BACKING - Winds are shifting counterclockwise.
BAROCLINIC BAND - an area of clouds and/or
precipitation caused by a difference in temperature between two air masses. A
front is an example of this.
CAA - Cold Air Advection. This will often
stabilize an air mass, and can cause windy conditions.
CAP - "THE MORNING SOUNDING INDICATES A CAP
AT 600MB", an inversion aloft that serves to put a lid on instability,
making thunderstorms less likely, unless MIXING breaks the
cap.
CAPE - Convective Available Potential Energy, a
scientific measure of energy available for thunderstorms. No CAPE, no thunderstorms. Lots of CAPE means a strong thunderstorm potential.
CB - Cumulonimbus clouds, thunderstorms.
CONVECTION - Usually means thunderstorms or
showers created by daytime heating.
CONVECTIVE TEMPERATURE - The temperature that
daytime heating needs to cause at the surface in order for convection to occur.
CONVERGENCE - Air molecules come together. The
convergent area becomes too crowded with air molecules and they must go up or
down depending on what level of the atmosphere convergence occurs at. Low level
convergence causes lift. Upper level convergence causes sinking in the
atmosphere.
CU - Cumulus clouds.
CWA - County Warning Area, NWS Boise’s CWA
includes most of Southwest Idaho and Southeast Oregon.
DEFORMATION - An area of stretching energy aloft
creating clouds or precipitation.
DIFFLUENCE - Usually used in context of upper
level flow. Diffluent flow splits apart and causes air from below to rise up and
produce lift in the atmosphere.
DIVERGENCE - Opposite of Convergence. Air
molecules become too sparse in a divergent area and need to be replenished by
having other air molecules rise up from below or sink from above. Upper level
divergence causes air from below to rise into the divergent area, causing lift.
Low level divergence causes air from above to sink into the divergent area,
stabilizing the atmosphere.
DYNAMICS - The forces and motions at play in a
weather system. Strong dynamics usually means plenty of energy in the atmosphere
to produce precipitation.
FA - Forecast Area. Same as CWA.
HEIGHTS - See 500MB.
INVERSION - A layer of warm air over a layer of
cooler air. This warm layer acts as a lid on the atmosphere, preventing
interaction between the air above and below the inversion.
LAPSE RATE - The change in temperature as you
increase in altitude. A strong lapse rate means an unstable atmosphere.
LIFTED INDEX (LI) -
A scientific measure of instability in the atmosphere. A "LI" above
zero is stable. A below zero "LI" implies an unstable atmosphere.
MARINE PUSH - An intrusion of low level moist
air from the Pacific. Fairly rare in our CWA.
MESOSCALE - A small scale weather system,
covering a portion of a state or region, often caused by local terrain.
MIXING - The process of mixing air from aloft down to the
surface and vice versa. Can cause gusty conditions as stronger winds aloft mix down to the surface. Usually caused by daytime heating
of the surface by the sun which leads to rising and sinking air currents.
500MB - The 500 millibar
pressure surface. This is a common level for analyzing upper level weather
features. It generally occurs around 18,000 feet above sea level, higher over
warm air masses and lower over cold air masses. The term HEIGHTS
("HEIGHTS RISING AS RIDGE BUILDS") usually refers to the height of the
500MB pressure level. For perspective 1000MB is near sea level, 850MB is around
5000 feet, 700MB is around 9000 feet and 250MB is around 30,000 feet, where the
jet stream is found.
NEGATIVELY TILTED - Usually used in referring to
an upper level trough. The base of the trough moves out ahead of the rest of the trough. This
is a good pattern for severe thunderstorms.
NVA - Negative Vorticity Advection; A kind of
transfer of vorticity that under certain conditions causes sinking of air in the
atmosphere and stabilization (see VORTICITY).
OMEGA BLOCK - A huge, slow moving upper level
ridge shaped like a Greek letter Omega. They are very hard to get rid of and
usually block weather systems from moving through, sometimes for weeks.
OROGRAPHIC - Pertains to mountains and how they
influence the weather.
POP - Probability of Precipitation.
PRECIPITABLE WATER - The amount of precipitation
an air mass could produce if there was lift available to squeeze all the water
out of it. High precipitable water means a moist air mass.
PVA - Positive Vorticity Advection; A kind of
transfer of vorticity that under certain conditions will cause lift in the
atmosphere.
QPF/QPS - Quantitative Precipitation Forecast.
"LOW QPF SHOWERS" means showers with not much measurable
precipitation.
REX BLOCK - A split flow pattern aloft featuring
a ridge to the north over an upper level low pressure to the south.
RIDGE - High pressure aloft, or high pressure at
the surface. Dry air masses are found under an upper level ridge. Generally
fair weather is found in a surface high pressure ridge.
RAOB - Radiosonde Observation. The
balloon
sounding of the atmosphere taken at 12Z (morning) and 00Z (late afternoon)
every day.
SHEAR - The difference in wind speed and/or
direction as altitude increases. As it pertains to thunderstorms, high shear can
mean stronger and possibly severe thunderstorms.
SHORT WAVE - A weak and often fast moving upper
level trough
or ridge.
SOUNDING - See RAOB
SPLIT FLOW - An upper level flow pattern where
energy splits between a northern track and a southern track.
SYNOPTIC - Large scale weather systems. Upper
level ridges
and troughs, large highs and lows.
TCU - Towering Cumulus clouds. If they keep
growing they become Cumulonimbus (CB).
THERMAL TROUGH - A surface low pressure trough
caused by surface heating which produces rising air and thus lower pressure.
Usually an axis of warmest temperatures (THERMAL AXIS). Also
called a THERMAL RIDGE
which is technically more correct since thermally the temperatures are highest
here. THERMALLY INDUCED PRESSURE TROUGH is the
proper label, but seldom used because it is too cumbersome to type out.
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