Transcript 20140915_DIFAX_Contourx

DIFAX Maps
• Weather maps generated by the NWS
• Before the Internet or AWIPS, these were the
basic weather analysis and forecast charts
used by meteorologists
• They were only available through a fax
machine connected to a dedicated landline
• DIFAX maps are gradually being phased out;
however, the most important ones are still
produced
• These maps are unique and contain
information which is priceless for operational
meteorologists
• All meteorology students benefit from
knowledge of these maps and their
interpretation
• If you understand how to interpret the
black&white DIFAX chart, you should have no
problem interpreting pretty colored charts
from other sources
DIFAX Map Access
National Weather Service:
http://weather.noaa.gov/fax/nwsfax.html
Colorado State Archive:
http://ldm.atmos.colostate.edu/
SUNY Albany:
http://www.atmos.albany.edu/weather/difax.html
Surface Charts
• Analyzed charts issued every 3 hours (00Z – 21 Z)
• Data includes
– Hourly synoptic stations
– Ship reports
– Buoy reports
• Maps can be found from the HPC:
http://www.hpc.ncep.noaa.gov/html/sfc2.shtml
Surface Charts
• Isobar analysis:
– 4 mb increments labeled with tens and units digits
– Lows and Highs labeled with L and H with the pressure
value labeled nearby (in whole mb)
• Frontal Analysis
http://www.hpc.ncep.noaa.gov/html/fntcodes2.shtml
• Used for current depiction of surface weather
features (most valuable weather chart)
Upper Air Analysis
• Generated every 12 hours with 00Z and 12Z
data
• Produced from the NAM Model analysis
– The NAM Model uses a first guess from the
previous model run 6 or 12 hours earlier as a basis
for constructing the analysis fields
– Data is incorporated into the first guess field and
the analysis is created via Optimal Interpolation
(OI) or 4-D Data Assimilation
– Actual data is plotted on the chart, but may not
agree with chart’s analysis field
850 mb Chart
• Isoheights (solid contours)
– 30 m intervals with 1500 m (150 decameters)
reference line
– Contour labels in decameters
– Plotted heights are in meters
• Isotherms (dashed contours)
– 5C intervals with 0C as reference line
850 mb Chart
• Uses:
– Low level jets
– Lower tropospheric temperature advection and
thermal profile (thermal ridges and troughs)
– Lower tropospheric moisture advection and
profiles (moist and dry tongues)
700 mb Chart
• Isoheights (solid contours)
– 30 m intervals with 3000 m (300 decameters)
reference line
– Contour labels in decameters
– Plotted heights are in meters
• Isotherms (dashed contours)
– 5C intervals with 0C reference line
700 mb Chart
• Uses:
– Mid-level jets
– Mid-tropospheric temperature advection and
thermal profile
– Elevated tropospheric moisture advection and
profiles
– Height changes
500 mb Chart (North America)
• Isoheights (solid contours)
– 60 m intervals with 5400 m (540 decameters)
reference line
– Contour labels in decameters
– Plotted heights are in decameters
• Isotherms (dashed contours)
– 5C intervals with 0C reference line
500 mb Chart (North America)
• Uses:
– Mid-tropospheric temperature advection and
thermal profile
– Mid-tropospheric moisture profile
– Wave pattern in the westerlies
• ID of longwaves and shortwaves
– LND and approximate steering level for surface
synoptic systems
– Height changes and wave motion
– Vertical and horizontal tilt of waves
500 mb Chart (Hemispheric)
• Contains same contours as the 500 mb North
American analysis, except void of data plots
• Additional Uses:
– Circumpolar vortex
– Planetary wave number and pattern
– Wave ID
300 mb Chart
• Isoheights (solid contours)
– 120 m intervals with 9000 m (900 decameter)
reference line
– Contour labels in decameters
– Plotted heights in decameters
• Isotachs (light dashed contours)
– 20 knot intervals with 10 knot reference line
– Stippled regions represent:
• 70-110 knot winds
• 150-190 knot winds
300 mb Chart
• Uses:
– Polar jet stream location/configuration/intensity
– 4-quadrant jet/divergence relationship
– Upper tropospheric wave pattern
– Regions of difluence and confluence
– Regions of upper-tropospheric vertical shear
1000-500 Thickness / MSLP Chart
• Thickness Values (usually dashed contours)
– Vertical distance in m between 1000mb and
500mb pressure levels
– Function of avg virtual temperature of 1000mb to
500mb layer
– Increments of 60 gpm
• MSLP (solid black contour)
1000-500 Thickness / MSLP Chart
• Uses
– Temperature advection
• Thickness is proportionally to temperature
• Use MSLP contours as proxy for wind (assume
geostrophic
– 5400 (540) line generally divides polar air from
mid-latitude air (rain-snow line)
General Rules For Drawing Contours
(see handout for more detail)
• Contour lines are drawn to identify constant values of an
atmospheric variable
• A contour is drawn through the station location only if the data for
that station has the exact value of the contour; otherwise, the
contour is drawn between stations
• Higher values are on one side of the contour and lower values on
the other side of the contour
• Contours never cross or touch each other
• More than one contour of a given value may appear on a given
map
• All contour lines must be clearly labeled
• Often easiest to find the highest value or the lowest value and
work from there
• Keep the surface wind in mind when drawing pressure contours.
– Relative to other stations, the stronger the wind, the stronger the
pressure gradient, thus the closer the isobars.
1016 mb
H
L
1016 mb
H
1012 mb
L
1016 mb
H
1012 mb
1008 mb
L
1016 mb
H
1012 mb
1008 mb
L
996 mb
1016 mb
H
1012 mb
1008 mb
L
996 mb
1000 mb
1016 mb
H
L
1012 mb
996 mb
1000 mb
1008 mb
1004 mb
1016 mb
H
L
996 mb
1012 mb
1000 mb
1008 mb
1004 mb