It is likely known that we can get GRIB formatted wind and pressure forecasts from numerical weather models such as GFS. But it is probably less known that we can get usable squall forecasts as well. We get this from the output parameter composite reflectivity (REFC), often called “simulated weather radar,” which is effectively what it is. Once we are in an area of squalls, we can indeed watch them and maneuver around or with them using our marine radar, but it is often valuable to know when they are likely, how severe they might be, and how they will move.
It was not that long ago that navigators beat themselves up chasing atmospheric instability parameters such as CAPE (convective available potential energy) and CIN (convective inhibition) and LI (lifted index) hoping to piece together a usable probability of squalls and their severity—with, I venture to guess, much the same success I had, minimal at best. Now we have a new generation of navigators who can skip all of that, and let the models do the stability analysis, and report it to us as a nice weather radar image right on our chart screens. It is in a sense like new navigators now never having to struggle with the Table 2 tide and current secondary-station corrections, which were, thank goodness, discontinued in 2021.
We can see what live weather radar looks like nationwide at radar.weather.gov. Our textbook Modern Marine Weather has an extended section on the interpretation of REFC.
Figure 1. Sample weather radar. From Modern Marine Weather.
Figure 2. Unofficial guidelines for relating dBZ to squall intensity. From Modern Marine Weather.
The units of reflectivity (Z) are complex and logarithmic (see noaa.gov/jetstream/reflectivity), so they have been simplified to decibels as dBZ. There is no official scale for squall wind intensity, but we made a rough correlation with thunderstorms (rain based) in Figure 2, which has proven practicable. We thus anticipate severe squalls for dBZ values above 40 or so. Squall conditions are most severe with fastest onset where the dBZ gradient is steep, meaning color change from blue to red is narrow.
Besides the global model GFS, the regional model HRRR also provides REFC. Gribs of both models are available by email request from Saildocs. REFC is also included in the high-res NAM models. A sample is shown in Figure 3.
Figure 3. REFC display from the NAM-Puerto-Rico model downloaded and displayed in LuckGrib (luckgrib. com). Forecasted squall winds of 32 kts, gusting to 36, in an area with REFC about 62 dBZ.
This is just a 4-hr forecast, but the general information would have been known earlier. These data are best in the regional models with higher resolution and more frequent updates. The GFS and NAM are only updated every 6 hr, but the HRRR is updated hourly, so it can be useful for near-live squall forecasting in local waters.
You can test these forecasts by looking at the actual weather radar for the same region and time, as shown in Figure 4.
Figure 4. Sample weather radar at about the same time and place as the NAM forecast in Figure 3.
To practice with this, look at the national radar map to find squalls (Florida has the most) and then compare to an HRRR REFC forecast for the area. The hourly updates of HRRR extend out 18 hr, except those run at the synoptic times (00, 06, 12, 18 UTC) that extend out to 48 hr.
Note in passing that the HRRR forecasts for all parameters might be your best local weather forecast available, especially in remote parts of the country.
