Ground School Part II Chapter 7

Ground School Part II Chapter 7
Interpreting Weather Data

Overview Section A: The Forecasting Process
Section B: Printed Reports and Forecasts Section C: Graphic Weather Products Section D: Sources of Weather Information

The Forecasting Process
Chapter 7A The Forecasting Process

Forecasting Methods Persistence Forecast
Persistence method involves predicting that the weather you are experiencing will continue to prevail Trend Forecast – assumption that weather that is moving in one direction will continue to move in that direction in the absence of outside force Example, if storm moving 30 mph to the east and it is 60 miles to the west, can predict it will hit in about 2 hours Analogue Forecast Combining historical data with other prevailing weather information can refine a forecast Compares the features of current weather chart w/ those of a historical chart can be used to predict future weather Not good for actual weather, but can help predict min and max temperature for a given day of the year

Forecasting Methods Meteorological Forecast – generally more accurate than other methods because it uses the forecaster’s scientific knowledge of atmospheric processes Can produce accurate 6-12 hour forecast by analyzing weather charts and data Numerical Weather Prediction – uses mathematical equations which relate atmospheric conditions with other variables System of equations is numeric prediction model; represents physical laws which govern behavior of atmosphere Computer applies model to predict weather for next 5-10 minutes; data re-fed over and over to predict next day or two; very good

Compiling and Processing Weather Data

Forecasting Accuracy and Limitations

Printed Reports and Forecasts
Chapter 7B Printed Reports and Forecasts

Printed Weather Reports
METAR (Aviation Routine Weather Report) Observation of surface weather in a standard report format

Type of Report METAR KTPA… - Two types, METAR, taken every hour, and SPECI; when significant change occurs in one or more elements Station Identifier METAR KTPA Z… - Each reporting station has its own four-letter ICAO ID; in contiguous US, K is the first letter; Alaska is PA, Hawaii is PH, Canada are CU, CW, CY, and CZ

Type of Report METAR KTPA Z 08020G38KT… - The date (day of the month) and time of the observation follows the station ID; time is given in UTC, or Zulu time. The report here was given on the 12th of the month at 2150 hours Zulu Modifier – when a METAR is created by a totally automated weather observation station, the modifier AUTO follows the date/time (e.g., …251955Z AUTO 30008KT…) - These stations are classified by the type of sensor equipment used to make observations, AO1 or AO2 will be noted in remarks section of report - AO1 means no precipitation discriminator - AO2 precipitation discriminator available - COR used to indicate a METAR has been corrected from previously disseminated report; if COR is used, AO1 or AO2 is removed if it would have been included normally

Wind Information …122150Z 08020G38KT 1/2SM R36L/2400FT… Read winds from 080 degrees at 20 knots gusting to 38 knots; calm is reported at 00000KT If wind direction varies 60º or more and speed > 6 KT, a variable group follows the wind group * Winds are reported in reference to true north

Visibility …122150Z 08020G38KT 1/2SM R36L/2400FT… Prevailing visibility greatest distance observer can see and id objects through at least ½ the horizon When prevailing vis varies from one area of sky to another, vis in majority of sky reported Runway visual range (RVR) may be reported following prevailing vis; RVR is based on what pilot should see when looking down runway in feet RVR above is runway 36 Left (R36L) 2400 feet visibility Prevailing visibility reported in statute miles (SM)

…1/2SM R36L/2400FT +TSRA SCT008 OVC012CB 20/18… Weather or obstructions to vision which are present at time of observation reported immediately after visibility in this order: - Intensity or proximity (-) is light (+) is heavy - When vicinity of airport, VC used (5-10 SM of airport) - Descriptor of precipitation; two-letter code TS – Thunderstorm DR – Low Drifting SH – Shower(s) MI – Shallow FZ – Freezing BC – Patches BL – Blowing PR – Partial - Precipitation RA – Rain GR – Hail (>1/4”) DZ – Drizzle GS – Small Hail / Snow Pellets SN – Snow SG – Snow Grains IC – Ice Crystals PL – Ice pellets UP – Unknown Precipitation

