Severe Weather Outlook – Aug 17th, 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: 17/08/2020 06 UTC – 18/08/2020 06 UTC
Issued by: Severe Weather Europe
Forecaster: Marko Korošec
Widespread severe storms, including supercells with a threat for large hail, severe winds, and torrential rainfall are expected across parts of northern Italy across Austria and Slovenia towards Czechia and Slovakia on Monday. Another area for severe storms is along the shallow front across northern Germany.
Overview of the risk areas across Europe
An upper ridge over western and northern Europe is collapsing while a large upper low over the North Atlantic significantly strengthens. An upper low over France opens into a long-wave trough and spreads towards central Europe. A large trough over western Russia pushes a surface frontal system towards the south.
+++ Italy, Austria, Czechia, Slovakia, Poland and Slovenia +++
ENH risk has been issued for northeastern Italy, Austria, northwestern Slovenia, eastern Czechia, western Slovakia into southern Poland with a threat for severe storms, capable of producing large hail, severe winds, and torrential rainfall with flash floods.
A short-wave moving from west to east will be the main focus for convective initiation during the afternoon and evening hours. Moderate to locally strong instability is present, coupled with marginal to moderate shear. Shear is higher over Slovenia and north Italy.
Organized storms, including a few supercells are likely to form by the afternoon, gradually growing while moving towards the east-northeast. Large hail and severe winds are possible with the most intense storms. High moisture should also contribute to torrential rainfall and locally flash floods.
Very low shear but strong instability will be particularly supportive of flash floods from eastern Austria across eastern Czechia and western Slovakia into southern Poland. Widespread storms are expected there.
Storms are likely to merge into a large cluster or two over the Alps in the evening hours and continue overnight towards Slovenia and southern Austria.
SLGT risk has been issued for areas surrounding the ENH risk across northern Italy, the Alps, southern Germany, Czechia, and Poland where isolated severe storms are possible. While slow-moving storms with flash floods are the primary threat north of the Alps, isolated supercells are more likely across northern Italy.
+++ north-central Germany +++
ENH/SLGT risks have been issued for north-central Germany, the Netherlands with a threat for severe storms, capable of producing severe winds, marginal hail, and torrential rainfall with flash floods.
A decaying surface front will continue moving towards the northeast and support additional storms from the afternoon into evening hours. Moderate instability remains, but the shear weakens. However, favorable forcing should allow numerous storms to form along the front, including intense multicells. Marginal hail and severe winds will be possible, but also torrential rainfall and locally flash floods threat.
MRGL risk has been issued for England into southern Scotland and France under the cold air aloft, allowing a few isolated storms to form. A marginal threat for some large hail and heavy rainfall exists.
+++ other areas +++
MRGL risks have been issued across northeastern Algeria into northern Tunisia and across western Russia where some isolated severe storms are possible. Mainly with a threat for large hail and severe winds.
TSTM risk areas have been placed across southern Norway, the Balkans, western Mediterranean 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.