1000–500 thickness
GFS · Run 00Z 1 Oct · +42 h · Valid Fri 18Z 2 Oct
GFS's 1000–500 thickness
Region Northern Plains
The readings under the map are for the whole map, not only the part shown.
GFS · Run 00Z 1 Oct · +42 h · Valid Fri 18Z 2 Oct
GFS's 1000–500 thickness
Thin lines are the thickness itself, every 6 dam. The heavy line is 540, the one forecasters read as the rain/snow boundary.
GFS's 1000–500 thickness at 18Z 2 Oct — the day you choose, always at the same hour so that successive days compare.
Methodology
The 1000–500 thickness is how far apart the 1000 mb and 500 mb pressure surfaces are — the depth of the lowest ~5 km of the atmosphere, which grows as that column warms. So it is a thermometer for the whole column rather than for the air at head height, and that is why a forecaster reaches for it: the 540 dam line is the classic rain/snow boundary, and it does not care that the surface reading is 35°F.
Computed as 500 mb height minus 1000 mb height 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, subtracted after every model is sampled onto one lattice by coordinate.
Thickness is in decametres (tens of metres), the unit every chart of it uses: 540 is 5,400 m. There is no metric variant of this map, because there is no second convention to offer.
540 is a boundary in the column, not a promise about precipitation type. It says the layer is cold enough through its depth for snow to survive the fall; what actually reaches the ground also depends on a warm nose, the surface layer and how hard it is raining. We do not draw a precipitation-type field, and this line is not one.
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.
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.