The message to proceed with caution also permeates aviation weather briefings. The FAA has decreed that a preflight weather briefing, whether a pilot gets information from an FAA flight service station, DUATS, or a commercial source, must begin with "adverse information." The National Weather Service has obliged by concocting a collection of weather "products." But without an understanding of when, how, and why adverse weather advisories are issued, a pilot can get scared into never leaving his tiedown spot.
Hurricane Advisories
Topping the list in significance are Hurricane Advisories, called WHs. Issued whenever a hurricane threatens, they herald a storm's location, expected movement, and wind speed. Details on associated hazards - ceilings, visibility, and related weather - are not included. They are listed in the pertinent forecasts and other advisories. If a hurricane is anywhere near your route of flight, this is an advisory to heed.
For example:
MIA WH 251010
HURCN JOE AT 1000Z CNTRD 30.3N 75.3W OR 350NM E OF ORLANDO FL EXPECTED TO MOV NE ABT 20 KT. MAX WNDS 150 KT OVR SML AREA NEAR CNTR. HURCN WNDS WITHIN 100-120 NM.
Hurricane Joe, at 1000 Z was centered at 30.30 degrees north, 75.30 degrees west - or 350 nautical miles east of Orlando Florida. It's expected to move northeast at about 20 knots. Maximum winds are at 150 knots over a small area near the hurricane's center, and hurricane force winds extend 100 to 120 nautical miles from the center.
Severe Weather Watch
A Severe Weather Watch comes in two versions - Bulletin and Alert messages, labeled WW and AWW respectively. The National Severe Storm Forecast Center issues these advisories when the formation of severe thunderstorms or tornadoes is likely, such as when a sharp cold front is scheduled to bulldoze through an unstable air mass.
A thunderstorm is classified severe when surface winds reach 50 knots, hail grows to three-quarters of an inch, or tornadoes sprout. The AWW - Alert Severe Weather Watch message - goes out as an advance notice, giving the general area of coverage and defining the precise locus in terms of distance on either side of an axis.
An AWW includes the possible hail and wind conditions, and the storm's general direction of movement. The WW - the actual Watch Bulletin - is issued shortly after the Alert message and gives more detailed information on the storm's location, phenomena, cause, and outlook. The Forecast Center issues additional status reports to keep tabs on the changing situation and a cancellation bulletin to sound the all-clear. Watches mean only that nasty weather is possible.
When tornadoes or severe thunderstorms are actually sighted, local Weather Service forecast and field offices post Severe Weather Warnings. In addition to being issued in text form, watches and warnings are usually outlined on the hourly radar summary charts broadcast by the NWS (National Weather Service). But because of formatting and transmission times, the graphic form may not be as up to date.
The importance of these advisories should be clear, and to be sure everyone gets the message, the advisories are, for the most part, issued in uncoded English.
MKC WW 081914 BULLETIN - IMMEDIATE BROADCAST REQUESTEDTORNADO WATCH NUMBER 56 NATIONAL WEATHER SERVICE KANSAS CITY MO 214 PM EST FRI NOV 11 1996
A. THE NATIONAL SEVERE STORMS FORECAST CENTER HAS ISSUED A TORNADO WATCH FOR MOST OF SOUTH CAROLINA PARTS OF SOUTHEAST VIRGINIAFROM 3PM EST UNTIL 10PM EST TODAY.
B. TORNADOES. LARGE HAIL. DANGEROUS LIGHTNING AND DAMAGING THUNDERSTORM WINDS ARE POSSIBLE IN THESE AREAS.
THE TORNADO WATCH AREA IS ALONG AND 79 STATUTE MILES EITHER SIDE OF A LINE FROM 30 MILES WEST OF AUGUSTA GEORGIA TO 30 MILES NORTHEAST OF ELIZABETH NORTH CAROLINA.
C. TORNADOES AND A FEW SVR TSTMS [severe thunderstorms] WITH HAIL SFC [surface] AND ALF [aloft] TO 3 IN. EXTRM TURBC [extreme turbulence] AND SFC WND [surface wind] GUSTS TO 70 KT. A FEW CBS [cumulonimbus clouds] WITH MAX TOPS TO 500 [50,000 feet]. MEAN WIND VECTOR 250060 [from 250 degrees at 60 knots].
