Subj : Wx Terms (F)(2)
To   : All
From : Daryl Stout
Date : Thu Sep 07 2017 12:06 am

First Law of Thermodynamics
The law of physics that states that the heat absorbed by a system either
raises the internal energy of the system or does work on the environment.

Flanking Line
A line of cumulus or towering cumulus clouds connected to and extending
outward from the most active part of a supercell, normally on the
southwest side. The line normally has a stair-step appearance, with the
tallest clouds closest to the main storm.

Flare
In solar-terrestrial terms, a sudden eruption of energy on the solar disk
lasting minutes to hours, from which radiation and particles are emitted.

Flash
A sudden, brief illumination of a conductive channel associated with
lightning, which may contain multiple strokes with their associated
stepped leaders, dart leaders and return strokes.

Flash Flood
A rapid and extreme flow of high water into a normally dry area, or a
rapid water level rise in a stream or creek above a predetermined flood
level, beginning within six hours of the causative event (e.g., intense
rainfall, dam failure, ice jam). However, the actual time threshold may
vary in different parts of the country. Ongoing flooding can intensify
to flash flooding in cases where intense rainfall results in a rapid
surge of rising flood waters.

Flash Flood Guidance
Forecast guidance, often model output, specific to the potential for
flash flooding (e.g., how much rainfall over a given area will be
required to produce flash flooding).

Flash Flood Statement
(FFS) In hydrologic terms, a statement by the NWS which provides
follow-up information on flash flood watches and warnings.

Flash Flood Table
In hydrologic terms, a table of pre-computed forecast crest stage values
for small streams for a variety of antecedent moisture conditions and
rain amounts. Soil moisture conditions are often represented by flash
flood guidance values. In lieu of crest stages, categorical
representations of flooding, e.g., minor, moderate, etc. may be used on
the tables.

Flash Flood Warning
Issued to inform the public, emergency management, and other cooperating
agencies that flash flooding is in progress, imminent, or highly likely.

Flash Flood Watch
Issued to indicate current or developing hydrologic conditions that are
favorable for flash flooding in and close to the watch area, but the
occurrence is neither certain or imminent.

Flash Multiplicity
The number of return strokes in a lightning flash.

Flashboards
In hydrologic terms, a length of timber, concrete, or steel placed on
the crest of a spillway to raise the retention water level but which
may be quickly removed in the event of a flood by a tripping device, or
by deliberately designed failure of the flashboard or its supports.

FLG
Falling

Float Recording Precipitation gage
In hydrologic terms, a rain gage where the rise of a float within the
instrument with increasing rainfall is recorded. Some of these gages
must be emptied manually, while others employ a self-starting siphon to
empty old rainfall amounts.

Floc
A cluster of frazil particles.

Floe
In hydrologic terms, an accumulation of frazil flocs (also known as a
"pan") or a single piece of broken ice.

Flood
Any high flow, overflow, or inundation by water which causes or
threatens damage.

Flood Categories
Terms defined for each forecast point which describe or categorize the
severity of flood impacts in the corresponding river/stream reach. Each
flood category is bounded by an upper and lower stage (see Example 1).
The severity of flooding at a given stage is not necessarily the same at
all locations along a river reach due to varying channel/bank
characteristics or presence of levees on portions of the reach.
Therefore, the upper and lower stages for a given flood category are
usually associated with water levels corresponding to the most
significant flood impacts somewhere in the reach. The flood categories
used in the NWS are:

*Minor Flooding* - minimal or no property damage, but possibly some
public threat.

*Moderate Flooding* - some inundation of structures and roads near
stream. Some evacuations of people and/or transfer of property to higher
elevations.

*Major Flooding* - extensive inundation of structures and roads.
Significant evacuations of people and/or transfer of property to higher
elevations.
*Record Flooding* - flooding which equals or exceeds the highest stage
or discharge at a given site during the period of record keeping.

Note: all three of the lower flood categories (minor, moderate, major)
do not necessarily exist for a given forecast point. For example, at the
level where a river reaches flood stage, it may be considered moderate
flooding. However, at least one of these three flood categories must
start at flood stage.

Flood Control Storage
In hydrologic terms, storage of water in reservoirs to abate flood
damage.

Flood Crest
Maximum height of a flood wave as it passes a certain location.

Flood Frequency Curve
In hydrologic terms,

(1) A graph showing the number of times per year on the average, plotted
as abscissa, that floods of magnitude, indicated by the ordinate, are
equaled or exceeded.

(2) A similar graph but with recurrence intervals of floods plotted as
abscissa.

Flood Loss Reduction Measures
In hydrologic terms, the strategy for reducing flood losses. There are
four basic strategies. They are prevention, property protection,
emergency services, and structural projects. Each strategy incorporates
different measures that are appropriate for different conditions. In
many communities, a different person may be responsible for each strategy.

Flood of Record
In hydrologic terms, the highest observed river stage or discharge at a
given location during the period of record keeping. (Not necessarily the
highest known stage.)

Flood Plain
In hydrologic terms, the portion of a river valley that has been
inundated by the river during historic floods.

Flood Potential Outlook
(ESF on AFOS) (FPO for Acronym): In hydrologic terms, An NWS outlook that
is issued to alert the public of potentially heavy rainfall that could
send area rivers and streams into flood or aggravate an existing flood.

Flood Prevention
In hydrologic terms, measures that are taken in order to keep flood
problems from getting worse. Planning, land acquisition, river channel
maintenance, wetlands protection, and other regulations all help modify
development on floodplains and watersheds to reduce their susceptibility
to flood damage. Preventive measures are usually administered by the
building, zoning, planning and/ or code enforcement offices of the local
government.

Flood Problems
In hydrologic terms, problems and damages that occur during a flood as a
result of human development and actions. Flood problems are a result from:

1) Inappropriate development in the floodplain (e.g., building too low,
too close to the channel, or blocking flood flows);

2) Development in the watershed that increases flood flows and creates a
larger floodplain, or;

3) A combination of the previous two.

Flood Profile
In hydrologic terms, a graph of elevation of the water surface of a river
in flood, plotted as ordinate, against distance, measured in the
downstream direction, plotted as abscissa. A flood profile may be drawn
to show elevation at a given time, crests during a particular flood, or
to show stages of concordant flows.

Flood Routing
In hydrologic terms, process of determining progressively the timing,
shape, and amplitude of a flood wave as it moves downstream to successive
points along the river.

Flood Stage
A gage height at which a watercouse overtops its banks and begins to
cause damage to any portion of the defined reach. Flood stage is usually
higher than or equal to bankful stage.

Flood Statement (FLS)
In hydrologic terms, a statement issued by the NWS to inform the public
of flooding along major streams in which there is not a serious threat
to life or property. It may also follow a flood warning to give later
information.



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