Categories
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People & offices
- SDM also: Senior Duty Meteorologist
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Senior Duty Meteorologist — the national on-duty coordinator for NWS operations.
The Senior Duty Meteorologist sits at NWS Central Operations in College Park, Maryland, and acts as the national duty officer for the whole Weather Service. They coordinate across forecast offices and national centres, decide when to declare a Critical Weather Day, direct where the movable satellite sectors point, and issue the administrative messages this site translates.
- NCO also: NWS Central Operations, NCEP Central Operations
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NWS Central Operations — runs the computing behind the forecast models.
NCO operates the supercomputers and data systems that produce NWS model guidance and move products around. When a message talks about dataflow, ingest or the production suite, NCO is the organisation doing it.
- NCEP
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National Centers for Environmental Prediction.
The umbrella for NWS’s national-scale centres — including the Storm Prediction Center, Weather Prediction Center, National Hurricane Center and the Environmental Modeling Center. NCEP produces the guidance that local forecast offices build on.
- WFO also: Weather Forecast Office
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Weather Forecast Office — one of 122 local NWS offices.
Each WFO is responsible for forecasts and warnings in its own area, typically a few dozen counties. The three-letter office code (TOP for Topeka, OUN for Norman) shows up throughout the API, especially in grid coordinates and product identifiers.
Observations
- RAOB also: rawinsonde, radiosonde, sounding, upper-air observation
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Radiosonde observation — the data from a weather balloon flight.
Twice a day, at 00Z and 12Z, roughly 90 US sites release a balloon carrying a radiosonde: a small instrument package that radios back temperature, humidity, pressure and wind as it rises. A flight lasts about two hours and can reach 100,000 feet before the balloon bursts. These soundings are the only routine direct measurement of the whole depth of the atmosphere, and they anchor the models’ starting conditions.
- TTAA
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The mandatory-levels part of a coded sounding.
A balloon sounding is transmitted in parts. TTAA carries the mandatory pressure levels — 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150 and 100 mb — up to 100 mb. If TTAA is missing, the backbone of the sounding is missing.
- TTBB
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The significant-levels part of a coded sounding.
TTBB carries the “significant levels”: the extra points between the mandatory levels where temperature or humidity changes sharply. Losing TTBB means losing the fine structure — inversions, dry layers, the details that decide whether storms fire — even though the basic profile survived.
- mb also: millibar, millibars, hPa
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Millibars — the unit of atmospheric pressure, also used as a height coordinate.
Sea level averages about 1013 mb. Because pressure falls predictably with height, meteorologists use pressure as a stand-in for altitude: 850 mb is roughly 5,000 ft, 500 mb roughly 18,000 ft, 250 mb roughly 34,000 ft. So when a message says a balloon was “short to 577 mb”, it means the flight ended around 15,000 ft — the sounding covers the lower atmosphere but says nothing about the jet stream level above it. Tropical products spell the same unit hPa (hectopascals) — the numbers are identical, so a hurricane’s 958 hPa central pressure is 958 mb. The lower it goes, the stronger the storm.
- METAR
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A routine surface weather report from an airport.
The standard hourly observation format used worldwide: wind, visibility, cloud layers, temperature, dewpoint and pressure, in a dense code. Most “current conditions” you see anywhere trace back to a METAR.
- PIREP
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A pilot report of conditions actually encountered in flight.
Pilots radio in what they are experiencing — turbulence, icing, cloud tops, ride quality. PIREPs are one of the few direct observations of the mid-atmosphere between balloon launches.
Models
- GFS
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Global Forecast System — the main American global weather model.
The GFS simulates the entire planet’s atmosphere out to 16 days, four times a day. It is the model behind most free forecast apps and the starting point for a great deal of American forecasting.
- NAM
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North American Mesoscale model — higher resolution, smaller area.
The NAM covers North America at finer resolution than the GFS but runs out to only 84 hours. Because its domain is smaller it can resolve terrain and smaller-scale weather features better.
- model production suite
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The scheduled sequence of model runs NWS executes every cycle.
Models run on a strict timetable: gather observations, run the analysis, run the forecast, post the output. Each step has a target time. When SDM says the suite is “running on time”, downstream products — including the forecasts in your weather app — will land when expected.
- ingest
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Taking observations into a model’s starting conditions.
Before a model can forecast it must first work out what the atmosphere looks like right now, by blending millions of observations into a coherent picture. Data that arrives after the cut-off simply misses the run — which is why “stations available for ingest” is a number worth watching.
Time
- Z also: UTC, Zulu
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Coordinated Universal Time — the single clock all weather data uses.
Weather is global, so meteorology runs on one clock: UTC, written with a trailing Z and pronounced “Zulu”. 12Z is noon UTC, which is 8am Eastern Daylight Time or 5am Pacific Daylight Time. Every time on this site is shown in your local time with the original Z time kept alongside.
