DIFAX Maps
• DIFAX maps are weather maps generated by the
National Weather Service.
• 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.
• The cost of the landline was very expensive and
the special wet electro-conductive paper was
expensive; thus only the FAA, NWS, and selective
universities and TV stations had access to this
information.
• The images were actually burned onto the
special FAX paper by electrical currents in
the FAX machine.
• The rotating helical blade in the fax
machine which conducted the electricity to
the paper was a deadly weapon:
– It was as sharp as a razor and many a person
was sliced open by this blade while changing
paper rolls or changing the blade itself.
• DIFAX maps, although gradually being phased
out, are still available on the internet.
• These maps are still unique and contain
information which is priceless for operational
meteorologists.
• All students of meteorology & forecasting will
benefit from knowledge of these maps and their
interpretation.
• Essentially, if you understand how to interpret a
black&white DIFAX chart, you should have no
problem interpreting pretty colored charts from
other sources.
DIFAX Map Descriptions
DIFAX maps fall under 2 general categories
• Basic data and analysis products
– Plotted data
– Analyses of basic variables
• Forecast products
– (generally produced at NCEP)
We will concentrate on the first category.
Surface Charts
• Analyzed charts issued every 3 hours (0000Z - 2100Z)
(http://www.hpc.ncep.noaa.gov/html/about_sfc.shtml)
• Data includes
– hourly synoptic stations
– ship reports
– buoy reports
• (Maps may be found at the Hydrological Prediction
Center)
– http://www.hpc.ncep.noaa.gov/html/sfc2.shtml
• Plotted data example
– http://www.hpc.ncep.noaa.gov/html/stationplot.shtml
Surface Charts (continued)
• Isobar analysis:
– 4 mb increments labeled with tens and units
digit (96, 00, 04, etc...)
– 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.s
html
• Use:
– Current depiction of surface weather features
(most valuable weather chart)
Plotted Data Only
• Plotted data is available hourly from the
HPC
– http://www.hpc.ncep.noaa.gov/sfc/sfcobs/sfcobs.shtml
Aviation Surface Analyses
• Contains analyzed surface charts and plots.
– http://www.hpc.ncep.noaa.gov/html/avnsfc.shtml
• Plots show only the temperature, wind,
cloud cover, current weather, and cloud
ceiling.
– http://www.hpc.ncep.noaa.gov/html/stationplot_awc.shtml
DIFAX Map Access:
• Many DIFAX maps are no longer available,
however some of the more important maps
are still being produced.
• They can be found at:
– South Alabama Synoptic Page
– www.southalabama.edu/meteorologyclub/synoptic/
National Weather Service:
– http://weather.noaa.gov/fax/nwsfax.html
SUNY Albany
– http://www.atmos.albany.edu/weather/difax.html
DIFAX Radar Composite Analysis
• Gridded from individual radars on a grid
(grid spacing roughly 48 km at 60N) (see
overhead)
• Entire grid box is assigned the largest value
observed anywhere within the grid box (see
overhead)
• Shading represents echo areas
• Contours are incremented Level 1-3-5 on a
scale of 0-6. (VIP Levels)
• Echo Tops (tens & hundreds digit
truncated: 540=54,000 ft top)
• Echo bases (tens & hundreds digit
truncated: 040=4,000 ft base)
• NE = No Echos
• NA = Not Available
• Movement of Areas and Lines:
– Pennants
540
040
From W at 45 kts
• Movement of Cells:
– Arrows and speed
45
From the WSW at 45 kts
• Precipitation Type and Change of Intensity
R
RW
TRW
S
SW
TSW
ZR
+
-
Rain
Rainshower
Thunderstorm
Snow
Snowshower
Thunder-snowshower
Freezing rain
New area/cell or increasing intensity
Decreasing intensity
• Special Severe Weather Designations
(Note: These are less common with Doppler radar network)
HOOK
HAIL
LEWP
WER
BWER
Hook echo (tornadic mesocyclone)
Radar-detected hail
Line Echo Wave Pattern (bow echoes)
Weak Echo Region
Bounded Weak Echo Region (supercell
mesocyclone)
• Watch Boxes
WS:
WT:
Severe Thunderstorm Watch
Tornado Watch
• Radar charts are issued nearly hourly
– Distributed approximately 1 h after valid time
– Data captured at 35 minutes after the hour
• Use
– Indicates areal coverage, type, intensity and
movement of precipitation
Real-Time Radar and Satellite Links
• National Weather Service Doppler Radar
– http://www.srh.noaa.gov/radar/national.html
• Geostationary Satellite Imagery:
– http://www.ghcc.msfc.nasa.gov/GOES/
Weather Depiction Chart
• Primarily for Aviation-sensitive weather
conditions
• Issued every 3 hours at 01, 04, 07, 10, 13,
16, 19, & 21Z) >>> i.e. 1-hour after
synoptic times
• Comprised of METAR, cloud, visibility,
and current weather information.
• Fronts are from the previous hour.
• Plotting guide (see handout).
• IFR:
Instrument Flight Rules
Ceiling < 1000 ft and or Visibility < 3 SM
• MVFR: Marginal Visible Flight Rules
1000 ft < ceiling < 3000 ft
3 SM < visibility < 5 SM
• VFR:
Visual Flight Rules
ceiling > 3000 ft
visibility > 5 SM
Maximum Temperature Chart
• Displays the highest temperatures recorded
for the 12 hours prior to 00Z each day.
Time period:
(d-1/12Z - d/00Z)
• Sometimes, the highest temperatures occur
at times other than within this 12-hour
window; these occurrences will not be
reflected on this chart.
– The most common “outside” occurrence of a
maximum temperature is heavily influenced by
frontal passages.
