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Aleutian LowA semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum instensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.BackflowIn hydrologic terms, the backing up of water through a conduit or channel in the direction opposite to normal flow. BaseflowIn hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as ground water flow, or dry-weather flow. Billow CloudA cloud consisting of broad parallel bands oriented perpendicular to the wind.Blocked FlowFlow approaching a mountain barrier that is too weak or too stable to be carried over the barrier.BlowingA descriptor used to amplify observed weather phenomena whenever the phenomena are raised to a height of 6 feet or more above the groundBlowing Dust or SandStrong winds over dry ground, that has little or no vegetation, can lift particles of dust or sand into the air. These airborne particles can reduce visibility, cause respiratory problems, and have an abrasive affect on machinery. A concentration reducing the visibility to ¼ mile or less often poses hazards for travelers.Blowing SnowBlowing snow is wind-driven snow that reduces surface visibility. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds. Blowing snow is usually accompanied by drifting snow.Blowing Snow AdvisoryIssued when wind driven snow reduces surface visibility, possibly, hampering traveling. Blowing snow may be falling snow, or snow that has already accumulated but is picked up and blown by strong winds.Channel InflowIn hydrologic terms, water, which at any instant, is flowing into the channel system form surface flow, subsurface flow, base flow, and rainfall that has directly fallen onto the channel.Closed LowA low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see Cutoff Low).Colorado LowA low pressure storm system that forms in winter in southeastern Colorado or northeastern New Mexico and tracks northeastward across the central plains of the U.S. over a period of several days, producing blizzards and hazardous winter weather.
Critical FlowIn hydrologic terms, a condition of flow where the mean velocity is at one of the critical values; ordinarily at Belanger's critical depth and velocity. Another important usage is in reference to the Reynolds' critical velocities which define the point at which the flow changes from streamline or nonturbulent to turbulent flow. Cutoff LowA closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression).
"Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.Downslope FlowA thermally driven wind directed down a mountain slope and usually occurring at night; part of the along-slope wind system.Dry Weather FlowIn hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel.
This is also referred to as baseflow, or ground water flow.Extratropical LowA low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity. Extremely Low Frequency (ELF)That portion of the radio frequency spectrum
from 30 to 3000 hertzFlow(abbrev. FLW) Wind. In meteorology, a qualitative reference of an air parcel(s) with respect to its direction of movement, sometimes specified at a certain height or pressure
elevation, e.g. westerly flow at 500 mb. In hydrology, the volumetric flow of water past a given point on a stream or river, usually in cubic feet per second (cfs)Flow Duration CurveIn hydrologic terms, a cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.Flow SeparationThe process by which a separation eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides.Flow SplittingThe splitting of a stable airflow around a mountain barrier, with branches going around the left and right edges of the barrier, often at accelerated speeds.Flowing WellIn hydrologic terms, a well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also
called an Artesian well.Ground Water FlowIn hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel.
This is also referred to as baseflow, or dry-weather flowGround Water OutflowIn hydrologic terms, the part of the discharge from a drainage basin that occurs through the ground water. The term "underflow" is often used to
describe the ground water outflow that takes place in valley alluvium (instead of the surface channel) and thus is not measure at a
gaging station.High Flow Magnitude (HFM)The magnitude of the maximum forecast streamflow conditions, measured in terms of annual exceedance probabilities.High Water FlowThe Office of Water Prediction (OWP) uses the term “high water” as an indicator of elevated flows for a given stream reach. This regionally varied threshold is used in many National Water Model (NWM) data services, such as maximum high flows, peak and high flow arrival times, rapid onset flooding, and flood inundation mapping. Filtering on flows that are greater than or equal to the high water threshold allows these services to focus on areas where flows are significant enough to monitor - where there is potential flooding, for example. High water thresholds are established for eleven clusters that categorize runoff efficiencies across the Continental United States (ConUS). Hawaii, Puerto Rico, and U.S. Virgin Islands domains utilize the 2.0 year recurrence interval for the high water threshold.Hydraulic FlowAtmospheric flow that is similar in character to the flow of water over an obstacle.Icelandic LowA semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during a Northern Hemisphere winter and early spring, centered over Iceland and southern Greenland, and is the dominate weather feature in the area. During the summer, it is weaker, less intense, and might divide into two parts, one west of Iceland, the other over the Davis Strait between Greenland and Baffin Island. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic.Inflow BandsBands of low clouds, arranged parallel to the low-level winds
and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air
into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands
that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a
mesocycloneInflow JetsLocal jets of air near the ground flowing inward toward the base of a tornado.Inflow NotchA radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).Inflow StingerA beaver tail cloud with a stinger-like shape. InterflowIn hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream
channels than runoff. This also called subsurface storm flow.Laminar FlowStreamline flow in which successive flow particles follow similar path lines and head loss varies with velocity to the first power.Leeside LowExtratropical cyclones that form on the downwind (lee) side of a mountain chain. In the United States, they frequently form on the eastern side of the Rockies and Sierra Nevadas.LowA region of low pressure, marked as "L" on a weather map. A low center is usually accompanied by precipitation, extensive cloudiness, and moderate winds. See Cyclone. Low Frequency(abbrev. LF) The portion of the radio frequency spectrum from 30 to
300 kHz.Low Level Jet(abbrev. LLJ)- A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).