- Obscuration to visibility FG – Fog PY – Spray BR – Mist SA – Sand FU – Smoke DU – Dust HZ – Haze VA – Volcanic Ash - Other weather phenomena SQ – Squall SS – Sandstorm DS – Duststorm PO – Dust/Sand Whirls FC – Funnel Cloud +FC – Tornado or Waterspout

Sky condition – describe amount of clouds, if any, heights, and in some cases, their types; vertical visibility may be reported if height of clouds cannot be determined Amount …+TSRA SCT008 OVC012CB 20/18… Amount of clouds in sky reported in eighths; or octas, of sky cover If sky clear, SKC reported in manual report or CLR in automated report; auto station cannot report clouds over 12,000’, so CLR means none below 12,000’ Few (FEW) = 0 to 2/8 of sky covered Scattered (SCT) = 3/8 to 4/8 Broken (BKN) = 5/8 to 7/8 Overcast (OVC) = > 7/8

Height, Type, and Vertical Visibility …+TSRA SCT008 OVC012CB 20/18… Height of clouds or vertical visibility reported with three digits in hundreds of feet above ground level (AGL) To determine cloud height, add two zeros to end of number in report; in above example, there is a scattered layer at 800 feet and overcast at 1200’ AGL Manual report may include cloud types if towering cumulus (TCU) or cumulonimbus (CB) present When more than ½ sky covered, a ceiling exists; a ceiling is the AGL height of lowest layer of clouds that is reported as BKN or OVC, or vertical visibility into an obscuration, such as fog or haze In controlled airspace, VFR conditions exist with a visibility of at least 3 SM and ceiling of 1000 feet or more Unlike clouds, obscuration does not have a definite base; may extend from surface for undeterminable height; vertical visibility used VV in hundreds of feet; for example, VV006 describes an indefinite ceiling at 600’ AGL

Temperature and Dewpoint …SCT008 OVC012CB 20/18 A2995… Temperature and dewpoint given in that order in Celsius; temp = 20, dewpoint 18; when prefixed with an M, means minus Altimeter Reported in inches of mercury in a 4 digit group prefaced by an A; in example, altimeter setting is 29.95” Hg.

Remarks …A2995 RMK TSB24RAB24 SLP134 Beginning of remarks portion annotated with RMK; remarks are made when weather may be significant to aircraft operations Beginning of an event annotated by B, followed by the time in minutes after the hour If event ended before report made, ending time indicated by E and minutes past hour In example, thunderstorm began at 24 past the hour and rain begin at 24 after hour Sea Level Pressure (SLP) is hectoPascals (hPa); add 10 to the front of the numbers given

Radar Weather Reports General areas of precip, especially thunderstorms, observed by radar on a routine basis; most radar stations issue routine radar weather reports (SDs) 35 minutes past each hour; with intervening reports as necessary Defines type, intensity, and location of echo top of the precip; also include movement (direction and speed) of cells; if base considered significant, may also be reported All heights in an SD are reported in hundreds of feet MSL

Pilot Weather Reports (PIREPs) Often confirm information such as height of bases and tops of cloud layers, in-flight visibility, icing conditions, wind shear, and turbulence When significant conditions reported in forecast, ATC required to solicit PIREPs Any time you encounter unexpected weather conditions, should give PIREP

Pilot Weather Reports (PIREPs) Standard Format

Printed Weather Forecasts
Terminal Area Forecast (TAF) Usually valid for 24 hour period scheduled 4 times a day; TAF codes similar to those used in METARs Times normally issued: 0000Z, 0600Z, 1200Z, 1800Z Elements: Type Location Issuance Date and Time Valid Date and Time Forecast

Type of Forecast TAF AMD KLFK… TAF normally routine forecast; amendments are given with AMD when current TAF no longer represents expected weather COR is corrected and RTD is delayed

Station Identifier and Issuance Date/Time TAF AMD KLFK Z … Four letter ICAO location ID code First two numbers are day of month, then time in Zulu Example 22nd day of month, 1555 Zulu hours

