What is your favorite "obscure" forecast parameter?

[Mike Johnston]

It would be interesting for folks to discuss one or more of their favorite forecast parameters that might be ones that aren't universally cited (such as CAPE, surface dewpoints, etc.)

One that I particularly like to check is the 500mb/850mb wind crossover. I guess it's not that obscure, but one that doesn't seemed to be cited real often in either SPC discussions or discussions on here. Ideally, I look for approximately a 45 degree angle between the two wind barbs. 500mb winds out of WSW, 850mb winds out of SSE for example. I suppose the value is that it seems to be a decent proxy for directional wind sheer. Also, perhaps it reprsents a favorable orientation of the respective troughs.

At any rate, it seems to be a measure that is decently forecast on the models and persistent enough that one can usually get a good forecats handle on. By contrast, a measure like helcity for example can be quite fleeting and fickle - a bullseye on one run, only to disappear on the next run. For me, it's just an anectdotal observation, but it seems like in a setup with otherwise favorable ingredients, areas with a good 500/850 crossover can narrow a target area down some.

Would love to hear some others discuss their own favorite parameters.

 

[Skip Talbot]

I like the 0-3km Cape and Cap Index plots on Earl's site. I see MLCAPE and SBCAPE cited most often, but that low level cape is great tornado indicator. There are times when the MLCAPE might otherwise preclude a forecaster from detecting a tornado threat, but large amounts of low level instability can lead to tornadic mini sups (cold core events come to mind). Additionally, if there is little low level instability, but other cape plots are still showing instability, much of that instability is probably in the midlevels and you're going to be dealing with non tornadic, elevated storms.

I like the cap index as opposed to the cinh plots because its a little friendlier to read and I actually think it does a better job at hinting at initiation potential and whether storms will stay surface based. The cap index is the width of the inversion whereas the cinh is the area of the inversion. Storms seem to have an easier time with tall, skinny inversions than they do short, fat ones, where the cinh would be the same for both scenarios, but the short, fat inversion would be highlighted be the cap index.

http://wxcaster.com/smallfiles_central_svr.htm

 

[Jesse Risley]

1. Surface features, vis-a-vis satellite maps and an old fashioned surface weather map. I think these are often overlooked in an era of high-resolution model imagery and radar software programs, but surface features, e.g., OFBs, wind patters, moisture pooling, pre-frontal troughs, etc., are often the key to how an event will unfold.

2. I always check out the Theta-E parameters, especially at 850 mb, as well as the temperature advection parameters. It's interesting to see how often there is a correlation between supercell activity proximal to a Theta-e ridge.

3. The amount/distribution of CAPE in the lowest 500 mb per SKEW-T soundings at a given point versus total CAPE, which can given an indication of the overall thermodynamic environment that might favor tornadogenesis, given favorable wind shear, in situations where the overall CAPE in the lowest 100 mb may otherwise appear to be deceptively small.

 

[Rob H]

Theta-e and low-level lapse rates for pinpointing convective initiation. Theta-e ridges are typically burst points for convection. Low-level lapse rates in certain scenarios can indicate where EML underrunning is taking place, which can infer localized processes that weaken capping and help start updrafts.

850 moisture transport - something I've started looking into more recently, after studying how storms break out on the dryline without focusing features at the surface like outflow boundaries or fronts. The moisture at this level also has a lot to do with alto cumulus development, and yet many people don't bother understanding how 850mb processes work.

 

[Matt Gingery]

There are several parameters that I look for in targeting during a chase, but usually narrow it down to the basics listed below. Keeping in mind that it really only takes a few select variables to effectively and efficiently chase storms. One in particular is SFC based CAPE coupled with moderately steep low/mid level lapse rates, a trigger (Fropah) and directional wind shear. Spreads are also very important as well in determining the LCL height, but not so much the case chasing up-slope, which might I add is my preference. Always keying in on any cyclogenisis however weak it may be that traverses a boundary and allows the winds to back moisture into developing updrafts. Could be a meso-low and all the right basic variables to create a small scale epic event.

Temperature Differential: One thing I picked up on during the 2010 upslope season, which I could also revert back to the Goshen, Wyoming tornado is the differential temperature on either side of a boundary (i.e. CF, DL, WF). During the Goshen event I believe there was a 5-7 degree swing in temperature as that cell peaked and produced, and a similar type set up with the Last Chance, Co. event in June of 2010, May 19 Bison, Ok also. The day after the Last Chance event, there was a much more extreme swing in temperature as a cell fired off the front range near Colorado Springs. Seems that there is a possible direct correlation with the type and nature of that supercell as opposed to the one just a day earlier, as it produced a prolific HP supercell that shrouded the tornadoes that touched down obscuring them from any high contrast view. IMO i feel that steeper decline in post frontal or boundary temperatures can create a less favorable environment for tornadoes due to undercutting, and higher concentrations of precipitation in and close to the main updraft and mesocyclone.

