Determining Moisture Advection

[Bob Hartig]

Maybe this is a question for the beginner's thread, but it seems a bit beyond anything I was thinking about when I first got into chasing. It's based on the following from the current SPC long range forecast:

[SIZE=-1]STG LOW LEVEL CYCLOGENESIS AND PRECURSORY
WAA ARE EXPECTED AHEAD OF THIS SYSTEM...HOWEVER LOW LEVEL RETURN
FLOW IN WAKE OF PRIOR FROPA SHOULD BE INSUFFICIENT FOR SIGNIFICANT
SVR EVENT WITH THIS WAVE.

[/SIZE]My question: how the heck can you tell what the WAA is going to be like following a system? I've seen everything from one system eating the next one's lunch in terms of moisture, to just the opposite, ushering in the requisite dewpoints for the ensuing system to be a contender. So, what determines whether a system is going to kill the dewpoints or assist them for the next trough down the pike?

 

[Derrick Herndon]

It depends on the depth of the system and the steering flow/orientation of the jet stream. A deep cold dry system being steered to the Southeast by deep northwest flow is going push the higher dewpoints deep into the Gulf or farther. Any wave following this pattern would have little moisture to work with. On the other hand a disturbance moving through nearly zonal flow (or maybe better, Southwest flow) may initiate moisture return by inducing cyclogenesis but be of insufficient strength to lift that moisture to the mid/upper levels thus leaving much of it behind. The steering flow carries the cool dry high to the East instead of pushing it more to the South thus leaving moisture pooled South of the high (perhaps right near the Gulf coast) to be tapped by a following and perhaps stronger disturbance. So in this scenario the first wave actually primes the environment for the second wave. If we are talking about a modified air mass over the Gulf there is an added advantage for the second disturbance in that the moisture depth may be increased through mixing in the boundary layer/evaporation from the Gulf over a longer period of time.

Maybe this is a question for the beginner's thread, but it seems a bit beyond anything I was thinking about when I first got into chasing. It's based on the following from the current SPC long range forecast:

[SIZE=-1]STG LOW LEVEL CYCLOGENESIS AND PRECURSORY
WAA ARE EXPECTED AHEAD OF THIS SYSTEM...HOWEVER LOW LEVEL RETURN
FLOW IN WAKE OF PRIOR FROPA SHOULD BE INSUFFICIENT FOR SIGNIFICANT
SVR EVENT WITH THIS WAVE.

[/SIZE]My question: how the heck can you tell what the WAA is going to be like following a system? I've seen everything from one system eating the next one's lunch in terms of moisture, to just the opposite, ushering in the requisite dewpoints for the ensuing system to be a contender. So, what determines whether a system is going to kill the dewpoints or assist them for the next trough down the pike?