trying to understand...... directional 180 wind

[jamesmseitz]

I have been a chaser for many yeas now but always did not completely understand the directional shear that is at 180 degrees. I realize that this is not great for tornados but why is it not great for storms as a whole.

Just looking for understanding and not really caring if I get laughed at I want to understand..... This is one area that i have not learned about yet

James Seitz

 

[Stan Rose]

You have to be more specific than that, how are you defining directional shear "at 180 degrees"? What levels are you referring to? Who said that 180 degrees of shear is "not great for storms"?? What does that mean?

 

[Greg Blumberg]

I think what James means is directional shear turning 180 degrees with height. For example, the surface winds are from the south and the upper level winds are coming from the north. (This is just a theroetical idea, a much more real example would be winds from the southeast at the surface and winds from the northwest in the upper levels.)

The problem with this is that when your storm forms, the precipitation created will not be blown away from the updraft, but blown onto it.

Imagine a positively buoyant (will rise in the environment) chunk of air at the surface being blown by surface winds from the southeast towards the northwest. As that air rises and condenses and forms precipitation, it encounters upper level winds from the northwest blowing towards the southeast. The precipitation from that chunk of air then gets blown back over where the air initially began to rise at the surface. The 180 degree directional shear then does nothing to separate the updraft from the downdraft. The cold, rainy downdraft chokes off the warm, moist updraft. It's like the same thing that happens to storms in an environment where there is no shear: the air goes up, cools, condenses, and sinks back down and cuts off the updraft.

Not gonna laugh at you dude. This one took some explaining for me too.

 

[Mikey Gribble]

Directional shear is great as long as you have the necessary updraft downdraft seperation. If you don't have that seperation you'll run into problems like Greg explained.
Its also important to consider things from a storm relative (also known as SR) point of view. That is something that can be really tough to envision in your mind. As far as the updraft/storm is concerned the shear profile is not the same as it would be to you standing at the ground. Its moving which totally changes how the winds in the atmosphere are relative to it. SR is whats really important. Thats why you'll see 1km, 5km, and 10km SR winds listed with forecast soundings all the time. All of them are important for different reasons, but they have a substantial impact on how storms will work in that environment so its important to understand SR winds at those levels and what they do.
As long as you can get good updraft downdraft seperation, 180 degrees of directional shear is great for tornadic storms.
btw thats not a stupid question. This is complicated stuff. I had to sit hear and think things through in my head so I don't sound stupid typing this. Not 100% sure it worked yet either lol.

 

[Stan Rose]

Mikey is absolutely right. That's why i asked "who said 180 deg is bad" because that is not necessarily the case. The storm motion and updraft propogation are important. Terrain can also factor--for example, in the Great Thompson flood in CO the inflow kept flowing upslope, directly under "the storm". The storm didn't get choked off, there was tons of CAPE and it kept dumping rain in a small location, ending up in flash flooding. That storm was certainly "great".

 

[Patrick Marsh]

...a much more real example would be winds from the southeast at the surface and winds from the northwest in the upper levels.

This is extremely typical of northwest flow setups. The southeast winds at the surface help to initiate convection in the higher terrain through orographic lift and the northwest flow aloft helps to move storms toward the southeast.

Or even more frequently, east winds at the surface (think just north of a surface boundary / warm front) and winds out of the west aloft.

The problem with this is that when your storm forms, the precipitation created will not be blown away from the updraft, but blown onto it.

Imagine a positively buoyant (will rise in the environment) chunk of air at the surface being blown by surface winds from the southeast towards the northwest. As that air rises and condenses and forms precipitation, it encounters upper level winds from the northwest blowing towards the southeast. The precipitation from that chunk of air then gets blown back over where the air initially began to rise at the surface. The 180 degree directional shear then does nothing to separate the updraft from the downdraft. The cold, rainy downdraft chokes off the warm, moist updraft. It's like the same thing that happens to storms in an environment where there is no shear: the air goes up, cools, condenses, and sinks back down and cuts off the updraft.

It's much more complex than this. A wind profile with 180 degrees of directional shear is not necessarily a bad thing. What is important is what happens to the wind profile (more precisely, the shear profile) throughout the depth of the storm. Probably the most famous hodograph in numerical modeling (this is prior to learning about the so-called sickle-shaped hodographs) is the idealized 20 May 1977 Del City, OK hodograph. This is a semi-circle hodograph, centered on (0,0). This hodograph features 180 degrees of directional shear, but over a large depth so that the updraft and downdraft still get separated.

This touches on another crucial thing regarding the hodograph and that is to determine over what depth the 180 degrees directional shear is occurring. Being 180 degrees out of phase over the surface-3km layer is different than over the surface-6km layer which is different than the surface-storm top layer.

 

[Greg Blumberg]

Thanks Patrick. I was generalizing because I was trying to go for the thought process of "why we say it's bad" answer.

I wasn't thinking of the east sfc winds/west aloft boundary problem. The few times I've had trouble chasing was because of the southeast sfc/northwest aloft scenario, but they've never been in areas of higher terrain.

I looked up the May 20th, 1977 storm you mentioned. That is a rather interesting sounding. I've heard of the storm, but never looked it up...here goes my task for the week.

And the explaining/learning continues!

 

[Mike Hollingshead]

I don't care what the orientation of things is, I've never had fun with 180 degrees of turning or more. The most you get is a storm that is ok for a while if there's good separtion. Eventually(usually fairly quickly) the storm gets hosed by itself(unless you're talking something more elevated in the first place). And in the meantime you're usually messing with rain in your view as it does so. More often than not, 180 degree chases are annoying as hell. I think pulsy action and wet views at that. So far from my chases I don't think it has ever failed to disappoint when chasing such setups. Slowly have learned to just swear off chasing them.

Granted, some great structure can happen if a storm dies in a rapid fashion like that. As the core fades, wala, you see the remaining structure. Seems that one along the Red River a year or so ago was such a setup and result, with some great structure down there. Lone sup travelling down the highway on the TX side. I didn't actually chase that day.