CyberAirVA

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Weather Reports

Weather is one of the most unpredictable phenomena that effects aviation. In this tutorial I will first go through the different types of weather reports available to pilots. Abbreviations can be found here.

METAR
METAR is the international standard Aviation Routine Weather Report. The METAR contains the following information:

Type of report (METAR or SPECI)
Station designator (ICAO)
Date and time of report
Wind
Visibility
Weather and obstruction to visibility
Sky conditions
Temperature and dew point
Altimeter settings
Remarks

Example 1

METAR ESMT 121520Z 30010KT 270V330 9999 SCT025 17/11 Q1001

So lets start going through the METAR above. This is the METAR for Halmstad (ESMT). It's been issued on the 12th day of the month at 1520Z (ZULU hours = UTC (coordinated universal time), for all intentions the same as GMT).The wind follows, blowing from 300 degrees at 10KT [30010KT] (if the wind is stronger than 99KT it will be three numbers, i.e. 110KT). The next group of letters are also the wind this tells us that the wind direction is variable [V] from 270 to 330 degrees. This will often be included in the first wind numbers, i.e. 270V33010KT. Next is visibility, we can see that this day is a beautiful day in Halmstad, visibility is over 9999 meters [9999]. There are some scattered clouds at 2500ft [SCT025] AGL (above ground level). The clouds can be: SKC (SKY CLEAR) by manual stations.

CLR (CLEAR) when reported by an automated station
FEW (FEW 1/8-2/8 coverage)
SCT (SCATTERED 3/8-4/8)
BKN (BROKEN 5/8-7/8)
OVC (OVERCAST 8/8)

If the sky is clear thought they will usually not put anything. Temperature is 17 degrees (Celsius) and the dew point is 13 degrees (Celsius). The dew point is at which temperature the air will be saturated with water. Note that the dew point will always be below the temperature. Altimeter setting is QNH1001 [Q1001]. There are no remarks.

METAR KEWR 172240 03545G60KT 0SM TSSHRA OVC015CB 23/21 A2983 RMK TSB10 SE MOVG ESE FQT LTGICCCCG VSBY HIR W RAB07

Well the first one was easy so lets go to the next one. It's a METAR for KEWR (Newark), it has been issued on the 17th day at 2253Z. Winds are blowing from 035 degrees at 45KT with gusts up to 60KT [45G60KT]. Visibility is 0 statute miles [0SM] (SM is used in US). The next part are the Weather and Obstruction to visibility part. TSSHRA stands for thunderstorms [TH], showers [SH] and rain [RA]. Next comes sky conditions, overcast at 1500 ft AGL [OVC015] of cumulonimbus.

TAF
TAF stands for terminal aerodrome forecast. An is, just as it sound like a forecast for the area around the airport. The forecast area extend to 5nm around the airport, this should be considered while analysing it. All the altitudes are in AGL. Well lets check what it should contain and then some examples:

Type: TAF, TAF AMD (amended) or TAF COR (corrected).

Location
Issuance time
valid time
Forecast
Wind
Visibility
Weather
Sky condition

MIA 121132Z 131212 VRB03KT P6SM SCT020 SCT080 BKN250
TEMPO 1214 BKN025 FM1600 18008KT P6SM VCSH SCT020 SCT250 PROB40 1822 3SM TSRA
BKN025CB FM0100 VRB03KT P6SM SCT025 SCT090 BKN250

So first things first, this is the TAF for KMIA (MIA = Miami Intl.) it was issued on the 12th day of the month at 1132Z [121132Z] and since it's a TAF it's valid for 24 hours [131212]. It first tells us the very near weather, variable wind direction at 3KT [VRB03KT], visibility is more than 6SM [P6SM], note that in most non US countries the visibility will be in meters, i.e. P6SM=9999. Clouds are scattered at 2000ft [SCT020], scattered at 8000ft [SCT080] and broken at 25000ft [BKN250]. Between 12Z and 14Z there can be temporary broken clouds at 2500ft [TEMPO 1214 BKN025].
>From 16Z to 00Z [FM1600] winds will be blowing from 180 deg at 8KT [18008KT], visibility will be as before [P6SM]. Showers are expected in the vicinity [VCSH]. Clouds will be scattered at 2000ft [SCT020], scattered at 25000ft [SCT250]. There is a probability (30-49%) [PROB40] for visibility 3SM [3SM], thunderstorm showers and rain [TSRA] with cumulonimbus clouds broken at 2500ft [BKN025CB], between 18Z and 22Z [1822].

From 01Z variable wind direction 03KT [VRB03KT], the same visibility as before [P6SM], clouds scattered at 2500ft [SCT025], scattered at 9000ft [SCT090] and broken at 25000ft [BKN250].