D. PARAMETERS N CAROLINAS VERY STG [strong] WITH STG LOW LVL [level] FLOW AND CNVRGC [convergence] ALG [along] WITH UNSTABLE AMS [air mass].
Convective SIGMETS
Severe thunderstorms also activate the National Aviation Weather Advisory Unit (NAWAU), which issues Convective SIGMETs - WSTs. This advisory is the result of actual and forecast severe thunderstorms as well as garden variety thunderstorms that are either embedded (hidden in a larger cloud mass and not visually discernible) or forming in lines. WST's also warn of storms categorized as intensity level four or higher, which affect at least 40 percent of a "widespread" area, defined as 3,000 square miles. The forecaster also may distribute a WST for any storm situation the forecaster feels is hazardous to all aircraft types, and let's face it - any thunderstorm is hazardous to all aircraft types.
WST's are issued for the western, central, and eastern conterminous U.S., delineated by longitudes passing approximately through Nashville (BNA) and Albuquerque (ABQ). They emerge on the weather circuit at five minutes before the hour and are sequentially numbered starting at 0000Z. Convective SIGMET 14E would be the 14th bulletin for a particular weather situation in the eastern states.
If the weather is clear, the message - CONVECTIVE SIGMET...NONE - will say so. The dispatch text contains an observation and/or forecast, valid for two hours, defined in reference to aviation fixes. This report details the extent, severity, trend, movement, and worst-case hazards. WST's are also posted on the radar summary chart put out by the NWS and most commercial vendors.
Like all weather products, WSTs follow a standard format.
MKCE WST 080955 CONVECTIVE SIGMET 21E VALID UNTIL 1155Z PA WV FROM 20ENE PIT-60SSE EKN-30SSE PIT-20ENE PIT AREA EMBD TS MOV FROM 220045KT. TOPS TO FL300.
The 21st consecutive WST of the day, it's valid until 1155Z and affects Pennsylvania and West Virginia. From 20 miles east-northeast of Pittsburgh; to 60 miles south-southeast of Elkins, West Virginia; 30 miles south-southeast of Pittsburgh; to 20 miles east-northeast of Pittsburgh; an area of embedded thunderstorms is moving from 220 degrees at 45 knots. Cloud tops reach to Flight Level 300 (30,000 feet).
Convective Outlook
The Convective Outlook (AC) is not exactly a warning, but it can be full of adverse weather. The AC is a 24-hour perspective on thunderstorm possibilities. It focuses on severe thunderstorms and thunderstorms that are approaching severe limits - those with 35 to 50-knot winds or half-inch diameter hail.
An AC describes the possibility of thunderstorms in terms of high, moderate, or slight risk. These criteria, in turn, are based on expected coverage within the specified area. A "Slight" risk implies a 2- to 5- percent chance; "Moderate" means a 6- to 10-percent chance; and "High" indicates more than a 10 percent chance. In other words, the forecaster expects storms in the area, so the question is not whether they will pop up, but when they will develop.
The National Severe Storm Forecast Center issues ACs twice daily, at 0700Z and 1500Z, and they are valid for the following 24-hour period, from 1200Z to 1200Z. The AC is a companion to the Severe Weather Outlook Chart, issued at 0800Z. The chart is a 48-hour forecast divided into two panels. The first 24-hour panel depicts areas of expected general and severe storms, the latter shown by hatched shading. The second panel depicts severe storms only.
MKC AC 0800600 CONVECTIVE OUTLOOK VALID 081200Z - 09120
THERE IS A SLGT RISK OF SVR TSTMS TO THE RIGHT OF A LINE FROM 30SSE SAV 50N AYS T0 ESE MCN 30ESE ATL.
GEN TSTMS ARE FCST TO THE RIGHT OF A LINE FROM 25WSW AGS 25W ABY 35ESE ATL AHN 200E AVL.
...SEVERE THUNDERSTORM FORECAST DISCUSSION...
- -SYNOPSIS - - HIGH-AMPLITUDE MID/UPR LVL TROF ORIENTED MERIDIONALLY ACRS MS VLY IS XPCD TO CONT MOVG EWD.......
