NWS snow forecast maps: expected, low-end, high-end, and impact risk

Start with the map label and valid period

The maps describe a single storm window—not the whole winter

NWS probabilistic snowfall products are designed to show a range of possible snowfall totals for an upcoming event. A local winter page may display the official expected amount beside experimental low-end, high-end, and exceedance products.

Before reading any number, confirm the map's issue time, ending time, units, and geographic coverage. Two maps can show different totals simply because they cover different hours. Recheck as the event approaches because the range can shift when forecasters receive new observations and model guidance.

Expected snowfall · Most likely outcome

The expected map is the official NWS snowfall forecast

The expected-snowfall map shows the amount NWS forecasters judge to be the most likely outcome for the stated period after evaluating computer guidance, satellite, radar, observations, and other information.

“Expected” is not a promise that every driveway, road, or elevation will receive exactly that amount. Snow bands, elevation, temperature, wind, compaction, and the timing of a change to rain or sleet can create sharp local differences. Use the expected amount as the center of the planning picture, then use the range maps to understand what could reasonably be lower or higher.

Reasonable low and high scenarios

Low-end does not mean minimum, and high-end does not mean maximum

Low-end amount

An unlikely lower scenario. The NWS explanation says there is a 9-in-10, or 90%, chance that more snow will fall and a 1-in-10 chance that less will fall.

Expected amount

The official most likely snowfall forecast for the map's stated period.

High-end amount

An unlikely upper scenario. There is a 1-in-10, or 10%, chance that more snow will fall and a 9-in-10 chance that less will fall.

These maps are planning bounds, not a guarantee that the final total must stay between them. If the difference between low-end and high-end amounts is large, uncertainty is meaningful: keep more flexibility in travel, staffing, plowing, or event decisions. If the range narrows, confidence in the amount has improved, but impacts can still depend on wind, temperature, timing, and road treatment.

Exceedance maps · A threshold question

“Chance of at least 6 inches” is different from “expected 6 inches”

Exceedance maps show the probability that snowfall will equal or exceed a specific threshold, such as 1, 4, 6, 8, or 12 inches, during the stated period.

Choose the threshold tied to your decision. A school, road crew, delivery route, or mountain trip may care about different amounts. Read the percentage at your location as the chance of reaching or exceeding that threshold—not the share of the area that will receive snow and not the expected amount.

The threshold series can answer questions that one expected-total map cannot: “How likely is enough snow to disrupt this route?” and “How much risk remains above the most likely total?”

Potential impact · Not an alert

The Winter Storm Severity Index adds an impact lens

WSSI estimates the potential severity of snow- and ice-related societal impacts. NWS says it does not depict official warnings and must be used with forecasts and warnings.

WSSI combines forecast information—including snow accumulation, blowing snow, wind, and temperature—with non-weather information such as land use, urban areas, and local snowfall climatology. That is why the same forecast snow amount can produce different impact levels in different places.

Read the overall impact color, then inspect the component driving it. A snow-load, blowing-snow, ground-blizzard, flash-freeze, ice-accumulation, or snow-amount signal can call for a different plan. Treat WSSI as context for potential impact, not as a road-condition report or a substitute for local emergency instructions.

Different products · Different decisions

A snow outlook, forecast, watch, and warning are not interchangeable

Outlook

A broad, longer-lead signal that winter weather may become important. Use it to monitor future plans; it is not a local accumulation forecast or an alert to act now.

Forecast

The expected conditions, timing, and amounts for a location or area. The expected, low-end, high-end, and exceedance maps help describe the forecast and its uncertainty.

Watch

Significant winter weather is possible. Review routes, shelter, supplies, and timing, and be prepared for an upgrade or change.

Warning

Significant winter weather is occurring or expected. Follow the official product's timing, affected area, hazards, and protective instructions.

An advisory can also signal hazardous winter conditions below warning criteria. When an advisory, watch, or warning includes your location, read that alert first; the probability and impact maps help explain the range, but the alert carries the official hazard message.

A practical check order

Move from amount, to range, to impact, to action

1. Confirm the place and period. Use the local NWS winter page or point forecast, and verify issue and valid times.
2. Read the expected amount. This is the official most likely snowfall forecast for that period.
3. Read the low and high scenarios. The width of the range helps you decide how much flexibility or contingency you need.
4. Check thresholds and WSSI. Ask whether a decision-relevant amount could be exceeded and which impact component is driving concern.
5. Finish with alerts and local instructions. A watch, advisory, or warning for your location changes the action; road closures and emergency directions belong to the responsible local authority.