All the above being said, I look for a weak variation in temperature change along boundary forcing, strong moisture transport via sufficient perpendicular wind component with height, steep lapse rates coupled with a strong westerly component aloft.

Basics: High CAPE, 0-1km EHI values of 1 or greater during the witching hour, but always looking for maximized 0-3 km EHI values as a indicating variable because even with high values at 0-3km, and nil at 0-1km in the early afternoon, this can quickly change as the day moves forward and diurnal cooling takes effect and the LLJ ramps up in the few hours prior to sunset cooling the inversion layer and breaching the CAP.

I look at these basic variables on chase day to determine targets: Forecast CAPE, 0-1Km and 0-3Km EHI, LCL's or Spreads, areas of concentrated Convective Inhibition to avoid, Lapse Rates, Backing winds, and the Triple Point. Lack of cloud deck is a determining variable, or convection that fires south of my target area. I monitor visible satellite until I am cross eyed trying to pick up any minor variation, such as a cirrus deck that can roll into a unstable capped region and help cool down the inversion layer, or subtle boundaries. Visible Satellite is my best friend when all other variables come together, trying to key in on OFB's and Cu fields. There are a tremendous amount of variables that a person can go over, but keeping it simple is the most effective IMO.

 

[V. Gensini]

It would be interesting for folks to discuss one or more of their favorite forecast parameters that might be ones that aren't universally cited (such as CAPE, surface dewpoints, etc.)

One that I particularly like to check is the 500mb/850mb wind crossover. I guess it's not that obscure, but one that doesn't seemed to be cited real often in either SPC discussions or discussions on here. Ideally, I look for approximately a 45 degree angle between the two wind barbs. 500mb winds out of WSW, 850mb winds out of SSE for example. I suppose the value is that it seems to be a decent proxy for directional wind sheer. Also, perhaps it reprsents a favorable orientation of the respective troughs.

At any rate, it seems to be a measure that is decently forecast on the models and persistent enough that one can usually get a good forecats handle on. By contrast, a measure like helcity for example can be quite fleeting and fickle - a bullseye on one run, only to disappear on the next run. For me, it's just an anectdotal observation, but it seems like in a setup with otherwise favorable ingredients, areas with a good 500/850 crossover can narrow a target area down some.

Would love to hear some others discuss their own favorite parameters.

I like the 850/500 crossover too! I have it on my page...(bottom of 'RUC' links)

http://vgensini.myweb.uga.edu/wx/

 

[Jeff Duda]

Isentropic surfaces. Very telling, but rarely shown.

 

[Josh Durkee]

As mentioned above, I prefer to keep it very simple and avoid the shopping spree of parameters to consider. However, you might consider short fuse composites "obscure" and I like to browse those over from time to time.

Here is a quick page of some that are available: http://people.wku.edu/joshua.durkee/wxanalysis/shortfuse/

The link at the top of the page is incorrect, but the cities should link you to SFC (not surface) pages from that NWS office.

 

[Bob Hartig]

I second Skip on the 0-3km CAPE. I've also been taking a look at normalized CAPE, which offers a feel for parcel acceleration--i.e. is the CAPE tall and skinny, or is it fat, suggesting rapid parcel acceleration? I haven't used this parameter a lot, but I do look into it from time to time as it suggests where the most rapid and robust surface-based storm development is likely to occur.

 

[David Wolfson]

I often use SRH (storm-relative helicity) and relative motion, especially the presentation you get from the regional Rapid Update Cycle. It gives a rather fine-scale view of fronts/boundaries and where systems are going in one picture.

 

[Paul Knightley]

Over the years I've tended to move back to a slightly broader view when picking a target region. That's because despite having some great hi-res modelling available these days, you simply cannot trust them to be able to break out precip/handle frontal/wind shift positions as well as you might want to. Yes, they can be very good at picking out trends, and development areas - just don't trust them down to grid point level detail.

I like to look at the broad picture regarding frontal zones, wind shift lines, dry punches, etc, and use that to pick out a reasonably broad region. Then I like to home in during the morning using surface obs/sat pics, etc. There has to come a point in the chase day when obs take precedence over model data.

However, it's hard to ignore the new HRRR model data, as it is initialised each hour with the latest obs/radar. Even then, it's only a simulation! Never get distracted from the real world!

 

[Bob Schafer]

SR Inflow. That's what it's all about.

(Caveat emptor: understand that storm motions can deviate)

 

[Greg Blumberg]

Victor, that's a fantastic diagnostic site you have. What do you use to generate those images? I saw some are RUC graphics, but are all the others objective analyses? If so, what scheme are you using?

 

[Patrick Marsh]

Greg, I'm pretty sure that is GEMPAK.

 

[CGardner]

Theta-E Advection seems to be that weird parameter that I trust on from time to time. Also, kind of folklore forecasting, but I sometimes hang around a major river or body of water in the target area. I don't know why, it always produced good results, chase-wise, when I did. Also, I like to fish, so it passes the time while I wait for initiation.