WPC Rainfall & Winter Outlooks
The Weather Prediction Center (WPC) issues national outlooks for heavy rainfall and winter precipitation. Like SPC outlooks, these are polygon-based — LMKwhen checks whether your location falls within an outlook area — and they give advance notice days before any NWS watch or warning is issued.
LMKwhen monitors two families: Excessive Rainfall Outlooks (flash-flood risk) and Winter Precipitation Probability.
Excessive Rainfall Outlook (ERO)
The ERO expresses the probability that rainfall will exceed flash flood guidance within about 25 miles of a point. It is a categorical outlook — each area carries a single risk rank.
| Rank | Label | Meaning (probability floor) |
|---|---|---|
| 1 | Marginal (MRGL) | At least 5% |
| 2 | Slight (SLGT) | At least 15% |
| 3 | Moderate (MDT) | At least 40% |
| 4 | High (HIGH) | At least 70% |
When creating an ERO rule, you set a minimum rank threshold — for example, Moderate (3) or higher alerts whenever your location falls within an MDT or HIGH area.
Outlook days
| Day | Typical issuance times (UTC) |
|---|---|
| Day 1 | ~01Z, ~12Z, ~16Z, ~18Z |
| Day 2 | ~12Z, ~18Z |
| Day 3 | ~12Z, ~18Z |
| Day 4 | ~18Z |
| Day 5 | ~12Z |
Days 4 and 5 cap at Slight — WPC does not issue Moderate or High risk at extended range, so the rank dropdown limits to Marginal / Slight when only a Day 4 or Day 5 product is selected.
In the rule wizard, choose the Excessive Rainfall outlook type under the SPC / outlooks path, pick the day(s), and set a minimum rank.
Winter Precipitation Probability
WPC issues probabilistic winter precipitation outlooks — the chance that snow or ice accumulation reaches a given threshold. LMKwhen covers two ranges with slightly different probability semantics.
Days 1–3 — snow & ice thresholds
For the near term, WPC bins the probability into Slight / Moderate / High ranges. LMKwhen alerts on the floor of the range you select.
| Threshold | Alert fires when the forecast band is… |
|---|---|
| Snow ≥ 4 in | 10% or higher (Slight), 40% (Moderate), or 70% (High) |
| Snow ≥ 8 in | same three bands |
| Snow ≥ 12 in | same three bands |
| Ice ≥ 0.25 in | same three bands |
| Probability floor | Band |
|---|---|
| 10% | Slight (10–39%) |
| 40% | Moderate (40–69%) |
| 70% | High (70–100%) |
Days 4–7 — extended range
At extended range, WPC publishes a true percentage for the chance of at least 0.25 in liquid-equivalent snow or sleet in a 24-hour period. You pick a minimum probability band directly:
| Probability | |
|---|---|
| 10% · 30% · 50% · 70% · 90% | Alert fires when the outlook reaches or exceeds your selection |
Creating a Winter Precip rule
In the rule wizard, choose the Winter Precip outlook type, then pick a precipitation family — Snow ≥ 4 in, Snow ≥ 8 in, Snow ≥ 12 in, Ice ≥ 0.25 in (all Days 1–3), or Winter precip (Days 4–7). Select the forecast day(s) and set your minimum probability. The Days 1–3 families use the Slight / Moderate / High floors; the Days 4–7 family uses the 10–90% bands.
Alert lifecycle
WPC outlook alerts follow the same lifecycle as SPC outlooks:
- Issued — your location enters an outlook area that meets your threshold
- Reconfirmed — a later issuance covers the same area at the same level
- Upgraded / Downgraded — the level changes; the old alert resolves and a new one fires
- Resolved — the outlook expires or a "no risk" issuance clears the area. When a later issuance drops your area entirely, the alert resolves and — if send all-clear is enabled on the rule — an all-clear notification is sent.
Use case
The ERO is the single best advance signal for flash-flood risk, often flagging a Moderate or High day 24–48 hours before any Flash Flood Watch. The winter probability outlooks give property and field teams a quantified, multi-day-ahead read on plowable snow and damaging ice — well before a Winter Storm Watch narrows the timing.
Seeing outlooks on the map
Both WPC outlook families can be displayed on the Locations map. Open the Layers panel and choose Rain for the Excessive Rainfall Outlook or Winter for the winter precipitation probabilities, then pick a day. Click any shaded area to see its category. See The Locations Map.