Minimum Temperature Chart
• Displays the lowest temperatures recorded
for the 12 hours prior to 12Z each day.
Time period:
(d/00Z - d/12Z)
• Sometimes, the lowest daily temperatures
occur at times other than within this 12-hour
window; these occurrences will not be
reflected on this chart.
– The most common “outside” occurrence of a
minimum temperature is during the winter
shortly after 12Z.
– Frontal influence is also a big factor
Record High and Low Temperatures
• Record high and low temperatures are noted
on the chart by the l being replaced by a
Y.
• The type of record is given in the box
(situated in the Gulf of Mexico):
HI vs LO
E:
Equalled
EX:
Exceeded
FM:
Record for entire month
DA:
Record for the day
AT:
SE:
SL:
All-time record
Record for so early in the year
(summer/fall freeze, etc...)
Record for so late in the year
(spring/summer freeze, etc...)
Examples:
HIEXDA
LOEAT
LOEXSE
24 Hour Precipitation Chart
• Shows liquid-equivalent precipitation totals
over a 24 hour period: (d-1/12Z - d/12Z)
• Precipitation is to the nearest hundredth of
an inch.
• Inch digit is larger than digits to the right of
the decimal.
• Additional precipitation totals are printed in
the column to the right.
Observed Snow Cover Chart
•
•
•
•
Seasonal chart (Sept - April)
Depicts snow cover at 12Z each day
Snow cover is in whole inches.
Recent accumulations (within the last 6
hours) is depicted by blocked numbers to
the right.
• Example:
16 9
Upper Air Analysis Charts
• Generated every 12 hours with 00Z and 12Z
data (charts are available on DIFAX 2 to 5
hours after this time).
• 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 chart, but may not
agree with the chart’s analyzed fields.
Upper Air Station Model
Decoding Heights
850mb: 150 = 1500m
700mb: 300 = 3000m
500mb: 540 = 5400m
300mb: 900 = 9000m
250mb: 000 = 10,000m
Dewpoint Depression
T: -5 C
DD: 12 C
Td = T – DD
Td = -17 C
DD < 5 C considered near Sat.
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 (generally add 1 in
thousands digit)
• Isotherms (dashed contours)
– 5oC intervals with 0oC 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)
– Height changes
• Caution:
– Sometimes underground near high terrain
Thermal Advection
• Advection – Horizontal movement of some atmospheric
property (Temp, Moisure, Thickness, Vorticity), usually by
the wind or atmospheric flow.
• Why is Thermal Advection important?
– Helps identify the location and movement of fronts
– Helps in identifying regions of slantwise upward & downward
vertical air motion (tied to isentropic lifting)
– Rapid moisture transport can provide additional instability for
thunderstorm development.
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 (generally add 2 or
3 in thousands digit)
• Isotherms (dashed contours)
– 5oC intervals with 0oC reference line
700 mb Chart
• Uses:
– Elevated tropospheric moisture advection and
profiles (elevated dry intrusions; moist tongues)
– Mid-tropospheric temperature advection and
thermal profile (thermal ridges and troughs)
– Mid-level jets
– Height changes
• Caution:
– Sometimes near surface in higher terrain
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 (add 0 in ones
digit for meters)
• Isotherms (dashed contours)
– 5oC intervals with 0oC reference line
500 mb Chart (North America)
• Uses:
– Wave pattern in the westerlies
• Identification of longwaves and shortwaves
– Height changes and wave motion
– Approximate steering level for surface synoptic
systems
– Vertical and horizontal tilt of waves
– Mid-tropospheric temperature advection and
thermal profile (warm and cold pools)
– Mid-tropospheric moisture profiles
500 mb Chart (Hemispheric)
• Contains the same contours as the 500 mb North
American analysis, except void of data plots
• Additional Uses:
– Circumpolar vortex configuration
– Planetary wave number and pattern
– Wave identification
• Internet Address (Environment Canada):
– http://weatheroffice.ec.gc.ca/data/analysis/sai_50.gif
300 mb Analysis
• Isoheights (solid contours)
– 120 m intervals with 9000 m (900 decameter)
reference line
– contour labels in decameters
– plotted heights are in decameters (add 0 in ones
digit for meters)
• Isotherms (dashed heavy contours)
– 5oC intervals with -50oC reference line
300 mb Analysis
• Isotachs (light dashed contours)
– 20 knot intervals with a 10 knot reference line
– Stippled regions represent:
• 70-110 knot winds
• 150-190 knot winds
300 mb Analysis
• Uses:
– Polar jet stream
location/configuration/intensity
• The 4-quadrant jet/divergence relationship
– Upper-tropospheric wave pattern
– Regions of Difluence and Confluence
(convection/severe weather)
– Regions of upper-tropospheric vertical shear
(tropical cyclones)
250 mb Analysis
• Isoheights (solid contours)
– 120 m intervals with 10,000 m (1000
decameter) reference line
– contour labels in decameters
– plotted heights are in decameters (add 0 in ones
digit for meters, and possibly a 1 for the tenthousands digit [if first plotted number is a 0])
• Isotherms (dashed heavy contours)
– 5oC intervals with -50oC reference line
250 mb Analysis
• Isotachs (light dashed contours)
– 20 knot intervals with a 10 knot reference line
– Stippled regions represent:
• 70-110 knot winds
• 150-190 knot winds
250 mb Analysis
• Uses:
– Sub-tropical jet stream
location/configuration/intensity
– the 4-quadrant jet/divergence relationship
– Upper-tropospheric wave pattern
– Regions of Difluence and Confluence
(convection/severe weather)
– Regions of upper-tropospheric vertical shear
(tropical cyclones)
– Tropopause folds and breaks