If You Had One Single Forecast Map to Use...

[Mike Johnston]

Just to drum up a little off-season conversation (although realizing these early winter systems seem to be peaking interest), would be interesting to see different opinions from the good forecasters on this forum re:

If you had one single 24-hour forecast map to use to determine your chase target, what would it be?

* Surface pressure
* 850 mb heights, winds
* 500 mb "
* 250 mb "
* Surface dewpoints
* Any other single dimension map

I'm always most interested in the rationale for analysis of forecasters. Realize the answer may be different depending on the nature of the system, time of year, etc., but it seems that some model forecast parameters can be more or less derived from others. So, if you really had to put your individual analysis skills to a test, what one piece of forecast information would you most rely upon?

 

[Mike Peregrine]

Interesting question ... I'm interested to hear what others say.

I guess if I was forced to weigh one against the other, I'd probably look at 500mb first. Just seems like the other maps may offer one bit of pertinent information each, where 500mb offers several.

 

[Shane Adams]

If I only had one map to look at, I'd choose h5 UVVs.

 

[Aaron Kennedy]

Sfc map with winds, MSLP, T and Td. That said, I wouldn't wanna forecast with just that

Aaron

 

[Andrew Khan]

850 mb Winds

Surface Charts

Cape Values

Helicity

 

[Michael P. Morris]

I'd go with the typical 850 mb temp/wind/height/RH map available from RAP. In a severe situation, you can diagnose low level warm advection/moisture advection, moisture depth, location of the LLJ, and the approximate location of the surface dryline or surface fronts. It does not give you info on the static stability or the shear, but from a synoptic scale prespective the 850 is rather useful.

 

[Robert Dewey]

Originally posted by Aaron Kennedy
Sfc map with winds, MSLP, T and Td. That said, I wouldn't wanna forecast with just that

Aaron

Hey now, I thought it was just ONE parameter, not a map with everything overlaid :lol:

Anyway, assuming you know nothing about the setup (i.e. where the best Td's are located, SFC features, etc.)... I would have to go with a plot of EHI. That would show you where the best instability and shear combination exists, but obviously leaves out very important information like the cap and any forcing/boundaries.

Like Shane, I thought of the UVV fields myself (more along the lines of 700MB though)... But then I remembered that strong UVV's can also be located over areas that aren't favorable for any type of severe weather (i.e. just north of strong baroclinic zones in winter storms). In fact, most of the time UVV fields line up pretty well with QPF fields, so you'd have just as much luck chasing areas of high QPF...

Either way, I'd rather take my chances with a high instability / shear combination than a strong forcing situation (strong UVV's)...

EDIT: Then again, I could program a map that blends together EHI with SFC to 925MB moist / wind convergence and low / mid level UVV's into one parameter... That'd be kind of interesting.

 

[David Wolfson]

I'd pick the 850Mb (or 925Mb if I can get it), so long as I can get good heights, winds, T, and Td. The first upper level avoids most of the boundary layer noise and shows advection well. It's close to a surface chart on the high plains. Problem is, lack of good data.

 

[Gabe Garfield]

I don't think I would choose an 850 mb map because I could just go outside and look up to see what it is doing. Warm air advection, moisture advection, etc. are easy to infer by looking at what the clouds are doing/how it feels outside.

Instability parameters are generally easy to figure out (i.e. are the clouds puffy or not?), so I certainly wouldn't waste my one map for that.

I'm probably going to go with Aaron on this one. Assuming there is a reason to chase (or at least, you believe there to be), a surface map is your greatest friend. Areas of locally backed winds, moisture pooling, dryline bulges, etc. are absolutely essential to nailing a forecast. The question is always "Where in this large area that is primed for supercells will the best storm be?" I think the surface map helps the most in this regard. Of course, a zillion other things can go wrong like the cap holding, storm mode problems, etc.

Now, if we didn't even know if a chase was in order, and we could only use one map, I'd definitely go for a 500 mb map. From this map you can determine where the most likely area for convection initiation is and a rough idea of the strength of the system you are dealing with (i.e. is it worth chasing?). Classic plains tornado situations come from classic plains troughs. To see this, you've got to look at the 500 mb map.

Gabe

EDIT: To comply with the single dimension qualifier, I will say surface wind vectors instead of surface map. If a boundary is present, moisture pooling can be inferrred, to a certain extent.

