South
Delta Area
NW NE
North
Central
SW SE
NW NE
Ozarks
BootHeel
SW Missouri
Lakes Area
Lake of the
Ozarks Area
Extreme
SRN
IL
SW SE
Central
NW NE
NW NE
SW SE
CentralIowa
SW SE
Extrm
SE
NW NE
Northeast Lakes
Region
Upper
Mississippi
Valley
Arrowhead
Lake SuperiorShoreline
WRN Upper MI
Cntrl
Upper
MI
ERN Upper MI
Door Peninsula
Mackinac Area
NRN Highlands
The
Thumb
Saginaw
Valley
Grand
Valley
Irish Hills
Near LakeMi
Extrm
N
NW NE
SW SE
Near Ohio River
SW
SE
West
NCntrl
Northern
Southern
CntrlEast
NE
West
Middle East Appalachian Mountains
SE
NENW
Along Ohio River
Extrm
NW
SWRNSouthern
W
Cntrl
E
Cntrl
N Third
S
Third
Near Lake Erie
Lee
Lake
Erie
Lower GreatLakes Basin
ExtrmNW
AlleghenyPlateau
NW
SW
S
Cntrl
NE
SE
N
Cntrl
Lee Lake Ontario
NRN
SRNNE
EasternShore
Coastal
Waters
NE
NW
SW
SE
Coastal Plain
NE
SE
Piedmont
West
Mtns
of
SC
Coastal Plain NW
MtnsHills
of
Alabama
West
Cntrl
East
Cntrl
ExtrmSE
Extrm Southern
Georgia
Mobile
Area FL W 85 Extrm
North
North
Central
Apalachee
Bay
South
Extrm
South
Upper
Keys
Lower Keys
Gulf Stream
Florida Straits
Coastal
Waters
Catskill
Mtns
E Cntrl Cape Cod
Coastal Plains
Hudson Valley
Mohawk
Valley
West of
Catskills
Adirondack
Mtns
Champlain
Valley
GreenMtns
N of Catskills White Mtns
NW NE
Coastal Section SE
SW
MN
IA
MI
IL IN
MO
AR
LA
MS AL
TN
GA
FL
SC
NC
VA
MD
DC
NJ
OH
WI
WV
PA
NY
ME
VT
NH MA
RI
CT
Coastal Waters–from
coast outward to the
flight information
region border
SW
SE
Northern
NE
Mtns
Central
Strait of Juan De Fuca
Olympic Mtns
Sacramento
Valley
Northwest
North Central
South Central
Lower
Rio
Grande
Valley
Lower
Coastal
Plain
Mid CoastalPlain
Upper CoastalPlain
Northwest
Texas
Puget
Sound
Cascade Mtns
Cascade Mtns
of Cascades
Mtns
NE WA
Columbia
BasinInterior
Valley
Columbia Gorge
Blue
Mtns
WallowaMtns
Mtns of
NE OR
Costal Range
Willamette Valley
Coast and
Costal Valley
High Plateau
East Slopes
WA
OR
ShastaSiskiyousNE CA
NRN
Sierra
Mtns
Coastal Mtns and Valleys
San Joaquin
Valley
SRN
Sierra
Mtns
Coastal Mtns and Valleys
Western
Nevada Eastern
Nevada
Coastal
Areas
Death
Valley
Lake Tahoe
Lake
Mead
Lake
Mojave
Mojave
Desert
Antelope
ValleyCoastal Range
SRN
Deserts
Santa
Barbara
Channel
Santa Monica
Bay
Coastal Plain
Gulf of
Santa Catalina
Coacnella
and
Imperial
Valleys
Great
Basin
Colorado
River
Valley
Gilariver Valley
Mogollon Rim
Chiricahua
Mtns
White
Mtns
Little
Colorado
Valley
Grand Canyon
Four
CornersArea
Extrm
SW
Uinta Basin
S Cntrl
Mtns
Great
Salt
Lake
Wasatch Mtns
West Central
Valleys
San Juan
Mtns
NW
Cntrl
Colorado
Mtns
So Platte Valley
NE
Palmer Lake Ridge
or Divide
Eastern Plains
Lower Arkansas Valley
SE
San Luis
Valley
North Park
Rio Grande Valley
Continental Divide
Sacremento
Mtns
Sangre De Cristo
Mtns
Eastern
New Mexico
Eastern
Plains
Green River
Basin So Central
Mtns
SW
WY
Laramie MtnsN Platte River Valley SE
WY
Black
Hills
NE WY
