Working with GRIB Data in qtVlm

Opening and Downloading GRIB Files

qtVlm can load GRIB files from two sources: files you've already downloaded to your computer, or files requested directly through the built-in download interface. Both methods end up in the same place — a loaded GRIB displayed on your chart — but the workflow differs.

To open an existing file, go to Grib → Open Grib (or use the keyboard shortcut Ctrl+G). Navigate to your .grb or .grib2 file and open it. qtVlm reads both GRIB1 and GRIB2 formats automatically. The forecast appears immediately as colored wind barbs overlaid on your chart area. If nothing appears, zoom or pan to the area covered by the GRIB — the data only exists within the file's geographic boundaries.

To download fresh data directly, go to Grib → Download Grib. This opens a dialog where you select the area (drag a box on the chart or enter coordinates), the model (GFS is the default and usually best choice), the resolution (0.25° or 0.5°), the forecast duration (how many days ahead), and which parameters to include (wind, pressure, waves, precipitation). Click download and qtVlm fetches the file from NOAA's servers. This is the fastest workflow when you have internet access.

qtVlm also supports the zyGrib/XyGrib file format and can open files from these popular GRIB viewers. If you've been using XyGrib for weather, your existing files work directly in qtVlm.

Time-Stepping Through a Forecast

A GRIB file isn't a single snapshot — it contains multiple time steps showing how the weather evolves over the forecast period. Learning to step through these time steps efficiently is essential. You're looking for patterns: when the wind fills in, when a front passes, when conditions deteriorate, when the best weather window opens.

The GRIB time bar at the bottom of the qtVlm window shows the full time range of your loaded data. The current time step is highlighted. Use the left/right arrow keys to step backward and forward through time, or click directly on the time bar to jump to a specific time. The date and UTC time of the current step are displayed prominently.

As you step through time, watch the wind barbs and colors update across the chart. You're building a mental movie of the weather evolution. Pay attention to how fast systems are moving, where calm zones form, and where wind accelerations develop. This animated view gives you an intuition for the forecast that numbers alone cannot — and it's the same data the routing algorithm will use to calculate your optimal path.

For detailed data at a specific point, right-click on the chart to see a tooltip showing the exact wind speed, direction, pressure, and any other loaded parameters at that grid point and time step. This is invaluable for checking conditions at a specific location — your departure point, a waypoint, or a concern area along the route.

Wind Display Options

qtVlm offers several ways to visualize wind data, and choosing the right display mode depends on what you're trying to understand. The key settings are found under Grib → Grib Display Options and in the toolbar.

Wind barbs are the classic meteorological display — small arrows on the chart where the flag direction shows where the wind is coming from and the barbs/flags on the tail encode speed (short barb = 5 knots, long barb = 10 knots, pennant = 50 knots). Wind barbs give you precise information at each grid point but can clutter the display at high zoom levels. They're most useful when you're examining a specific area in detail.

Color fill paints the entire chart area with colors representing wind speed — typically blue for light air through green, yellow, orange, and red for storm-force winds. This gives you an instant overview of the wind field: where the strong wind is, where the calms are, where gradients are steep. The color legend in the corner shows the speed-to-color mapping. Color fill is the best mode for getting the big picture at a glance.

Streamlines or arrows show wind direction as flowing lines or small vectors across the chart. These are excellent for understanding flow patterns — convergence zones, divergence, circulation around highs and lows — that aren't immediately obvious from wind barbs alone. Combining streamlines with color fill gives you both direction and speed information in a highly readable format.

You can toggle these displays independently and overlay them. A common setup is color fill for speed with streamlines for direction, plus wind barbs turned on only when you zoom in close. Isobars (pressure contour lines) can also be overlaid from the same menu — they help you see the high and low pressure systems driving the wind patterns.

Multiple GRIB Slots and Layering

qtVlm provides two GRIB slots — Grib Slot 1 and Grib Slot 2 — allowing you to load two different GRIB files simultaneously. This is a powerful feature that supports several important workflows.

The most common use is loading a global model in Slot 1 (GFS at 0.5° covering your full passage area) and a regional model in Slot 2 (ICON or NAM at higher resolution covering your departure or landfall area). The routing algorithm uses Slot 1 by default, but you can view both simultaneously to check whether the regional model shows important detail that the global model misses — coastal effects, thermal winds, or stronger gradient flows.

Another use is comparing forecast models. Load GFS in Slot 1 and ECMWF (if available) in Slot 2. Where the models agree, you can have higher confidence. Where they diverge — and they often do beyond 4-5 days — you know the forecast is uncertain and should plan conservatively. Model disagreement is itself information.

You can toggle each slot's display on and off independently using the Grib menu or the toolbar buttons. The display settings (barbs, color fill, isobars) can be configured separately for each slot, so you might show Slot 1 as color fill and Slot 2 as wind barbs. To switch which slot the routing algorithm uses, check the routing settings — by default it uses Slot 1, but you can specify Slot 2 or blend them.