Probability of precipitation

Last updated

Probability of precipitation (PoP) is a commonly used term referring to the likelihood of precipitation falling in a particular area over a defined period of time, which is commonly a day, half day, or hour.

Contents

The PoP measure is meaningless unless it is associated with an interval of time. Forecasts commonly use PoP defined over 12-hour periods (PoP12), though 6-hour periods (PoP6) and other measures are also published. A "daytime" PoP12 means from 6 am to 6 pm. [1]

Probabilities are often calculated by ensemble forecasting and represents the number of simulations that show rain occurred. [2]

PoPs are generally not statistically independent. A good example of an event that has a strongly dependent hour-to-hour PoP is a hurricane. In that case, there may be a 1 in 5 chance of the hurricane hitting a given stretch of coast, but if it does arrive there will be rain for several hours, with the effect that a one-hour PoP for the same region and period would be similar: about 1 in 5. Localized thunderstorms may be less dependent, with the effect that the one-hour PoPs may be somewhat less than the one-day PoP. [3]

Definitions

U.S. National Weather Service

According to the U.S. National Weather Service (NWS), PoP is the probability of exceedance that more than 0.01 inches (0.25 mm) of precipitation will fall in a single spot, averaged over the forecast area.

The NWS provides hourly forecasts. [4] The hourly PoP is the probability that more than 0.01 inches (0.25 mm) of precipitation would occur if conditions like those in the given hour continued for twelve hours. It can be similar to the daily PoP and vary little, or it can vary dramatically. the daily PoP will be the average of the hourly PoP across the whole day.

Other US forecasters

AccuWeather's definition is based on the probability at the forecast area's official rain gauge. There is also a probability of precipitation for every location in the United States for every minute for the next two hours. This is also known as a minute-cast. The Weather Channel's definition may include precipitation amounts below 0.01 inch (0.254 mm) and includes the chance of precipitation 3 hours before or after the forecast period. This latter change was described as less objective and more consumer-centric. [5] The Weather Channel has an observed wet bias – the probability of precipitation is exaggerated in some cases. [6]

Environment Canada

Environment Canada reports a chance of precipitation (COP) that is defined as "The chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period." [7] The values are rounded to 10% increments, but are never rounded to 50%. [8]

UK Met Office

The UK's Met Office reports a POP that is rounded to 5% and is based on a minimum threshold of 0.1 mm of precipitation. [9]

Alternative expressions

The probability of precipitation can also be expressed using descriptive terms instead of numerical values. For instance, the NWS might describe a precipitation forecast with terms such as "slight chance" meaning 20% certainty and "scattered" meaning 30–50% areal coverage. [10] The precise meaning of these terms varies. [11]

The UK's Met Office replaced descriptive terms, such as "likely", with percentage chance of precipitation in November 2011. [12]

Public understanding

Probability of precipitation may be widely misunderstood by the general public. [13]

The Plain English Campaign objected to the Met Office's use of the phrase "probability of precipitation" in 2011. [14] [15] The Met Office explained that the proposed alternative, "chance of rain", would not describe all the forms of precipitation included in the forecast. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Precipitation</span> Product of the condensation of atmospheric water vapor that falls under gravity

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.

<span class="mw-page-title-main">METAR</span> Format for weather reports commonly used in aviation

METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting. Today, according to the advancement of technology in civil aviation, the METAR is sent as IWXXM model.

<span class="mw-page-title-main">Weather Prediction Center</span> United States weather agency

The Weather Prediction Center (WPC), located in College Park, Maryland, is one of nine service centers under the umbrella of the National Centers for Environmental Prediction (NCEP), a part of the National Weather Service (NWS), which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Government. Until March 5, 2013 the Weather Prediction Center was known as the Hydrometeorological Prediction Center (HPC). The Weather Prediction Center serves as a center for quantitative precipitation forecasting, medium range forecasting, and the interpretation of numerical weather prediction computer models.

A winter storm warning is a hazardous weather statement issued by Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States to alert the public that a winter storm is occurring or is about to occur in the area, usually within 36 hours of the storm's onset.

A winter weather advisory is a hazardous weather statement issued by local Weather Forecast Offices (WFO) of the National Weather Service in the United States when one or more types of winter precipitation—snow, rain and snow mixed, freezing rain, sleet, graupel, etc.—presenting a hazard, but not expected to produce accumulations meeting storm warning criteria, are forecast within 36 hours of the expected onset of precipitation or are occurring in the advisory's coverage area.

