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Severe Weather Outlook – Aug 14th, 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: 14/08/2020 06 UTC – 15/08/2020 06 UTC
Issued by: Severe Weather Europe
Forecaster: Marko Korošec
 

SUMMARY

 
Severe storms with a threat for large hail, severe winds, and torrential rainfall with flash floods are again expected across a large part of central Europe, especially around the Alps into the northern Balkans. A short-wave should provide enough lift for storm initiation under strongly unstable air mass and enhanced shear on Friday.
 

Overview of the risk areas across Europe

 
weather forecast

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

 
An upper ridge begins strengthening over western Europe and North Atlantic while a very deep low moves from Greenland towards Scandinavia. A new upper low is moving towards the Iberian peninsula. Remnants of the upper low over France act as a short-wave trough across the Alps into northern Balkans. The upper low over western Russia begins weakening while moving east.

FORECAST DISCUSSION

 

+++ Austria, Germany, Switzerland, Slovenia, Italy, Czechia, Hungary, Croatia and Bosnia +++

 
weather forecast area 1
 

ENH risk has been issued for southeast Germany, Czechia, Austria into east-central Slovenia, western Hungary, Croatia and northern Bosnia with a threat for severe storms, capable of producing severe winds, large hail, and torrential rainfall with flash floods.

SLGT risk has been issued for areas surrounding the ENH risk area including Switzerland, north-central Italy, west-central Balkans, western Slovakia into southwestern Poland with more isolated threat for severe winds, torrential rainfall, and marginally large hail.

Moderately to locally strong instability is expected to build up within weak to moderate wind shear. Shear increases towards the south. Therefore, areas south of the Alps could face a few supercells with large to very large hail and severe winds. Conditions should support scattered to widespread convective initiation by the early/mid-afternoon and continue into the evening hours. 

A still moderately unstable air mass within weak shear north of the Alps should support a rather widespread multicell activity with primary torrential rainfall and flash floods threat. Slow-moving storms are expected and result in high rainfall sums locally. 

+++ France, Germany, Benelux into England +++

 
weather forecast area 2
 

SLGT risk has been issued for northeast France and north-central Germany with a threat for severe storms, capable of producing severe winds, marginally large hail, and torrential rainfall with flash floods.

MRGL risk has been issued for eastern France into southern England and Wales with only an isolated threat for severe storms. Limited shear and instability preclude higher threat and storm coverage. 

Overall, yet another day of widespread convective activity within a moderately unstable but weakly sheared environment. Local topography and pre-existing outflow boundaries from the overnight activity will serve as the focus for storm initiation.

Again, mainly multicell storms and small clusters are expected, producing locally lots of rain and flash floods threat due to their slow-moving nature. Storms could extend into the evening hours.

+++ northeast Spain +++

 
weather forecast area 3
 

SLGT risk has been issued for northeast Spain with a threat for isolated severe storms, including supercells. Those will be capable of producing large hail, torrential rainfall, and severe winds.

Moderately strong instability builds up with a moisture recovery under an approaching new upper low. At the same time, wind shear also increases. This should support organized storms again. A few isolated supercell storms are likely to bring large hail and severe winds in the afternoon.
 

+++ other areas +++

 

TSTM risk areas have been placed across southern Sweden, southern Ireland, west Bay of Biscay, 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.

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

Severe Weather Outlook - Aug 13th, 2020
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