Valid Period …221555Z KT… Normally, forecast valid for 24 hours; first 2 digits represent valid date, next is beginning hour of valid time in Zulu; and last two digits are ending hour This example, on 22nd day of month, valid beginning at 1600 and ending at 1200 of next day If airport open part time, amendments not issued after closing time If airport closes at 0500Z, TAF may include statement such as NIL, AMD NOT SKED AFT 0500Z; when part time airports closed, word NIL may appear

… KT P6SM OVC015 TEMPO SM –TSRA BKN010CB… Body of TAF has codes for forecast wind, vis, weather and sky condition similar to METAR Weather, including obstructions to vis, is added to forecast only when it is significant to aviation In TAF, expected vis > 6 SM are forecast using a P (plus) In example above, winds are 060 degrees at 6 knots, visibility greater than 6 SM, OVC layer at 1500’ AGL In TAF, predicated low level wind shear not associated with convective activity may be included using code WS, followed by height (up to and including 2000’) a slant (/) and winds at height indicated; for example WS015/30045KT indicated low-level wind shear expected at 1500’ with winds from 300 degrees at 45 kts

Forecast Change Groups …FM KT P6SM SCT012 BKN020… …FM KT P6SM OVC015… …FM0400 VRB05KT 3SM BR SCT010 OVC020 BECMG 0610 P6SM NSW When significant permanent change to weather conditions expected during valid time, change group is used; from FM time of change, conditions listed after FM exist until next change group time A more gradual change in weather, taking about 2 hours, coded as BECMG, followed by beginning and ending times of change period

Temporary Conditions …KLFK Z KT P6SM OVC015 TEMPO SM –TSRA BKN010CB FM KT P6SM SCT012 BKN020 TEMPO 1923 VRB08G18KT 3SM TSRA OVC010CB… Wind, visibility, weather or sky conditions that are expected to last less than an hour are described in a temporary group (TEMPO), followed by beginning and ending time The first group in example predicts that between 1600 and 1900, visibility is expected to be reduced to 4 SM in light rain associated with a thunderstorm

Probability Forecast …FM KT P6SM OVC015 PROB SM TSRA OVC010CB FM0400 VRB05KT 3SM BR SCT010 OVC020… TAFs may include the probability of thunderstorms or precip events with the associated wind, vis, and sky conditions. A PROB group is used when probability of occurrence is between 30 and 49% The percentage is followed by the beginning and end of time precip is expected In example, there is a 30% chance of thunderstorm with moderate rain and 2 SM vis between 0000Z and 0400Z

Aviation Area Forecast (FA) covers general weather conditions over a wide region and is a good source of enroute weather information Also helps determine weather at airports that do not have TAFs FAs issued 3 times a day in 48 contiguous states, amended as required FA has several sections Communications and product header Precautionary statement section Two weather sections: synopsis and VFR clouds and weather section

Communication and Product Headers SLCC FA SYNOPSIS AND CLDS/WX SYNOPSIS VALID UNTIL CLDS/WX VALID UNTIL …OUTLK VALID ID MT NV UT WY CO AZ NM SLC – Salt Lake City C – product contains clouds and weather forecast FA – means area forecast – forecast issued on 14th day on month at 1045Z * Since these forecasts are rounded to nearest whole hour, valid time for report is 1100Z Synopsis valid until the 15th at 0500 Clouds and weather valid until 14th at 2300, outlook valid from 14th 2300 until 15th at 0500 Last line indicates states included in Salt Lake City forecast area

Precautionary Statements SEE AIRMET SIERRA FOR IFR CONDS AND MTN OBSCN. TSTMS IMPLY PSBL SVR OR GTR TURBC SVR ICG LLWS AND IFR CONDS. NON MSL HGTS ARE DENOTED BY AGL OR CIG. First statement alerts you to check AIRMET SIERRA, which describes areas of mountain obscuration Thunderstorms imply possible severe or greater turbulence severe icing low-level wind shear and IFR conditions Last statement indicates that NON-MSL heights are denoted by AGL or CIG (ceiling)

