NOAA's National Weather Service Weather Forecast Office

 
H

HADS (Hydrometeorological Automated Data System):  Software that replaced GDDS to process and distribute the GOES DCP data and CADAS data collected from DCP's and LARCS.
 
Hail (GR): Precipitation in the form of balls or lumps usually consisting of concentric layers of ice. A thunderstorm is classified as severe when it produces hail 3/4 of an inch or larger in diameter. The following table shows hail size estimates:
 

Hail Index (HI):  This WSR-88D radar product displays an indication of whether the thunderstorm structure of each identified by the storm series algorithm is conducive to the production of hail.  A green triangle indicates areas where the algorithm thinks that there is hail.  It will also display an estimated hail size.  This does not mean that there is actually hail occurring or the hail is the size that the radar is indicating, but it does attract the radar operator's attention to check out the thunderstorm more closely.

Hail Spike: When looking at a WSR-88D Cross-Section, one will occasionally see a distinctive spike above the actual top of the thunderstorm. This is due to the high reflective properties of hail. When the side lobe energy hits the hail, it reflects enough energy back to be detected by the radar receiver even when the radar beam 1/2 power width has exceeded the top of the storm. The radar receiver "thinks" that the detected energy is coming from the center of the radar beam as a result it extends the actual top of the storm in the form of a spike. This "hail spike" has been associated with many hail producing thunderstorms.

Haines Index:  This is also called the Lower Atmosphere Stability Index.  It is computed from the morning (12Z) soundings from RAOB stations across North America. The index is composed of a stability term and a moisture term. The stability term is derived from the temperature difference at two atmosphere levels. The moisture term is derived from the dew point depression at a single atmosphere level. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior. The Haines Indexes range from 2 to 6 for indicating potential for large fire growth:
 
 

Halos: Rings or arcs that encircle the sun or moon. These are caused by refraction of light through ice crystals that make up cirrus clouds.

Hanging (Ice) Dam:  A mass of ice composed mainly of frazil or broken ice deposited underneath an ice cover in a region of low flow velocity.
 
Haze (HZ): A concentration of salt particles or other dry particles not readily classified as dust or other phenomenon. Occurs in stable air usually only a few thousand feet thick, but may extend as high as 15,000 feet. Haze layers often have definite tops above which the visibilities are good. However, the visibility in the haze layer can be very poor.

HDRAIN:  An Hourly Digital Rainfall Product of the WSR-88D.

Head:  The difference between the pool height and tailwater height. Usually expressed in feet of head, or in lbs./sq. inch
 
Head Loss:  The decrease in total head caused by friction.
 
Head Race:  A channel which directs water to a water wheel; a forebay.
 
Headward Erosion:  Erosion which occurs in the upstream end of the valley of a stream, causing it to lengthen its course in such a direction.
 
Headwaters:  Streams at the source of a river.
 
Headwater Advisory Program (ADVIS):  A Program which uses the Antecedent Precipitation Index (API) method of estimating runoff, unit hydrograph theory and stage-discharge ratings to produce hydrologic forecasts for headwater basins.
 
Headwater Advisory Table:  A table developed by a River Forecast Center for a Headwater Guidance Point; a pre-computed matrix of values allows a forecaster to ascertain an anticipated crest or rise on a small river or stream for a variety of rainfall events and soil moisture conditions.

Headwater Basin:  A basin at the headwaters of a river. All discharge of the river at this point is developed within the basin.
 
Heat Advisory:  This product is issued by the National Weather Service when excessive heat may pose a hazard or is life threatening if action is not taken.  The criteria for this advisory varies from state to state.  In Michigan, the criteria is a heat index of 105 degree F or higher for a period of 3 hours or more.

Heating Degree Day: see Degree Day

Heat Index: The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. To find the Heat Index (HI), look at the Heat Index (HI) Chart below. As an example, if the air temperature is 90^oF (found at the left side of the table) and the Relative Humidity (RH) is 70% (found at the top of the table), the Heat Index (HI)--or how hot it actually feels--is 106^oF. This is at the intersection of the row 90^oF and the 70% column.

