Understanding Ensemble Plumes
An ensemble runs a forecast model many times with slightly different starting
conditions or physics. Where the runs agree the forecast is confident; where they fan out, several
outcomes are possible.
SREF and REFS
SREF (26 members, to 87 hours, runs at 03/09/15/21Z, ready ~5h20m later) retires
on October 6, 2026. Its successor is REFS, the RRFS ensemble at
3km (runs at 00/06/12/18Z, ready ~3.5h later). NOAA doesn't publish individual REFS members, so the
REFS view shows the deterministic RRFS run as an hourly line and the
REFS ensemble mean with a band of mean ± spread (3-hourly to 60 hours),
plus a precipitation-type timeline. Switch models in the header; the latest available run is
picked automatically.
Reading the charts
- Thin lines are individual members. SREF has two cores: ARW
(solid) and NMB (dashed); when both agree, confidence is higher.
- Bands show the middle 50% (darker) and 80% (lighter) of outcomes.
- The heavy line is the ensemble mean, usually the best single guess. In REFS
the dashed line is the deterministic RRFS run.
- Compare overlays the means of the three previous runs, so you can see
whether the forecast is trending up or down.
Summary statistics
Each chart lists the mean and the range across the ensemble (SREF: member max/min; REFS: the
10th-90th percentile of the band). A smaller spread means more agreement.