Sea-level pressure, how far apart
GFS, ECMWF, ECMWF AIFS, GEM · Run 00Z 14 Oct · +114 h · Valid Sun 18Z 18 Oct
Gap between the highest and lowest sea-level pressure of these models
Region Northwest
United States
Europe
The readings under the map are for the whole map, not only the part shown.
GFS, ECMWF, ECMWF AIFS, GEM · Run 00Z 14 Oct · +114 h · Valid Sun 18Z 18 Oct
Gap between the highest and lowest sea-level pressure of these models
Widest gap 22.4 hPa at 48.5N 135.0W · unshaded ground is where all 4 land within 2 hPa of each other
Hatched ground is above 1000 m. Sea-level pressure there is not measured or forecast: each model works it out below the ground from its own forecast, its own way, so the models disagree there before any forecast has happened. That ground is left out of the shading, the lines and the widest gap.
Thin lines are sea-level pressure itself, every 4 hPa — the per-cell median of the 4, so a pattern no single model forecast. Read the highs and lows off them; the shading is where that pattern is in doubt.
Shows where the models disagree about sea-level pressure at 18Z 18 Oct — the day you choose, always at the same hour so that successive days compare.
Methodology
Across-model mean sea-level pressure spread from GFS, ECMWF, ECMWF AIFS, GEM — four models and three centres, named because a roster is a fact about a chart: two of these are ECMWF's, and where those two agree they agree partly for family reasons. Uncorrected model output on every side. GFS's is its standard reduction to sea level (PRMSL), the same kind ECMWF's is, rather than the smoother one its own pressure maps here draw (MSLET): measured over high ground at the analysis, the standard one sits within a hectopascal of the other models' where the other sits four below them.
Above 1000 m of GFS's own terrain the models already disagree by 2 to 4 hPa at the analysis — before any forecast has happened — because each extrapolates sea-level pressure below the ground its own way. That is a difference of arithmetic, not of weather, so it is not shaded. For the pattern over the West, read the 500 mb spread, which is above all of it.
The bands were cut against what this field actually does: over six runs in mid-October the models sat within 2 hPa of each other on seven tenths of the ground below 1000 m, and past 8 hPa on about one cell in fifty. Spread grows with lead — a day-2 map is mostly unshaded, and that is the picture.
Nothing here is smoothed. The shading, the isobars and the widest gap are all read from the models' own grids at their own resolution, because averaging a cell into its neighbours fills in exactly the extremes this map exists to show — a deep low is where four models at four resolutions disagree most. The widest gap comes from the drawn frame only.
Valid at 18Z and the same hour every day so successive maps compare — the other spread maps' hour, kept so a reader switching between them compares one moment.
Shaded ground is a disagreement rather than an error — none of these models is the truth here, and the map cannot say which is right. For who has actually been closest, the model leaderboards are the place.
This map is drawn from model output we store ourselves, not from anyone's picture of it, and is based on data and products of NOAA, the European Centre for Medium-Range Weather Forecasts (ECMWF) and Environment and Climate Change Canada. The per-city read of the same day — which cities the models are fighting about, and what the discussion makes of it — is on the national page. Every model's full forecast for any of these cities, or anywhere else, is in the beta: request an invite; it's free during the beta.
Sources and licences
- NOAA: GFS, from NOAA's NOMADS server. US government data, not subject to copyright (NOAA/NWS terms of use).
- ECMWF: IFS and AIFS Single, from ECMWF's open data. This service is based on data and products of the European Centre for Medium-Range Weather Forecasts (ECMWF). Source: www.ecmwf.int. This ECMWF data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0). ECMWF does not accept any liability whatsoever for any error or omission in the data, their availability, or for any loss or damage arising from their use.
- Environment and Climate Change Canada: GEM, from the MSC Datamart. Data Source: Environment and Climate Change Canada, under its Data Services End-use Licence.
What we change: each field is cropped to the frame these maps share when we download it, and each map then smooths, contours, differences or ranks those numbers as its Methodology says. These are our maps of the providers' numbers, not their products, and none of them has reviewed or endorsed this site.
Coastlines, borders, lakes and place names: Natural Earth, public domain.