Randy Jennings
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- Joined
- May 18, 2013
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- 780
One of things I most often get wrong when trying to forecast a target area is lift. I saw a very good discussion on of the topic of drylines and lift for thunderstorm initiation in a forecast discussion written today by Dennis Cavanaugh of NWS Forth Worth. I though I would share for others to learn from:
"The problem with relying on drylines alone for lift and thunderstorm initiation is that the lift along a dryline in the afternoon typically works like a backwards front. With any strong front...the tight thermal gradient along the front forces a vertical circulation in an attempt to balance out the temperature gradient. That circulation results in lift on the warm side of the front...and sinking motion on the cool side. Lift in the atmosphere provides cooling...so lifting the warm air is the atmosphere`s attempt to cool off the warm air. The opposite is true for sinking motion on the cool side of the front. While this is pretty straight forward in cold fronts...you get lift in the warm moist air ahead of the front...resulting in thunderstorms/rain near the front...this circulation does not always cause storms to develop along drylines.
With drylines...the lift occurs on the back side of the boundary...in the very hot...but very dry air that typically exists west of the boundary. In the warm...moist... potentially unstable air to the east...a downward motion is typically present. So how thunderstorms typically develop along a dryline is that some mid-level moisture exists over the surface dryline circulation. The lift interacts with this moisture resulting in very high based updrafts that move east off the dryline and attempt to ingest the really moist and unstable low-level air to the east.
The problem is...how strong is the downward moving air on the moist...somewhat cooler side of the dryline? If the downward air is too strong...a strong inversion sets up preventing the moist low-level air from being ingested into the mid-level based updraft. Unfortunately there’s no real way to know if dryline initiations will be successful or not. Typically the deciding factor between these mid-level thunderstorm initiation attempts failing (typically referred to as turkey towers...based on the classic anvil and skinny dissipating updraft remnants that visually mark failed thunderstorm attempts for storm spotters) or not (resulting in full fledged thunderstorms) is the presence of ambient large scale lift which serves to weaken the capping inversion to the east of the dryline. Because that lift does not appear to be present on Thursday...we are siding with the dryline not initiating storms Thursday afternoon at this time."
"The problem with relying on drylines alone for lift and thunderstorm initiation is that the lift along a dryline in the afternoon typically works like a backwards front. With any strong front...the tight thermal gradient along the front forces a vertical circulation in an attempt to balance out the temperature gradient. That circulation results in lift on the warm side of the front...and sinking motion on the cool side. Lift in the atmosphere provides cooling...so lifting the warm air is the atmosphere`s attempt to cool off the warm air. The opposite is true for sinking motion on the cool side of the front. While this is pretty straight forward in cold fronts...you get lift in the warm moist air ahead of the front...resulting in thunderstorms/rain near the front...this circulation does not always cause storms to develop along drylines.
With drylines...the lift occurs on the back side of the boundary...in the very hot...but very dry air that typically exists west of the boundary. In the warm...moist... potentially unstable air to the east...a downward motion is typically present. So how thunderstorms typically develop along a dryline is that some mid-level moisture exists over the surface dryline circulation. The lift interacts with this moisture resulting in very high based updrafts that move east off the dryline and attempt to ingest the really moist and unstable low-level air to the east.
The problem is...how strong is the downward moving air on the moist...somewhat cooler side of the dryline? If the downward air is too strong...a strong inversion sets up preventing the moist low-level air from being ingested into the mid-level based updraft. Unfortunately there’s no real way to know if dryline initiations will be successful or not. Typically the deciding factor between these mid-level thunderstorm initiation attempts failing (typically referred to as turkey towers...based on the classic anvil and skinny dissipating updraft remnants that visually mark failed thunderstorm attempts for storm spotters) or not (resulting in full fledged thunderstorms) is the presence of ambient large scale lift which serves to weaken the capping inversion to the east of the dryline. Because that lift does not appear to be present on Thursday...we are siding with the dryline not initiating storms Thursday afternoon at this time."
