700mb temperatures as a rough estimation of the "cap"
regarding initiation of severe convection by Jon Davies
These apply in the Central U.S. east of the High Plains (east of approximately 100 deg W longitude)
The following table of 700mb temperatures can serve as a very
rough estimation or "ball park" guideline for estimating areas
with a capping inversion strong enough to inhibit severe convection east of the
High Plains. These are areas where temperatures aloft are generally
too warm to allow strong surface-based convection to form, based on experience
and a database of archived cases 1992-2002. Do not use these values in the
High Plains (west of roughly 100 deg W longitude) where surface heating and
upslope winds can initiate storms in areas of 700 mb temperatures warmer than shown due to
Values warmer than these seasonally-indicated guidelines suggest the
presence of a significant "cap" that will likely inhibit surface-based
convection. BUT, there are many caveats when using this table, so it serves as only
a starting point in assessing the "cap"! Relevant values can vary
considerably during seasonal transition months like March-April and September-October.
That's why it is important to pay attention to where convection occured on previous
days relative to 700 mb temperatures. A good April 2008 case study illustrating this
is shown on my blog here: http://davieswx.blogspot.com/2008/04/assessing-cap-42408-in-kansas.html
Also, with very strong surface heating (e.g., temperatures mid 90's to around 100 deg F), storms can form within areas of warm or seemingly "capped" 700mb temperatures, but the storms will tend to be very high-based, and will typically weaken quickly near or after sunset. As a result, the main threats in such situations will be strong wind reports as a result of the high-based/high LCL storms and possibly hail; tornadoes are generally limited to areas to the north of these guideline values. And, as noted above, in the High Plains and westward, the guidelines above often do not apply in the warm season because of heating over elevated terrain and/or upslope wind conditions. Here is a July 2009 case study that illustrates how 700 mb temperatures can aid in a severe convective forecast in the Plains east of roughly 100 deg W longitude, but may not be useful farther west due to intense surface heating in the High Plains: http://www.jondavies.net/700mbTcap/071009cap_kc.htm
Below are a couple older cases I put together as examples back in 2003.
One problem in applying 700 mb temperature guidelines regarding the
"cap" is numerical forecast model handling of temperatures, which can
vary. RUC guidance often tends to be a little warm, while Eta and NAM-WRF guidance
can sometimes be a little cool at 700 mb. The following example from 00 UTC on
5/7/02 illustrates this (click on images to enlarge):
<SPC 700mb analysis (from RUC) <Eta 700mb analysis <DDC observed sounding
Notice that the observed 700 mb temperature at Dodge City (DDC) was 10
deg C, and fell between the RUC and Eta analyzed or short-term forecast values.
Using the warmer RUC guidance might suggest that the DDC area was somewhat
"capped" for the time of year (early May), when in fact a tornadic supercell
developed in strong convergence just east of DDC. The temperatures at 700 mb farther
south over western Oklahoma were notably warmer, and that area was clearly
"capped" for thunderstorms:
<00 UTC 5/8/02 visible satellite image
The above case is 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 specific model's handling of the 700 mb temperature field may give a rough idea of the capping "values" for that model at that particular time of year and month. In the 5/7/02 case above, morning storms had developed near the Kansas/Oklahoma border, but did not develop farther south, suggesting a very strong cap south of the Kansas border (see satellite image above) that was associated roughly with 10-11 deg C and warmer isotherms at 700mb.
The following example from mid June (6/13/01) shows storms that
developed in northwest Texas in an area that appeared possibly "capped"
according to Eta 700 mb temperatures. But that area also had strong surface heating,
with temperatures near 100 deg F (not shown):
<Eta 700mb analysis temperatures <2345 UTC 6/13/01 IR satellite image
As mentioned earlier, thunderstorms can certainly develop with very strong daytime surface heating in areas where 700 mb temperatures appear "too warm" or indicate "cappng", particularly in and close to the High Plains. However, in the 6/13/01 Texas case above, the storms were very high-based (surface T/Td spreads near 40 deg F) and dissipated near dark, which is typical of storms that manage to develop in strong surface heating within an area of warmer 700 mb temperatures aloft. The main severe threat with such high-based storms is isolated downburst winds and possibly hail, and they are unlikely to produce tornadoes. In the 6/13/01 case above, storms farther north near the front in southeast Nebraska at the sharp northwestern edge of the cap (12-13 deg C at 700 mb estimated 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.
===>A final note: 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 forecasters, these guidelines can increase awareness for potential "cap bust" situations.
Jon Davies - originally posted 2003 - updated August 2009 back to Case Studies/Briefs page