1000–500 thickness, and the 540 line
Widest gap 12.3 dam, 40 mi NNE of Durango, Colorado · unshaded ground is where all 4 land within 3 dam of each other
Thin lines are the thickness itself, every 6 dam — the per-cell median of the 4, so a depth no single model forecast. 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.
Shows where the models disagree about how deep the lowest 5 km of the atmosphere is at 18Z 9 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.
The bands were cut against what this field actually does rather than against a round guess: over twenty valid times the four sat within 3 dam of each other on about half the frame, and past 10 dam on one cell in a hundred. It is not the 500 mb height spread wearing another name — the two agree far less than they look like they should, and least at the short leads a reader actually uses.
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.