Pressure and Fronts are synoptic overlays for the big picture: where the highs, lows and frontal boundaries sit across North America for the selected MAP TIME hour. They are drawn as light vector lines, so they switch on instantly at any zoom and read best zoomed out over a large part of the continent. Both can be combined with each other and with the Wind, Thermal, Cloud, Rain and Smoke maps.
Lines of equal mean sea-level pressure every 4 hPa (heavier lines every 16 hPa around 1012), labelled in hPa, with H and L marking pressure centres. The data follows the map model picked in the toolbar:
Isobars are generated on the server when each model run is ingested: the field is smoothed (about 40 km) so lines show the synoptic pattern rather than terrain noise, then contoured and simplified. Over high terrain, sea-level pressure is an extrapolation, so treat small wiggles over the mountains with caution. Hours outside the model run (for example beyond HRRR’s forecast range) show no isobars.
Surface fronts and troughs from the NOAA Weather Prediction Center (WPC), used for both the US and Canada regardless of the map model. The fronts nearest to the selected hour are shown: the latest analysis for past and current hours, and WPC’s forecast charts (roughly every 12 hours, out to about 2½ days) for later hours. The legend shows which chart is displayed and dims when it is more than 3 hours from the map time.
Coverage: the WPC analysis covers all of Canada. The WPC forecast charts stop at about 60°N and 59°W, so in forecast hours fronts over the northern territories, northern Quebec and Labrador, and Newfoundland may be missing or end abruptly at that edge.
Pressure updates with each HRRR or HRDPS run. Fronts are checked twice an hour; WPC issues a new analysis every 3 hours and new forecast charts a few times a day. Neither layer replays archive days from Hist. Data.