Satellite
- MDS also: mesoscale sector, mesoscale domain sector, meso sector
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Mesoscale Domain Sector — a small, movable, rapid-scan satellite window.
Each GOES satellite scans a fixed full-disk view and a continental view on a set schedule, plus two small “mesoscale” boxes about 1,000 km across that can be aimed anywhere in view and are photographed roughly every 60 seconds. Forecasters point them at the day’s highest-stakes weather — a developing tornado outbreak, a hurricane’s eye, a fast-moving wildfire.
- GOES also: GOES-East, GOES-West
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The American geostationary weather satellites.
GOES satellites sit 22,236 miles above the equator, orbiting at exactly the speed the Earth turns, so each one stares at the same face of the planet forever. GOES-East watches the Americas and the Atlantic; GOES-West watches the western US and the Pacific.
Operations
- Critical Weather Day also: CWD
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An internal NWS posture that freezes non-essential system changes.
When NWS expects high-impact weather, the Senior Duty Meteorologist can declare a Critical Weather Day. Scheduled software, hardware and network changes at the affected offices are postponed so nothing is disturbed while forecasters work. Emergency changes still happen, with senior approval. It says something about how NWS is protecting its own systems — not about the forecast where you live.
- NWSTG
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NWS Telecommunication Gateway — the hub that routes NWS products.
Nearly every NWS product passes through the Gateway on its way to the public, the FAA, broadcasters and international partners. When the Gateway has trouble, products can be delayed or missing across many systems at once.
- EMWIN
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Emergency Managers Weather Information Network.
A low-cost satellite and internet broadcast of NWS warnings and products, designed so emergency managers can keep receiving data even when normal internet is unavailable.
- NOAAPort
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The satellite broadcast that pushes NWS data to bulk users.
NOAAPort delivers a firehose of NWS data by satellite to universities, broadcasters, private weather companies and emergency managers — the pipe most large weather data consumers actually drink from.
- IDSS
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Impact-based Decision Support Services.
NWS’s term for working directly with emergency managers and other officials — briefings, phone calls, on-site support — rather than only publishing forecasts and warnings.
- IMET
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Incident Meteorologist — a forecaster deployed to a wildfire.
IMETs deploy to the field with a mobile weather station and provide forecasts directly to fire incident commanders. When an SDM message mentions IMETs, there is a significant fire being actively worked.
Products
- ADM
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Administrative Message — the routine SDM status product.
The product code for the Senior Duty Meteorologist’s regular operational messages: model production status, balloon reports, satellite sector moves and Critical Weather Day status.
- ADA
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Alert Administrative Message — the higher-urgency SDM product.
Used for outages, dataflow problems and Critical Weather Day declarations. Worth noting: the message header, not the product code, is what actually tells you whether a message is an alert — ADM-coded products sometimes carry an ALERT header.
- AFD
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Area Forecast Discussion — the forecaster’s reasoning, in their own words.
Several times a day each forecast office publishes an unvarnished explanation of how they read the situation, which models they trust today and what they are unsure about. It is written for other meteorologists but is readable and often fascinating.
API concepts
- gridpoint also: forecast grid
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One square of a forecast office’s editing grid, roughly 2.5 km across.
Modern NWS forecasts are not written as text first. Forecasters edit a grid of numbers covering their area, and the text forecast is generated from it. The API exposes both: the grid itself, and the readable forecast built from it.
- zone also: forecast zone, fire weather zone
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A named area that forecasts and warnings are written for.
NWS divides the country several different ways at once: public forecast zones, county zones, fire weather zones and marine zones. They overlap and are not the same shapes, which is why a warning sometimes covers a different area than the forecast for the same town.
- GeoJSON
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The default format api.weather.gov returns.
A JSON format for geographic data, where each item carries both its data and its shape on the map. The NWS API returns GeoJSON by default; you can ask for JSON-LD instead with an Accept header.
Tropical cyclones
- invest also: invest area, invest number, area of interest
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A numbered area a warning centre is watching for possible development.
When forecasters want to run models and collect data on a disturbance that is not yet a tropical cyclone, they give it an invest number: 90 to 99, followed by a basin letter — L for the Atlantic, E for the eastern Pacific, C for the central Pacific, W for the western Pacific. So 96L is simply the Atlantic area currently holding the number 96 — not the ninety-sixth storm of the season. The numbers are recycled within a season, which is why guidance filed under one may belong to an earlier system. An invest is an area being watched, not a tracked storm: it has no name, no advisory number and no forecast track.
- formation chance also: formation risk, genesis probability, 2-day chance, 7-day chance, formation potential
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The published probability that an area becomes a tropical cyclone, within a stated window.