Big Horn MtnsBig Horn
Basin
Wind River Mtns
TetonMtnsSnake River Valley
Central
Mtns(Sawtooth) Yellowstone
Park
Bitterroot Range
Flathead Valley
East
Slopes of
Cont Dvd
SWRN
Mtns
Fort Peck Reservoir
Wind River
Basin
E of Cont Dvd
Pecos River
West of
Pecos
Big Bend
Area
East of
Pecos
Southwest
Panhandle
Northeast
Southeast
Coastal Bend
Southwest Southeast
Panhandle
Northeast
High Plains
Flint
Hills
Marais des Cygnes Basin
Kansas River Valley
Blue River ValleyLowerRepublican
Valley
Cheyenne River
James
River
NE Lakes
Region
Rosebud
Country
Pine
Ridge Area
Big BendReservoir
North of
Black
Hills
Lake Lewis and
Francis Case Clark
Dame
Reservoir
Missouri
Slope
SourisRiver Valley
Sand Hills
Central Nebraska
Nebraska
pan
Handle
Platte River Basin
SE
NE
CA
NV
ID
MT
ND
SD
NE
KS
OK
TX
WY
UT
CO
AZ
NM
Lake Powell
Red River Valley
Lower Mississippi Valley
CHI Chicago BOS Boston
MIA MiamiSFO
San Francisco
SLC Salt Lake City
DFW
Dallas/Fort Worth
Lake Mich Shoreline
Figure 13-8. Area forecast region map.
2000Z, the forecast calls for scattered thunderstorms with rain
developing and a few becoming severe; the CB clouds have
tops at flight level (FL) 450 or 45,000 feet MSL.
It should be noted that when information is given in the area
forecast, locations may be given by states, regions, or specific
geological features such as mountain ranges. Figure 13-8
shows an area forecast chart with six regions of forecast,
states, regional areas, and common geographical features.
Inflight Weather Advisories
Inflight weather advisories, which are provided to en route
aircraft, are forecasts that detail potentially hazardous
weather. These advisories are also available to pilots prior
to departure for flight planning purposes. An inflight
weather advisory is issued in the form of either an AIRMET,
SIGMET, or convective SIGMET.
AIRMET
AIRMETs (WAs) are examples of inflight weather advisories
that are issued every 6 hours with intermediate updates
issued as needed for a particular area forecast region. The
information contained in an AIRMET is of operational
interest to all aircraft, but the weather section concerns
phenomena considered potentially hazardous to light aircraft
and aircraft with limited operational capabilities.
An AIRMET includes forecast of moderate icing, moderate
turbulence, sustained surface winds of 30 knots or
greater, widespread areas of ceilings less than 1,000 feet
and/or visibilities less than three miles, and extensive
mountain obscurement.
Each AIRMET bulletin has a fixed alphanumeric designator,
numbered sequentially for easy identification, beginning with
the first issuance of the day. Sierra is the AIRMET code used
to denote IFR and mountain obscuration; Tango is used to
denote turbulence, strong surface winds, and low-level wind
shear; and Zulu is used to denote icing and freezing levels.
