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Severe Weather Outlook – Aug 22nd, 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: 22/08/2020 06 UTC – 23/08/2020 06 UTC
Issued by: Severe Weather Europe
Forecaster: Marko Korošec
 

SUMMARY

 
Severe storms are expected to form along the cold front associated with the large trough over northern Europe. Conditions should support isolated supercells from Baltic into the Alpine region on Saturday afternoon.
 

Overview of the risk areas across Europe

 
weather forecast
 

Explanations for abbreviations (TSTM, SLGT, ENH, etc.)
 
SYNOPTIC OVERVIEW

 
A large upper trough is moving across Northern Atlantic towards norther Europe with a surface cold front extending across central Europe and the Alps. An elongated upper-level ridge is stretched from northwestern Russia across eastern into southern Europe.

FORECAST DISCUSSION

 

+++ France, Italy, Switzerland, Austria, Slovenia, Hungary into Czechia, Slovakia and Poland +++

 
weather forecast area 2
 

ENH risk has been issued for the east-central Alps, including northeastern Italy, north-northeast Slovenia into east-central Austria, and extreme northwest Hungary with a threat for severe storms, capable of producing large hail, severe winds, and torrential rainfall.

Storms will mainly form along the moving surface cold front from the early afternoon onwards. Around 1000-1500 J/kg of MLCAPE, coupled with moderate 30-40 knots deep-layer shear should favor strong multicells and isolated supercell storms. Those will be capable of producing large hail and severe winds.

Storms will likely cluster into the MCS in the evening hours, traveling off the southeast Alps towards east-southeast.

SLGT risk has been issued for areas surrounding the ENH risk from southeast France across the Alps towards Poland. Only isolated severe storms with large hail and severe winds will be possible.

+++ Norway, Denmark, Sweden and Finland +++

 
weather forecast area 1
 

SLGT risk has been issued for part of Norway, Sweden, Finland, and Denmark with a threat for severe storms, capable of producing severe winds, marginal hail, and heavy rainfall.

A large long-wave trough is moving across northwestern Europe with a broad area of cool maritime air mass spread into Scandinavia. Marginal CAPE but moderate shear could again support some organized storms with marginal hail, severe winds, and heavy rainfall throughout the day.

+++ other areas +++

 
MRGL risks have been issued for northern Morocco, northeastern Algeria, northeast Turkey, and Georgia with a threat for isolated severe storms, capable of producing severe winds and large hail.

Marginal shear and instability could result in some better-organized storms, but coverage is strongly limited due to a lack of large-scale forcing.
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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.

Read more: Explanations for abbreviations (TSTM, SLGT, ENH, etc.)

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