Aviation Forecasts (Part Two)

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.

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]

Figure 13-9. Winds and temperature aloft forecast.

Figure 13-9. Winds and temperature aloft forecast.

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.