Glossary
The following terms appear throughout this guide. Definitions are written in the IEM context — what each term means for someone using this website, not as standalone meteorological or regulatory definitions.
AFOS — Automation of Field Operations and Services. The legacy message-routing system the NWS used before the internet era, and whose product identifiers it never stopped using. On IEM, you will encounter AFOS whenever you are working with raw NWS text products. Every product — Area Forecast Discussion, Hazardous Weather Outlook, Local Storm Report, public zone forecast — has an AFOS PIL (see PIL) that uniquely identifies it. IEM’s text product archive at /wx/afos/p.php is organized around these identifiers. If you know the PIL, you can retrieve any NWS product IEM has ever received.
ASOS — Automated Surface Observing System. The primary airport weather observation network in the United States, operated jointly by the NWS, FAA, and Department of Defense. ASOS stations report every hour and, at most sites, every minute, measuring temperature, dew point, wind speed and direction, visibility, precipitation, sky condition, and pressure. On IEM, ASOS is the workhorse: it is the network behind most of the hourly download portals, most of the climate charts in Autoplot, and most of the long-term station records available for your airport. The IEM ASOS archive runs from approximately 1973 at the longest-serving stations. Understanding ASOS’s known limitations — particularly its tendency to undercount light snowfall and its whole-degree Fahrenheit internal temperature storage — matters when you are doing precision climatology.
AWOS — Automated Weather Observing System. A family of FAA-operated automated stations at smaller airports that do not qualify for full ASOS instrumentation. AWOS stations vary significantly in what they measure: AWOS-1 gives only wind and altimeter; AWOS-3 adds visibility and sky; AWOS-3P adds a precipitation sensor. IEM ingests AWOS data alongside ASOS but the two networks have different capabilities, different QC histories, and different archive depths. When you pull data from a small general aviation airport and find gaps or suspect readings, the first question is whether it is an AWOS site and which tier.
CAI — Climate Averages and Indices. A term IEM uses for the derived climatological products computed from long-period COOP station records. When you are working with IEM’s normals data — 30-year averages for temperature, precipitation, heating degree days — you are working with CAI products. The distinction matters when you are downloading data: CAI fields are computed summaries, not raw observations.
CF6 — NWS Climate Form 6. The monthly summary climate product that NWS forecast offices issue at the end of each month for each of their official climate stations. A CF6 contains the daily maximum and minimum temperature, precipitation, snowfall, snow depth, and departures from normal for every day of the month, plus monthly totals and records. IEM archives CF6 products as text and in database form. When there is a discrepancy between what a station officially reported in a given month and what appears in ASOS hourly data, the CF6 is often the authoritative record because it reflects any observer corrections applied after the automated data was ingested.
CDD — Cooling Degree Day. A measure of how much cooling demand a day creates, calculated as the number of degrees by which the daily mean temperature exceeds a base of 65°F. A day with a mean temperature of 80°F produces 15 CDDs. On IEM, CDDs appear in Autoplot charts focused on agricultural and energy applications, alongside HDDs and GDDs. IEM computes daily degree days from ASOS and COOP station data and archives them in the daily summary database, accessible through the daily download portal at /request/daily.phtml.
CLI — CLImatological report. The daily climate product issued each morning by NWS forecast offices for their official climate stations. A CLI includes yesterday’s high and low temperature, precipitation, snowfall, and comparisons to the record and normal for that date. IEM receives and archives CLI products as they come off the LDM. The text products are retrievable by AFOS PIL (typically CLI followed by the three-letter city identifier, such as CLIMDT for Harrisburg).
CoCoRaHS — Community Collaborative Rain, Hail and Snow Network. A citizen science precipitation observation network operated through Colorado State University, in which volunteers take daily precipitation readings using standardized gauges at precise measurement times. On IEM, CoCoRaHS data is ingested from the national CoCoRaHS database and presented alongside the COOP and ASOS networks. The key advantage of CoCoRaHS is density: in a typical Pennsylvania county there may be one ASOS station and two COOP stations, but twenty or more CoCoRaHS observers. For studying spatial variability in precipitation — particularly snowfall, which ASOS notoriously undermeasures — CoCoRaHS is often the most valuable network IEM aggregates.
