Severe Weather Outlooks FAQ

Day 1 and Day 2 Severe Weather Outlooks

Day 1 and Day 2 Outlooks feature risk areas of organized severe weather with risk levels and severe weather threats. Six risk categories (TSTM, MRGL, SLGT, ENH, MDT and HIGH) stand for the coverage and intensity of organized severe weather (supercells, multicells, squall lines, mesoscale convective systems, wind storms, flooding). Pulse storms, known as weakly forced storms, are usually included in the TSTM risk category, but if severe threat is expected with them, those are included in the threat levels.

Severe weather threats include:
– Large hail ( of at least 2 cm in diameter)
– Tornado (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)

TSTM – this level is issued for areas where convective storms are expected with >50% probability of occurring in a 40 km radius from a location.

MRGL – this level is issued for areas where convective storms are expected with <2% probability of organized severe weather occurring in a 40 km radius from a location.

SLGT – this level corresponds to 5-15% probability of organized severe weather occurring in a 40 km radius from a location. Additionally, 5% or higher probability of extremely severe weather is expected. Organized severe weather is expected, but an isolated nature of events and with low coverage.

ENH – this level corresponds to 15-30% probability of organized severe weather occurring in a 40 km radius from a location. Additionally, 5% or higher probability of extremely severe weather is expected. Organized severe weather is expected, but scattered nature of events and with moderate coverage.

MDT – this level corresponds to 30-45% probability of organized severe weather occurring in a 40 km radius from a location. Additionally, 15% or higher probability of extremely severe weather is expected. Organized severe weather is expected, moderate intensity and coverage.

HIGH – this level corresponds to 45-60% or higher probability of severe weather occurring in a 40 km radius from a location. Additionally, 30% or higher probability of extremely severe weather is expected. A HIGH risk area will include a more detailed forecast than TSTM, SLGT, ENH or MDT risk areas.

SIG – this level corresponds to 60% or higher probability of severe weather occurring in a 40 km radius from a location. Normally, a major severe weather outbreak is expected with high coverage of severe weather (such as the August 2008 Poland tornado outbreak, hurricane force windstorms, catastrophic flash floods or similar).

WNTR – this level corresponds to an enhanced threat for excessive snowfall when >50 cm of fresh snow is expected. This threat does not have additional severity threats, however. All details are included in the Outlook discussion.

See forecast text below the forecast map for a detailed discussion of severe weather threats in individual threat areas.

Mid-range Severe Weather Outlooks / Mesoscale Discussions

These products include shaded risk areas for days following Day 1 and 2. No threat levels are defined. Severe weather threats are included in the description below the forecast map. Additionally, Mesoscale Discussions are issued which include a broad overview of a developing pattern and threats. Such discussion usually includes model maps and current observations where more attention on possible severe weather threat is needed.