SW flow vs NW flow

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Aug 27, 2009
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The worst thing about a seasonal hobby like storm chasing is that I tend to forget much of what I learn between the years. Lately there have been talk about SW flow and NW flow and I would like to clarify if I remember it correctly - and what to look for.

First of all, "NW/SW flow" refers to 500 mb, right?

As far as I understand, a SW flow probably means you have a Low to the west (such as over Colorado today) and that it drags the flow from underneath and "releases" it to NE, so to say (hard to describe). In comparison NW flow should mean that you have the Low to the east, right?

SW flow is the "normal" flow for good storms and together with some NW flow above and SE flow from the gulf at the surface you get the rotational, shear, right?

How is NW flow played in that case? I assume you need the surface flow from W and the upper flow from like straight N? As the moisture wouldn't really come from the West, I would assume this needs the moisture to already be in place. Am I understanding this correctly?
 

Jeff Duda

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The terms "NW flow" and "SW flow" are very broad and generic, so your uncertainty is appropriate. Yes, in general, when people refer to "[Direction]-flow" events as they pertain to severe weather, they are referring to the mid-level (generally 500 mb) synoptic scale wind direction.

To help your understanding, don't forget the basics. You can assume winds above 700 mb everywhere in the CONUS are geostrophic, and (generally from 925 and above on the Plains, with small exceptions). So southwest flow implies a geopotential height gradient from NW-SE (in order of increasing heights). While that doesn't necessarily imply there is a closed low nearby, it does suggest there is a height minimum/trough somewhere to your west/northwest/north (depending on the exact direction of flow and the shape of the height field, which is what you really should look at instead of the wind pattern itself). Northwest flow, on the other hand, means the geopotential height gradient is rotated 90 degrees clockwise, so that the gradient is NE-SW (again, in order of increasing height). That can indicate a height minimum somewhere to the east/northeast/north, again with details depending on the structure of the height field itself.

Oftentimes when you have SW flow and a height minimum/trough to your west, even northwest, it tends to move east or northeast, as dictated by the planetary scale westerly flow in the mid-latitudes. The reason a low is more likely to move northeast than southeast is due to the occlusion process - when that happens, the low tends to pull back towards the colder air as it becomes dislocated with the synoptic scale warm sector. Given warmer air is always south, that means it pulls north (while still getting pulled east).

When the mean synoptic flow has a substantial northerly component to it, lows can move to the southeast. That is the crux behind a NW flow event - a southeastward sinking trough with a southeastward sinking surface low and cold front.

In typical southwest synoptic scale flow, the shear for severe storms comes from the discrepancy between low level southerlies or southeasterlies that are typically forced by a surface pressure trough in the lee of the Rockies and the response to the mass divergence ahead of the upper level trough that promotes southerly low-level geostrophic flow with the southwest mid-level flow itself. In the US, the Rocky Mountains, and in particular, the contintental divide and gradual/nearly homogeneous downward terrain slope east of there artificially drives pressure falls several hundred miles east of the divide, where the best combination of warming from downsloping winds meets the edge of Gulf moisture advecting from the southeast. That is why you can get "lee troughs" without much of a synoptic scale trough above (because really all that is required is "cross-barrier" flow, which in this part of the world is westerly flow, which is generally a daily occurrence regardless of whether a trough or ridge are present, and stable lapse rates, which are also very common), and hence additional forcing along the Great Plains dryline (but this also can drive surface winds to turn from where they normally would be in a flat-land scenario).

In NW flow, on the other hand, everything is rotated 90 degrees, so you usually have southwest surface winds interacting with the northwest mid-level flow to get directional turning and shear for severe storms. As a NW flow trough approaches, you will typically see a SLP pattern with a SE-NW pressure gradient or even a S-N one, which, when combined with the turning due to surface friction, typically leads to southwest surface winds rather than southerly or southeast surface winds. However, if a closed surface low forms in a NW flow event (strong enough lift concentrated over a small area), you can get backed winds (SSE or SE) very close to the low itself. It is also possible that if the surface low picks up a surface baroclinic zone, you could end up with a SE-NW oriented warm front that approximates the future track of the surface low, and north of that front you can end up with southeasterly surface winds as well. While it can be more difficult for quality moisture to move in to a setup featuring NW flow, NW flow events typically occur during the mid-summer when a mT air mass has already been in place beforehand, so little additional moisture advection is needed for sufficient instability. Even so, if you can find an air parcel trajectory that gets back to a moisture source (like the Gulf of Mexico), you can still get moisture advection even with seemingly unwanted surface wind direction.

The shape/structure of the geopotential height field and shape of the mid-level vorticity include details that make the difference between the various cases described above. You should look for these details and examine how they impact things. For example, compact shortwave troughs tend to birth compact surface lows which can be a lot of fun to analyze for severe weather and/or chase, as they can give you excessively backed surface winds and semi-circular hodographs without destroying the thermodynamic profile! Large/broad troughs, on the other hand, tend to birth broad surface troughs that may not even have a point-minimum MSLP center, but rather an elongated trough (or sometimes, double-minima in MSLP with a trough connecting them...sometimes referred to as "banana lows"). In those cases you're more likely to wind up with a sweeping cold front no matter what else is above it. However, you also tend to wind up with a much larger area with sufficient flow and shear for severe weather, and if the warm sector is large enough, you can still end up with a severe weather outbreak if you can get storms to fire ahead of the synoptic front (e.g., from small-scale boundaries due to terrain or OFBs from prior convection, or if a "lead impulse" can set up some kind of pre-frontal surface trough).