Synopsis – a brief description of location and movement of fronts, pressure systems, and circulation patterns in the FA area over an 18 hour period When appropriate, forecasters may use terms describing ceilings and visibilities, strong winds, and other phenomena VFR Clouds and Weather Usually long, broken down by states or geographical regions Describes clouds and weather which could affect VFR ops over an area 3,000 square miles or more Valid for 12 hours, and is followed by 6 hour categorical outlook (18 hours in AK) When vis to be <= 6 SM, vis and obstructions indicated in forecast When precip, thunderstorms, and sustained winds of 20 kts or greater included Term OCNL (occasional) used when there is a 50% chance or greater probability of cloud of vis conditions which may affect VFR fligh Percentage of area covered by showers or thunderstorms indicated by: ISOL (isolated, meaning single cells) WDLY SCT (widely scattered, less then 25% of area) SCT or AREAS (25% to 54%) NMRS or WDSPRD (numerous or widespread, 55% + of area) ISOL may also indicate areas of ceilings or vis < 3,000 square miles

Winds and Temperatures Aloft Forecast (FD) – provide an estimate of wind direction in relation to true north, wind speed in knots and temperature in degrees Celsius for selected altitudes Usually given for nine levels from 3,000’ to 39,000’ FD does not include levels within 1500’ of station elevation Temperatures are not forecast for the 3,000’ level or for a level within 2,500’ of the station elevation First two numbers indicated true direction from which wind is blowing Example: Winds from 260 degrees at 35 knots, temp is negative 8 degrees C Often need to interpolate between two altitudes and/or two locations All temps above 24,000’ are negative To decode winds between 100 and 199 knots, subtract 50 from the two-digit direction code and multiply by 10, then add 100 to the two-digit wind speed code. If the code 9900 is used, means winds are light and variable (less than 5 knots) Example: ( = 77 for wind direction, and 101 – 100 = 01 for wind speed) winds are out of 270 degrees at 101 knots

Severe Weather Reports and Forecasts
Hurricane Advisory (WH) Issued when a hurricane is at least 300 nm offshore, but threatens coastline Gives storm center, expected movement, and maximum winds in and near storm center Convective Outlook (AC) National forecast of thunderstorm activity covering two, 24-hour periods: Day 1 and Day 2 ACs describe areas in which there is a slight, moderate, or high risk of severe thunderstorms, as well as general thunderstorm activity Severe thunderstorm criteria include surface winds of 50 kts or greater, hail 3/4 “ in diameter or greater, and tornadoes Severe Weather Watch Bulletin (WW) Defines areas of possible severe thunderstorms or tornadoes WWs issued on an unscheduled basis and are updated as required Published to general public as well To alert forecasters and weather briefers that a severe weather watch bulletin is being issued, a preliminary message, called an alert severe weather watch (AWW) is sent

AIRMET (Airmen’s Meteorological Information) (WA) – issued every 6 hours with amendments issued, as necessary, for weather phenomena which are of operational interest to all aircraft These weather conditions are potentially hazardous to light aircraft or aircraft having limited capability AIRMETs issued for: Moderate icing Moderate turbulence Sustained winds of 30 knots or more at surface Ceiling less then 1000’ and or vis less than 3 miles affecting over 50% of an area Extensive mountain obscurement AIRMET identifiers: Sierra for IFR conditions and mountain obscuration Tango for turbulence, strong surface winds and low-level wind shear Zulu for icing and freezing levels After first issuance of the day, AIRMETs are numbered sequentially

SIGMET (WS) – Significant Meteorological Information Criteria Severe icing Severe or extreme turbulence Clear air turbulence (CAT) Duststorms and sandstorms lowering vis to less than 3 miles Volcanic ash SIGMETs unscheduled forecasts that are valid for 4 hours, but if relates to hurricanes, valid for 6 hour SIGMETs use consecutive alphanumeric designators November through Yankee

