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: 24/09/2020 06 UTC – 25/09/2020 06 UTC
Issued by: Severe Weather Europe
Forecaster: Marko Korošec
SUMMARY
Severe storms including supercells with torrential/excessive rainfall, severe winds and marginal hail are likely across the northern Mediterranean on Thursday. A lot of rain is expected over northern Italy. Severe winds will spread in the wake of surface low across western Europe.
Overview of the risk areas across Europe
SYNOPTIC OVERVIEW
A very large and deep long-wave upper trough is moving into western Europe. A strong upper ridge develops over the Azores and northwestern Russia. A short-wave enter Turkey from the west. At the surface, a deep low forms over Ireland deepens while moving towards the southeast. It reaches Benelux on Thursday night. Cold front is moving into central Europe.
FORECAST DISCUSSION
+++ Spain, France, Italy, Switzerland, Germany, Austria, Slovenia, Hungary and Croatia +++
ENH risks have been issued for northeast Italy into southern Austria, western Slovenia, northwestern Croatia, and the northern Apennines with a threat for severe storms, capable of producing severe winds, marginal hail, and torrential/excessive rainfall.
SLGT risk has been issued for the areas surrounding the ENH risk across the northern Mediterranean and central Europe with a more isolated threat for severe storms, capable of producing severe winds, marginal hail, and torrential/excessive rainfall.
Large scale forcing along with the approaching upper trough will result in rather widespread organized storms, expected to form within a sheared and moderately unstable environment. Supercells are possible as well, especially across northeast Italy in the late afternoon and evening hours.
With time, shear will increase and therefore strongly organized storms are likely to form. While the primary threats will be intense rainfall and flooding threat, supercell storms could also bring severe winds and some hail.
Rainfall sums could reach 100-150 mm, potentially even close to 200 mm across the ENH risk areas. A combination of both strong orographic and convective rainfall could lead to flash floods.
The threat will be gradually enhance while spreading east-northeast with an arriving upper trough, moving into the Alpine region. Storms could cluster into larger systems over the northern Mediterranean region overnight to Friday.
+++ the Bay of Biscay, France, Ireland and England +++
SLGT risk has been issued for the Bay of Biscay, extreme south Ireland, southwestern England, and northwestern France with a threat for severe (non-convective) winds.
Associated with the deep surface low moving over the Ireland and UK, mainly non-convective severe winds are likely across the SLGT risk area. Peak gusts could exceed 100 km/h.
Some convective storms will also be possible to the north of the SLGT risk area, within very cold mid-levels of the maritime air mass.
+++ other areas +++
SLGT risk has been issued for southern Turkey with isolated threat for severe storms, capable of producing severe winds, large hail and torrential rainfall.
TSTM risks have been issued for parts of eastern Europe and the Balkans, eastern Turkey into Georgia, and across Ireland and England 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.)