Identifying a jet core with surface data

[Cris Schroeder]

I've been sharpening my surface analysis skills this spring, and I'm struggling with finding the location of upper level jet cores/maxima using surface data. Model data is nice, but I'm looking for measured data to help locate localized areas of ascent.

Is the best method to locate the jet max using upper air charts in the morning, and then project where it will be in the afternoon? I believe it was Tim V's book that discussed using local altimeter falls in the surface data to locate areas of decreasing pressure. If this is a viable method, how much of a drop should I be expecting to look for, and is there any correlation between the surface pressure drop and the velocity of the upper level jet?

 

[Tim Vasquez]

ALSTG pressure falls of about 0.01 to 0.03 inches per hour are normal on a severe weather day, and values of 0.04-0.05 inches per hour are a fuzzy boundary for being significant. The actual values may be influenced somewhat by the usual diurnal pressure tides (strong falls in heated terrain are quite normal during the afternoon). Hourly trends and movements in fall centers, and consistency, are vastly more important than simple spot checks of isallobaric maxima at a given time. There's not really any correlation with upper level speeds since there can be other processes removing mass from the column.

I've had hit or miss luck with isallobaric fields... sometimes it helps, and other times you don't see anything nor see a correlation. About all it tells you is where there might be upper lift, possible cap weakening, mesolow formation, and processes that might locally strengthen shear. Also make sure you avoid using SLP (SLP pressure change often masquerades as 3 hr pressure change, which in the METAR/airways group is obtained from SLP). Using SLP instead of ALSTG introduces T-12 hr temperature bias (a front that came through an area 12 hours ago, for example, can make SLP isallobars screwy), so you're better off taking your chances with pressure artifacts due to present weather & present heating/cooling.

Tim

 

[Tim Vasquez]

Also I should add that pressure falls don't give information on jet max/streak location, but might indicate that one is in the vicinity. Ageostrophic motions around jet streaks that cause vertical motion are often on the periphery and may be at some distance away. Also you may be dealing with something like a short wave trough or just an area of strong heating rather than a jet max.

All in all, a strong pressure fall should simply serve as a diagnostic indicator and tell you that you should look at more data and consider what process might be at play and how it will influence destabilization, boundaries, and shear in your target areas.

Tim

 

[Cris Schroeder]

Thanks for the info, Tim. Am I putting too much emphasis on locating a jet max/streak? It sounds like I would be better off using the morning upper air data to project a rough location of the jet max in the afternoon, and then focus on the fronts, dryline, and outflow boundary intersections for initiation.

I'm getting better at building a synoptic-scale picture of the atmosphere, but I'm still struggling with the meso- and storm-scale features that seem to make or break a chase.

 

[Jared Farrer]

One way of finding areas of possible upper level interest is paying close attention to surface to 850mb wind speeds. Usually LLJ and sometimes surface wind speeds and direction can be an indication that Jet dynamics are taking place above. As long as you keep all of the other causes in mind and rule them out occordingly. And as Tim mentioned also keep a good eye on pressure falls, most people I know disregard this tool but it can be useful.

 

[Tim Vasquez]

Beyond 6-12 hours out the models are going to beat human prediction pretty consistently. So looking at 12Z morning data for afternoon convection you want to go with model output. Some experienced forecasters will look for initialization errors, though from what I've seen in recent years most models are initialized pretty well in the upper levels and it's hard to second-guess those fields. The models also have the benefit of initialization from previous runs, which might have been picking something up during the previous 24 hours.

The type of subjective analysis/diagnosis detective work done with isallobars and so forth is better for refining short term positions and trends; that is, when you're doing forecasting less than 3-6 hours in advance, such as just before initiation.

During the morning hours the time is best spent searching for any and all boundaries that might be capable of doing something at initiation time, and looking at model data for clues as to whether they're being resolved and how they might shape up later given the large scale weather pattern. And of course don't forget the all-important goal of sifting through all upper/surface/sounding charts to build a 3-D conceptual model of what's going on; right after the new 12Z upper-air data is in is where that begins on a chase day.

Tim

 

[Cris Schroeder]

Thanks for the help, guys, and I appreciate the advice.