National Weather Service United States Department of Commerce

Wintry Mix in the Northeast; Critical Fire Weather in Southern California

A large storm will bring snow and rain over parts of the Midwest into the Northeast. Heavy lake effect snow is expected down wind from Lakes Erie and Ontario and into New England. Gusty offshore winds will drive several days of critical fire weather in southern California. Above-average to record-breaking temperatures are expected across much of Southern California into Nevada and Arizona Read More >

Drawing of airplane flying through clouds


A Pilot's Guide to Aviation Weather Services





This brochure is designed to help you use the NATIONAL AVIATION WEATHER SYSTEM to the fullest extent possible. The information and services described here are available from the National Weather Service (NWS), the Federal Aviation Administration (FAA), and information service companies.

For More Information


For more detailed weather information consult AVIATION WEATHER (FAA Advisory circular 00-6A); AVIATION WEATHER SERVICES (FAA Advisory Circular 00-45); and the AIRMAN'S INFORMATION MANUAL, (Chapter 7, Safety of Flight). These publications are available at local government bookstores and from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.



The following is a list of abbreviations and acronyms used in this brochure:


A/FD Airport/Facility Directory
AC Advisory Circular
AGL Above Ground Level
ASOS Automated Surface Observing System
ATIS Automatic Terminal Information Service
AWOS Automated Weather Observing System
AWW Alert Weather Watch
CWA Center Weather Advisory
CWSU Center Weather Service Unit
DVFR Defense VFR (Flight Plan)
EFAS Enroute Flight Advisory Service
FA Area Forecast
FAA Federal Aviation Administration
FD Wind and Temperature Aloft Forecast
FSS Flight Service Station
FT Terminal Forecast
HIWAS Hazardous In-flight Weather Advisory Service
IFR Instrument Flight Rules
LIFR Low IFR Weather Conditions
MSL Mean Sea Level
MVFR Marginal VFR Weather Conditions
NOAA National Oceanic Atmospheric Administration
NOTAM Notice to Airmen
NWS National Weather Service
PATWAS Pilot's Automatic Telephone Weather Answering Service
PBS Public Broadcasting Service
TIBS Telephone Information Briefing Service
TWEB Transcribed Weather Enroute Broadcast
VFR Visual Flight Rules




Preparation for your weather briefing:

A.M. WEATHER is a 15-minute television weather program designed for pilots and can be seen on more than 300 PBS stations Monday through Friday mornings. Check your local TV listings for exact time and station.

Meteorological and aeronautical information is provided by continuous recorded Transcribed Weather Broadcasts (TWEB), the Pilot's Automatic Telephone Answering Service (PATWAS), and the Telephone Information Briefing Service (TIBS). Complete weather information is available by telephone call or visit to the nearest FAA Flight Service Station (FSS) or designated NOAA Weather Service Office. Information is also available from private commercial vendors. During periods of marginal weather, briefers are busy and telephone delays may occur. While waiting for the briefer, you may get basic information from TWEBs, PATWAS, or TIBS - - but do continue to wait for the briefer. The latest hourly aviation weather observations from distant stations are normally available by 5 minutes past the hour.

Pilots may obtain Federal pre-flight weather briefings tailored to your individual needs. Any one of three types of briefings may be requested: standard, abbreviated or outlook.

A standard briefing should normally be requested even when you have received prerecorded or mass media weather information (e.g., TWEB, A.M. WEATHER, etc.). After giving the briefer the necessary background information, you will automatically receive the following:

  • adverse conditions
  • whether VFR flight is not recommended*
  • synopsis of prevailing weather systems
  • current conditions
  • en route forecast
  • destination forecast
  • winds aloft
  • Notices to Airmen (NOTAMs)*
  • any expected delays*
  • any additional information the pilot requested.

* Not provided by NWS briefers

An abbreviated briefing should be requested if you have used prerecorded or mass media weather information to make a go/no-go decision and only selected additional information is required. You should provide the pertinent background information, tell the briefer what previous information source you have used, and ask for an abbreviated briefing with specified observation or forecast products.

An outlook briefing should be requested for long-range flight planning. This briefing contains forecasts for a flight beginning more than 6 hours in the future. An abbreviated or standard briefing should then be obtained when closer to the time of departure.

For your pre-flight weather briefing, give the briefer the following background information:

Type of flight VFR, IFR or DVFR

  1. Aircraft identification or pilot's name
  2. Aircraft type
  3. Departure point
  4. Route-of-flight
  5. Destination
  6. Altitude(s)
  7. Estimated time of departure
  8. Estimated time en route or time of arrival

After receiving weather information, either for short or long-range flights, consider carefully if conditions are suitable for your intended flight.


