front in the open warm sector may keep thunderstorms a bit more isolated. CAPE/shear space supports long-lived supercells with all hazards possible inlcuding giant hail, severe to damaging wind gusts and excessive rain. Despite somewhat high LCLs during the afternoon and evening hours, the environmental conditions for tornado evelopment constantly improve during that time and forecast soundings show strongly curved hodographs. Significant and long-lived tornadoes are possible. The final magnitude of tornado activity depends on how many storms can stay discrete for some time. Betimes this activity also grows upscale into a cluster (potential severe MCS) during the night, which races to the N/NE with all kind of hazards possible.

As extreme events are possible and due to the chance for a long-lived MCS (derecho) event or an outbreak of strong tornadoes (depending on the final storm mode), a level 3 was issued.

Given the favorable upper-level pattern and the strength of the short-wave, we went far north with the level areas to indicate the chance for a long-lived intense MCS leaving Estonia north and approaching SE-Finland thereafter. Decreasing CAPE may be easily offset by strong LL and deep-layer shear so a constant risk of organized convection exists. In addition, surface based convection is possible all night long, so we would not be surprised to see a severe overnight MCS over parts of Finland with severe wind gusts (isolated extreme events are still possible), large hail and excessive rain. Discrete cells also pose a tornado threat next to isolated embedded spin-ups.

... E-Poland to the Adriatic and Ionian Sea ...

The combination of a quasi-stationary frontal boundary and the approaching large-scale trough create a favorable environment for scattered to widespread CI over a broad area. BL moisture is maximized along the front and in general over offshore areas beneath the upper trough with values in the 500 to 1200 J/kg range. DSL of 15-20 m/s is enough for organized updrafts and multicells/isolated supercells with large to very large hail, severe wind gusts and excessive rain are forecast. An isolated tornado risk exists mainly along the front itself but also offshore and along the cost with augmented LLCAPE. A favorable upper-level pattern with strong divergence indicates the chance for a large cluster with an augmented excessive rainfall risk over parts of the CNTRL-Balkan States.

... Atlas Mountains ....

Scattered thunderstorms are forecast during the day . 15-20 m/s DLS and 400-800 J/kg MLCAPE support multicells with large hail and excessive rain. The activity gradually weakens during the night.