This is an interesting topic of which I have gone around and around with our local DEQ regarding storm intensity vs the NRCS design storms. In chapter 1 of the TR-55 manual, it states "Therefore, the rainfall distributions
were designed to contain the intensity of any
duration of rainfall for the frequency of the event
chosen. That is, if the 10-year frequency, 24-hour
rainfall is used, the most intense hour will approximate
the 10-year, 1-hour rainfall volume."
I guess it is all in how you define your thunderstorm in terms of rainfall intensity and storm duration. The NRCS storms are based on a 24-hour event, and are volume-driven, not necessarily intensity-driven. A brief, intense thunderstorm may produce a short-lived, high flow runoff, but in terms of an NRCS storm (total rainfall) it may be much less significant than you would think.
A better comparison would be to the IDF curves for your area. Depending on where you are, this info may be available on line through NOAA.
You will need to have (1) the rainfall record for the thunderstorm and (2) intensity-duration-frequency information for the region.
There is probably special software (and I think you may be able to do this within the SWMM program) but I have used Excel spreadsheet for this type of comparison as follows. Import the storm record to Excel. For this discussion let's say the precip was logged at 1-min intervals. Next setup columns to compute compute running totals for rainfall over 5-min, 10-minute, 15-minute, 20-minute, ..., 60 minute durations. For example, if the rainfall is in Column B, the formula for the 10-minute accumulation would be something like =SUM(B1:B10) which you then fill down
Fill down these accumulation formulas and find the maximum in each column. This represents the maximum accumulation during that interval. Let's say the maximum value for the 10-minute accumulation is 0.15 inches. Then the 10-minute intensity would be 0.15 in/10 min * 60 min/hr = 0.9 in/hr. You can then compare this to the published IDF data for the region and see how it compares to the 2-yr or 25-yr intensities (for the same duration), etc.
It can take a while to set up the accumulation formulas, but once you create a template you can use it for future analyses.
The SCS/NRCS rainfall distributions were esentially developed using IDF data, and contain intensity information for all events UP TO 24-hour duration. Roughly speaking, if you take the middle 10-minutes of the 24-hour storm, it will give you the 10-minute intensity. The middle 20-minutes will give you the 20-minute intensity, etc.
This means that a short duration thunderstorm will be contained (nested) within the 24-hour storm. Of course, you will be studying this in the context of a longer 24-hour event, with the corresponding rainfall depth. If you want to evaluate the effects of the thunderstorm ALONE, then you would need to create an appropriate rainfall distribution, either devived from and IDF curve or based on an actual measured event.