Convective SIGMET (WST) issued for hazardous convective weather which is significant to safety of all aircraft Always imply severe or greater turbulence, severe icing, low-level wind shear, these items not specified in the advisory WSTs include the following: Tornadoes Lines of thunderstorms Thunderstorms over a wide area Embedded thunderstorms Hail greater than or equal to ¾” in diameter and/or wind gusts to 50 kts or greater WSTs issued for eastern (E), central (C), and western (W) US SIGMETs numbered sequentially for each area (01-99) each day Bulletins issued 55 min past each hour unless special update bulletin required SIGMET valid for 2 hours from the time of issuance or until superceded by next hourly issuance If criteria not met for a convective SIGMET at time of issuance, the message CONVECTIVE SIGMET…NONE is sent

Graphic Weather Products
Chapter 7C Graphic Weather Products

Graphic Reports Surface Analysis Chart
Also called Surface Weather Analysis Chart Shows weather conditions as of the valid time shown on the chart Picture of atmospheric pressure patterns at the earth’s surface Provides surface weather observations for large number of reporting points Each circle represents a reporting station

Graphic Reports TYPE DEFINITION MAP SYMBOL
COLD FRONTS A front where cold air A blue line with blue barbs replaces warm air pointing in the direction of the cold air flow. WARM FRONTS A front where warm air A red line with red half moons replaces cold air pointing in the direction of warm air flow. STATIONARY A front with little A line with alternating red warm FRONTS lateral movement front symbols and blue cold front symbols, pointing in opposite directions to symbolize little frontal movement. OCCLUDED A front where the cold A front with purple (combined red FRONTS front has overtaken and and blue) half moons and barbs on merged with the warm the same side, pointing toward the front direction of frontal motion.

Graphic Reports

Graphic Reports BINOVC Breaks in Overcast Missing or
Partial Obscuration

Graphic Reports Weather Depiction Chart
Provides an overview of favorable and adverse conditions for the chart time For flight planning, the weather depiction chart is most useful for determining general weather conditions and quickly locating areas of adverse weather Plotted information derived from aviation routine weather reports (METARs) Transmitted every 3 hours Valid for time of plotted data Unlike Surface Analysis, pressure patterns and winds not depicted Simplified station models provided; depicted by circles A station circle on a weather depiction chart shows visibility, cloud height or ceiling, and weather and/or obstruction to vision

Graphic Reports

Graphic Reports Radar Summary Chart
Depict location of precipitation along with the type, intensity, and cell movement Produced 35 minutes past each hour Graphically depicts collection of Radar Weather Reports (SDs) Signals sent out by transmitting pulses of radar energy in a specific direction with a rotating antenna; when signals encounter precipitation, they are reflected back to the antenna; reflected signals, also called echoes, are presented on radar screen showing strength and location of the radar return Depicts location, size, shape, and intensity of returns as well as direction of movement; also shows lines and cells of hazardous thunderstorms as well as echo heights of the tops and bases of precipitation areas

Graphic Reports

Graphic Reports Radar Summary Chart
Height of precipitation (not cloud) tops and bases are displayed in hundreds of feet MSL. Tops are printed above a short line while any available bases are shown below the line Absence of a base figure indicate echo base at or near ground; not unusual since precip usually reaches ground Top height is highest in the indicated area A rectangular dashed-line box denotes a severe weather watch area Letters WS indicate severe thunderstorm watch WT indicates tornado watch Numbers tell which severe weather watch issued in that year If no report from a radar site, depicted as NA If no echoes detected by radar site, depicted as NE A heavy dark line is used to indicate echoes reported as a line. If there is at least 8/10 coverage, a line of echoes is labeled solid (SLD) at both ends Individual cell movement is shown by an arrow with the speed in knots entered as a number at the top of the arrow head. Little movement is indicated by the letters LM Intensity is shown on the chart by contours. The six levels displayed on a radar display are combined into three contours. The first contour represents levels 1 and 2 (light to moderate precip), second contour represents levels 3 and 4 (heavy to very heavy precip), and the third levels 5 and 6 (intense to extreme precip) Precipitation abbreviations are used to denote rain, snow and thunderstorms, but not same as in METAR (R, RW, TRW)

