Severe Weather Outlook – Aug 18th, 2020
Severe weather outlook – forecast across Europe. This forecast features areas of organized severe weather with risk levels and severe weather threats across the European continent.
SEVERE WEATHER OUTLOOK – DAY 1
Valid: 18/08/2020 06 UTC – 19/08/2020 06 UTC
Issued by: Severe Weather Europe
Forecaster: Marko Korošec
Another day with widespread severe storms, including supercells with a threat for large hail, severe winds, and torrential rainfall is expected from northeast Germany across Poland, Czechia, Slovakia into Austria and the north-central Balkan peninsula. A powerful extratropical cyclone forms over the North Atlantic.
Overview of the risk areas across Europe
A large deep upper low over the North Atlantic continues strengthening, while an unseasonably strong surface extra-tropical cyclone forms. Remnants of the ex-tropical storm Kyle arrive in Ireland and England. To its east, a broad channel of low geopotential spreads across central Europe and the Balkans. A shallow frontal system is moving south across northeast Europe.
+++ Italy, Slovenia, Croatia, Hungary, Bosnia and Serbia +++
ENH risk has been issued for northeastern Italy, southern Slovenia, Croatia, Bosnia into central Serbia and Kosovo with a threat for severe storms, capable of producing large hail, severe winds, and torrential rainfall with flash floods.
A short-wave continues east across the northern Balkans and will again serve as the main focus for convective initiation during the day. Moderate to locally strong instability is still present, coupled with marginal to moderate shear. Shear is decreasing from north to south.
Organized storms, including numerous multicells and a few supercells, are expected to form by the afternoon, gradually growing while moving towards the east-southeast. Bowing segments are possible. Large hail and severe winds are likely with the most intense storms. High moisture should also contribute to torrential rainfall and locally flash floods, especially if training cells would occur (western Balkans).
Storms are likely to merge into a large cluster or two in the evening hours and continue over the Balkans overnight.
SLGT risk has been issued for areas surrounding the ENH risk across northern Italy, the Alps, Austria, Hungary, western Romania, and Serbia with more isolated severe storms. Similar conditions should support mainly multicell storms, and possibly a supercell or two as well. Primary threats will be large hail, severe winds, and torrential rainfall with flash floods.
+++ north-central Germany +++
ENH/SLGT risks have been issued for northeastern Germany, southern Denmark, Czechia, Slovakia, southern Poland into western Ukraine with a threat for severe storms, capable of producing severe winds, marginal hail, and torrential rainfall with flash floods.
A decaying wave will continue moving towards the northeast and support additional storms from the afternoon into evening hours. Moderate instability remains, but the shear is low. Some forcing should allow scattered storms to form along with the frontal wave, including intense multicells. Marginal hail and severe winds will be possible, but torrential rainfall and locally flash floods will remain the highest threat.
+++ other areas +++
TSTM risk areas have been placed across England and Scotland, Bay of Biscay, France, Benelux, southern Scandinavia, Belarus, and Georgia with a threat for daytime driven storms. Limited shear is present, so the storms should remain sub-severe.
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Understanding Severe Weather Outlook
Severe Weather Outlook features areas of organized severe weather with risk levels and severe weather threats. Risk levels are divided into seven categories:
TSTM – Thunderstorms
MRGL – Marginal risk
SLGT – Slight risk
ENH – Enhanced risk
MDT – Moderate risk
HIGH – High risk
SIG – Significant risk
WNTR – Winter risk
Risk categories stand for the coverage and intensity of organized severe weather. Those could include supercells, squall lines, mesoscale convective systems, wind storms, flooding, snowstorms, or ice storms.
Severe weather threats include:
- large hail (of at least 2 cm in diameter)
- Tornadoes (including waterspouts)
- Wind gusts (convective or non-convective) above 25 m/s (or above 90 km/h)
- Torrential convective precipitation / Flash floods
- Excessive rainfall (100 mm within 12 hours) / snowfall (50 cm within 12 hours)
Extremely severe weather threats include:
- Large hail (of at least 5 cm in diameter)
- Tornadoes of F2 intensity or stronger
- Wind gusts (convective or non-convective) above 33 m/s (or above 119 km/h) or 12 Bft
- Torrential convective precipitation / Flash floods
- Excessive rainfall (150 mm within 12 hours or above ) / snowfall (above 100 cm within 24 hours)
Categories in the forecast represent the chance of severe weather occurring within a 40 km radius from a location. The used level is based on the conversion table of probabilistic risk into the outlook categories. A threat level is upgraded into a higher category if probabilities meet the threshold criteria for the specific threat (e.g. tornado, wind, hail, or rainfall threat).
Each individual threat area includes a detailed forecast map and discussion on the potential of severe weather threats.