The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.Low Pressure SystemAn area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern
Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone. Low Water AdvisoryAn advisory to describe water levels which are significantly below average levels over the Great Lakes, coastal marine zones, and any tidal marine area, waterway, or river inlet within or adjacent to a marine zone that would potentially be impacted by low water conditions creating a hazard to navigation. Lowland FloodingIn hydrologic terms, inundation of low areas near the river, often rural, but may also occur in urban areas.Mean Low Water(MLW) - A tidal datum. The average of all the low water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch.Mean Lower Low Water(MLLW) - A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch. Meridional FlowLarge-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.Mesolow(or Sub-synoptic Low) - A mesoscale low-pressure center. Severe weather potential often increases in the area near and just ahead of a mesolow. Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.Minor Tidal OverflowMinor flooding caused by high tides that results in little if any damage. Offshore FlowOccurs when air moves from land to sea, and is usually associated with dry weather.Onshore FlowOccurs when air moves from sea to land, and is usually associated wtih increased moisture.OutflowAir that flows outward from a thunderstorm.Outflow BoundaryA storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin.
New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).Outflow ChannelIn hydrologic terms, a natural stream channel which transports reservoir releases.Overland FlowIn hydrologic terms, the flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a watercourse it becomes runoff.Peak Flow Arrival Time (PFAT)The time at which a stream reach is forecast to achieve its maximum flow within the forecast period.Plow WindA term used in the midwestern United States to describe strong, straight-line winds associated with the downdrafts spreading out in advance of squall lines and thunderstorms. Resulting damage is usually confined to narrow zones like that caused by tornadoes; however, the winds are all in one direction (straight-line winds). Return FlowSouth winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).Shallow FogFog in which the visibility at 6 feet above ground level is 5/8ths statute mile or more and the apparent visibility in the fog layer is less than 5/8ths statute
mile.Sheet FlowIn hydrologic terms, flow that occurs overland in places where there are no defined channels, the flood water spreads out over a large area at a uniform
depth. This also referred to as overland flow. Snow Pillow1) A window of snow deposited in the immediate lee of a snow fence or ridge.
or
2) In hydrologic terms, an instrument used to measure snow water equivalents. Snow pillows typically have flat stainless steel surface areas. The pillow
below this flat surface is filled with antifreeze solution and the pressure in the pillow is related to the water-equivalent depth of the
snow on the platform. One great advantage of snow pillows over a snow survey is the frequency of observations, which can be as
high as twice per day.Split FlowA flow pattern high in the atmosphere characterized by diverging winds. Storms moving along in this type of flow pattern usually weaken.StreamflowIn hydrologic terms, water flowing in the stream channel. It is often used interchangeably with discharge. Sub-synoptic LowEssentially the same as mesolow.Subsurface Storm FlowIn hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream
channels than runoff. This also called interflow.Terrain Forced FlowAn airflow that is modified or channeled as it passes over or around mountains or through gaps in a mountain barrier.Thermal LowArea of low pressure that is shallow in vertical extent and produced primarily by warm surface temperatures.TUTT LowTropical Upper-Tropospheric Trough or TUTT Low - A semipermanent trough extending east-northeast to west-southwest from about 35N in the eastern Pacific to about 15 to 20N in the central west Pacific. A similar structure exists over the Atlantic Ocean, where the mean trough typically extends from Cuba toward Spain.UnderflowThe lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream
channels than runoff. This also called subsurface storm flow.Upslope FlowSame as Orographic Lifting; air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.Very Low Frequency (VLF)That portion of the radio frequency spectrum from
3 to 30 kHzWarm Core LowA low pressure area which is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern. Unlike cold core lows, these lows produce much of their cloud cover and precipitation during the nighttime.Yellow SnowSnow given a golden or yellow appearance by the presence in it of pine, cypress pollen, or anthropogenic material or animal-produced material.Zonal FlowLarge-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.
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