1000–500 thickness, and the 540 line
Thin lines are the thickness itself, every 6 dam. 540 — the line forecasters read as the rain/snow boundary — is off this map: every cell on the frame is deeper than that, which is what early autumn looks like here.
ECMWF's 1000–500 thickness at 18Z 12 Sep — 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. Raw 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.
Every map on this page is drawn from model output we store ourselves, not from anyone's picture of it. 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.