Example:
BOSS WA 211945
AIRMET SIERRA UPDT 3 FOR IFR AND MTN OBSCN
VALID UNTIL 220200
AIRMET IFT…ME NH VT MA CT RI NY NJ AND CSTL
WTRS FROM CAR TO YSJ TO 150E ACK TO EWR TO
YOW TO CAR OCNL CIG BLW 010/VIS BLW 3SM
PCPN/BR. CONDS CONT BYD 02Z THRU 08Z
AIRMET MTN OBSCN…ME NH VT MA NY PA
FROM CAR TO MLT TO CON TO SLT TO SYR TO CAR
MTNS OCNLY OBSCD BY CLDS/PCPN/BR. CONDS
CONT BYD 02Z THRU 08Z
Explanation:
AIRMET SIERRA was issued for the Boston area at 1945Z
on the 21st day of the month. SIERRA contains information
on IFR and/or mountain obscurations. This is the third
updated issuance of this Boston AIRMET series as indicated
by “SIERRA UPDT 3” and is valid until 0200Z on the
22nd. The affected states within the BOS area are: Maine,
New Hampshire, Vermont, Massachusetts, New York, and
Pennsylvania. Within an area bounded by: Caribou, ME;
to Saint Johns, New Brunswick; to 150 nautical miles east
of Nantucket, MA; to Newark, NJ; to Ottawa, Ontario;
to Caribou, ME. The effected states within Caribou, ME
to Millinocket, ME to Concord, NH to Slate Run, PA to
Syracuse, NY to Caribou, ME will experience ceilings
below 1,000 feet/visibility below 3 SM, precipitation/mist.
Conditions will continue beyond 0200Z through 0800Z.
SIGMET
SIGMETs (WSs) are inflight advisories concerning non-
convective weather that is potentially hazardous to all
aircraft. They report weather forecasts that include severe
icing not associated with thunderstorms, severe or extreme
turbulence or clear air turbulence (CAT) not associated with
thunderstorms, dust storms or sandstorms that lower surface
or inflight visibilities to below three miles, and volcanic ash.
SIGMETs are unscheduled forecasts that are valid for 4 hours
unless the SIGMET relates to a hurricane, in which case it
is valid for 6 hours.
A SIGMET is issued under an alphabetic identifier, from
November through Yankee. The first issuance of a SIGMET
is designated as an Urgent Weather SIGMET (UWS).
Reissued SIGMETs for the same weather phenomenon are
sequentially numbered until the weather phenomenon ends.
Example:
SFOR WS 100130
SIGMET ROME02 VALID UNTIL 100530
OR WA
FROM SEA TO PDT TO EUG TO SEA
OCNL SEV CAT BTN FL280 AND FL350 EXPCD
DUE TO JTSTR.
CONDS BGNG AFT 0200Z CONTG BYD 0530Z .
Explanation:
This is SIGMET Romeo 2, the second issuance for this
weather phenomenon. It is valid until the 10th day of the
month at 0530Z time. This SIGMET is for Oregon and
Washington, for a defined area from Seattle to Portland to
Eugene to Seattle. It calls for occasional severe clear air
turbulence between FL280 and FL350 due to the location of
the jet stream. These conditions will begin after 0200Z and
continue beyond the forecast scope of this SIGMET of 0530Z.
Convective Significant Meteorological Information
(WST)
A Convective SIGMET (WST) is an inflight weather
advisory issued for hazardous convective weather that affects
the safety of every flight. Convective SIGMETs are issued
for severe thunderstorms with surface winds greater than 50
knots, hail at the surface greater than or equal to ¾ inch in
diameter, or tornadoes. They are also issued to advise pilots
of embedded thunderstorms, lines of thunderstorms, or
thunderstorms with heavy or greater precipitation that affect
40 percent or more of a 3,000 square mile or greater region.
Convective SIGMETs are issued for each area of the
contiguous 48 states but not Alaska or Hawaii. Convective
SIGMETs are issued for the eastern (E), western (W), and
central (C) United States. Each report is issued at 55 minutes
past the hour, but special Convective SIGMETs can be
issued during the interim for any reason. Each forecast is
valid for 2 hours. They are numbered sequentially each day
from 1–99, beginning at 00Z time. If no hazardous weather
exists, the convective SIGMET is still issued; however, it
states “CONVECTIVE SIGMET…NONE.”
Example:
MKCC WST 221855
CONVECTIVE SIGMET 20C
VALID UNTIL 2055Z
ND SD
FROM 90W MOT-GFK-ABR-90W MOT
INTSFYG AREA SEV TS MOVG FROM 24045KT. TOPS
ABV FL450. WIND GUSTS TO 60KTS RPRTD.
TORNADOES…HAIL TO 2 IN… WIND GUSTS TO
65KTS
POSS ND PTN
Explanation:
Convective SIGMET was issued for the central portion of
the United States on the 22nd at 1855Z. This is the 20th
Convective SIGMET issued on the 22nd for the central
United States as indicated by “20C” and is valid until 2055Z.
