What I Check Before Reading a Strong El Niño Snow Map

Why I looked this up

I wanted to know what a “strong El Niño snow map” actually tells me about a U.S. winter. Does a colored area represent expected snowfall, or something that happened in previous El Niño years?

The distinction became the focus of my reading. NOAA has historical snowfall maps, while the Climate Prediction Center explains seasonal precipitation outlooks. I would not read those as interchangeable products.

These are my notes on interpreting the maps—not a claim that a strong El Niño is currently occurring or a forecast for the coming winter.

The main thing I learned

A historical snowfall departure is not a predicted snow total.

NOAA’s snowfall maps covering 1950–2009 compare October–April snowfall during the 10 strongest El Niño winters with snowfall during all 21 El Niño winters in that study. Blue represents more snow than average; brown represents less.

Those colors describe departures from average across a selected set of past winters. I cannot turn a blue patch into a number of inches expected at my address this winter.

The selection matters, too. A map labeled “strongest” does not necessarily use the same winters as one labeled “moderate-to-strong.”

My checklist before interpreting a map

I put together this reading order from the NOAA analyses and CPC’s explanation of seasonal outlooks:

What I check	Why it changes my interpretation
Historical analysis or forecast?	Past snowfall patterns and probabilities for a future season answer different questions.
Snowfall or total precipitation?	CPC describes seasonal total precipitation categories, not snowfall totals.
Which months?	NOAA’s older comparison covers October–April; its newer snowfall analysis examines January–March.
Which El Niño events?	All El Niño winters, the strongest winters, and moderate-to-strong winters are different selections.
What does the color measure?	An average departure and a count of below-average winters are different summaries.
What baseline and adjustments were used?	NOAA’s newer departure maps use the 1991–2020 average and remove long-term trends.

My stopping rule is straightforward: when a map’s caption does not establish these details, I leave its local implication unresolved rather than treating the color as a forecast.

What I looked into further: average versus frequency

NOAA’s October 26, 2023, snowfall analysis uses 1959–2023 data. Alongside average January–March snowfall departures, it shows how many of the 13 moderate-to-strong El Niño winters had below-average snowfall.

That gave me two separate questions to ask: “What was the average departure?” and “How often was snowfall below average?” I find that distinction worth preserving instead of reducing both maps to “less snow.”

I also checked what a seasonal precipitation percentage means. CPC defines above-, near-, and below-normal categories using thirds of the historical distribution. A probability of being above normal is not a percentage increase in precipitation—and it is not a snowfall amount.

What I take away

I now read a strong El Niño snow map as a labeled piece of evidence, not a standalone winter prediction. I check the period, event selection, measurement, and baseline before drawing a conclusion.

The historical NOAA pages remain useful for that purpose, but their study dates stay attached to the findings. Nothing in these notes turns an older map into an outlook for the coming winter.