The National Hurricane Center publishes two windows for each area it is watching: the chance of formation within 2 days and within 7 days. They answer different questions and the 7-day figure is always the larger, because it contains the 2-day one — a 20% 2-day and 60% 7-day chance means most of the risk is later in the week, not that two sources disagree. Each percentage also carries the centre’s own band: Low below 40%, Medium 40–60%, High above 60%. JTWC publishes a single roughly 24-hour potential instead, usually as a band (poor, fair, good) rather than as a percentage. A window a source did not issue is shown as absent rather than as zero, and an area a source has not classified reads Unknown — which is the absence of a published risk, not a low one.
- cone of uncertainty also: forecast cone, cone
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The probable path of the storm’s centre — not the size of the storm.
The cone is drawn so that the centre of the storm stays inside it about two thirds of the time, based on how far the centre has historically strayed from forecasts of the same lead time. That is all it says. It is not the size of the storm, not the reach of its winds, and not the extent of its hazards: rain, surge, wind and tornadoes regularly occur well outside it, and a place outside the cone can still be hit. The cone is official GIS data published by the National Hurricane Center for the storms it forecasts. JTWC does not publish one, and a cone must never be manufactured by drawing a buffer around a forecast track — a shape that looks like a cone but is not one is worse than no cone at all.
- wind radii also: wind radius, wind-field radii
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How far out from the centre a given wind speed reaches, by quadrant.
A warning centre reports the reach of 34-kt (gale), 50-kt and 64-kt (hurricane-force) winds separately in each of the four compass quadrants, in nautical miles, because a tropical cyclone is rarely symmetric — the strongest side is usually the one the storm is moving toward. Wind radii are the closest thing published to the size of the storm, and they are the reason the cone is not it. A source that reports none has not reported a small storm; it has reported nothing about extent, and the difference is not something a reader or a renderer may fill in.
- kt also: knot, knots
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Knots — one nautical mile per hour, the unit tropical winds are measured in.
Warning centres issue wind speeds in knots because the thresholds that define a tropical cyclone are set there: 34 kt is tropical-storm force, 64 kt is hurricane or typhoon force, 96 kt is a major hurricane. One knot is about 1.15 mph, so 65 kt is about 75 mph. Converted figures are rounded and the knot value is the one the centre actually published — which is why both are shown rather than one replacing the other.
- nm also: nautical mile, nautical miles
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Nautical mile — about 1.15 statute miles, or 1.85 km.
One nautical mile is one minute of latitude, which is what makes it the natural distance unit on a chart: sixty of them is one degree of latitude anywhere on earth. Wind radii and forecast-error distances are published in nautical miles for that reason.
- tau also: forecast hour, lead time
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The lead time of a forecast point, in hours from the cycle it was initialized on.
A forecast point at tau 48 is the position forecast for 48 hours after the forecast was initialized — not 48 hours after the advisory was issued, and not 48 hours from now. The distinction is real money on NHC storms: the centre initializes its forecast on the synoptic hour (00, 06, 12 or 18Z) and issues the advisory built on it up to six hours later, so the earliest points of a forecast can already be in the past by the time the advisory is published.
- advisory also: tropical cyclone advisory
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One numbered issuance about one storm — the unit a forecast is published in.
The National Hurricane Center issues an advisory for each storm every six hours, numbered from 1, with intermediate issuances lettered (4A, 4B) when watches or warnings change in between. JTWC issues warnings on the same idea. The number, not the timestamp, is how everyone refers to a particular issuance: “Advisory 4” is a thing a forecast discussion, a broadcaster and a reader can all point at. The last one a centre issues for a system is its final advisory, and it says so in its own words.
- unclassified also: unknown risk, not classified
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The source stated no value — which is not the same as a low one.
A forecast point can carry a position and a wind with no development stage attached, and an area being watched can carry no risk band at all. Both are shown as an explicit absence — an em dash, or the word Unknown — rather than being filled in with a plausible value or quietly left blank. A centre that classified a point in terms this site does not carry is treated the same way: the value is not published as text, because “Unknown” printed beside a stated wind and a stated position reads as a broken page rather than as the hedge it is.
Radiosonde status codes
RAOB recaps often report a bare five-digit code next to a station — 10142,
10159 and so on. These are internal NWS codes and, as far as we can find,
NWS does not publish a legend for them anywhere public. We checked the NWS
upper-air program pages, the NCO radiosonde quality-control reports and the message archive
itself.
So we only define a code when the messages themselves define it, and we show every other code as a raw code with an honest label. Guessing would be worse than useless here: it would be believed.
- 10142
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Ground equipment failure — the station’s tracking/receiving equipment was down, so no usable sounding was produced.
How we know: The SDM messages themselves print this code alongside the text "Ground equipment failure", so the meaning comes straight from the source.
- 10145, 10158, 10159 and others
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Meaning not published.
These appear regularly and clearly indicate some problem with the launch or the data, but we have no verifiable source for what each one specifically means. They are shown on message cards as unrecognised codes rather than translated.