The 3-kilometer NAM

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The Rapid Refresh (RR) and High-Resolution Rapid Refresh (HRRR) aren't the only "mesoscale models" available. The National Centers for Environmental Prediction also run high-resolution, convection-allowing versions of models you're already familiar with, which also have use in mesoscale forecasting.

One such model is the NAM. For its high-resolution output, the NAM employs "one-way" smaller nests within the larger outer model domain. Within each nest, the model computes forecasts concurrently with the 12-km NAM parent run. For the record, "one-way nested" means that the inner (nested) model domain receives its lateral boundary conditions from the outer domain, but it does not feed back any information to the outer domain. In other words, the outer domain is not affected by the nest.

The parent 12-kilometer domain of the NAM, along with its three-kilometer nests for CONUS, Alaska, and Hawaii / Puerto Rico, respectively. The smallest rectangles represent very high-resolution nests for predicting fire weather.

Credit: National Centers for Environmental Prediction

The nested domains within the parent NAM have higher resolutions, with three-kilometer nests covering the contiguous U.S., Alaska, Hawaii and Puerto Rico (shown above). The resolution of the internal nests of the NAM is sufficiently high to realistically simulate convection, so while convection is parameterized in runs of the parent 12-km NAM, it's not in the higher-resolution forecast nests. In case you're wondering, the small unlabeled boxes in the image above represent small nests with even higher resolution that are used for predicting fire weather.

The GFS, on the other hand, actually runs on a dynamic model core called the "FV3" ("Finite Volume Cubed Sphere"), which runs on a "flexible" grid. The flexible grid gives modelers options for running higher-resolution versions that can realistically simulate convection over parts of the globe. The model also has the ability to run higher resolution "two-way" nests within its global domain (two-way nests receive their lateral boundary conditions from the outer domain and can feed back some information to the outer domain).

So, are the high-resolution versions of the NAM and FV3 every bit as useful as the HRRR? Not exactly. There's a key difference between the two. While the HRRR is initialized every hour, the high-resolution FV3 and NAM are still only initialized every six hours (06Z, 12Z, 18Z, and 00Z). The high-resolution FV3 and NAM do have forecast intervals of one hour, but they do not get infused with hourly surface observations, which makes them less viable for predicting the small-scale rapidly changing environments that may favor the initiation of thunderstorms.

While the high-resolution FV3 and NAM produce forecasts with realistic-looking convective structures (like in the example below), the same caveats that went along with HRRR forecasts apply. Just because the forecasts look realistic doesn't mean they're accurate, and remember, the fact that the high-resolution FV3 and NAM are only initialized every six hours is a notable drawback. On the flip side, one advantage to these models is that their forecasts go out a few days into the future, which is longer than forecasts from the RR and HRRR. Like with the HRRR, the timing and exact location of individual thunderstorms are often incorrect in high-resolution FV3 and NAM forecasts, but they can still give useful insights into the general coverage and structure of thunderstorms.

The 31-hour forecast of radar reflectivity and MSLP valid at 19Z on May 11, 2023 (from the run initialized at 12Z on May 10) from the high-resolution version of the FV3. Since the high-resolution FV3 does not parameterize convection, its forecasts include realistic-looking convective structures.

Credit: Tropical Tidbits

For comparison with the forecast prog above, the corresponding forecast of radar reflectivity and MSLP from the high-resolution NAM had general similarities to the high-resolution FV3 forecast, but lots of differences in the finer details of convective placement and structure.

Given the differences that regularly occur in high-resolution model output, high-resolution ensemble forecasts can also be of great use, and indeed, NCEP has developed the High-Resolution Ensemble Forecast (HREF) system for mesoscale forecasting. The HREF is comprised of HRRR forecasts, along with high-resolution versions of the NAM, FV3, and other convection-allowing models primarily used by the research community. So, mesoscale forecasters have multiple options for convection-allowing guidance and even a convection-allowing ensemble of models!

If you're interested in accessing forecasts from high-resolution, convection-allowing models, check out the Explore Further section below. Otherwise, we'll wrap up our introduction to mesoscale meteorology with a brief Case Study of a tornado outbreak, which illustrates the connections between spatial scales and the utility of real-time mesoscale model analyses. Read on.