Sunday, April 27, 2008

Assessing the cap: 4/24/08 in Kansas

A strong capping inversion with warm air aloft pre-empting thunderstorms over the southern 2/3rds of Kansas was a big issue last Thursday afternoon (4/24/08). Many storm chasers targeted south-central Kansas waiting for convection to develop in convergence along the dryline there, but no storms developed. Looking back at some forecast and analysis maps from last Wednesday and Thursday, there were some definite clues that most of Kansas would remain capped on Thursday.

It is often helpful to reference 700mb temperatures (roughly 10,000 ft MSL) as a crude ballpark guideline for where the cap is setting up, particularly if convection occurred on the previous day and can be used as a comparison guide. For example, above, examine the 700 mb forecast temperatures from the NAM/WRF model 12-hour forecast valid the day before (Wednesday evening) at 00z 4/24/08. Then compare that with the observed composite radar graphic next to it at the same time (00z). Notice that temperatures above 7 or 8 deg C (the 7.5 deg C isotherm is marked by a dotted line on both images) seemed to delineate an area south of which storms did not develop. As such, that might be a good first guess for locating the cap using the next day’s models.

The 12-hour and 18-hour NAM/WRF 700mb temperature forecasts on Thursday valid at 00z 4/25/08 and 06z 4/25/08 are also shown side by side above, with the 7.5 deg C isotherm marked similar to the previous graphic. Although there is a “dip” in the 7.5 deg C isotherm over central Kansas at 00z, notice that the general pattern suggests a broad “tongue” of warm air aloft over most of Kansas, and that by 06z, the 7.5 deg C isotherm has moved northward close to the Nebraska border. This suggested that the cap would build in strong over Kansas during the evening, limiting convection to areas near the Nebraska border as an upper disturbance moved across overnight. The 02z radar composite (also shown above) confirms that evening convection was indeed limited to northwest Kansas where a supercell was approaching Hill City (see my earlier blog post about the ensuing nighttime tornado environment).

Also shown above are mixed-layer CAPE and CIN at 20z (3 pm CDT) on Thursday from the SPC mesoanalysis, and 700 mb temperatures at the same time. This is a good example showing that CIN values alone should not be relied on to identify a cap. Notice that the 20z CIN graphic shows little or no CIN was present in the Alva-Medicine Lodge area and near Kinsley KS, incorrectly suggesting an absence of capping along the dryline. However, the SPC 700mb temperature graphic suggests that all of south-central Kansas was capped with temperatures 7 to 8 deg C or above over a broad area, when using the previous day’s values as guidance.

Shawna Helt and I ventured west to the Salina area on Thursday afternoon, but when we checked data around 3 pm CDT, it became clear from satellite and SPC graphics that daytime thunderstorms over central or north-central Kansas probably would not happen. The strength of the cap discussed above, and the slowness of the upper disturbance coming out of Colorado (not shown), sent us on back to Kansas City before dark due to a tight schedule.

It is also important to understand that ballpark values of 700 mb temperatures suggesting the location of an inhibiting cap in the plains vary by situation and time of year (for example, 4 or 5 deg C in March to as high as 13 or 14 deg C in July). So be careful and don't use them as a "concrete guideline". Also, though such convection typically is very high-based, it is important to remember that strong heating and convergence over elevated terrain (such as the high plains west of 100 W lon) can pop storms within what looks like a “capped” area at 700 mb. This is a very common occurrence in the summer months. So there are lots of caveats to the above where experience is the tool that really helps.

Hope this is helpful regarding recognizing some future potential cap “busts” :-).

Jon Davies 4/28/08

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