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Surface
Temp/ Winds
Temperature
and dew point temperatures displayed are
extrapolated to a "minimum" topography
field to give values more representative of
valley stations in mountainous areas, where
surface stations are usually located.
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3
Hour Forecast Surface Precipitation Type
Categorical precipitation
types - rain/snow/ice pellets/freezing rain - These
yes/no indicators are calculated from the explicit cloud microphysics
in the RUC2 model. (from NCAR/Penn State MM5 model).
These values are not mutually exclusive. More than
one value can be yes (1) at a grid point. Here is how the diagnostics
are done (Diagnostic logic for precipitation types):
Snow - There are a few ways to get
snow.
- If fall rate for snow mixing ratio at ground is
at least 0.2 x 10**-9 g/g/second, snow is diagnosed.
- If fall rate for graupel mixing ratio at ground
is > 1.0 x 10**-9 g/g/s and
- Surface temp is < 0 deg C, and max rain
mixing ratio at any level < 0.05 g/kg or the graupel rate at the
surface is less than the snow fall rate, snow is diagnosed.
- Surface temp is between 0 - +2 deg C
Rain - If the fall rate for rain
mixing ratio at ground is at least 0.01 g/g/second, and the
temperature at the surface is > or = 0 deg C, then rain is
diagnosed. The temperature used for this diagnosis is that at the
minimum topography, described above.
Freezing rain - Same as for rain,
but if the temperature at the surface is < 0 deg C and
some level above the surface is above freezing, freezing rain is
diagnosed.
Ice pellets - If the graupel fall
rate at the surface is at least 1.0 x 10**-9 g/g/s and the surface
temp is < 0 deg C and the max rain mixing ratio in the column
is > 0.05 g/kg and the graupel fall rate at the sfc is greater
than that for snow mixing ratio, then ice pellets are diagnosed.
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MAPS
Mean Sea Level Pressure/ Winds
This reduction is the one used in
previous version of MAPS/RUC using the 700 mb temperature to
minimize unrepresentative local variations caused by local surface
temperature variations. This method has some improvement over the standard reduction
method in mountainous areas and gives geostrophic winds that are
more consistent with observed surface winds. |
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3
Hour Surface Pressure Change
These fields are determined by
differencing surface pressure fields at valid times separated by 3
h. Since altimeter setting values (surface pressure) are used in
the MAPS analyses, this field reflects the observed 3-h pressure
change fairly closely over areas with surface observations. It is
based on the forecast in data-void regions.
The 3-h pressure change field during the first 3 h of a model forecast
often shows some non-physical features, resulting from gravity
wave sloshing in the model. After 3 h, the pressure change field
appears to be quite well-behaved. The smaller-scale features in
this field appear to be very useful for seeing predicted movement
of lows, surges, etc. despite the slosh at the beginning of the
forecast. |
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Surface
Soil Moisture
Cycled continuously since 26 April
1996. There are 6 levels in the 20-km MAPS soil model, extending
down to 3 m deep, but the field shown is for the top 2 cm of soil
only, so this field responds quickly to recent precipitation or
surface drying and may not be indicative of deep soil moisture.
The variable displayed is the soil volumetric moisture content,
the ratio of water volume to total volume in the soil. Values of
0.25 or so are relatively high values. |
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12
Hour Surface Snow Accumulation
Snow accumulations are calculated
using a 10 to 1 ratio between snow depth and liquid water
equivalent. Of course, this ratio varies in reality, but the ratio
used here was set at this constant value so that users will know
the water equivalent exactly. The snow accumulation is not
diagnosed based on temperature, but is explicitly forecast through
the mixed-phase cloud microphysics in the MAPS model. |
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3
Hour Surface Snow Water Depth
This field is the current estimated
snow depth (using a 2.5 to 1 ratio between snow and liquid water
equivalent). It evolves in the RUC2 1-h cycle, increasing from
accumulation from the explicit snowfall in the RUC2 from the cloud
microphysics, and decreasing from melting depending on an energy
budget in the snow layer in the RUC2 model. This field has been
evolving since the beginning of the snow season in fall 1997. Due
to a few small outages at FSL, some snow events were missed, but
generally the field shows fairly reasonable accuracy. This
accuracy will be still better from the RUC2, since it runs more
reliably than the 40-km MAPS at FSL. As of early January 1998, the
maximum depth in the RUC2 domain was 2 meters over the mountains
of British Columbia, equivalent to 0.8 meters of liquid water
using the 2.5 - 1 ratio. |
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| Current
Upper Air Data & Indices |
Cloud
Base Height
Lowest level at which combined
cloud and ice mixing ratio exceeds 10**-6 g/g. |
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Cloud
Top Height
Top level at which combined cloud and ice mixing ratio exceeds 10**-6 g/g. |
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Potential
Convective Cloud Top Height
From RUC CAPE/CIN routine. This is
the level at which negative buoyant energy cancels out the CAPE
below the equilibrium level. It is also equivalent to the height
at which vertical velocity goes to zero (assuming no entrainment).
