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|Storm Index Definition – The storm index covers weather guidance for pulse thunderstorms, multi-cell thunderstorms and more organised mesoscale convective systems or plume events.
Storms of this nature can range from a weak pulse storm that forms in a weak wind-shear, low/moderate cape environment which is not conditional on any external factors to stronger and more organised moderate wind-shear and high helical multicell storm modes capable of more frequent lightning, moderate size hail, locally damaging winds and flashing flooding.
Storms may also be capable of producing weak rotation induced by low level surface convergence and/or low level moisture and SBCAPE. Only covers funnel risk and not tornadoes.
Blue (Very Low Risk) 10%-20% – Isolated storms possible or issued for preliminary watches based on conditional storm potential.
Yellow (Low Risk) 25-45% - Scattered storms are likely or issued for higher confidence preliminary watches based on conditional storm potential.
Amber (Moderate Risk) 50-65% - Scattered/Frequent more organised storms are likely or issued for higher confidence preliminary watches based on conditional storm potential.
Red (High Risk) 70-80% - Frequent organised storms are expected or issued for very high confidence preliminary watches based on conditional storm potential.
Red (Very High Risk) 85%+ - Very Frequent organised storms are expected or issued for extremely high confidence preliminary watches based on conditional storm potential.
|Severe Storm Index Definition – The severe storm index covers weather guidance for the more extreme thunderstorm attributes such as bow echoes/derechoes/tornadoes/squall lines and tornadoes. Storms of this nature can range from a moderate strength linear squall line in moderate wind-shear and low/moderate cape environment which is conditional or not conditional on external factors to stronger squall lines, bow echoes, derecho, mesocylcones (supercells) and tornadoes, all associated with higher wind-shear, higher helical values and high-end CAPE. These dangerous storms will produce frequent lightning, large or very large size hail, often damaging winds, flashing flooding and tornadoes. Tornadoes are covered in the index and can occur in a squall line, a strong multicell storm or more significantly within a supercell. Severe storms differ from standard storms in intensity of wind, hail and the additional threat of more frequent lightning and damaging tornadoes.
Yellow 2% Severe – Marginally severe characteristics such as small/moderate size hail, winds gusting 30-40mph, isolated weak tornadoes, and frequent lightning. Often associated with strong multi-cells or mesocyclones LT supercells.
Amber 5% Severe - Severe characteristics such as moderate size hail, winds gusting 40-50mph, weak-moderate strength tornadoes and frequent lightning. Often associated with linear squall lines, some bow echoes or LEWP and supercells.
Red 10% Severe - Severe characteristics such as large hail, winds gusting 60mph-70mph, moderate/strong tornadoes and frequent lightning. Associated with bow echoes/LEWP, strong damaging squalls, derecho and tornadic supercells.
Pink 15% Very Severe - Very Severe characteristics such as large or even very large hail, damaging winds gusting over 70mph, strong long-tracked tornadoes and frequent lightning. Associated with strong bow echoes/LEWP, powerful and damaging squall lines and derecho and tornadic long track supercells.
Pink 25% Extremely Severe Extremely Severe characteristics such as all encompassed within 15% but with giant hail, winds gusting in excess of 90mph, long tracked devastating tornadoes and dangerous lightning frequency. Seldom issued in the UK.