ESSA 121610Z 130018 33005KT 6000 -RA SCT008 BKN012
TEMPO 0018 4000 RA BKN008 PROB30 0018 1000 +RA BR BKN004

Above we have the TAF for ESSA (Arlanda, Sweden). Issued on the 12th day, at 1610Z. [121610Z], valid until 0018 the next day [130018] Winds are blowing from 330 deg at 5KT [33005KT], visibility 6000 meters [6000], light rain showers [-RA]. Clouds scattered at 800ft [SCT080], broken at 1200ft [BKN012]. Temporary [TEMPO] conditions between 00Z and 18Z [0018], visibility 4000 meters [4000], rain [RA], broken clouds at 800ft [BKN008]. A probability (20-39%) [PROB30] between 00Z and 18Z [0018] of visibility 1000 meters [1000], heavy rain [+RA], mist [BR] with broken clouds at 400ft [BKN004].

AIRMET (WA)
The AIRMET is a weather advisory, it is updated four times a day. It's valid for six hours. It is split into three parts: SIERRA (describes areas of IFR conditions and mountains obscurement), TANGO (forecasts turbulence, low level wind shear (LLWS) and strong surface winds) and ZULU (shows location and intensity of icing and freezing levels). The AIRMET covers at least an area of 3000 square miles, a big area. An AIRMET is issued if any of the following phenomena is expected to occur (develop):

Moderate icing
Moderate turbulence
Sustained wings of 30 knots or more at the surface
Ceiling less than 1000ft and/or visibility less than 3 miles
Extensive mountain obscurement
So lets see an example of the AIRMET, lets go through the different parts one at a time.
SIERRA

SFOS WA 261345
AIRMET SIERRA UPDT 2 FOR IFR AND MTN OBSCN VALID UNTIL 262000
.
AIRMET IFR...CA
FROM 70 WSW MFR TO UKI TO SNS TO 20W SNS TO 30W UKI TO FOT TO
70WSW MFR
OCNL CIG BLO 10 VSBY BLO 3F CONDS ENDG 17Z-19Z.
.
AIRMET MTN OBSCN...CA
FROM 40W SAC TO 20 NNW SBA TO 50WNW PSP TO 40E SAN TO 10S SAN
TO LAX TO 40W SBA TO 40NW SFO TO 40W SAC
CSTL MTNS OBSCD CLDS/FOG. CONDS ENDG CNTRL CA PTN AREA 19Z-
20Z..CONTG SRN CA BYD 20Z THRU 02Z.

So what does all this mean. First it's the AIRMET [WA] from SFO [SFOS], It's issued on the 26th day of the month, at 1345Z [261345]. It also lets us know that the SIERRA AIRMET [AIRMET SIERRA], update 2 [UPDT 2], and what it is, meaning of SIERRA. And that it's valid till 2000Z [26200]. It then reports the IFR part, it's for California [CA]. Next it describes an area between a number of VOR beacons [UKI TO SNS]. It also uses distances from the VORs [70WSW MFR]. It's easier if you draw the locations on a map. Then comes the forecast: Occasional ceilings below 1000ft [OCNL CIG BLO 10], visibility below 3SM because of fog [VSBY BLO 3F] and the conditions are expected to end between 17Z and 19Z [CONDS ENDG 17Z-19Z]. The next part of the AIRMET is the mountain obscurement part. It's also for California [CA], it also defines an area. It forecasts costal mountains obscured due to clouds or/and fog [CSTL MTNS OBSCD CLDS/FOG]. The conditions are expected to end for the central Californian area between 19Z and 20Z [CONDS ENDG CNTRL CA PTN AREA 19Z-20Z]. But is expected to continue in southern California beyond 20Z and through 02Z [CONTG SRN CA BYD 20Z THRU 02Z].
TANGO

MIAT WA 121345
AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL 122000
.
NO SGFNT TURB EXP EXC INVOF CNVTV ACT.

So this is the TANGO AIRMET for Miami [MIAT], issued 12th day, at 1345Z [121345]. It's the second update for turbulence [UPDT 2 FOR TURB] and is valid until 2000Z [122000]. No significant turbulence is expected except in and over the convection area [NO SGFNT TURB EXP EXC INVOF CNVTV ACT]. If there was something to report it would have given the area, intensity, altitudes and how long, see the example below (not commented).

CHIS WA 201040
AIRMET TANGO UPDT 1 FOR TURBC VALID UNTIL 201400
FROM ISN TO MQT TO COU TO ACT TO SJN TO FMN TO DEN TO ISN
OCNL MDT TURBC BLO 150 WI 10 ML OF COLD FNT. CONDS CONTG BYD
14Z THRU 20Z.

ZULU

NDFWZ WA 141945
AIRMET ZULU UPDT 3 FOR ICE AND FRZLVL VALID UNTIL 150200.
NO SGFNT ICE EXPCD OUTSIDE OF CNVTV ACT..FRZLVL...140-160 THRUT.