 

[Jeff Snyder]

Tough call... I think it matters if we know ahead of time that tomorrow is a chase day. If it's like we've been following the model output up to today, then it's one thing since we know the general synoptic setup. On the other hand, if it's like we have no access to anythign for a month, then asked to request a map for any particular day (say, tomorrow), then it's another thing completely. If the latter case is correct (we have absolutely no prior info about a day), then I'd probably take either 250mb or 500mb, both of which can give you an idea about large-scale vertical motion (through divergence via transverse circulations, DPVA leading to vertical motion, etc), which in terms can give you clues about the surface response (e.g. divergence or DPVA aloft often yields surface pressure falls). Of course, if it's May and we see a 250mb chart with a huge west-coast trough and a jet streak rounding the base of that trough, we can make a pretty darn good guess as to what the low-levels look like.

A lower-level chart would be nice as well, since we can infer some things about the upper-level flow by looking at a low-level chart (e.g. tight baroclinic zone would imply, via the thermal wind relationship, relatively strong flow aloft nearly parallel to that boundary). However, we've all seen many days with warm, juicy low-levels that are under very weak flow aloft. Heck, take any June or July day in the southern plains and you'll likely see warm, moist low-levels. We've all seen plenty of days with extreme instability but very weak flow aloft and/or no 'trigger' mechanism. We may be able to get a glimpse at the cap potential with, for example, a 700mb chart (though I hate using temps at one level to infer a cap strength), but an upper-level chart can let us infer strong upward vertical motion and thus column cooling (likely weakening whatever cap is there). Upper-level charts don't tell anything about the moisture in the low-levels, but again, if we are given a time of year, a synoptic set-up (e.g. west coast trough) can possibly shed some light on the low-level environment that we would likely find associated (via climatology, pattern recognition, etc) with that upper-level synoptic environment (e.g. lee trough, broad southerly flow in the case of the western US trough).

Of course, as we all know, no single-level chart can give us a good view of the severe weather potential, but that's just my reasoning. I'll take a 250mb heights chart, with a 2nd choice of 500mb heights.

 

[Andy Wehrle]

Originally posted by Jeff Snyder
if it's May and we see a 250mb chart with a huge west-coast trough and a jet streak rounding the base of that trough

Well we can always dream.

I might go with the dewpoint chart just because from that you can derive the location of synoptic boundaries such as fronts and drylines, although that's assuming it's a scenario wherein synoptic boundaries are likely to be the main focus for convection, which I probably don't know if I don't know anything else about the setup.

 

[Glen Romine]

Seems a lot of folks don't want to play by the rules.... one chart, at one height or pressure level, with ONE variable displayed on it. As Jeff mentioned, would be nice to have a clue about what was going on ahead of time, time of year, etc..., but without a clue of anything else - I'll take the 500 mb heights. This gives insight into the time of year, large-scale synoptic pattern, shortwave activity/location and a clue as to whether sufficient deep layer shear is present to support supercells. This will lead to a crappily soft target choice - but without a clue about shear, I don't really care how pretty the surface boundaries are. If I have a decent shortwave emerging out of southwest flow onto the plains at a reasonable time of year - there is a pretty good bet that convection will occur - and I'd prefer that to a suntan.

Glen

 

[Justin Turcotte]

I'll take either surface winds (speed + direction) or a SLP chart. Either one of these with proper resolution will allow you to see convergence boundaries, low pressure center, and perhaps some mesoscale features. My next favorite would be H70 vorticity (which is not readily available on the net). There are many subtle shortwaves that reveal themselves at this level. If it is early in the season I might consider a Td chart as this would also indicate boundaries. Quantity of moisture is also often what makes or breaks an otherwise favorable early season set-up. The Td chart in my opinion would be less beneficial to others I mentioned during peak season as the dews are usually sufficiently high for solid convection.

 

[Marko Korosec]

I'd pick up a 500mb heights/temps chart or like Jeff said, 250mb heights/temp chart, love these charts

 

[brody_clifton]

I'd pick the 500mb heights/temps map as well. Going only on that,
chase targets would be SSE to SE of any neutral or neg. tilt trough centers, assuming that in those cases a surface low/850 LLJ is more probable to exist or develop. During the warm/hot season my targets would be under diffluence and between the 576 and 582 height contours a couple hundred or so miles ahead of the base of the trough traced along that height. Same for the mild/cool season except between the 570 and 576 contours and maybe 300 miles ahead of the base, assuming the systems are moving through more quickly. Tricky part would be wondering whether boundary moisture is/will be/is not/will not be in place, with the odds of success higher in the warm/hot season.