Severe Weather Outlook – Aug 16th, 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: 16/08/2020 06 UTC – 17/08/2020 06 UTC
Issued by: Severe Weather Europe
Forecaster: Marko Korošec
Severe storms with a threat for large hail, severe winds, and torrential rainfall are expected along the front across parts of France and Benelux into southern England. Slow-moving storms with torrential rainfall and flash floods are expected from the eastern Alps into Slovakia and southern Poland on Sunday.
Overview of the risk areas across Europe
An upper ridge persists over the North Atlantic and expanding into northern Europe. To its west, a new low is deepening and moving south. A deep low over northern Scandinavia is gradually weakening and ejects into the Arctics. A shallow upper low is rotating over Slovakia and Czechia. Short-wave moves into the eastern Black Sea region.
+++ England, France, Belgium and the Netherlands +++
ENH/SLGT risks have been issued for England into northeast France, Belgium, western Germany, and the Netherlands with a threat for severe storms, capable of producing large hail, severe winds, and torrential rainfall with flash floods.
A rather well-defined surface front races north-northeast during the afternoon and evening hours, being the main focus for organized storms. Quite strong instability is likely to build up ahead of the front, while shear around 25-40 knots is present.
This should allow organized storms to form along the front, including intense multicells and a few discrete supercells. Large hail and severe winds will be possible, but also torrential rainfall and locally flash floods threat. As shear and frontal boundary are almost parallel, storms could maintain over the same areas for a longer period and bring lots of rain.
Further south across France, storms could be severe but should be more isolated in nature due to lack of instability and upper-level forcing. Those could potentially bring some marginal hail and heavy rainfall. SLGT risk has been extended into northwest France where lots of rain is possible on the wake of surface low.
+++ Austria, Slovenia, Hungary, Slovakia, Czechia and Poland +++
SLGT risk has been issued for eastern Austria, northeast Slovenia, northern and western Hungary, eastern Czechia, west-central Slovakia and southern Poland with a threat for severe storms, capable of producing severe winds, marginal hail, and torrential rainfall with flash floods.
Another weak upper low will be the main focus for a rather widespread convective activity. Moderate CAPE builds up under very low shear. Storms are expected to form over local topography features with torrential rainfall and flooding threat.
Weak capping should allow storms to start by early afternoon. Small storm clusters are possible, producing locally lots of rain and flash floods threat due to their slow-moving nature. Storms could continue into the evening and night hours.
+++ Algeria, Tunisia, Sicily, the central Balkans and Georgia +++
MRGL risks have been issued for northern Algeria into northern Tunisia and Sicily, south-central Balkans, and Georgia with a threat for isolated severe storms with large hail, torrential rainfall with flash floods, and severe winds.
Moderate shear and strong instability bring another day with isolated threat for a few supercells over Algeria and Tunisia, those could even reach Sicily.
More slow-moving storms are expected over the Balkans, with primarily a flash floods threat. High rainfall sums are possible.
A short-wave passage over Georgia and surroundings could bring a few strong to severe storms, fueled by weakly to moderately unstable air mass and shear.
+++ other areas +++
TSTM risk areas have been placed across northeast Ukraine into western Russia, Italy and the Alps 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.