Chase Cases
prepared by Tim Vasquez

Want to try your hand at forecasting for a historically important storm event? Presented here are various surface charts that cover events that occurred in the 1970s, 1980s, and 1990s. You'll have the opportunity to scrutinize each chart in detail, and won't know what the date or event is until you're ready. A text narrative is presented for each map, which will provide hints. Experienced chasers should NOT read these narratives until after you pick your target area, as it will influence your thinking.

AREA: Sorry, these are all Southern Plains maps. I may post some Northern Plains or Midwest maps in a future installment.

PRESSURE: All pressures are plotted as altimeter settings. This measure of pressure is the same as that reported on NOAA Weather Radio and on TV, and is depicted on this map as units, tenths, and hundredths of an inch of mercury. Therefore "993" is 29.93, and "005" is 30.05. You will plot isobars every 0.05 inches. Altimeter settings were used by severe weather forecasters on a widespread basis back in the 1980s due to the lack of sea-level pressures at about 30% of weather stations. Why? The widespread overnight station closures in the pre-ASOS era made 12-hour temperature changes unavailable, which in turn made r-factors for SLP computations unavailable (and in some cases r-factors were not developed for various stations). SLP values are much more common nowadays, however the use of 12-hour temperature data in order to dampen out the effects of heating actually masks some mesoscale disturbances. Therefore it's not at all a bad thing to continue analyzing altimeter settings.

PRINTING: Test out these graphics on your printer before you print a lot of them. Some browsers have an annoying tendency to split large graphics across multiple pages. If this happens, copy the graphic to the clipboard with a right-click of the mouse, and paste it in your favorite graphics program. Use it to print the map.

DOING GOOD ANALYSIS: Some tips to do a better job analyzing the maps:
  • Spend less time in high pressure areas, where contrasts will flatten out, and more time in low pressure, where contrasts and baroclinicity will intensify.
  • Start by lightly sketching out where the boundaries (fronts, drylines, wind shift lines, and outflow boundaries) appear to be.
  • Isobars should always kink into frontal boundaries. There are clear meteorological reasons why this is so. The kinking is not necessary with drylines and other features.
  • The primary characteristic of a front is temperature contrasts. The front must be on the south edge of the contrast zone. If anything, you should walk away with a firm understanding of frontal placement!
  • The primary characteristic of a dryline is dewpoint temperature. The dryline must be on the moist edge of the contrast zone. If it is unclear where to place a dryline in a broad zone, do not place it any further east of the zone in which convection could be supported; but do not place it west in an area too dry to support convection. Obviously this is a judgement call sometimes!
  • Never ignore a data point. Your isobars should accept each point of data unless there is good reason. What looks strange may amplify as you draw into an important feature, and can make you think more about what is happening there. If you are convinced it is faulty data, circle the parameter.
  • Certain stations contain systematic errors. For example, Guadalupe Pass TX (GDP); Junction TX (JCT); Marfa TX (MRF); and sometimes Lufkin TX (LFK) all transmit pressures that don't fit well on the map. These can be ignored for the time being. Encountering them on this map
  • If you are trying to do an exceptionally high-quality analysis job, place a piece of Kleenex underneath your drawing hand. This will prevent pencil lines from smudging and will keep skin oils from degrading the paper.
  • Chase Case A
  • Chase Case B
  • Chase Case C
  • Chase Case D
  • Chase Case E
  • Chase Case F
  • Chase Case G
  • Chase Case H
  • Chase Case I
  • Chase Case J
  • Chase Case K
  • Chase Case L

  • Shallow moisture (rapid vertical mixing)
  • Strong westerly component in troposphere (moisture advection suppressed)
  • Time of maximum heating (enhanced vertical mixing)
  • Deep moisture (slow vertical mixing)
  • Weak westerly component in troposphere (moisture advection supported)
  • Time of minimum heating (suppressed vertical mixing) FAST WARM FRONTAL MOVEMENT:
  • Shallow or mildly cold air, or highly sloped frontal surface