Ingredients necessary for severe convective straight-line winds : 

 

In addition to the ingredients necessary for thunderstorm formation (moisture, instability, lift), the production of severe convective straight-line winds will be favored with the presence of these following additional atmospheric ingredients :

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• Particularly high atmospheric instability - When the instability of the airmass is very high (i.e. > 2000 J/kg), this favors a strong thunderstorm updraft. Strong updrafts often imply strong downdrafts because of the important precipitation loading that the strong updraft produces.

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• Moderate to strong vertical wind shear - This allows the updraft and downdraft of a thunderstorm cell to remain separate from each other, thereby strenghtening both of them. Also, particularly strong low-level jets can be forced down to the ground by strong convective downdrafts (downward transport of horizontal momentum). Moderate to strong vertical wind shear also implies a fast storm movement. This forward translation speed is an additive aggravating factor since convective straight-line wind gusts are composed of both the downdraft speed and the forward translation speed of the storm.

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• Dry air at mid-levels and/or below the forecasted cloud base - This allows for significant evaporational cooling of downdraft air as part of the precipitation shaft evaporates within the dry air layer. This process increases the temperature difference between the downdraft air and the ambient air thereby helping accelerate the downdraft towards the ground. Be watchful for inverted-V forecast or proximity sounding profiles and for dry layers visible between 850 and 700 hPa often associated with elevated mixed layers (i.e. Spanish Plumes). Thunderstorms evolving in environments with inverted V soundings are known to favor dry microbursts.

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• An elevated freezing level  - This favors the melting of hail within the thunderstorm downdraft which acts to cool the air further and can help accelerate it to the ground. This can aid in the development of wet microbursts, especially when coupled with a very humid tropospheric sounding profile and a thick warm-cloud layer (> 3 km).

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In addition to the above pre-convective ingredients, the following nowcasting hints should be utilized to nowcast severe convective straight-line wind gusts while thunderstorms are already in progress :

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The presence of a bowing segment along a convective squall-line or a bowed-out thunderstorm cell on radar reflectivity imagery -  This is usually indicative that the thunderstorm cell or cells are "gusting out" and have a higher than average risk of producing severe straight-line winds. In addition, bowed-out storms known as "bow echoes" are often associated with "rear-inflow jets" which can significantly strenghten an already strong downdraft.

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The presence of high/strong radar radial velocities behind a bowed-out convective reflectivity structure (rapidly approaching or receeding radial velocities depending on the position of the bow echo relative to the radar) - This is usually indicative of a strong to severe straight-line wind potential at the surface and can be confirmed with ground station data. When analyzing radial velocity data for individual radars, please keep in mind that the further the radar signature is from the radar, the higher up in the troposphere it is located. Rear inflow jets typically originate anywhere between 850-700 hPa in the diabatically heated layer where the low-level meso-low forms in the trailing stratiform region behind the bowed-out reflectivity structure. Strong radial velocity signatures in this zone should be taken seriously.

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The identification of a Descending Reflectivity Core (DRC) on radar reflectivity or velocity vertical cross-sections - When visible, this can be an indication that the precipitation cascade of a thunderstorm cell is in the process of collapsing to the ground. When associated with an airmass thunderstorm, this is usually a result of the updraft weakening. The presence of a DRC can be an imminent precursor to the production of a wet downburst/ microburst by the storm, susceptible to produce severe convective straight-line winds. This occurrence can also sometimes precede the bowing out phase of a supercell storm, just before it starts to "gust out".​

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