...GENERAL THUNDERSTORM FORECAST DISCUSSION...
SUF MSTR/INSTBY WL BE IN PLACE TO SUPPORT WDLY SCTD TSTMS ARE PSBLE ALG/AHD CDFNT. THIS ACTVTY SHOULD BE GENLY NON-SVR...
There is a slight risk of severe thunderstorms to the right of a line from 30 miles south-southeast of Savannah, Georgia, to 50 miles north of Waycross, Georgia, to east-southeast of Macon, Georgia, to 30 miles east-southeast of Atlanta.
General thunderstorms are forecast to the right of a line from 25 miles west-southwest of Augusta, Georgia, to 25 miles west of Albany, Georgia, to 35 miles east-south east of Atlanta and Athens, to 20 miles east of Asheville, North Carolina.
A high-amplitude mid and upper level trough is oriented meridionally across the Mississippi Valley and it's expected to continue moving eastward.
Sufficient moisture and instability will be in place to support widely scattered thunderstorms possible along and ahead of the cold front. This activity should be generally non-severe.
SIGMETS
Thunderstorms are not the only weather hazard that can prompt a SIGMET. SIGMETs (WS) also are issued to warn pilots of non-convective conditions hazardous to all aircraft. These hazards must be "widespread" - again, pertaining to an area of at least 3,000 square miles. However, only a portion of this area may be affected at any one time.
To warrant a SIGMET, the weather must include severe icing; severe, extreme, or clear-air turbulence; volcanic eruptions; volcanic ash; sand storms; or dust storms. The latter weather might seem localized and easily avoided, but ash and dust can quickly fill vast areas, causing havoc with airplane engines and other parts.
SIGMETs are valid for up to four hours and are labeled November through Yankee, excluding Sierra and Tango, which are reserved for warnings we'll cover later. Bulletins for the same set of circumstances are consecutively numbered.
The NWS issues SIGMETs along the same lines as the Area Forecast (FA). Boston (BOS) handles Ohio, the Mid-Atlantic, and Northeast. Miami (MIA) is responsible for the Carolinas, Georgia, and Florida. Chicago (CHI) encompasses the north central and north plains states, and Dallas (DFW) covers the south central and gulf coast. Salt Lake City (SLC) comprises the mountain and western states, excluding Washington, Oregon, and California, which belong to San Francisco (SFO).
The first SIGMET for a particular condition, and subsequent numbers at the forecaster's discretion, are tagged URGENT (UWS), which makes the SIGMET a warning with a bullet.
WS 081100 SIGMET QUEBEC 5 VALID UNTIL 081500
ME NH VT MA CT NY PA NJ
FROM 50N CAR TO 50NE MLT TO ACY TO HAR TO YOW TO 50N CAR
MOD OCNL SEV TURBC BLW 10. LLWS CONDS ALSO POSS. CONDS CONTG BYD 15Z.
SIGMET Quebec 5 is valid until 1500Z on the eighth day of the month and covers Maine, New Hampshire, Vermont, Massachusetts, New York, Pennsylvania, and New Jersey. It covers an area bounded by points 50 miles north of Caribou, Maine, 50 miles northeast of Atlantic City, New Jersey, Harrisburg, Pennsylvania, Ottawa, Ontario, Canada, to 50 miles north of Caribou. Pilots can expect moderate, occasional severe turbulence below 10,000 feet. Low-level wind shear conditions are also possible. Conditions will continue beyond 1500Z.
AIRMETS
Conditions that pose a threat to small aircraft or those flying VFR are announced by AIRMETs. Issued every six hours, with updates and corrections as necessary, AIRMET criteria are moderate icing, moderate turbulence, sustained 30-knot surface winds, ceilings less than 1,000 feet, visibility less than three miles, and mountain obscuration.
An AIRMET designated Sierra warns of instrument conditions (IFR) and mountain obscuration. AIRMET Tango addresses turbulence, winds, and low-level wind shear, and AIRMET Zulu is for icing. An AIRMET is valid for six hours, but it can have an outlook, which is valid for an additional six hours, that describes the expected trend.
This AIRMET is from Boston.