The affected states are North and South Dakota, from 90
nautical miles west of Minot, ND; to Grand Forks, ND; to
Aberdeen, SD; to 90 nautical miles west of Minot, ND. An
intensifying area of severe thunderstorms moving from 240
degrees at 45 knots (to the northeast). Thunderstorm tops will
be above FL 450. Wind gusts up to 60 knots were reported.
FB KWBC 151640
DATA BASED ON 151200Z
VALID 151800Z FOR USE 1400-2100Z
TEMPS NEGATIVE ABV 24000
FB 3000 6000 9000 12000 18000 24000 30000
AMA 2714 2725+00 2625-04 2531-15 2542-27 265842
DEN 2321-04 2532-08 2434-19 2441-31 235347
Figure 13-9. Winds and temperature aloft forecast.
Also reported were tornadoes, hail to 2 inches in diameter, and
wind gusts to 65 knots possible in the North Dakota portion.
Winds and Temperature Aloft Forecast (FB)
Winds and temperatures aloft forecasts (FB) provide wind
and temperature forecasts for specific locations throughout
the United States, including network locations in Hawaii
and Alaska. The forecasts are made twice a day based on the
radiosonde upper air observations taken at 0000Z and 1200Z.
Altitudes through 12,000 feet are classified as true altitudes,
while altitudes 18,000 feet and above are classified as
altitudes and are termed flight levels. Wind direction is
always in reference to true north, and wind speed is given in
knots. The temperature is given in degrees Celsius. No winds
are forecast when a given level is within 1,500 feet of the
station elevation. Similarly, temperatures are not forecast for
any station within 2,500 feet of the station elevation.
If the wind speed is forecast to be greater than 99 knots but
less than 199 knots, the computer adds 50 to the direction
and subtracts 100 from the speed. To decode this type of data
group, the reverse must be accomplished. For example, when
the data appears as “731960,” subtract 50 from the 73 and
add 100 to the 19, and the wind would be 230° at 119 knots
with a temperature of –60 °C. If the wind speed is forecast to
be 200 knots or greater, the wind group is coded as 99 knots.
For example, when the data appears as “7799,” subtract 50
from 77 and add 100 to 99, and the wind is 270° at 199 knots
or greater. When the forecast wind speed is calm, or less than
5 knots, the data group is coded “9900,” which means light
and variable. [Figure 13-9]
Explanation of Figure 13-9:
The heading indicates that this FB was transmitted on the
15th of the month at 1640Z and is based on the 1200Z upper
air data. The valid time is 1800Z on the same day and should
be used for the period between 1400Z and 2100Z. The
heading also indicates that the temperatures above FL240
are negative. Therefore, the minus sign will be omitted for
all forecast temperatures above FL240.
A four-digit data group shows the wind direction in reference
to true north and the wind speed in knots. The elevation at
Amarillo, Texas (AMA) is 3,605 feet, so the lowest reportable
altitude is 6,000 feet for the forecast winds. In this case,
“2714” means the wind is forecast to be from 270° at a speed
of 14 knots.
A six-digit group includes the forecast temperature aloft.
The elevation at Denver (DEN) is 5,431 feet, so the lowest
reportable altitude is 9,000 feet for the winds and temperature
forecast. In this case, “2321-04” indicates the wind is forecast to
be from 230° at a speed of 21 knots with a temperature of –4 °C.
Weather Charts
Weather charts are graphic charts that depict current or
forecast weather. They provide an overall picture of the
United States and should be used in the beginning stages of
flight planning. Typically, weather charts show the movement
of major weather systems and fronts. Surface analysis,
weather depiction, and significant weather prognostic charts
are sources of current weather information. Significant
weather prognostic charts provide an overall forecast weather
picture.
Surface Analysis Chart
The surface analysis chart depicts an analysis of the current
surface weather. [Figure 13-10] This chart is transmitted
every 3 hours and covers the contiguous 48 states and
adjacent areas. A surface analysis chart shows the areas of
high and low pressure, fronts, temperatures, dew points, wind
directions and speeds, local weather, and visual obstructions.