COOP — Cooperative Observer Program. The NWS volunteer observer network that has been collecting daily temperature and precipitation data across the United States for well over a century. COOP observers take a once-daily reading — at a locally consistent time, traditionally early morning — of maximum temperature, minimum temperature, precipitation, snowfall, and snow depth. The network’s value on IEM is its historical depth: COOP stations in Pennsylvania often have continuous records back to the 1880s, far predating any ASOS data. That longevity comes with tradeoffs: observer changes, station relocations, instrument upgrades, and time-of-observation differences all create inhomogeneities that require care when building long-period trends.
DCP — Data Collection Platform. The generic term for NWS-operated automated river, stream, and environmental monitoring sites that transmit data via GOES satellite. On IEM, DCP refers to the network of hydrological observation stations — stream gauges, reservoir sensors, soil moisture sites — that report through the HADS system. DCP data on IEM includes river stage, streamflow, precipitation from co-located rain gauges, and in some cases soil temperature and moisture. Archive depth varies widely by station: some Pennsylvania DCP sites have continuous records going back to the 1970s; others are recently installed.
ETN — Event Tracking Number. The sequential number assigned by a WFO to each VTEC event within a phenomena-significance category within a calendar year. An ETN resets to 1 every January 1 at each office. On IEM’s warning search, an ETN is what allows you to track a single warning event across multiple WFO-issued updates — the initial issuance, any extensions, and the cancellation or expiration all share the same ETN. Combining WFO identifier + phenomena code + significance code + ETN + year gives you a globally unique identifier for any NWS warning event, which is why IEM’s VTEC search is far more powerful than searching by date or county alone.
GDD — Growing Degree Day. A heat accumulation unit used in agricultural science to predict crop and pest development. Calculated as the number of degrees by which a day’s mean temperature exceeds a crop-specific base temperature (commonly 50°F for corn, 32°F for wheat), subject to minimum and maximum caps that prevent extreme days from distorting the calculation. IEM is deeply invested in GDD data because of its land-grant agricultural mission: the Autoplot chart library includes multiple GDD charts, and the IEM Pest Maps and Ag Weather tools are built directly on accumulated GDD values. For weather enthusiasts in Pennsylvania, GDD also serves as a proxy for seasonal progression — you can compare GDD accumulation to normal as a way of quantifying how “behind” or “ahead” a spring is running.
G-AIRMET — Graphical AIRMET. The graphical successor to the text-format AIRMET (Airman’s Meteorological Information) product issued by the NWS Aviation Weather Center for significant en-route flight hazards below 18,000 feet MSL. G-AIRMETs cover turbulence, icing, and instrument meteorological conditions across geographic polygons rather than the textual regions used in the legacy format. On IEM, G-AIRMETs are available as downloadable GIS shapefiles at /request/gis/awc_gairmets.phtml, making them useful for anyone studying aviation weather patterns or correlating flight delay data with atmospheric conditions.
HADS — Hydrometeorological Automated Data System. The NWS system that ingests, processes, and distributes GOES-transmitted DCP observations to end users. HADS is the plumbing behind the DCP network: when a stream gauge transmits its reading to a GOES satellite, HADS routes that data to NWS systems and eventually to IEM. On IEM, references to HADS typically appear in the DCP network context — if data is tagged as coming through HADS, it originates from a GOES-transmitting sensor site.
HDD — Heating Degree Day. The mirror of CDD: the number of degrees by which a day’s mean temperature falls below the 65°F base. A day with a mean of 30°F produces 35 HDDs. HDD accumulation through a winter is a standard proxy for heating energy demand. IEM archives HDDs in the daily summary database and includes them in Autoplot temperature and energy charts. Like CDDs and GDDs, HDDs are available for download at /request/daily.phtml.
