DAMBREAK FLOODING – Dambreaks present a unique flood warning problem. Dams may erode and empty relatively slowly. In these situations, when river forecasts are available, flood warnings and flood statements are used. Dams may also fail catastrophically during a heavy rainfall event or earthquake, resulting in a very dangerous flash flood-type situation. For these events, Flash Flood watches, warnings and statements will be used. Similar action may be required for the failure of certain levees.
DEFORMATION ZONE (DFRMTN ZN) – An area in the atmosphere where winds converge along one axis and diverge along another. Deformation zones (or axis of deformation as they are sometimes referred to) can produce clouds and precipitation.
DENSE FOG – A cloud, with its base on the surface, which reduces visibility to 1/4 of a mile or less.
DEW – Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.
DEWPOINT – The temperature to which the air must be cooled for water vapor to condense.
DEWPOINT DEPRESSION (TDD)– The positive numerical distance between the temperature and the dewpoint.
DIFFERENTIAL ADVECTION- The increasing or decreasing of advection with height. Vorticity advection increasing with height is favorable for uplift as well as warm air advection increasing when moving from aloft to the surface.
DIFFLUENCE– A 2-D process in which airstreams spread apart.
DIGGING– Deepening, becoming more amplified with time. Often in reference to a trough deepening and building to the South.
DIRTY HIGH OR RIDGE – High pressure usually brings clear weather. A high pressure is termed “dirty” if it has a canopy of clouds associated with it. These clouds can form due to orography, a saturated PBL (with low level fog and/or stratus building under a middle level inversion), or lifting mechanisms within the region of high pressure (WAA, moisture advection).
DISTURBANCE – a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.
DIURNAL EFFECTS– A reference to an effect that has its origins due to daytime heating, such as afternoon cumulus cloud development or the formation of a lake/sea breeze. These phenomena dissipate once the sun goes down and surface heating is lost.
DIVERGENCE – refers to the spreading out of winds.
DIVQ– Diverging Q vectors. Indicates air will sink due to low level cold air advection and/or upper level convergence
DPNG – Deepening. Usually in reference to a low pressure intensifying.
DOPPLER RADAR – A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere.
DOWNBURST – A strong downdraft of air which induces an outburst of damaging winds on or near the ground.
DRIFTING SNOW – An uneven distribution of snowfall/snow depth caused by strong surface winds, but does not reduce surface visibility.
DRIZZLE – Fairly uniform precipitation composed exclusively of fine drops very close together. Drizzle appears to float while following air currents; however unlike fog droplets, it falls to the ground. Drizzle drops are too small to disturb appreciably still water puddles.
DRY SLOT– Refers to a influx of dry air (especially toward a mid-latitude cyclone)
DYNAMICS– Refers to upward forcing caused by jet streak divergence or PDVA
DUST DEVIL- A small, rapidly rotating wind that is made visible by the dust, dirt, or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.
DUST STORM – an area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile.