GM1 MET.TR.250(d) SIGMET

• ED Decision 2022/004/R found in: ATM/ANS Provision of Services (2017/373) (Feb 2023)

GM1 MET.TR.250(d) SIGMET ED Decision 2022/004/R CRITERIA RELATED TO PHENOMENA (a)       An area of thunderstorms and cumulonimbus clouds is considered: (1)       obscured (OBSC) if it is obscured by haze or smoke or cannot be readily seen due to darkness; (2)       embedded (EMBD) if it is embedded within cloud layers and cannot be readily recognised; (3)       isolated (ISOL) if it consists of individual features which affect, or are forecast to affect, an area with a maximum spatial coverage less than 50 % of the area concerned (at a fixed time or during the period of validity); and (4)       occasional (OCNL) if it consists of well-separated features which affect, or are forecast to affect, an area with a maximum spatial coverage between 50 and 75 % of the area concerned (at a fixed time or during the period of validity). (b)       An area of thunderstorms is considered frequent (FRQ) if within that area there is little or no separation between adjacent thunderstorms with a maximum spatial coverage greater than 75 % of the area affected, or forecast to be affected, by the phenomenon (at a fixed time or during the period of validity). (c)       Squall line (SQL) indicates a thunderstorm along a line with little or no space between individual clouds. (d)       Hail (GR) is used as a further description of the thunderstorm, as necessary. (e)       Severe and moderate turbulence (TURB) refers only to: low-level turbulence associated with strong surface winds; rotor streaming; or turbulence whether in cloud or not (CAT). Turbulence is not used in connection with convective clouds. (f)        Turbulence is considered: (1)       severe whenever the peak value of the EDR equals or exceeds 0.45; and (2)       moderate whenever the peak value of the EDR is equal to or above 0.20 and below 0.45. (g)       Severe and moderate icing (ICE) refers to icing in other than convective clouds. Freezing rain (FZRA) should refer to severe icing conditions caused by freezing rain. (h)       A mountain wave (MTW) is considered: (1)       severe whenever an accompanying downdraft of 600 ft/min (3.0 m/s) or more and/or severe turbulence is observed or forecast; and (2)       moderate whenever an accompanying downdraft of 350–600 ft/min (1.75–3.0 m/s) and/or moderate turbulence is observed or forecast. GM1 MET.TR.250(d) SIGMET ED Decision 2022/004/R CRITERIA RELATED TO PHENOMENA (a) An area of thunderstorms and cumulonimbus clouds is considered: (1) obscured (OBSC) if it is obscured by haze or smoke or cannot be readily seen due to darkness; (2) embedded (EMBD) if it is embedded within cloud layers and cannot be readily recognised; (3) isolated (ISOL) if it consists of individual features which affect, or are forecast to affect, an area with a maximum spatial coverage less than 50 % of the area concerned (at a fixed time or during the period of validity); and (4) occasional (OCNL) if it consists of well-separated features which affect, or are forecast to affect, an area with a maximum spatial coverage between 50 and 75 % of the area concerned (at a fixed time or during the period of validity). (b) An area of thunderstorms is considered frequent (FRQ) if within that area there is little or no separation between adjacent thunderstorms with a maximum spatial coverage greater than 75 % of the area affected, or forecast to be affected, by the phenomenon (at a fixed time or during the period of validity). (c) Squall line (SQL) indicates a thunderstorm along a line with little or no space between individual clouds. (d) Hail (GR) is used as a further description of the thunderstorm, as necessary. (e) Severe and moderate turbulence (TURB) refers only to: low-level turbulence associated with strong surface winds; rotor streaming; or turbulence whether in cloud or not (CAT). Turbulence is not used in connection with convective clouds. (f) Turbulence is considered: (1) severe whenever the peak value of the EDR equals or exceeds 0.45; and (2) moderate whenever the peak value of the EDR is equal to or above 0.20 and below 0.45. (g) Severe and moderate icing (ICE) refers to icing in other than convective clouds. Freezing rain (FZRA) should refer to severe icing conditions caused by freezing rain. (h) A mountain wave (MTW) is considered: (1) severe whenever an accompanying downdraft of 600 ft/min (3.0 m/s) or more and/or severe turbulence is observed or forecast; and (2) moderate whenever an accompanying downdraft of 350–600 ft/min (1.75–3.0 m/s) and/or moderate turbulence is observed or forecast.