The big picture

The large-scale drivers forecasters watch for DC winter weather: the blocking patterns that steer storms and cold air, the stratospheric polar vortex, and the tropical and seasonal background. Updated twice a day; built .

Blocking and the storm track

The three daily indices that matter most for East Coast winter storms. Observed values (ink) run through ; after that, the 31 GEFS ensemble members (thin) and their mean (bold) from the run. Big DC snowstorms usually come with a negative NAO and AO (blocking that slows storms and holds cold air) and a positive PNA (a western ridge that carves an eastern trough).

500 mb: the pattern itself

500 mb heights (contours every 6 dam, bold every 12) and how far they sit from the 1991–2020 normal. Blue is a trough (cold, stormy aloft), red a ridge. For a DC snowstorm you typically want a trough digging into the Ohio Valley and Southeast, inside the outlined box, with ridging over the West and blocking near Greenland. Lead 0 of each run is that model's own analysis.

Every model at the same time

Valid UTC, from each model's latest run (runs differ by up to 12 hours). Models whose runs don't reach that far are left out: ICON stops at about 7.5 days and UKMET at 7.

The polar vortex

The westerly wind circling the pole 30 km up. When it collapses, in a sudden stratospheric warming (SSW), the cold air it holds often spills south 1–4 weeks later and the AO and NAO tend to turn negative. The shaded bands are the middle 50% and 80% of 1991–2020 (NCEP/NCAR Reanalysis); the ink line is this season from GFS analyses.

The tropics: Madden–Julian Oscillation

The MJO is a pulse of tropical thunderstorms that circles the globe every 30–60 days. Where it is steers the jet stream downstream. The diagram traces the last 40 days: distance from the center is strength (inside the circle is weak), and the angle is where the convection sits.

In published composites for the eastern US in winter, phases 8, 1 and 2 lean cold and phases 4–6 lean mild, typically with a 1–2 week lag. The link is loose, and it only means much when the amplitude is above 1.

Observed RMM index from the Australian Bureau of Meteorology (via the IRI Data Library).

El Niño and DC snow

Each dot is one winter since 1950. The shaded zone marks strong El Niños (ONI ≥ 1.5): . A strong El Niño loads the southern jet with moisture, but DC also needs cold air, which is why the blocking indices above matter so much.