The various parameters available
for each sounding are: TIME - The
exact time of the sounding, (when the GOES sounder was scanning the particular
location). In this example, the time is 2009Z (20 hours, 9 minutes), or
3:09am EST, and is the same for the GOES sounding and the AVN forecast.
NOTE: The AVN forecast is available at 6 hour intervals. Therefore, two
forecast times (in this case, 18Z and 00Z) are linearly interpolated to
the time of the GOES scan. ELEV - The surface elevation
above sea level, in meters (not necessarily the same as the surface elevation
of the station). The actual location of the sounding can be up to 30 nautical
miles from the station; this can be critical in regions of greatly varying
terrain. This information is given below the sounding plot. PARP - Pressure of a "parcel
of air" used to determine some stability parameters. An initial parcel
is determined by examining the lowest three levels of the atmosphere. The
pressure level with the most potential buoyancy (i.e., the highest equivalent
potential temperature) is determined to be the best parcel for determining
stability. Using this parcel, rather than a simple surface parcel is critical,
especially for morning soundings when a surface inversion can reflect unrealistically
stable conditions. Note that in the example above, while the surface pressure
is 1019mb, the parcel chosen is from the 950mb level. PART - The temperature (C) of
the parcel being lifted for stability calculations. This is simply
the actual profile temperature at the PARP pressure level. PARD - The dewpoint temperature
(C) of the parcel being lifted for stability calculations. It is important
to note that the moisture is mixed from the surface to the parcel level.
Therefore, while the temperature of the parcel (PART) is exactly the same
as the profile temperature at the parcel pressure, the dewpoint of the
parcel (PARD) may very well not be equal to the profile dewpoint at the
parcel pressure level. TSKIN - The surface skin temperature(C).
This is the estimated temperature of the ground surface as derived from
the GOES satellite (note that no value is available for the AVN). PW - The total precipitable water
(millimeters) derived from the soundings. The TPW is a measure of the liquid
content of a vertical column through the atmosphere.In this example, the
AVN forecast TPW is 30mm, and the GOES sounding TPW is 36mm. This is consistent
with the increased dewpoint temperatures of the GOES sounding in the profile.
Precipitable water estimates are one of the most important products generated
from GOES sounder data. While the changes to the AVN forecast temperature
profile using GOES sounder data are usually small, large changes to the
AVN forecast dewpoint profile and resulting TPW occur frequently. Overall
statistics as well as case studies have consistently
showed the GOES TPW values to be more accurate than the first guess TPW.
As such, examining the TPW values from the GOES and AVN is useful in adjusting
forecasts of severe weather, cloud cover and precipitation. L.I. - The Lifted Index is calculated
by lifting (frontal, orographic, upper air dynamics, etc.) a parcel of
air dry adiabatically while conserving moisture until it reaches saturation.
At that point the parcel is lifted moist adiabtically up to 500mb. The
Lifted Index is the ambient air temperature minus the lifted parcel temperature
at 500mb. If the parcel is warmer than the environment (negative L.I.),
it has positive buoyancy, and will tend to continue to rise, favoring convection.