This index was devised for shady, light wind conditions. Exposure to full sunshine can increase Heat Index (HI) values by up to 15^oF. Also strong winds, particularly with very hot, dry air, can be extremely dangerous. Any value Heat Index (HI) greater than 105^oF is in the Danger Category. This is denoted by a thick line on the chart. Any number below or to the right of the thick line is in this category. When the Heat Index is between 105-115^oF for 3 hours or more, a Heat Advisory will be issued by the local National Weather Service Forecast Office (NWSO).  If the Heat Index is greater than 115^oF for 3 hours or more, an Excessive Heat Warning  will be issued by the local National Weather Service Forecast Office (NWSO).
 

The following table describes the symptoms of the various heat disorders in the previous table.  It also tells what to do for each heat disorder:
 

See Apparent Temperature.

Heat Lightning: Lightning that occurs at a distance such that thunder is no longer audible.

Heavy Snow Discussion (HSD):  This message discusses the potential for heavy snowfall in the contiguous United States.  The threshold value in this product for heavy snow is 4 inches or more in a 12-hour period.  However, this product discusses all potential snowstorms, including those not expected to attain the threshold.  Although the focus is on the meteorological reasoning for the forecast, the impact of numerical model forecasts and model differences are also explained.  This narrative is a 3-part product.  The first part is a synopsis and short-term forecast for the few hours before the main 12-hour forecast period.  The second part includes the 12-hour forecast and the accompanying meteorological reasoning.  The third part is a meteorological discussion of the outlook period that extends 12 hours beyond the forecast period.  There is an accompanying graphic for each forecast under the AFOS identifier 93S.  The HSD is issued 4 times a day from September 15 to May 15 and other times as needed as described below.  A revised or amended HSD may be issued as necessary.  Routine issuances are as follows:

1) Around 9 PM EST (10 PM EDT), a forecast is issued for 1 AM EST (2 AM EDT) to 1 PM EST (2 PM EDT), and the outlook is issued for 1 PM EST (2 PM EDT) to 1 AM EST
    (2 AM EDT)    the next day.

2) Around 2 AM EST (3 AM EDT), a forecast is issued for 7 AM EST (8 AM EDT) to 7 PM EDT (8 PM EDT), and the outlook is issued for 7 AM EST (8 AM EDT) to 7 PM EST (8 PM EDT).

3) Around 9 AM EST (10 AM EDT), a forecast is issued for 1 PM EST (2 PM EDT) to 1 AM EDT (2 AM EDT) the next day, and the outlook is issued for 1 AM EST (2 AM EDT) to
    1 PM EST (2 PM EDT).

4) Around 2 PM EST (3 PM EDT), a forecast is issued for 7 PM EST (8 PM EDT) to 7 AM EDT (8 AM EDT) the next day, and the outlook is issued for 7 AM EST (8 AM EDT) to
    7 PM EST (8 PM EDT).

Heavy Surf: Large waves breaking on or near the shore resulting from swells spawned by a distant storm.

Helicity: A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve.  Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion.  The helicity is the area on a hodograph that is enclosed by a line from the tip of the storm motion vector to the surface wind vector, then following the hodograph curve to 3 km level, then back to storm motion vector. This value allows the forecaster to determine the rotational tendency of a thunderstorm. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones).  Extreme values can exceed 600 m2/s2. It is dependent on the local environmental wind profile in which a thunderstorm develops and the thunderstorm motion.

Heliophobia:  The fear of the sun.

Hemispheric Map Discussion (HMD):  This discussion is issued once a day around 1 PM EST (2 PM EDT) and is primarily intended to provide insight into the hemispheric circulation patterns over the next 5 days.  This includes a discussion of the 5-day mean circulation pattern.  Comparisons, differences, and continuity among the numerical models are highlighted, and preferred solutions are proposed with an explanation of why a solution is preferred.  This includes any reasons why the preferred solution differs from any model.  In cases where certain models are not universally available, an attempt will be made to describe that model's solution to an extent that a reader can understand it's important aspects.  Although actual or forecast sensible surface weather conditions are not a focus of this discussion, some discussion, some discussion may be made to correlate them to circulation patterns.