You should look for embedded "impulses" within larger troughs. They are actually quite common, if you know how to identify them. Nearly imperceptible ripples in the synoptic scale flow can trigger a dynamical instability and develop into their own embedded surface lows which can impact the coverage and intensity of convective storms on any given day in a way that would not be present without them.
 
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Aug 27, 2009
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Jeff: Thanks for the VERY thorough explanation! I have to admit, I had a bit of a hard time transferring that info to an image in my head but I understand it better now. Thanks!
 
@Jeff Duda something just clicked in my mind while reading this, but I want to ensure that I'm understanding correctly. I'd always wondered why NW flow events in the Midwest tend to have a number of successful events. Based on your explanation, and the fact that I'm more or less a walking atlas, the thing that benefits Midwestern NW flow events the way they often do is the fact that, even though the surface winds are usually from the SW, they're still pulling from areas that are saturated in terms of Td. So that means the shear vectors are still there, *and* they're able to pull in ample moisture to make it happen. Am I correct, at least in part, in that line of thinking? Once I thought of it in that terms, especially me having grown up in Eastern Oklahoma where it's significantly more humid than central or western OK and knowing how much further east that Illinois and Indiana are, it made a lot of sense to me.
 
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@Jeff Duda something just clicked in my mind while reading this, but I want to ensure that I'm understanding correctly. I'd always wondered why NW flow events in the Midwest tend to have a number of successful events. Based on your explanation, and the fact that I'm more or less a walking atlas, the thing that benefits Midwestern NW flow events the way they often do is the fact that, even though the surface winds are usually from the SW, they're still pulling from areas that are saturated in terms of Td. So that means the shear vectors are still there, *and* they're able to pull in ample moisture to make it happen. Am I correct, at least in part, in that line of thinking? Once I thought of it in that terms, especially me having grown up in Eastern Oklahoma where it's significantly more humid than central or western OK and knowing how much further east that Illinois and Indiana are, it made a lot of sense to me.
I think you've nailed qualitatively the reason why tornadoes with SW surface winds are more common east of the Plains. As one moves W of I-35 and dryline setups increasingly dominate the tornado climatology, SW surface winds are likely to advect in drier air and also to have a meaningful downslope component, particularly if SW winds are occurring over a large surrounding area (i.e., if a representative parcel trajectory comes from the SW over a long distance upstream).

It isn't impossible to get a tornadic supercell in the heart of the Plains with surface winds veered beyond 180°, but that would be more likely in a subtle, weakly forced setup with abundant, deep moisture in place (think late spring/summer), and perhaps a boundary in play. In that case, advection may not have an overwhelmingly negative influence on the low-level thermodynamic environment.
 

Jeff Duda

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That's largely what I was getting at with my comment about trying to find air parcel trajectories that can be traced back to a moisture source such as the Gulf or the western Atlantic - even with SW surface winds in a place like IA/IL/WI/IN, you can usually still find some trajectories that bend across the central plains and back out over the Gulf. And even if you can't, the area where NW flow events are more common (Midwest) so so far removed from the Gulf that, if you have a large moisture-laden air mass upwind of the target area that doesn't contain a moisture source, advection from that over-land moisture-laden air mass can still provide sufficient moisture return so as to give you the necessary instability.

That is...if you're looking to chase a NW flow even in IL, you don't really care how thermodynamically unstable it is in OK at the same time. Let that moisture in OK come to you in IL.

I acknowledge this would probably be very easy to explain if I had visual aids, but I'm too lazy to prepare them.
 
Brett and Jeff, that makes absolute sense, although I will readily admit that me being a walking atlas makes it much easier to visualize than it probably is for most people. Thanks to both of you for confirming my line of thinking. I have a lot of family connections in IN, so I used to spend summers up there watching many of these NW flow events unfold. Just for some reason, it had never clicked in my mind until I was reading this over last night.
 
Aug 27, 2009
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I have thought of NW flow as something that is not really optimal but if that's the case, that is the case, and you just have to work with what you have got. But, looking at it from a probability point of view, we still get more tornadoes and severe weather with SW flow, right? For example, if you would pick a chase week you would generally pick a week of SW flow over NW flow - if you were to chase in Tornado Alley that is?
 

Jeff Duda

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I have thought of NW flow as something that is not really optimal but if that's the case, that is the case, and you just have to work with what you have got. But, looking at it from a probability point of view, we still get more tornadoes and severe weather with SW flow, right? For example, if you would pick a chase week you would generally pick a week of SW flow over NW flow - if you were to chase in Tornado Alley that is?
I have no idea whether that is a true statement or not, but I see it as irrelevant because you will rarely, if ever, see a SW flow and NW flow event present themselves on the same day in the CONUS. And even if they did, chances are they'd be close enough to each other for you to be able to pick the day of. I'd still go for the SW flow stuff just for familiarity and openness of chase terrain unless you want to avoid large crowds.
 
I honestly couldn't pick out an example of having a SW and NW flow event packed together within a few days of each other. It's probably happened at some point, but there's nothing that jogs the memory. So to answer your question, yes, SW flow is preferable over the "traditional" chase territory (central and southern plains). With that said, up in the Midwest, I would say that chances of success are close to even between the two types of events. Given that Illinois, and much of Indiana as well, has a paved road grid, those chases might actually be easier in some regards, with the exception of storm motions generally being more rapid for Midwestern events. You certainly don't have to be as cognizant of the type of vehicle you are in when deciding whether to go down a road that's not a state or US highway like you do across most of the plains.