During marginal VFR or IFR conditions, keep a particularly close check on en route, terminal and alternate airport weather. Routine weather information is available by radio from any FAA FSS. Selected FSSs broadcast In-flight Advisories (SIGMETs, Convective SIGMETs, Center Weather Advisories [CWAs], Alert Weather Watches [AWW] and AIRMETs) and severe weather information. TWEBs also can be received while airborne. Pilots should monitor Hazardous In-flight Weather Advisory Service (HIWAS) weather broadcasts routinely. See the Airport/Facility Directory (A/FD) and aeronautical charts for frequencies. Do not hesitate to request specific information from the En Route Flight Advisory Service (EFAS) - "Flight Watch" - on 122.0 MHZ below 18,000 feet MSL. See the A/FD for high altitude frequencies. In-flight briefing procedures are explained in detail in the Airman's Information Manual.

Before Landing

Prior to descent, request current weather for the terminal area and conditions at your destination airport. At many airports, this information is continuously broadcast on ATIS, ASOS, AWOS, or selected navigation aids. These broadcasts reduce pilot and controller communications workloads.

Aviation Weather Support Products

TERMINAL FORECASTS (FT) contain information for specific airports. They are issued 3 times a day, amended as needed, and are valid for a 24-hour period. The last six hours of each period is covered by a categorical forecast indicating that VFR, MVFR, IFR, or LIFR conditions are expected. Terminal forecasts are written in the following form:
AIRPORT IDENTIFIER: 3 or 4 alphanumeric characters.
MESSAGE TYPE: RTD (Delayed), COR (Corrected), or AMD (Amended).
CEILING: Identified by the letter "C" prefix.
CLOUD HEIGHTS: In hundreds of feet above the airport (AGL).
CLOUD AMOUNT: CLR (Clear), SCT (Scattered), BKN (Broken), OVC (Overcast), or X (Obscured).
VISIBILITY: In statute miles (6+ indicates unrestricted).
WEATHER AND OBSTRUCTION TO VISIBILITY: Standard weather and obstruction to visibility symbols are used.
SURFACE WIND: In tens of degrees and knots. Omitted when less than 6 knots. Gusts indicated by a "G" followed by maximum speed.




 Category  Ceiling (feet)    Visibility (miles)
 LIFR   less than 500  and/or  less than 1
 IFR  500 to 1000  and/or  1 to 3
 MVFR  1000 thru 3000  and/or  3 thru 5
 VFR  more than 3000  and  more than 5





DCA 221010 10 SCT C18 BKN 5SW- 3415G25 OCNL C8 X ½ SW.

12Z C50 BKN 3312G22.




Decoded Example: Washington National Airport for the 22nd of the month valid from 10Z to 10Z. Scattered clouds at 1000 feet, ceiling 1800 feet broken, visibility 5 miles in light snow showers, surface wind 340 degrees at 15 knots, gusts to 25 knots. Occasional ceiling 800 feet, sky obscured, visibility one-half mile in moderate snow showers. By 12Z becoming ceiling 5000 feet broken, surface wind 330 degrees at 12 knots, gusts to 22 knots. The categorical outlook for the last 6 hours beginning at 04Z calls for marginal VFR conditions due to ceiling.

AREA FORECASTS (FA) provide an 18-hour synopsis of expected weather patterns; a 12-hour forecast of VFR cloud cover, weather and visibility; and a 6-hour categorical outlook. FAs are prepared 3 times a day (4 times a day in Alaska and Hawaii) and are supplemented and updated by SIGMETs, AIRMETs, and by FA amendments. Heights in the FA are above mean sea level (MSL) unless stated as above ground level (AGL). Ceilings (CIG) are always AGL.

WIND and TEMPERATURE ALOFT FORECASTS (FD) are 6, 12, and 24-hour forecasts of wind direction, speed, and temperatures for selected altitudes to 53,000 feet MSL at specified locations. Direction is relative to true north rounded to the nearest 10 degrees. Speed is in knots. Temperatures aloft (in degrees Celsius) are included with wind data for all but the 3000-foot MSL level and those levels within 2500 feet of the ground. Temperatures above 24,000 feet MSL are always negative. Winds at other locations and altitudes can be obtained by interpolation.




FT 3000 6000 9000 etc.
ACY 2925 2833+02 2930-03 etc.



Decoded example: For Atlantic City, N.J., at 6000 feet MSL wind from 280 degrees true at 33 knots, temperature 2 degrees Celsius.

