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 Analysishttp://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.

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