Northwest Flow

[Bob Hartig]

Chase Case #4 has brought up the issue of northwest flow. This is a new issue for me. From the get-go, I've been innately leery of seeing upper levels coming from the northwest, but I can't really say why. Seems to me like the real consideration is what the winds are doing relative to each other throughout the atmosphere. Is there veering with height? Good shear? Decent helicity? And of course, is there adequate moisture and CAPE?

The notorious Plainfield, IL, tornado was a northwest flow event. Then again, so have been a gazillion garden variety squall lines.

So my question is: what makes northwest flow so undesirable?

 

[Danny Neal]

I've learned to enjoy NW flow, since I have no other choice. Gotta live with it. Roanoke in July of 2004 was a NW flow event too. NW flow is usually the bringer of late night MCS and lightning ops. Usually NW flow events are clusters of severe storms spotted all over the mid west. Most of the time NW flow aren't real big tornado producers which most people probably won't want to drive hundreds of miles for wind or hail.

 

[Adam Lucio]

Well in the winter they can bring some nasty cold spells to our neck of the woods...taking a look at the GFS right now makes me want to hurl...especially seeing whats progged for next week. Thats more of a local bias though.

As far as chaseable setups go. The main trough is generally positioned in a less favorable area. The troughs tend to dig over the eastern conus rather than the west. The ideal chase setups have the west coast trough spinning up those "colorado" lows and ejecting them back to the NE as they cross the plains. Theyre closer to gulf and have more mositure and heating to work with as well.

A NW flow can still produce, though, like you mentioned Plainfield was a NW flow event, and so was july 13 2004 here in IL...the day the Parsons plant got knocked out by an F4. Its just allot harder for such a flow pattern to utilize the other ingredients needed.

 

[Mike Peregrine]

I've seen some good stuff in NW flow before, but it's definitely a hit-or-miss thing. I think the thing that makes me more suspect of these events is just the fact that it's tough to adapt my mentality to the motion during the actual chase. If I'm moving northeast following a storm, I just know where to be and how I'm going to have to use the road network. Going SE throws the system off and makes it hard to leap frog ahead of the meso. Plus, the mesos end up in weird places and it can feel disorienting for some reason.

 

[Chad Cowan]

Seems like at least once a summer there is a good supercell in Nebraska under NW flow that only Mike H is on. What's really interesting is the shape of the hodographs in these environments.

 

[Skip Talbot]

The northwest flow events I've seen were lacking in directional shear. The Roanoke event of '04 had rather poor directional shear. SPC only went with a 5% tor on one of the day 1's. That was THE ONLY tornado too, as the day was a big derecho event for the most part. Perhaps because northwest flow precedes a trough and SLP that can provide good low level shear? Has there been a northwest flow string of pearls event like you can get when a broad area of southwest flow intersects the dryline?

 

[Dustin Wilcox]

NW flow events are far from uncommon across NE during the Summer Months. I'd prefer a WNW flow event over a NW flow event, but both yield some extremely photogenic supercells; I love chasing the isolated flying eagles across the open fields with no other chasers in sight. One of the main problems with a pure NW flow event is the downdraft disrupting the updraft, it seems a good deal of NW flow chases I find myself repeatedly moving SW as the updraft keeps struggling and restrengthening off to the SW.

 

[Danny Neal]

Has there been a northwest flow string of pearls event like you can get when a broad area of southwest flow intersects the dryline?

Correct me if I am wrong but wasn't May 30th, 2003 a NW flow event?

 

[mikegeukes]

Couple of Journal Articles:
A Synoptic Climatology of Northwest Severe Weather Outbreaks: Part 1
A Synoptic Climatology of Northwest Severe Weather Outbreaks: Part 11

Mike
http://geukesweather.blogspot.com -Home of Hundreds of Weather Links

 

[Gene Moore]

Chase Case #4 has brought up the issue of northwest flow. So my question is: what makes northwest flow so undesirable?

For tornadoes, I despise early season NW flow, especially if it's strong. Generally the surface fronts will outrun their upper level support. This may lead to a condition where there are no storms, or if storms do form a narrow (solid) squall is likely. The time of the year to be most wary of NW flow is March and April when systems appear very strong (on paper). I'm not saying there cannot be tornadoes, but the overall probabilities are much lower.

The odds begin to change in May and sometimes it can be good, each system varies and needs careful scrutiny. Then in June-August NW flow can be a blessing. This time the overspreading of cooler 700 MB temperatures allow the strong late season cap to break easier. Also the directional shearing from SE at the surface to NW at 300 MB can make for beautiful sculpted supercells. Additionally some NW flow supercells have the mesocyclone/ flanking line around the back side of the storm behind the rain....because that is the upshear or upwind side of the storm. So take your classic supercell model and rotate it 90 degrees clockwise.

One of my favorite loves is late season NW flow in Iowa.

 

[Bob Hartig]

Mike, right now I've just glanced at the links a bit, but from what I've seen, they look like a goldmine of info. Thank you! And thanks as well to the rest of you who have shared your input.

EDIT: Just saw Gene's response. I wondered if that wasn't the case at 700mb with cold air advection! Makes perfect sense. Gosh, I guess I've actually learned a few things these past few years. It's an interesting and helpful response, bringing in seasonal distinctions.

 

[B. Kendrick]

For my personal opinion, here in the Tx Panhandle Northwest flow is pretty good to chase in June. For some reason this is when i see the wealth of my tornadic storms. Also these storms are really good to photograph. Some of my best photos come from these storms. And I at least see 1 tornado out of northwest flow here. I know that it is not the most desirable pattern to chase but where iam in high school still i cant chase the ideal storm in april and may so i take what i can get. Also it is hard not to chase this pattern especialy when it is in your own back yard here in canyon tx.

 

[Steve Miller TX]

I agree with Gene Moore regarding the seasonal aspect of NW flow events. Here in the Texas Panhandle, West Texas, E NM and E CO, NW flow in later May and through July can be awesome. I simply look for an impulse (not too strong or you tend to get MCS or clusters), southerly moist boundary layer flow, and perhaps a boundary. Any decent capping inersion is a bonus too to keep the cells isolated. With the NW flow, I prefer 20-30 knot mid level flow at most (considering you have strong CAPE) with strong low level flow "force feeding" storms as they move SE or even south.

I have seen NW flow aloft with SE boundary layer flow that just doesn't seem to work very well as storms forward flank precip area contaminates the updraft region as the storm moves into rain cooled air. That's why I prefer S flow or even a bit of a SW component.

Last June, I had a field day for a little over a week out here with some vicious supercells. One of which produced the largest hail I've ever seen...grapefruits. The structure was awesome too.

On June 2, 2007, one of the craziest supercells I've seen erupted along with a few others that moved due south and produced a few tornadoes and wild structure.

NW flow events aren't known to be prolific tornado producers, but can produce some spectacular structure and gorilla hail. They are easy to chase as long as you remember a more southerly intercept strategy versus the traditional easterly approach.

 

[Dean Baron]

I dont have a ton of experience chasing or forecasting NW flow events but I will say that the best storm structure I got last year came on a NW flow event (May 31st). Until last year I was so confused on chasing these types of setups. We are used to seeing and wanting winds out of the south or southeast for a SW flow event, but with a NW flow you want to see winds out of the SW, winds that we are used to calling veering winds and winds that we usually want to stay away from with SW flow systems. Everything is just turned 90 degrees clockwise, plus you dont get hybrid HP monsters.

 

[Bill Schintler]

RE: northwesterly upper-level flow

Go back and look at May 27, 2001... I was chasing that storm, or should I say, was chased by it - bill