IN-FLIGHT ADVISORIES warn pilots of potentially hazardous weather. They include SIGMETs, CONVECTIVE SIGMETs, AIRMETs, and Center Weather Advisories (CWA). SIGMETs warn of hazardous conditions of importance to all aircraft i.e. severe icing or turbulence, duststorms, sandstorms, and volcanic ash. AIRMETs warn of less severe conditions which may be hazardous to some aircraft or pilots. SIGMETs are issued as needed. AIRMET bulletins are issued routinely and supplement the Area Forecast (FA). CONVECTIVE SIGMETs are issued hourly for thunderstorms in the continuous U.S. Center Weather Advisories, issued as needed, are detailed advisories of conditions which meet or approach SIGMET or AIRMET criteria.







Decoded Example: SIGMET OSCAR 2 is valid until 2100Z on the 5th day of the month. For Kansas and Nebraska from Pawnee City VORTAC to Oswego VORTAC to Liberal VORTAC to Pawnee City VORTAC. Severe turbulence below 6000 feet expected due to strong northwesterly flow behind a cold front. Conditions continuing beyond 2100Z.

TRANSCRIBED WEATHER BROADCASTs (TWEB) are continuous broadcasts of recorded NOTAM and weather information prepared for a 50-nautical mile wide zone along a route and for selected terminal areas. TWEBs are broadcast over selected NDB and VOR facilities and generally contain a weather synopsis, in-flight advisories, route and/or local vicinity forecasts. Winds Aloft Forecasts, current weather reports, NOTAMs, and special notices. TWEB outlets are listed below by state:


ACE 277 kHz PEE 305 kHz LQ 382 kHz
AIX 323 kHz RWO 394 kHz TUK 194 kHz
AMF 403 kHz SHH 265 kHz Michigan
ANC 114.3 MHz SIT 344 kHz DL 379 kHz
ANI 359 kHz SMA 230 kHz FCM 111.8 MHz
AQH 114.7 MHz SQM 529 kHz IN 353 kHz
BCC 212 kHz SSR 114.0 MHz MS 266 kHz
BET 114.1 MHz TAL 116.6 MHz Nevada
BGQ 112.5 MHz TKA 116.2 MHz BAM 112.2 MHz
BIG 114.9 MHz UMM 326 kHz ELY 110.6 MHz
BKA 116.0 MHz YAK 113.3 MHz HZN 114.1 MHz
BTT 116.0 MHz Arizona ILC 116.3 MHz
CMQ 338 kHz ENZ 394 kHz LWL 114.2 MHz
CUN 257 kHz RYN 338 kHz SDO 114.3 MHz
DJN 347 kHz California TPH 117.2 MHz
EAV 391 kHz ACV 110.2 MHz New Mexico
ENM 117.8 MHz CEC 109.0 MHz ILT 247 kHz
ENQ 117.6 MHz FIM 112.5 MHz RO 305 kHz
FAI 108.2 MHz FJS 109.6 MHz Ohio
FDV 529 kHz FOT 114.0 MHz CL 344 kHz
FTO 242 kHz GLJ 111.0 MHz Oregon
FYU 114.4 MHz MOG 382 kHz AST 114.0 MHz
GAM 369 kHz PBT 338 kHz CVO 115.4 MHz
GKN 115.6 MHz POM 110.4 kHz EUG 112.9 MHz
HHM 356 kHz SXC 111.4 MHz PDT 114.7 MHz
HNS 245 kHz VNY 113.1 MHz UBG 117.4 MHz
HOM 114.6 MHz Idaho   Texas
HPB 115.2 MHz MQG 108.2 MHz EL 242 kHz
IIK 115.9 MHz Illinois FT 365 kHz
IWW 379 kHz DPA 108.4 MHz GLS 206 kHz
LVD 116.5 MHz ME 350 kHz Washington
MND 332 kHz Iowa ALW 111.8 MHz
OCC 305 kHz DV 353 kHz PSC 108.4 MHz
ODK 117.1 MHz Louisiana PUW 109.0 MHz
OME 115.0 MHz GNI 236 kHz Wisconsin
OSE 251 kHz Maine GM 242 kHz
OTZ 115.7 MHz LNT 344 kHz



VOR facilities (108.00 - 117.95 MHZ) have line-of-sight range.

NDB (L/MF) facilities (190 - 535 kHz) have varying ranges.



PILOT'S AUTOMATIC TELEPHONE WEATHER ANSWERING SERVICE (PATWAS), and the TELEPHONE INFORMATION BRIEFING SERVICE (TIBS), provide continuous recordings of weather and aeronautical information. The information may include area and/or route briefings, airspace procedures, and special announcements. Telephone numbers for PATWAS and TIBS locations are found in the Airport/Facility Directory.

TWEBs, PATWAS, and TIBS are for preflight or inflight planning and should not be considered a substitute for formal preflight briefings.