L.I. values less than -5C indicate very unstable conditions. A positive
L.I. value indicates negative parcel buoyancy, and the parcel will tend
to sink. This is representative of stable conditions where convection is
unlikely. Increasingly negative numbers correspond to increasing instability
and likelihood of severe weather. At times, very high (stable) lifted index
values in cold air are indicative of frozen or freezing precipitation versus
rain during warm advection events. The extreme stability does not allow
air to lift out, resulting in cold air "damming", which restricts the advance
of warm air at the surface. CAPE - Convective Available Potential
Energy, a measure of the cumulative buoyancy of a parcel as it rises, in
units of Joules per kilogram. CAPE values larger than 1000J/kg represent
moderate amounts of atmospheric potential energy. Values exceeding 3000J/kg
are indicative of very large amounts of potential energy, and are often
associated with strong/severe weather. Graphically, the CAPE is the
positively buoyant area (shaded purple) on the skew-t diagram. It
is important to note, however, that for the purposes of this CAPE calculation
only the lowest positively buoyant region is included. There may
be times when a small negatively buoyant region may break up two positive
areas. This is critical, especially if the lower positive area is
larger than the negative area. In this case, disregarding other outside
influences, the parcel would have enough buoyancy from passing through
the lower positive region to successfully pass through the entire negative
region and back into the high positive area. In a case such as this,
the listed CAPE value will be deceivingly low and visual examination of
the areas of positive and negative buoyancy are very important. NCAP - Normalized CAPe is a measure
of the structure of the positively buoyant parcel. The NCAP is equal
to the CAPE divided by the depth of the positive area. This value
is equal to the acceleration of the parcel (cm/s^2). Therefore, a
high CAPE value over a great depth will result in a slowly accelerating
parcel, whereas, a high CAPE value over a shallow depth will result in
a much greater parcel acceleration. The NCAP value can be helpful
in determining the potential for tornadic activity. High NCAP values
(>20) are typical for such severe cases; lower NCAP values (roughly, 10-15cm)
in combination with high CAPE values typically indicate conditions more
conducive for heavy rain and, possibly, hail. MXHAIL - MaXimum HAIL, is a rough
estimate of the maximum hail size that can be expected (cm). Given
the acceleration (NCAP), disregarding outside vertical forcing, one can
calculate the parcel speed at the top of the positively buoyant layer of
the atmosphere. The fall velocity of hail can be roughly estimated
as a function of size. As such, using a fall velocity equal to the
parcel's maximum velocity can yield a prediction of hail size. Of
course, with various unknowns in this calculation (outside vertical forcing,
formation level of the hail, etc.), it is a very rough approximation and
is intended to be on the high side, representing the maximum possible hail
size under the given conditions. Note that under conditions of Convective
Inhibition (see below) greater than 20 J/kg, forcing required for convection
is great enough such that the MXHAIL parameter is not produced. CINH - Convective INHibition,
a measure of negative buoyancy below the layer of positive buoyancy (if
it exists), in Joules pre kilogram. Below the "positive area" which defines
the CAPE, there can exist some negative area, where the parcel is colder
than the environment. The atmosphere in these situations are sometimes
referred to as "capped". In these cases, either lifting of a parcel through
some forcing mechanism, or heating of the lower atmosphere to eliminate
the negative buoyancy area is need for initiation of convection. Dynamically,
once the parcel gets through this negative area it is free to rise through
the positive area. Thus, occasionally a sounding may have more than one
negative region, but only the lowest negative area is considered the Convective
Inhibition. Since CINH is not reported unless some CAPE is present, the
CINH values are typically fairly low. CINH values above 50 J/kg are typically
enough to inhibit convection, unless dynamic forcing is extreme.
Values from 25-50 J/kg require significant forcing, but can be overcome
with reasonable dynamics or heating. Values from 10-25 also require
a decent amount of forcing. CINH values under 10 indicate a requirement
for only minimal forcing. K.I. - The K-Index is a simple
index using data from discreet pressure levels, instead of a lifted parcel.