HIC:  1) Hydrologist in Charge of an RFC. 2) The Hydrometeorological Information Center of the Office of Hydrology (OH).
 
High:  A region of high pressure, marked as "H" on a weather map.  A high is usually associated with fair weather.  See Anticyclone.

High Clouds: These clouds have bases between 16,500 and 45,000 feet in the mid latitudes. At this level they are composed of primarily of ice crystals. Some clouds at this level are cirrus, cirrocumulus, and cirrostratus.

High Risk of Severe Thunderstorms:  Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak. Also, see slight risk, moderate risk, and convective outlook.

High Seas:  The major oceans of the world including, for National Weather Service purposes, the coastal and offshore areas.  Areas of responsibility for the United States are determined by international agreements under the auspices of the World Meteorological Organization (WMO).  The United States is responsible for that portion of the Atlantic and Pacific oceans which extends from 20 to 40 nm off the Western and Eastern US coasts and extends to 35W in the Atlantic ocean and to 160E in the Pacific Ocean. The area includes both the coastal and offshore waters.

High Seas Forecasts (HSF):  This National Weather Service (NWS) marine forecast are designed to meet the needs of ships making ocean transits; therefore, the primary focus is on major weather systems and sea states affecting oceangoing vessels.  NWS units issuing High Seas Forecasts are the Marine Forecasting Branch of the National Meteorological Center (NMC), the Tropical Satellite Analysis and Forecast Unit of the Tropical Prediction Center (formerly called the National Hurricane Center), and the marine forecast section of the WFO Honolulu.

High Wind Advisory:  This product is issued by the National Weather Service when high wind speeds may pose a hazard.  The criteria for this advisory varies from state to state.  In Michigan, the criteria is sustained non-convective (not related to thunderstorms) winds greater than or equal to 30 mph lasting for one hour or longer, or winds greater than or equal to 45 mph for any duration.

High Wind Watch:  This product is issued by the National Weather Service when there is the potential of high wind speeds developing that may pose a hazard or is life threatening.  The criteria for this watch varies from state to state.  In Michigan, the criteria is the potential for sustained non-convective (not related to thunderstorms) winds greater than or equal to 40 mph and/or gusts greater than or equal to 58 mph.

High Wind Warning:  This product is issued by the National Weather Service when high wind speeds may pose a hazard or is life threatening.  The criteria for this warning varies from state to state.  In Michigan, the criteria is sustained non-convective (not related to thunderstorms) winds greater than or equal to 40 mph lasting for one hour or longer, or winds greater than or equal to 58 mph for any duration.

Hinge Crack:  Crack caused by significant changes in water level.

HOD:  1) The Hydrologist on Duty at an RFC.  2) The Hydrologic Operations Division of the Office of Hydrology (OH).
 
Hodograph: A polar coordinate graph which shows the vertical wind profile of the lowest 7000 meters of the atmosphere. These plots are used to determine the advection patterns aloft, whether a thunderstorm will rotate, and the type of thunderstorms that you will likely see that day. On the graph, only the tip of wind vectors are plotted on this graph. The tips are denoted by a dot. As the distance between the dot and the center of the graph increases, the magnitude of the wind will also increase. These dots are sequentially connected together by a line beginning with the first wind reported and ending with last wind reported. This is similar to a dot to dot book. Normally, they are plotted every 500 meters from the surface to 7000 meters. Another interesting feature of this graph is that the axes are rotated 180 degrees. This means that 180^o is located on the top of the graph, 270^o is located on the right side of the graph, 0^o is located on the bottom of the graph, and finally 90^o is located on the left side of the graph. Therefore, if you are going to plot a west wind, it would be located to the right of the center of the graph.  Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).

Homichlophobia:  The fear of fog.

Homodyning:  The transfer of signal intelligence from one carrier to another by mixing of signals at different frequencies.

