Northwesterly Flow Storms

[Tony Perkins]

In reference to June 17th and other times in the past, I've seen mentions that NW flow storms can produce great structure. What is it about NW flow vs SW flow that may enhance structure?

 

[Jeff House]

NW flow situations can have less moisture than SW flow set-ups all else equal, allowing better chances of Classic or LP supercells as opposed to HP. Visibility is improved too. My post is pretty general, and may or may not apply to 17 June. In traditional SW flow one would want surface winds backed to the southeast for good shear. In NW flow, a southwest surface wind gives plenty of shear. Everything is just turned clockwise 90 degrees from surface to jet stream. That southwest surface wind is drier than a southeast wind, promoting better visibility. Of course one still needs enough moisture to get severe storms. NW flow in April will not get the job done, but in June just seasonable amounts of moisture is enough. Someone else may have a different angle, but that's my two cents worth.

 

[Donald Rosenfeld]

http://ams.allenpress.com/archive/1520-0493/110/11/pdf/i1520-0493-110-11-1653.pdf

http://ams.allenpress.com/archive/1520-0493/112/3/pdf/i1520-0493-112-3-449.pdf

 

[Andrew Revering]

Similar to what Jeff suggested, I think a big part of it is water loading. With a northwest flow your precipitable water values typically will be less than 1.25" or even less than 1.00". so you have a better chance of reducing water loading so updrafts can be maximized given whatever amount of instability is left, leaving nicer towers and cleaner updrafts due to reduced moisture to work with. This is also a key part to help with those elevated hailers you get in northwest flow with seemingly little traditional ingredients.

 

[Mike Hollingshead]

I don't know that I'd be able to argue NW flow produces any better structure more frequently than SW flow. Seems to me if anything it's worse.

I don't know that I buy the less moisture thing either. Seems like a good chunk of the nw flow setups are over summer juice in the corn belt.

I can't think of any great structure I've seen in nw flow in the mid-levels. I guess maybe the Alvo storm June 13, 2004 might have been nw flow, I know the upper jet was nw orientated. July 12, 2004 was more westerly flow at 500mb than nw. Ainsworth July 24, 2000 I am not sure on either, that might have been nw flow...or it could have been more westerly mid-levels too.

 

[Jay McCoy]

I can't think of any great structure I've seen in nw flow in the mid-levels.

Try the Texas panhandle. Cant speak for Nebraska but some of our most picturesque storms are NW flow supercells. Also they can be big rotators since we have built in shear with SE or south winds feeding into NW 500mb. Just look at some of Steve Miller's or Jason Boggs pictures from the last week.

I also dont buy the less moisture so LP instead of HP. Some of the panhandles best rains are from NW flow systems coming off the NM mountains. Our DP depressions are higher since it is warmer so the bases are higher but we still get prolific rainmakers. We got hit 2 days in a row in amarillo by flooding rains. over 2 inches in less than an hr on the east side on Thursday and then on Friday 2.5-3" on the west side in about 45 minutes. Major street flooding and wild lighting.

 

[Mike Hollingshead]

Try the Texas panhandle. Cant speak for Nebraska but some of our most picturesque storms are NW flow supercells. Also they can be big rotators since we have built in shear with SE or south winds feeding into NW 500mb. Just look at some of Steve Miller's or Jason Boggs pictures from the last week.

Do you think it is better on average than your SW flow storms though?

 

[Dennis Sherrod]

Try the Texas panhandle. Cant speak for Nebraska but some of our most picturesque storms are NW flow supercells. Also they can be big rotators since we have built in shear with SE or south winds feeding into NW 500mb. Just look at some of Steve Miller's or Jason Boggs pictures from the last week.

I also dont buy the less moisture so LP instead of HP. Some of the panhandles best rains are from NW flow systems coming off the NM mountains. Our DP depressions are higher since it is warmer so the bases are higher but we still get prolific rainmakers. We got hit 2 days in a row in amarillo by flooding rains. over 2 inches in less than an hr on the east side on Thursday and then on Friday 2.5-3" on the west side in about 45 minutes. Major street flooding and wild lighting.

Mike,
I can only speak from my own experience, but I got some of my best pictures so far this year in the TX panhandle, W OK, and SW KS. on these storms. Most all of them came this last week at the same time Steve and Jason were getting them also. I love the storms in that area.

 

[Jay McCoy]

Do you think it is better on average than your SW flow storms though?

Not for tornados but for structure shots I think its as good if not better. Usually a bit higher bases and great rotation striations. You get good layer cakes and barber poles along with incredible bell shaped bases and wall clouds. Not ground hugging ragged wall cliuds like in april and may. Instead clean smooth clouds. It just depends on your favorite medicine. Also we tend to get more right movers tha move due south. These can also be major hailers so caution is warranted.

Like I said look at Steves pics..

http://texastailchaser.com/blogger/

 

[Donald Rosenfeld]

"They find that the additional positive buoyancy due to release of this latent heat of fusion more than offsets the negative buoyancy due to water loading. Therefore, both contributions, or neither contribution, should be considered when assessing parcel buoyancy."

from: sect 3-E of http://www.nssl.noaa.gov/csi/csimwr.html