BOS WA 090845 AIRMET ZULU 1 FOR ICG AND FRZLVL VALID UNTIL 091500
AIRMET ICE...ME NH VT MA CT RI NY OH PA NJ WV VA MD DE DC
FROM CAR TO 180S ACK TO ACY TO TRI TO HNN TO CVG TO FWA TO DTW TO CAR
OCNL MOD RIME/MXD ICG ICGIP. CONDS BTN 040 AND 160W OF SYR-HAR LN. SLPG TO LAYER BTN 100 AND FL200 OVR NEW ENGLAND. CONDS CONT BYD 15Z THU 21Z.
AIRMET Zulu 1 for icing and freezing level valid until 1500Z on the ninth day of the month. The AIRMET for ice applies to the listed states. It covers an area bounded by Caribou, Maine, to 180 miles south of Nantucket, Massachusetts, to Atlantic City, New Jersey, to Bristol, Tennessee, to Henderson, West Virginia, to Cincinnati, Ohio, to Fort Wayne, Indiana, to Detroit, Michigan, and back to Caribou.
Occasional moderate rime and mixed icing in precipitation. These conditions exist between 4,000 and 16,000 feet west of a line between Syracuse, New York, and Harrisburg, Pennsylvania. They slope to a layer between 10,000 feet and Flight Level 200 (20,000 feet) over New England. The conditions are expected to continue beyond 1500Z through 2100Z.
Center Weather Advisory
Each Air Route Traffic Control Center (ARTCC) has a Center Weather Service Unit (CWSU). The meteorologists work closely with their NWS brethren, but they create and issue their own Center Weather Advisories (CWA) "...for use in anticipating and avoiding adverse weather conditions in the en route and terminal areas."
The CWA is not a flight planning forecast, but a nowcast for conditions beginning within the next two hours. A CWA is valid for two hours, but if conditions are expected to continue past the valid period, a statement is included in the advisory.
Center Weather Advisories reiterate existing warnings - with one difference. If a pilot report (PIREP, or UA) indicates the presence of some nasty weather the NWS forecasters didn't anticipate, the CWSU can beat them to the punch with a CWA. Looking at CWAs is a good way to double-check the other warnings, advisories, and PIREPs.
A typical CWA looks like this:
ZDC1 UCWA 01 092025-092225
FROM 70 WNW BWI TO BWI TO RIC TO 30NE GSO TO 70WSW LYH TO 70WNW BWI
AREA LVL 1/2 SHRA/SHSN ISOL LVL 3 TS MOV FM 22025KT. TOPS TO FL 200. TS HAVE FEW CG LTNG MNLY VC KIAD. SHRA/SHSN AND ISOL TS SHUD CONT MOV NE AFFECTING MD.. DC..VA..ERN W.V.
The CWA covers an area bounded by points 70 miles west-northwest of Baltimore-Washington International, to BWI, to Richmond, Virginia, to 30 miles northeast of Greensboro, North Carolina, to 70 miles west-southwest of Lynchburg, Virginia, back to the starting point. In this area are Level I and 2 rain showers and snow showers, and isolated Level 3 thunderstorms moving from 220 degrees at 25 knots. The cloud tops reach to Flight Level 200 (20,000 feet). Thunderstorms have a few cloud to ground lightning strokes, mainly in the vicinity of Washington-Dulles Airport. Rain showers, snow showers, and isolated thunderstorms should continue to move northeast, affecting Maryland, Washington, D.C., Virginia, and eastern West Virginia.
VNR Statement
A VFR-not-recommended statement is not a weather "product," but it is an advisory appended to briefings and broadcasts. When conditions are reported or forecast to be below VFR minimums, or when visual flight might be less than prudent (squeaking between high mountains and low clouds etc.) the flight service specialist is required to advise a pilot that "VFR flight is not recommended." This isn't an official end to your VFR plans. The pilot in command has the "final authority and ultimate responsibility" and makes all the decisions regarding his flight. But it makes sense to investigate the VNR advisory and find out what weather caused it to be appended to your briefing.
Like life, aviation is filled with warnings, cautions, and advisories. Some may seem frivolous, but the intent is to help you fly safely. Knowing what the weather is - and will do - is important on every flight, so we'd like to close with one final caution - WARNING! NEVER IGNORE ANY WARNINGS!
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