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.