It is based on vertical temperature changes, moisture content of the lower
atmosphere, and the vertical extent of the moist layer. The formula for
K.I. is: K.I.=(T850-T500)+(TD850-(T700-TD700))
where: T850=Temperature at 850mb It is more correlated to convective
activity in general as opposed to severe weather. The higher the K-Index
the more conducive the atmosphere is to convection. K.I. values below 20
imply little support for thunderstorm activity, while values exceeding
30 are quite supportive of thunderstorm activity. Values in the Central
and Eastern U.S. typically need to be slightly higher than in the Western
U.S. in order to indicate the same level of potential thunderstorm activity. TT - The Total Totals Index,
like the K-Index, is computed using discreet pressure level information,
but is more indicative of severe weather potential. It's formula is: TT=(T850+TD850)-2(T500) Generally, TT values below 40-45 are
indicators of little or no thunderstorm activity, while values exceeding
55 in the East and Central or 65 in the West are indicators of considerable
severe weather, including the potential for tornadic activity. Total Totals
values tend to be somewhat higher over higher elevations, therefore higher
TT values in the Western U.S. are required to indicate the same level of
storm severity as lower TT values in the Central and Eastern U.S. SHOW - The Showalter Index is a parcel-based
index, calculated in the same manner as the Lifted Index, using a parcel
at 850mb. That is, the 850mb parcel is lifted to saturation, then moist
adiabatically to 500mb. The difference between the parcel and environment
at 500mb is the Showalter Index. Again, the calculation is environment
minus parcel, so negative numbers indicate instability. The SHOW values
are similar to the LI values as far as references for severe weather (negative
is unstable, below about -5C is highly unstable). SWEAT - The SWEAT index is calculated
only for the GOES Sounding, using the gradient wind information. Current
processing contraints preclude the use of AVN forecast winds. The SWEAT
index is computed from five terms that contribute to severe weather potential: Low-level moisture (850mb dewpoint
temperature) he formula used to calculate
the SWEAT Index is: SWEAT=12(TD850)+20(TT-49)+2(WS850)+WS500+125(S+0.2),
where: TD850 = Dewpoint temperature (Degrees
C) at 850mb WS850 = 850mb wind speed in knots WD500=500mb wind direction NOTE: The entire wind shear term [125(S+0.2)
is set to zero when any of the following conditions are met: WD850 is between 130 and 250 degrees NOTE: No term in the formula may be
negative. Higher SWEAT indices correspond to
greater potential for severe weather, given some triggering mechanism.
SWEAT values under 200 indicate little potential for severe weather, values
> 300 are adequate for severe weather, and tornadic activity is possible
with SWEAT values exceeding 400. The greatest value of the SWEAT
index is that it is the only one of the various stability parameters on
the chart which includes a term to account for wind shear (which can enhance
tornadic potential). LR8-5 - The 850 to 500mb lapse
rate (C/km). CVT - Convective Temperature,
the temperature at which convection will begin without any aid from lifting.
That is, if the surface air temperature reaches the convective temperature,
the initial parcel will become buoyant, regardless of whether or not any
lifting mechanism is present. LCL - The Lifting Condensation
Level is the pressure at which a parcel, when lifted, will reach saturation.
This is determined by lifting a parcel dry adiabatically while conserving
moisture (constant mixing ratio). The LCL is defined as the pressure at
which the saturation mixing ratio of the parcel equals the parcel mixing
ratio (i.e., the parcel is saturated). LFC - The Level of Free Convection
is the pressure at which a parcel becomes buoyant. This is found by raising
a parcel to the LCL and then continuing to lift it moist adiabatically.
The pressure where the parcel temperature becomes greater than the environmental
(profile) temperature is the level at which the parcel is buoyant. In a
stable atmosphere a raised parcel may never become buoyant and, therefore,
may not have a level of free convection. In some complex profiles
there may be more than one buoyant area, broken up by a negatively buoyant
area. In such cases the first (lowest) LFC is output. EL -The Equilibrium Level is
defined as the pressure at which a buoyant parcel becomes cooler that the
environment. If the environment is stable and there is no level of free
convection, there is obviously no equilibrium level. In some complex
profiles there may be more than one buoyant area, broken up by a negatively
buoyant area. In such cases the first (lowest) EL is output. ELT - The Equilibrium Level Temperature
is the actual environment (profile) temperature at the above defined Equilibrium
Level (EL). CCL - The Convective Condensation
Level is the pressure at which condensation will occur in a parcel providing
the only mechanism for convection is heating. A parcel with the convective
temperature, rather than the actual temperature, is lifted normally (physically,
this lifting would be from heating alone) until saturation. The pressure
level of saturation, found in the exact same manner as the LCL, defines
the CCL. MCL - The Mixing Condensation
Level is the pressure at which the CAPE is equal to the negative potential
energy (Convective Inhibition), following complete vertical atmospheric
mixing. As lifting occurs to produce convection, the negative buoyancy
area acts as an inhibitor until it is "mixed" with the positive area. That
is, if there are 100 J/kg of negative potential energy beneath the area
of positive potential energy, the MCL is the pressure at which 100 J/kg
of positive potential energy is reached. -20C - The -20C level is the
height in the atmosphere at which the temperature reaches -20C. Because
water droplets can become supercooled before freezing, it is common to
locate the level of either -15C or -20C temperatures to aid in hail prediction.
