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 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 up themselves 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 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.
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 that 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.
This is just a four-hour 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 six hours, 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.
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 hours, except those run at the synoptic times (00, 06, 12, 18 UTC) that extend out to 48 hours.
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.
— David Burch is the Director of the Starpath School of Navigation and the author of Modern Marine Weather and the Starpath online course in Electronic Chart Navigation, which covers optimum sailboat routing using polar diagrams and numerical weather model forecasts.