Surface weather observations for reporting points across
the United States are also depicted on this chart. Each of
these reporting points is illustrated by a station model.
[Figure 13-11] A station model includes:
• Sky cover—the station model depicts total sky cover
and is shown as clear, scattered, broken, overcast, or
obscured/partially obscured.
• Sea level pressure—given in three digits to the nearest
tenth of a millibar (mb). For 1,000 mbs or greater,
prefix a 10 to the three digits. For less than 1,000 mbs,
prefix a 9 to the three digits.
• Pressure change/tendency—pressure change in tenths
of mb over the past 3 hours. This is depicted directly
below the sea level pressure.
• Dew point—given in degrees Fahrenheit.
• Present weather—over 100 different standard weather
symbols are used to describe the current weather.
• Temperature—given in degrees Fahrenheit.
• Wind—true direction of wind is given by the wind
pointer line, indicating the direction from which the
wind is blowing. A short barb is equal to 5 knots of
wind, a long barb is equal to 10 knots of wind, and a
pennant is equal to 50 knots.
Figure 13-10. Surface analysis chart.
1. Total sky cover: Overcast
2. Temperature/dew point: 34 °F/32 °F
3. Wind: From the northwest at 20 knots (relative to true north)
4. Present weather: Continuous light snow
5. Sea level pressure: 1014.7 millibars (mb)
Note: Pressure is always shown in 3 digits to the nearest tenth of a millibar.
For 1,000 mb or greater, prefix a “10” to the 3 digits
For less than 1,000 mb, prefix a “9” to the 3 digits
6. Pressure change in past 3 hours: Increased steadily or unsteadily by 2.8 mb
Examples of Wind Speed and Direction Plots
NE/5 kts SW/10 kts N/15 kts W/50 kts S/60 ktsCalm
147
28 /
34
32
**
Pressure change in past 3 hours
Pressure tendency
Sea level pressure
Wind speed
Wind direction
Temperature
Total sky cover
Present weather
Dew point
Figure 13-11. Sample station model and weather chart symbols.
Figure 13-12. Weather depiction chart.
Weather Depiction Chart
A weather depiction chart details surface conditions as
derived from METAR and other surface observations. The
weather depiction chart is prepared and transmitted by
computer every 3 hours beginning at 0100Z time and is valid
data for the forecast period. It is designed to be used for flight
planning by giving an overall picture of the weather across
the United States. [Figure 13-12]
The weather depiction chart also provides a graphic display
of IFR, VFR, and marginal VFR (MVFR) weather. Areas of
IFR conditions (ceilings less than 1,000 feet and visibility
less than three miles) are shown by a hatched area outlined
by a smooth line. MVFR regions (ceilings 1,000 to 3,000
feet, visibility 3 to 5 miles) are shown by a nonhatched area
outlined by a smooth line. Areas of VFR (no ceiling or ceiling
greater than 3,000 feet and visibility greater than five miles)
are not outlined. Also plotted are fronts, troughs, and squall
lines from the previous hours surface analysis chart.
Weather depiction charts show a modified station model
that provides sky conditions in the form of total sky cover,
ceiling height, weather, and obstructions to visibility, but
does not include winds or pressure readings like the surface
analysis chart. A bracket ( ] ) symbol to the right of the station
indicates the observation was made by an automated station.
Significant Weather Prognostic Charts
Significant weather prognostic charts are available for low-
level significant weather from the surface to FL 240 (24,000
feet), also referred to as the 400 mb level and high-level
significant weather from FL 250 to FL 630 (25,000 to 63,000
feet). The primary concern of this discussion is the low-level
significant weather prognostic chart.
The low-level chart is is a forecast of aviation weather
hazards, primarily intended to be used as a guidance product
for briefing the VFR pilot. The forecast domain covers the 48
contiguous states, southern Canada and the coastal waters for
altitudes below 24,000 ft. Low altitude Significant Weather
charts are issued four times daily and are valid at fixed times:
0000, 0600, 1200, and 1800 UTC. Each chart is divided on
the left and right into 12 and 24 hour forecast intervals (based
on the current NAM model available).