IBW — Impact-Based Warning. An NWS warning format introduced in the early 2010s that adds structured tags to tornado and severe thunderstorm warnings indicating the expected intensity, likely damage, and who is at risk. IBW tags appear in the text of the warning product and are also coded in the VTEC header. On IEM, IBW tags are captured in the warning database, so Autoplot charts and VTEC searches that work with tornado and severe thunderstorm warnings can filter or color-code by tag level. If you are doing post-event analysis and want to know whether a warning that produced significant damage was tagged accordingly, IEM’s VTEC archive is where to check.
IEMRE — IEM Reanalysis. IEM’s own gridded analysis product, which blends station observations from the ASOS, COOP, and other networks onto a 0.125 × 0.125 degree grid covering the contiguous United States. IEMRE is updated daily and extends back to 1950, making it one of the longer gridded datasets covering the period before the modern reanalysis era. It is most useful when you need a gridded estimate of temperature or precipitation at a location without a nearby station — or when you want to compare a specific station’s recent values against a regional background field. IEMRE’s resolution and methodology differ from products like PRISM and ERA5; it is an IEM-specific product, which means its strengths and limitations are distinct from more widely used gridded datasets.
LDM — Local Data Manager. The Unidata software system that powers IEM’s real-time data ingestion. The LDM is a distributed data subscription and relay network: IEM subscribes to feeds from the NWS Family of Services, ASOS networks, and other providers, and the LDM delivers those data streams in real time to IEM’s database systems. From a user perspective, the LDM is invisible — it is the reason IEM has NWS products within seconds of issuance and ASOS observations within minutes of the observation time. For advanced users, IEM exposes a data request interface at /request/ldm.php that allows bulk retrieval of LDM-delivered product streams.
LSR — Local Storm Report. The structured NWS text product issued by forecast offices to document significant weather observations: storm damage, large hail, measured wind gusts, tornadoes, flash flooding, heavy snowfall. LSRs are issued in real time during events and are the primary source for IEM’s storm event database. On IEM, LSRs are searchable at /lsr/ and downloadable as shapefiles at /request/gis/lsrs.phtml. Because LSRs are issued by human forecasters making real-time decisions about what to document, they are a qualitative record rather than a complete inventory: an unmeasured hail event in a cornfield gets no LSR; a 1.75-inch stone that breaks a windshield in a parking lot often does.
MCD — Mesoscale Convective Discussion. The Storm Prediction Center’s real-time product issued when a watch is possible or a developing weather situation warrants detailed analysis but does not yet rise to the level of a watch. MCDs are the SPC forecasters’ running commentary during active weather — often dense with technical meteorological reasoning about shear profiles, CAPE, and boundary locations. On IEM, MCDs are archived and downloadable as GIS shapefiles at /request/gis/spc_mcd.phtml. They are particularly useful for post-event analysis because they document SPC’s thinking at the time, not after the fact.
MOS — Model Output Statistics. A statistical post-processing technique that translates raw numerical model output into station-specific forecast guidance by applying regression equations derived from years of matched model-output/observation pairs. On IEM, MOS is the entire content of the Datasets → MOS section: a deep archive of operational MOS guidance going back decades, covering multiple NWS forecast models. What makes IEM’s MOS archive genuinely useful is its historical depth — you can retrieve GFS-MOS guidance issued on any date back to the 1990s and compare it against what actually happened, which is the foundation of any serious forecast verification project. Models archived include AVN, GFS, ETA, NAM, NBS, NBE, ECM, LAV (LAMP), and MEX.
OGC — Open Geospatial Consortium. The standards body that defines interoperability protocols for geographic information systems, including WMS (Web Map Service), WFS (Web Feature Service), and WCS (Web Coverage Service). On IEM, OGC refers specifically to the /ogc/ endpoint that exposes IEM’s spatial data layers in standard OGC formats. If you use QGIS, ArcGIS, or any other GIS software that can consume WMS or WFS feeds, the IEM OGC service lets you load live and historical IEM data — warning polygons, NEXRAD composites, station observations — directly into your mapping environment.
PDS — Particularly Dangerous Situation. An NWS designation applied to tornado watches when atmospheric conditions suggest an elevated risk of long-track, violent tornadoes — EF4 and EF5 events. PDS watches carry the phrase “Particularly Dangerous Situation” in the watch product text and are coded in the VTEC header. On IEM, PDS status is captured in the VTEC warning database, so you can filter the historical tornado watch archive to identify PDS events for any WFO’s area. They are rare: a PDS tornado watch in a mid-Atlantic WFO’s area may appear only once or twice per decade.