This article describes severe weather terminology used by the National Weather Service (NWS) in the United States. The NWS is a government agency operating as an arm of the National Oceanic and Atmospheric Administration (NOAA) branch. It defines precise meanings for nearly all of its weather terms.

<span class="mw-page-title-main">Lake Storm Aphid</span> 2006 severe snowstorm in Buffalo, New York

The October 2006 Buffalo storm was an unusual early-season lake effect snow storm that hit the Buffalo, New York, area and other surrounding areas of the United States and Canada, from the afternoon of Thursday, October 12 through the morning of Friday, October 13, 2006. It was called Lake Storm Aphid by the National Weather Service office in Buffalo, in accordance with their naming scheme for lake-effect snowstorms for that year, which related to insects, though locals never used that terminology and have simply referred to it as the October Surprise or the October Storm or Arborgeddon.

<span class="mw-page-title-main">Quantitative precipitation forecast</span> Expected amount of melted precipitation

The quantitative precipitation forecast is the expected amount of melted precipitation accumulated over a specified time period over a specified area. A QPF will be created when precipitation amounts reaching a minimum threshold are expected during the forecast's valid period. Valid periods of precipitation forecasts are normally synoptic hours such as 00:00, 06:00, 12:00 and 18:00 GMT. Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail. Starting in the mid-to-late 1990s, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States. Forecast models show significant sensitivity to humidity levels within the planetary boundary layer, or in the lowest levels of the atmosphere, which decreases with height. QPF can be generated on a quantitative, forecasting amounts, or a qualitative, forecasting the probability of a specific amount, basis. Radar imagery forecasting techniques show higher skill than model forecasts within 6 to 7 hours of the time of the radar image. The forecasts can be verified through use of rain gauge measurements, weather radar estimates, or a combination of both. Various skill scores can be determined to measure the value of the rainfall forecast.

<span class="mw-page-title-main">Automated airport weather station</span> Automated sensor suites

Airport weather stations are automated sensor suites which are designed to serve aviation and meteorological operations, weather forecasting and climatology. Automated airport weather stations have become part of the backbone of weather observing in the United States and Canada and are becoming increasingly more prevalent worldwide due to their efficiency and cost-savings.

The Early December 2007 North American winter storm was a major winter storm which affected the majority of the United States and portions of southern Canada from November 29 to December 5, hitting the Intermountain West and Midwestern United States, the Great Lakes region and the Northeast. The storm brought significant snows to portions of the Upper Midwest, Great Plains and Great Lakes regions of the United States and Canada on December 1 with a major winter storm for Quebec, Ontario and parts of the Northeast region on December 2 and 3 as well as the Canadian Maritimes on December 4 and 5. The system was also responsible for a major ice storm across the Midwestern states which caused disruptions to several major cities including Des Moines, Chicago, Detroit, Milwaukee and Toronto. The storm was blamed for at least 16 deaths across nine US states and one Canadian province. 10 traffic deaths had been reported, as of 2 December 2007.

Montreal is the second largest city in Canada and the largest city in the province of Quebec, located along the Saint Lawrence River at its junction with the Ottawa River. The city is geographically constrained, with the majority on the Island of Montreal in the Hochelaga Archipelago, and has several prominent features, including the eponymous Mount Royal. The region experiences four distinct seasons and is classified as a humid continental climate, with very cold and snowy winters and warm and humid summers.

<span class="mw-page-title-main">Rain</span> Precipitation in the form of water droplets

Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.

A weather warning generally refers to an alert issued by a meteorological agency to warn citizens of approaching dangerous weather. A weather watch, on the other hand, typically refers to an alert issued to indicate that conditions are favorable for the development of dangerous weather patterns, although the dangerous weather conditions themselves are not currently present.

<span class="mw-page-title-main">Mubi North</span> LGA in Adamawa State, Nigeria

Mubi North is a Local Government Area of Adamawa State, Nigeria. The town is the location of the Adamawa State University and the Federal Polytechnic, Mubi.

Wet bias is the phenomenon whereby some weather forecasters report an overestimated and exaggerated probability of precipitation to increase the usefulness and actionability of their forecast. The Weather Channel has been empirically shown, and has also admitted, to having a wet bias in the case of low probability of precipitation but not at high probabilities of precipitation. Some local television stations have been shown as having significantly greater wet bias, often reporting a 100% probability of precipitation in cases where it rains only 70% of the time.