Used in National Weather Service Aviation Products

ALF aloft NM nautical mile(s)
AMS air mass NMRS numerous
ACFT aircraft MRTM maritime
ACTV active MSL mean sea level
AGL above ground level MXD mixed
ARND around NOTAM notice to airmen
ASL above sea level OBSC obscure
BKN broken OCLN occlusion
BLZD blizzard OCNL occasional, occasionally
BRF brief OCR occur
BTWN between OTLK outlook
BTR better OTRW otherwise
BYD beyond OVC overcast
CAT clear air turbulence PCPN precipitation
CAVOK ceiling and visibility OK PIREP pilot report
CHG change PRST persist
CIG ceiling PBL probable
CONT continue PSBL possible
CSDRBL considerable PSG passing/passage
CVR cover PTLY partly
DCR decrease PVL prevail
DMSH diminish RA rain
DNS dense RAREP radar weather report
DRZL drizzle RESTR restrict
DSIPT dissipate RDG ridge
DVLP develop RGD ragged
EMBDD embedded RMN remain
EXTRM extreme RPD rapid
EXTSV extensive RTE route
FCST forecast RUF rough
FLRY flurry SCT scattered
FQT frequent SCTR sector
FROPA frontal passage SHFT shift
FROSFC frontal surface SHWR shower
GNDFG ground fog SKC sky clear
GRDL gradual SLGT slight
HGT height SMK smoke
HLSTO hailstones SNW snow
HND hundred SQLN squall line
HURCN hurricane STBL stable
HVY heavy STG strong
ICGIC icing in clouds SVR severe
ICGIP icing in preparation THN thin
IMDT immediate TSHWR thundershower
INCR increase TSTM thunderstorm
INDEF indefinite TURBC turbulence
INTSFY intensify TWD toward
ISOLD isolated UPSLP up slope
JTSTR jet stream VRBL variable
KT knot(s) VSBY visibility
LGT light WDLY widely
LMT limit WEA weather
LVL level WK weak
LWR lower WV wave
LYR layer WX weather


Conversion tables for temperature, time, wind speed, and pressure

Diagram of U.S. Standard Armosphere - Note the uniform change of temperature with height.  At 18,000 feet, air pressure is approximately half the sea-level pressure.

Diagram showing that the uncorrected altimeter reads too high when the air is colder, and too low when the air is warmer than the U.S. Standard Atmosphere.

Icing/Turbulence Reports

REPORTS INCLUDE: Aircraft Identification
  Coordinated Universal Time (UTC) or Z
  Intensity and Type of Icing or Turbulence
  Altitude/Flight Level
  Aircraft Type
  Indicated Air Speed

Duration of Icing or Turbulence
REPORTING DURATION: Occasional - Less than 1/3 of the time.
  Intermittent - 1/3 to 2/3.
  Continuous - More than 2/3.

Reporting Airframe Icing



Trace Ice becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. It is not hazardous even though deicing/anti-icing equipment is not utilized, unless encountered for an extended period of time (over 1 hour).
Light The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used.
Moderate The rate of accumulation is such that even sort encounters become potentially hazardous and use of deicing/anti-icing or diversion is necessary.
Severe The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.

Diagram showing that ice forms when the temperature is below freezing and there is visible moisture.

Icing diagram - Probable icing conditions in these two examples of cold fronts are dissimilar because of different air mass temperatures.

Diagram of freezing rain under a warn front.


Rime Ice: Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.
Clear Ice: A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets.


From 50 miles south of Albuquerque to 30 miles north of Phoenix, 1210Z to 1250Z, occasional Moderate Rime Ice, 10,000 feet, PA34.




Light Turbulence that momentarily causes slight erratic changes in altitude and/or attitude (pitch, roll, yaw). Report as Light Turbulence.* OR Turbulence that causes slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as Light Chop. Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service many be conducted and little or no difficulty is encountered when walking.
Moderate Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed. Report as Moderate Turbulence.* OR Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as Moderate Chop. Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
Severe Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Report as Severe Turbulence.* Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking are impossible.
Extreme Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage. Report as Extreme Turbulence.*  
*High level turbulence (normally above 15,000 feet MSL) not associated with cumuliform cloudiness, including thunderstorms, should be reported as CAT (Clear Air Turbulence) preceded by the appropriate intensity, or light or moderate chop.


NOTE: Pilots should report location(s), time (UTC or Z), intensity, whether in or near clouds, altitude, type of aircraft and when applicable, duration of turbulence. Duration may be based on time between two locations or over a single location. All locations should be readily identifiable.


Over Omaha, 1232Z, Moderate Turbulence, in cloud, Flight Level 310, B767.