PIL — Product Identifier Locator. The AFOS identifier that uniquely identifies any NWS text product: a six-character string combining a three-character category code (such as AFD for Area Forecast Discussion) and a three-character location identifier (such as CTP for State College). On IEM, PILs are how you retrieve specific NWS text products from the archive. The text product search at /wx/afos/p.php takes a PIL and returns the full text of the product, including historical versions. Knowing the PIL structure — and the fact that it is consistent across all NWS offices — is what makes IEM’s text product archive navigable rather than a flat dump of thousands of files.
PIREP — Pilot Report. An aviation weather observation made by a flight crew and transmitted to the NWS Aviation Weather Center. PIREPs report actual in-flight conditions: turbulence intensity and altitude, icing conditions and altitude, sky condition, and visibility aloft. Unlike surface observations, PIREPs provide direct measurement of atmospheric conditions at altitude — which is why they remain valuable for verifying numerical model forecasts of upper-tropospheric turbulence and icing. On IEM, PIREPs are available as a GIS shapefile download at /request/gis/pireps.php and appear in the Datasets menu.
QC — Quality Control. In the IEM context, the suite of automated and manual processes applied to incoming observation data to detect and flag erroneous values. IEM uses multiple QC layers: automated range checks, consistency checks between neighboring stations, MADIS network QC from the NWS, and human review logged in the public QC ticket system at /QC/tickets.phtml. Data that has failed one or more QC checks is flagged rather than deleted; the flag allows downstream users to decide whether to include or exclude the value. Understanding IEM’s QC flags — and the distinction between “flagged” and “removed” data — matters whenever you are building a dataset for analysis.
SHEF — Standard Hydrometeorological Exchange Format. The data encoding format used throughout the NWS hydrology community to transmit river and environmental sensor observations. SHEF encodes time series of observed values with physical element codes and quality flags in a compact text format. On IEM, SHEF appears primarily in the DCP/HADS context: river gauge and soil sensor data often arrives at IEM encoded in SHEF, and some IEM download portals allow retrieval in SHEF format for users whose analysis software expects it.
SIGMET — Significant Meteorological Information. An aviation weather advisory issued by the NWS Aviation Weather Center for meteorological conditions considered hazardous to all aircraft: severe or extreme turbulence, severe icing not associated with thunderstorms, widespread dust storms or sandstorms, and volcanic ash. In the U.S., SIGMETs also include Convective SIGMETs for large convective areas, embedded thunderstorms, and tornadoes. On IEM, SIGMETs are archived and downloadable as GIS shapefiles at /request/gis/awc_sigmets.phtml.
SPC — Storm Prediction Center. The NWS national center in Norman, Oklahoma, responsible for convective outlooks, tornado and severe thunderstorm watches, and mesoscale discussions. On IEM, SPC products are woven throughout: Autoplot has multiple charts derived from SPC watch and outlook data, the GIS shapefile downloads include SPC convective outlooks, SPC watches, and SPC MCDs, and the VTEC archive integrates SPC-issued watch polygons alongside WFO-issued local warnings. The IEM VTEC system treats SPC watches as VTEC events in their own right, with ETNs that track each watch from issuance through cancellation.
SPS — Special Weather Statement. The NWS text product used for weather situations that are significant but do not rise to the level requiring a formal watch or warning. An SPS might cover a brief period of locally dense fog, a wind advisory situation that does not quite meet advisory thresholds, or significant but not severe weather. On IEM, SPS products are in the text archive and are also downloadable as GIS shapefiles at /request/gis/sps.phtml. The shapefile version is useful because SPS polygons, like all VTEC-encoded NWS products, carry geographic boundaries that let you map where the statement applied.