Graphic Reports Satellite Weather Pictures
Visible – determine presence of cloud, as well as cloud shape and texture Infrared (IR) – depicts heat radiation emitted by the various cloud tops and earth’s surface; difference between cloud temps can be used to determine cloud height Visible Satellite Infrared Satellite

Graphic Forecasts US Low-Level Significant Weather Prog Chart
Valid from the surface up to 400 millibar pressure level (24,000 feet) Designed to help plan flights to avoid areas of low visibility and ceilings as well as regions where turbulence and icing may exist Issued 4 times a day with valid time printed in lower margin of each panel 2 left panels forecast weather for 12 hours from issue time, 2 right panels forecast weather 24 hours ahead Significant weather panels Upper panels show areas of nonconvective turbulence and freezing levels as well as IFR and MVFR Areas of forecast nonconvective moderate or greater turbulence are shown using a heavy dahsed contour line; numbers in these areas give the height of the turbulence in hundreds of feet MSL Figures below the line show the expected base; above line represent top of turbulence Since thunderstorms always imply moderate + turbulence, areas of possible thunderstorm turbulence are not outlined Forecast freezing levels are depicted at 4,000’ intervals with dashed lines; labeled in hundreds of feet MSL A zig-zag line labeled SFC shows the surface location of the freezing level

Graphic Forecasts Four panels; left panels issued at 00Z, right panels issued at 12Z; the two Lower panels are 12 and 24 hour forecasts of surface weather conditions, while The upper panels are 12 and 24 hour forecasts of weather between surface and 24,000 feet

Graphic Forecasts US Low-Level Significant Weather Prog Chart
Surface Prog Panels Two lower panels are surface prog panels Contain standard symbols for fronts and pressure centers Areas of forecast precip, as well as thunderstorms, are outlined

Graphic Forecasts Convective Outlook Chart
48 hour forecast for thunderstorm activity Day 1 panel depicts outlook for general thunderstorm activity and severe thunderstorms for 24 hour period from 1200Z to 1200Z following day; issued 5 times a day Day 2 panel depicts forecast for second day beginning at 1200Z Both panels outline areas of convective activity Line with arrowhead depicts forecast general thunderstorm activity When facing direction of arrow, thunderstorm activity is expected to right of line Areas of greater risk are outlined and associated risk is indicated inside the outlined region SLGT implies well-organized thunderstorms, in small numbers or low coverage MDT moderate HIGH high SEE TEXT – indicates a situation where a SLGT risk was considered at time of forecast, was not warranted

Graphic Forecasts

Graphic Forecasts Forecast Winds and Temperature Aloft Charts (FD)
12 hour forecasts valid at 0000Z and 1200Z daily Charts contain 8 panels, each corresponding to a forecast level: 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, 39,000 MSL Predicted winds shown using an arrow eminating from a station circle to show direction w/in 10º Second digit of the wind direction is printed at the end of the arrow to help you pinpoint the forecast direction Pennants and/or barbs at the end of the arrow depict forecast wind speed similar to a surface analysis chart When calm or light and variable winds are expected, the arrow is eliminated and a 99 entered below the station circle Predicted temperatures are shown as whole degrees Celsius near the station circle

Graphic Forecasts

Graphic Forecasts Volcanic Ash Forecast and Dispersion Chart (VAFTAD)
When a volcanic eruption is reported Developed with input from National Centers for Environmental Prediction (NCEP) using a model which focuses on hazards to aircraft flight operations with emphasis on ash cloud location Concentration of ash is forecast over 6 and 12 hour time intervals, beginning at 6 hours after eruption VAFTAD uses 4 panels in column for each valid time period Top 3 panels in a column reflect the ash location and relative concentrations for an atmospheric layer Bottom panel ini column shows total ash concentrations from surface up to 55,000 feet (FL 550) For flight planning purposes only; not intended to replace SIGMETs regarding volcanic eruptions and ash

Graphic Forecasts

Ground School Part II Chapter 7

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