PWAT relative to normal
18.8% of the map is past its 90th percentile for 00Z · 7.5% is under its 10th
17.8% of the map is past its 90th percentile for 06Z · 8.0% is under its 10th
16.6% of the map is past its 90th percentile for 12Z · 9.1% is under its 10th
14.7% of the map is past its 90th percentile for 18Z · 9.8% is under its 10th
13.5% of the map is past its 90th percentile for 00Z · 10.1% is under its 10th
13.0% of the map is past its 90th percentile for 06Z · 9.8% is under its 10th
12.7% of the map is past its 90th percentile for 12Z · 9.7% is under its 10th
12.8% of the map is past its 90th percentile for 18Z · 8.7% is under its 10th
13.5% of the map is past its 90th percentile for 00Z · 8.0% is under its 10th
13.9% of the map is past its 90th percentile for 06Z · 7.8% is under its 10th
14.6% of the map is past its 90th percentile for 12Z · 7.4% is under its 10th
14.8% of the map is past its 90th percentile for 18Z · 5.3% is under its 10th
16.3% of the map is past its 90th percentile for 00Z · 4.8% is under its 10th
17.4% of the map is past its 90th percentile for 06Z · 6.3% is under its 10th
19.5% of the map is past its 90th percentile for 12Z · 5.2% is under its 10th
21.2% of the map is past its 90th percentile for 18Z · 4.5% is under its 10th
22.3% of the map is past its 90th percentile for 00Z · 3.9% is under its 10th
22.2% of the map is past its 90th percentile for 06Z · 3.2% is under its 10th
22.4% of the map is past its 90th percentile for 12Z · 3.5% is under its 10th
23.0% of the map is past its 90th percentile for 18Z · 3.6% is under its 10th
23.6% of the map is past its 90th percentile for 00Z · 3.2% is under its 10th
One GFS run, issued 00Z 14 Sep — played forward 6 hours at a time, from the analysis to 120 hours out.
Methodology
PWAT — precipitable water — is how much water vapour the air overhead holds, as the depth of water it would make if it all fell at once. It is the moisture a storm has to work with, which is why forecasters look at it before heavy rain.
A column that is ordinary in Miami is extraordinary in Salt Lake City, so this map compares each place with itself: the column is ranked against that place's own columns at the same hour of the day, within 15 days of the date, over 2021–2026 of the ERA5 reanalysis. Past the 90th percentile means wetter than nine in ten of those. That is not the same number as a day's peak against peaks, which is how a heavy-rain threshold is usually set: an afternoon's column sits a few millimetres under that day's peak.
Two models, because only two file this field: GFS as PWAT and ECMWF as total column water vapour, the same quantity. ECMWF's AI model files only total column water, which counts cloud and rain as well and reads high exactly where it rains; GEM and NOAA's AI model file neither. Raw model output, ranked against a reanalysis rather than against each model's own climate, so a model that runs moist will clear its 90th percentile more often than one day in ten.
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.