Hook or Hook Echo: A pendant or hook on the right rear of a radar echo that often identifies mesocyclones on the radar display. The hook is caused by precipitation drawn into a cyclonic spiral by the winds, and the associated notch in the echo is caused by precipitation-free, warm, moist air flowing into the storm.  A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.

Hourly Precipitation Data (HPD):  This National Climatic Data Center (NCDC) publication is produced monthly by state or region.  It contains data on nearly 3,000 hourly precipitation stations (National Weather Service, Federal Aviation Administration, and cooperative observer stations) in inches to tenths or inches to hundredths at local standard time. HPD includes maximum precipitation for nine (9) time periods from 15 minutes to 24 hours, for selected stations.

HMT (Hydrometeorological Technicians):  Individuals who, at the technical level, have knowledge in meteorology and hydrology. Among their duties are data collection, quality control, gage network maintenance, as well as the gathering and disseminating of data and products.

HP (High Precipitation) Storm or HP (High Precipitation) Supercell:  High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often
including hail) falls on the trailing side of the mesocyclone. Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events. Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous. See Bear's Cage.

HPC:  An acronym for the Hydrometeorological Prediction Center.  See Hydrometeorological Prediction Center.

HRL:  The Hydrological Research Laboratory at the Office of Hydrology (OH).
 
HSA (Hydrologic Service Area):  A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.
 
HSB:  The Hydrologic Systems Branch in the Office of Hydrology (OH).
 
HTC:  The Hydrometeorological Training Council
 
Humidity:  Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

Hummock:  A hillock of broken ice which has been forced upward by pressure.
 
Hummocked Ice:  Ice piled haphazardly one piece over another to form an uneven surface.
 
Hurricane:  A warm-core tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 kph) or more. The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian.  It has a diameter of 250 to 500 miles and a cyclonic circulation typically extending to near 50,000 feet. It is called a Typhoon in the western Pacific north of the Equator and west of the International Dateline, a Cyclone in the Indian Ocean, and Baguio in the Philippines area. See Saffir-Simpson Hurricane Intensity Scale.

Hurricane Local Statement (HLS):  This products is issued by a local National Weather Service office when it is in or near an area threatened by a tropical storm or a hurricane.  This statement will take the place of Special (SPS) and Severe (SVS) Statements, Flash Flood/Flood (FFS) Statements, Coastal Flood Statements, and Marine Weather (MWS) Statements.  This statement does not replace the tropical storm or hurricane advisory from a hurricane center; rather, it complements the advisory with crucial local information.  It is normally issued whenever the local National Weather Service warning area of responsibility is under one or more of the following:  a tropical storm or hurricane watch or warning, evacuation orders, or rumors which the local station manager feels should be clarified.  Inland offices close to the coast may use HLSs if tropical storm or hurricane conditions are forecasted or observed.  It is optional to include the probability of hurricane/tropical storm conditions from the hurricane center's advisory.  This product contains information that affects the local county or parish warning area such as weather conditions, areas that should be evacuated, other precautions to protect life and property, and any other relevant information.  They are issued at regular and frequent intervals.  When a tropical storm or hurricane is close to the coast, HLSs may be issued every 2 or 3 hours, but more frequently if information and conditions warrant.

Hurricane Model:  The Geophysical and Fluid Dynamics Laboratory(GFDL) developed the hurricane model in order to improve hurricane landfall forecasts. The hurricane model is centered on the eye of the hurricane for each run of the model. Since hurricanes do not always form over the same locations, the geographical location of the model's forecast varies from run to run. This is called a relocatable model.  The model does not forecast for large distances away from the hurricane because its main focus is the development and the movement of the hurricane.  As a result of forecasting for small horizontal distances, the resolution of the hurricane model is 10 kilometers. The model got a very good test of its skill during its first year of operation in the hurricane season of 1995, which was one of the most active hurricane seasons in history. The results show a 20% improvement in hurricane landfall forecasts 48 to 72 hours in advance. This improvement gives people living in the regions expected to be hit by a hurricane more time to prepare or evacuate. Another file gives more details on the hurricane model.