TAF — Terminal Aerodrome Forecast. The NWS aviation point forecast issued for airports, covering a 24- or 30-hour period in a structured format that specifies expected wind, visibility, weather phenomena, and sky condition in coded groups. TAFs are issued by NWS Weather Forecast Offices four times daily and are the primary aviation forecast product for instrument flight planning. On IEM, TAF data is archived and downloadable at /request/taf.php, making it possible to retrieve historical TAF guidance for any airport and compare forecast conditions against the ASOS observations that were actually recorded.
Trace — The IEM convention for precipitation amounts too small to be measured but not zero. IEM represents trace precipitation as 0.0001 inches in numerical fields — enough to be nonzero in calculations and queries, and distinguishable from missing data, but below the 0.01-inch threshold of measurability. This convention is important when you are working with precipitation frequency statistics: days with trace amounts should typically be excluded from analyses of “measurable precipitation” but should not be treated as dry days. When you download ASOS or COOP precipitation data from IEM and see 0.0001 in a daily or hourly column, that is a trace observation, not a rounding artifact.
UGC — Universal Geographic Code. The NWS coding system for identifying the forecast zones and counties to which warnings and advisories apply. A UGC code is a six-character identifier: a two-letter state abbreviation, a one-letter type indicator (Z for forecast zone, C for county/parish), and a three-digit number. On IEM, UGC codes are the mechanism by which VTEC warnings are linked to geographic areas: when you search IEM’s warning archive by county or zone, you are querying on UGC codes. Understanding UGC is important when you want to analyze warning statistics for a specific county or group of counties, because the county and the forecast zone do not always overlap cleanly.
USCRN — U.S. Climate Reference Network. The NWS network of high-precision automated climate monitoring stations sited specifically to provide long-term climate change detection with minimal siting bias and no urban heat island contamination. USCRN stations use triple-redundant temperature sensors, weighing-bucket precipitation gauges, and solar radiation sensors, and are sited in rural locations away from human infrastructure. On IEM, USCRN data is available at /uscrn/ and downloadable at /request/uscrn.php. For climate trend analysis where siting quality matters, USCRN is the most defensible network IEM provides access to — the tradeoff is that there are far fewer stations than ASOS or COOP.
VTEC — Valid Time Event Code. The structured coding system the NWS embeds in every watch, warning, and advisory product. A VTEC string encodes the issuing office, the action being taken (new issuance, update, cancellation, expiration), a two-letter phenomena code (TO for tornado, SV for severe thunderstorm, FF for flash flood), a one-letter significance code (W for warning, A for advisory, Y for watch), and an Event Tracking Number. On IEM, VTEC is the backbone of the entire warning archive: every event has a unique identifier assembled from these components, which is what makes it possible to search for all tornado warnings issued by a specific office in a specific year, or to retrieve the full lifecycle — issuance, updates, verification, expiration — of any individual warning event. Understanding VTEC is what separates using IEM’s warning tools from just reading storm reports on a news site.
WFO — Weather Forecast Office. The local NWS offices responsible for issuing watches, warnings, advisories, and forecasts for their county warning areas. The contiguous U.S. has 122 WFOs. On IEM, the WFO is the fundamental unit of the warning archive: every VTEC event belongs to one WFO, warnings are organized by WFO, and most Autoplot charts that show warning statistics present data at the WFO level. Pennsylvania falls under four WFOs — State College (CTP), Philadelphia (PHI), Pittsburgh (PBZ), and Mount Holly, NJ (PHI) — with BWI occasionally relevant for southernmost counties. Knowing your WFO’s identifier is the starting point for most VTEC searches on IEM.
WPC — Weather Prediction Center. The NWS national center in College Park, Maryland, responsible for national quantitative precipitation forecasts (QPF), excessive rainfall outlooks, and Mesoscale Precipitation Discussions (MPDs). On IEM, WPC products appear in two places: the MPD shapefiles, downloadable at /request/gis/wpc_mpd.phtml, which are essential for analyzing the forecaster reasoning around significant precipitation events; and the Autoplot chart library, which includes charts derived from WPC QPF data. For anyone studying heavy precipitation events in Pennsylvania — a region where WPC watches closely because of its complex terrain and frequent high-impact rainfall — the WPC MPD archive on IEM is a frequently underused resource.