<span class="mw-page-title-main">2015 Texas–Oklahoma flood and tornado outbreak</span> Wind and rainstorm in the south-central United States and Mexico

Preceded by more than a week of heavy rain, a slow-moving storm system dropped tremendous precipitation across much of Texas and Oklahoma during the nights of May 24–26, 2015, triggering record-breaking floods. Additionally, many areas reported tornado activity and lightning. Particularly hard hit were areas along the Blanco River in Hays County, Texas, where entire blocks of homes were leveled. On the morning of May 26, the National Weather Service issued a flash flood emergency for southwest Harris County and northeast Fort Bend County. The system also produced deadly tornadoes in parts of Mexico, Texas, and Oklahoma. This flood significantly contributed to the wettest month ever for Texas and Oklahoma.

<span class="mw-page-title-main">2011–12 North American winter</span>

The 2011–12 North American winter by and large saw above normal average temperatures across North America, with the Contiguous United States encountering its fourth-warmest winter on record, along with an unusually low number of significant winter precipitation events. The primary outlier was Alaska, parts of which experienced their coldest January on record.

<span class="mw-page-title-main">2010–11 North American winter</span>

The 2010–11 North American winter was influenced by an ongoing La Niña, seeing winter storms and very cold temperatures affect a large portion of the Continental United States, even as far south as the Texas Panhandle. Notable events included a major blizzard that struck the Northeastern United States in late December with up to 2 feet (24 in) of snowfall and a significant tornado outbreak on New Year's Eve in the Southern United States. By far the most notable event was a historic blizzard that impacted areas from Oklahoma to Michigan in early February. The blizzard broke numerous snowfall records, and was one of the few winter storms to rank as a Category 5 on the Regional Snowfall Index. In addition, Oklahoma set a statewide low temperature record in February.

<span class="mw-page-title-main">Climate of Seattle</span>

The climate of Seattle is temperate, classified in the warm-summer (in contrast to hot-summer) subtype of the Mediterranean zone by the most common climate classification although some sources put the city in the oceanic zone. It has cool, wet winters and warm, dry summers, covering characteristics of both. The climate is sometimes characterized as a "modified Mediterranean" climate because it is cooler and wetter than a "true" Mediterranean climate, but shares the characteristic dry summer and the associated reliance upon cooler-season precipitation. The city is part of USDA hardiness zone 9a, with surrounding pockets falling under 8b.

References

  1. Enyedi, Angie (27 August 2010). "What are PoPs?". National Weather Service, Jacksonville, FL. Archived from the original on 10 June 2015. Retrieved 27 November 2018.
  2. https://www.rmets.org/metmatters/what-does-30-chance-rain-mean [ bare URL ]
  3. Gorski, Chris (21 August 2014). "How Do Daily Weather Forecasts Relate To Hourly Forecasts? It Depends". Inside Science. Retrieved 4 December 2018.
  4. National Weather Service. "Get your hourly weather forecast from the NWS". Weather.gov. Retrieved 4 December 2018.
  5. Bialik, Carl (2008-12-09). "Deciphering a 20% Chance of Rain". The Wall Street Journal.
  6. Silver, Nate (2012-09-07). "The Weatherman Is Not a Moron". The New York Times.
  7. "Weather and Meteorology - Glossary: Chance of Precipitation (COP)". Environment Canada. Retrieved 2015-06-07.
  8. "Guide to Environment Canada's Public Forecasts: Chance of Precipitation". Environment Canada. Retrieved 2015-06-07.
  9. "The science of 'probability of precipitation'". Met Office. 2014-08-14. Archived from the original on 2018-10-16.
  10. "Forecast Terms". National Weather Service, Binghamton, NY Weather Forecast Office. Retrieved 2015-06-07.
  11. The NWS alone has published other sets of terms, such as those in NOAA Technical Memorandum NWS AR-44 and NWS Instruction 10-503 (p. 18).
  12. "Chance of a shower? You decide, as Met Office launches new style weather forecasts". The Telegraph. 2011-11-10.
  13. Joslyn, Susan; Nadav-Greenberg, Limor; Nichols, Rebecca M. (February 2009). "Probability of Precipitation: Assessment and Enhancement of End-User Understanding". Bulletin of the American Meteorological Society. 90 (2): 185–193. Bibcode:2009BAMS...90..185J. doi: 10.1175/2008BAMS2509.1 .
  14. "2011 Golden Bull Awards". Plain English Campaign. Retrieved 2015-06-07.
  15. "Plain English award for Met Office 'gobbledygook'". BBC News. 2011-12-09. Retrieved 2015-06-07.
  16. "A golden conundrum". Met Office. 2011-12-09.