Hurricane Season:  The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central Pacific basin runs from June 1 to November 30.

Hurricane Warning:  A warning that sustained winds 64 kt (74 mph or 119 kph) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

Hurricane Watch:  An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a possible threat, generally within 36 hours

Hydraulic Fill Dam:  A dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water.

Hydraulic Grade Line:  A line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a given fluid flow path, such as along a pipeline or a ground water streamline.
 
Hydraulic Head:  1) The height of the free surface of a body of water above a given point beneath the surface.  2) The height of the water level at the headworks, or an upstream point, of a waterway, and the water surface at a given point downstream.  3) The height of a hydraulic grade line above the center line of a pressure pipe, at a given point.
 
Hydraulic Mean Depth:  The right cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called Hydraulic Radius.
 
Hydraulic Permeability:  The flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
 
Hydraulic Radius:  The right cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called Hydraulic Mean Depth.
 
Hydrograph: A graph showing the water level (stage), discharge, or other property of a river with respect to time. The common hydrographs in National Weather Service usage depict stage vs. time and flow vs. time.

Hydrographic Survey:  An instrumental survey to measure and determine characteristics of streams and other bodies of water within an area, including such things as location, areal extent, and depth of water in lakes or the ocean; the width, depth, and course of streams; position and elevation of high water marks; location and depth of wells, etc.
 
Hydrograph Separation: The process where the storm hydrograph is separated into baseflow components and surface runoff components.

Hydrologic Budget:  An accounting of the inflow to, outflow from, and storage in, a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project.
 
Hydrologic Cycle: The constant movement of water above, on, and below the Earth's surface. Processes such as precipitation, evaporation, condensation, infiltration, and runoff comprise the cycle. Within the cycle, water changes forms in response to the Earth's climatic conditions.

Hydrologic Equation:  The water inventory equation (Inflow = Outflow + Change in Storage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage.
 
Hydrologic Model:  A conceptual or physically-based procedure for numerically simulating a process or processes which occur in a watershed.
 
Hydrologic Service Area (HSA):  A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.
 
Hydrologic Services:  A general Term referring to the operations, products, verbal communications, and related forms of support provided by the NWS for the Nation's streams, reservoirs, and other areas affected by surface water.

Hydrologic Unit:  A geographical area representing part or all of a surface drainage basin or distinct hydrologic feature such as a reservoir, lake, etc.

Hydrology:  The applied science concerned with the waters of the earth, their occurrences, distribution, and circulation through the unending hydrologic cycle of: Precipitation, consequent runoff, infiltration, and storage; eventual evaporation; and so forth. It is concerned with the physical and chemical reaction of water with the rest of the earth, and its relation to the life of the earth.

Hydrometeor:  A particle of condensed water (liquid, snow, ice, graupel, hail) in the atmosphere.

Hydrometeorological Prediction Center (HPC):  This is one of 9 centers that comprises the National Centers for Environmental Prediction (NCEP, formerly the National Meteorological Center). This national center provides basic hydrometeorological analysis and forecasts for National Weather Service Field Offices and the entire meteorological community. HPC meteorologists serve as experts in quantitative precipitation forecasting and numerical model interpretation. Products provided by the HPC include surface analyses, outlooks for heavy rain and snow, as well as guidance weather forecasts through five days.

Hydrometeorologists:  Individuals who have the combined knowledge in the fields of both meteorology and hydrology which enables them to study and solve hydrologic problems where meteorology is a factor.
 
Hydrometeorology:  The interdisciplinary science involving the study and analysis of the interrelationalships between the atmospheric and land phases of water as it moves through the hydrologic cycle.
 
Hydrosphere:  The region that includes all the earth's liquid water, frozen water, floating ice, frozen upper layer of soil, and the small amounts of water vapor in the earth's atmosphere.
 
Hydrostatic Head:  A measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce the same pressure.
 
Hyetograph:  A graphical representation of rainfall intensity with respect to time.
 
Hygrometer: An instrument which measures the humidity of the air.