Height above sea level. |
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MAPS
Precipitable Water
Integrated precipitable water vapor
from surface of MAPS model to top level (~50 mb). This field is
influenced by all available moisture data and the dynamic and
physical processes in the ongoing MAPS cycle. The 40km MAPS data
run at FSL includes GOES precipitable water observations as of 11
June 1997. The RUC2 run at NCEP includes both GOES precipitable
water observations and also those from SSM/I. Precipitable water
reflects the amount of water contained in a vertical column above
the surface if it were all precipitated out. This is a good
indicator of how much rain or snow might fall as a result of a
thunderstorm or low pressure system. |
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CAPE/
CIN
Convective Available Potential
Energy (CAPE)
Another instability tool, indicates energy available for
buoyant parcel from native RUC2 hybrid-b level with maximum
buoyancy within 180 mb of surface (changed to 300 mb on 6 May
1999). Before the most buoyant level is determined, first an
averaging of potential temperature and water vapor mixing ratio is
done in the lowest 7 RUC native levels (about 40 mb). CAPE
represents the amount of energy a parcel might have if it were
lifted. Often this reflects the strength of updrafts within a
thunderstorm. CAPE values of greater than 2000 represent enough
energy to produce thunderstorms. A value greater than 3000
represents enough energy to produce strong thunderstorms. Values
< 1000 denote a reletively stable atmosphere. < 300 Very weak
convection,
300-1000 Weak,
1000-2500 Moderate,
2500-3000 Strong,
3000+ Wizard of Oz.
Convective Inhibition (CIN)
Indicates negative buoyancy in layer through which a
potentially buoyant parcel must be lifted before becoming
positively buoyant. Thresholds are shown at 75 W/m*m (marginally
strong capping inversion, depicted with loose cross-hatching) and
100 W/m*m (strong cap, depicted with tight cross-hatching).
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Helicity/
Storm Motion/ CAPE
This is a contour plot of the amount
of storm relative rotation/shear in the atmosphere. Helicity is used
to indicate where rotation/shear is high enough to allow
thunderstorms to organize into severe or supercell storms. In the
lack of helicity, storms develop vertically and the precipitation
will snuff out the updraft killing the thunderstorm. Severe storms
need helicity to maintain an organized structure allowing the storm
to develop to severe limits. Values in the 150-900 range can
correspond with tornadic thunderstorms. A value of 400-500 is often
needed to produce severe storms. Helicity is basically a measure of
the low-level shear, so in high shear situations, such as behind
strong cold fronts or ahead of warm fronts, the values will be very
large maybe as high as 1500. High negative values are also possible
in reverse shear situations. Often this is used in conjunction with
CAPE to determine severe storm location. |
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Tropopause
Pressure
Diagnosed from 2.0 isentropic
potential vorticity unit (PVU) surface. The 2.0 PVU surface is
calculated directly from the native isentropic/sigma RUC grids.
Low tropopause regions correspond to upper-level waves and give a
quasi-3D way to look at upper-level potential vorticity. They also
correspond very well to dry (warm) areas in water vapor satellite
images, since stratospheric air is very dry. |
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500
mb Heights/ Vorticity
Vorticity is measurement of the
spin of air parcels. In meteorology, we are most concerned with
the spin of horizontally flowing air about a vertical axis. Of
most interest are localized regions of much higher vorticity
values, called positive vorticity maxima (generally above 16
X10e-5 s^-1). East of a vorticity max., upward motion helps to
strengthen surface low pressure centers and induce precipitation.
Conversely, west of a vorticity maximum, air is generally sinking,
resulting in fair weather. |
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700
mb Vertical Velocity/ Heights
700 mb charts depict conditions in
the middle troposphere (roughly 3000 meters). This is also
referred to as the steering level, since most weather systems are
"steered" by the winds at this level. 700 mb vertical
velocity is simply the velocity of air moving through the 700 mb
surface in a vertical direction. Upward motion has a positive
velocity and downward motion has a negative velocity.
The vertical velocity field is
valid for the time shown in the image. Therefore, the upward
motion shows where precipitation is forming or will likely form in
the near future and where precipitation will likely move. Downward
motion, however, tends to suppress vertical development of clouds
and precipitation, resulting in fair weather conditions. |
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