For example, if the CAPE value were very high, but over a very shallow
layer (thus making the NCAP quite high and, likely, making 15TH - The distance (thickness)
between 1000 mb and 500mb pressure levels. As a general rule of thumb,
5400m (540dm) is used as a typical rain versus snow line, where thicknesses
below 5400m indicate snow and thicknesses above indicate rain. Since the
thickness is based on a mean temperature profile in the layer, relatively
warm air in the upper levels of this layer can still be below freezing,
but still result in thicknesses above 5400m, supporting snowfall. Likewise,
a shallow, warm boundary layer can result in rain with thicknesses below
5400m. Generally, the GOES soundings show little change from the AVN first
guess since the temperature profiles exhibit little modification. However,
significant changes can occur with subtle temperature adjustments and major
moisture adjustments, as thickness calculations are based on virtual temperature,
which is slightly impacted by moisture. Thickness adjustments are typically
on the order of one to ten meters, but changes as high as about fifty meters
have been seen. 87TH - 850mb - 700mb thickness.
At very high elevations (low surface pressure), a thickness from 1000mb-500mb
cannot be calculated. In these cases, an 850mb-700mb thickness is more
appropriate (though it is still calculated for the lower elevation stations).
The same assumptions apply for this thickness as for the 1000mb-500mb thickness;
a general rule of thumb regarding snow/rain line is 1555m. FRZL - Freezing Level, defined
as the lowest pressure at which the temperature drops below freezing. Because
of the small temperature differences between the GOES and AVN first guess
temperature profiles, the GOES and AVN values for the freezing level are
often extremely close. WBFR - The Wet Bulb Freezing
Level is the lowest pressure at which the wet bulb temperature drops below
freezing. Frequently, the wet bulb temperature profile is more useful than
the dry bulb (normal) temperature profile when attempting to determine
precipitation type. Also, because of the differences in dewpoint profiles
between the GOES and AVN first guess, the wet bulb temperature profiles
are frequently noticeably different. TADV - The Thermal Advection
is the mean advection in the 850-500mb layer, in degrees C/hour. The gradient
winds derived from the soundings are used to calculate the mean temperature
gradient for the 850-500mb layer. PCPT - The Precipitation Type
as indicated from the sounding, and is derived primarily from the wet bulb
temperature profile. Since GOES soundings are not available in cloudy areas,
the precipitation type indicator is obviously present only in locations
where precipitation is not falling. However, it can still be useful several
hours prior to the onset of precipitation, providing overcast conditions
have not developed. It is especially useful if the GOES Sounding is indicating
a different precipitation type than the first guess.
T500=Temperature at 500mb
TD850=Dewpoint temperature at 850mb
T700=Temperature at 700mb
TD700=Dewpoint temperature at 700mb
Instability (Total Totals Index)
Low-level winds (850mb wind speed)
Upper-level winds (500mb wind speed)
Warm advection (Veering of the wind
between 850mb and 500mb)
TT = Total Totals Index (Degrees C)
(If TT < 49, the term 20(TT-49) is set to zero
WS500 = 500mb wind speed in knots
S = SIN(WD500-WD850), where:
WD850=850mb wind direction
WD500 is between 210 and 310 degrees
(WD500-WD850) is positive
WS850>15 knots AND WS500>15 knots
the MXHAIL parameter rather high)
and that shallow layer was quite low in the atmosphere, the top of the
positively buoyant parcel of air (the EL) may not extend up to the -20C
level. As a result, in this instance, hail formation would be unlikely,
regardless of the MXHAIL indicator.