Using 700mb temperatures as an estimation of the "cap" and to limit tornado potential   by Jon Davies

The following table of 700mb temperature values (click on image) works reasonably well as a way of estimating where the "cap" is located (area where temperatures aloft are too warm to allow surface-based convection to form), based on experience and a database of archived cases 1992-2002:
                                                                                                                

Values warmer than these seasonally-indicated guidelines suggest the presence of a significant cap.  With strong surface heating or an elevated warm moist layer, storms can form within areas of warm or "capped" 700mb temperatures, but the storms will tend to be very high-based or possibly elevated, and typically weaken quickly near or after sunset.  As a result, tornadoes are generally limited to areas to the north of these guideline values.

A problem in applying these guidelines is numerical forecast model handling of temperatures, which can vary.  RUC guidance often tends to be a little warm, while Eta guidance tends to be a little cool at 700 mb.  The following example from 00 UTC on 5/8/02 illustrates this (click on images to enlarge):
<SPC 700mb analysis (from RUC-2)    <Eta 700mb analysis    <DDC observed sounding

Notice that the observed 700 mb temperature at Dodge City (DDC) is 10 deg C, and falls between the RUC and Eta analyzed or short-term forecast values.   Using only the warmer RUC guidance might suggest that the DDC area is somewhat "capped" for the time of year, when in fact a tornadic supercell developed in strong surface boundary convergence just east of DDC:
                                                                                               <00 UTC 5/8/02 visible satellite image

That's why the guideline 700 mb temperature values shouldn't be applied as "thresholds"... common sense and other information are required.  For example, if there is morning convection, check the southward extent of where it developed... matching that up with a particular model's handling of the 700mb temperature field may give you a rough idea of the capping "values" for that model on that day or time of year.  In the 5/7/02 case above, morning storms had developed near the Kansas/Oklahoma border, but did not develop further south, suggesting a very strong cap south of the Kansas border (see satellite image above), that was associated roughly with the 10-11 deg C isotherms at 700mb, depending on the model used.

The following example from 00 UTC 6/14/01shows storms that developed in northwest Texas in an area that had surface temperatures near 100 deg F, but appeared rather capped according to Eta 700 mb temperatures:
<Eta 700mb analysis temperatures    <2345 UTC 6/13/01 IR satellite image

However, concerning tornado potential, the storms were very high-based (surface T/Td spreads near 40 deg F), which is typical of storms that manage to develop further south in strong surface heating within an area of warmer 700 mb temperatures aloft. This would reduce tornado potential based on recent tornado research.   Comparatively, storms near the front in southeast Nebraska at the sharp northwestern edge of the cap (12-13 deg C at 700 mb for June) were lower-based (surface T/Td spread < 15 deg F), and were tornadic in a favorable shear-CAPE environment with lower LCL heights.

Please don't use the 700 mb temperature guidelines presented here as "thresholds" or a hard-and-fast "rule"...  They are only guidelines intended as a tool with other information and common sense when monitoring where an inhibiting cap may be located.  For storm chasers, these guidelines can help in assessing whether a "cap bust" is possible or likely.

Jon Davies - 3/12/03                     back to Jon Davies page