The AIRMET ZULU was issued on the 14th day, at 1945Z [131945], it's the third update and is valid until the 15th day, at 0200Z [150200]. No significant icing expected outside of convection activity [NO SGFNT ICE EXPCD OUTSIDE OF CNVTV ACT]. Freezing levels are throughout 14000ft and 16000ft [FRZLVL...140-160 THRUT]. If icing exists the AIRMET will say were, type, altitude and for how long.

SIGMET (WS)
A SIGMET is also a weather advisory but it usually covers a greater area than the AIRMET. The SIGMET is issued when one of the following phenomena is expected:

Severe icing
Severe or greater non-convective turbulence
Moderate or greater clear air turbulence (CAT)
Widespread dust storms, sandstorms or volcanic ash that reduce visibility below 3 miles over
an area of at least 3 000 square miles

So lets see an example:

SFON WS 152130
SIGMET NOVEMBER 3 VALID UNTIL 160130
CA
FROM FOT TO 50NW RNO TO 50NE BFL TO SBA TO 40W SBA TO 30W SFO
TO FOT
OCNL SVR TURBC BLO 100 XCP BLO 150 VCNTY SIERRAS. STG UDDFS
VCNTY MTNS AND LLWS POTENTIAL BLO 20 AGL. CONDS CONTG BYD
0130Z.

This is the SIGMET [WS] for SFO [SFON], it has been issued on the 15 day at 2130Z [152130] and it is valid until day 16 at 0130Z [160130]. The following part is the area the SIGMET is for, in this case California [CA], the next part is the area effected, designated by VORs. Occasional severe turbulence is expected below 10000ft MSL, except below 15000ft MSL in the vicinity of the Sierra mountains [OCNL SVR TURBC BLO 100 XCP BLO 150 VCNTY SIERRAS]. Strong updrafts and downdrafts in the vicinity of mountains plus low-level wind shear are anticipated below 2000ft AGL [STG UDDFS VCNTY MTNS AND LLWS POTENTIAL BLO 20 AGL]. Conditions are expected to continue beyond the end of the advisory period [CONDS CONTG BYD 0130Z].

TWEB
TWEB stands for transcribed weather broadcast. The most common TWEB is the route forecast. This is as the name implies a weather forecast for the flight routes. The route forecast covers a corridor of 50 nm along the route.

There can also be TWEB synopsis, this contains a short description of fronts, pressure systems and local climate or terrain factors that can affect the route. So lets see a couple of examples, we will start with the route forecast.

115 TWEB 150820 KMIA-KEYW. ALL HGTS AGL XCP TOPS. KMIA-50SW KMIA
TIL 17Z P6SM FEW025 WITH ISOLD BLO 3SM +SHRA AND +TSRA
BKN008...AFT 17Z P6SM SCT025 WITH LCL BKN025 BKN060 AND NMRS
BLO 3SM +SHRA AND +TSRA BKN008 WITH ISOLD SFC WND G30KT.
50SW KMIA-KEYW P6SM FEW-SCT025 WITH ISOLD BLO 3SM +SHRA AND
+TSRA BKN008.=

This is the route forecast [TWEB] for route 115 and it is valid until day 15, valid from 08Z to 20Z [150820]. Note that the TWEBs are issued three times a day (morning, afternoon and evening), this is the morning TWEB. It then tells us that all heights are in AGL, except for tops [ALL HGTS AGL XCP TOPS], It first describes the weather for a part of the route from Miami intl. to 50 miles southwest of Miami intl. [KMIA-50SW KMIA]. Then comes the weather, till 17Z visibility is plus 6SM, few clouds at 2500ft with isolated visibility below 3SM, heavy rain showers and heavy thunderstorm rain, and broken clouds at 800ft. [TIL 17Z P6SM FEW025 WITH ISOLD BLO 3SM +SHRA AND +TSRA BKN008]. After 17Z the visibility will be the same, scattered clouds at 2500ft, with local broken clouds at 2500ft. Broken at 6000ft and numerous visibility below 3SM, heavy rain showers and heavy thunderstorm rain. Broken clouds at 800ft with isolated surface winds gusting to 30kt [AFT 17Z P6SM SCT025 WITH LCL BKN025 BKN060 AND NMRS BLO 3SM +SHRA AND +TSRA BKN008 WITH ISOLD SFC WND G30KT].
>From 50 miles southwest of Miami intl. to Key west [50SW KMIA-KEYW] the visibility will be plus 6SM, clouds will be few to scattered at 2500ft with isolated visibility below 3SM. Heavy rain showers, heavy thunderstorm rain and broken clouds at 800ft can be expected during the whole forecast period [P6SM FEW-SCT025 WITH ISOLD BLO 3SM +SHRA AND +TSRA BKN008].

So lets take an example of a synopsis:

LAX SYNS 191203 UPR RDG ALG W CST WITH NLY FLO ALF. SFC HI CONTS
TO BLD OVER GT BASIN FOR CONTG SANTA ANA CONDS.

As we can see this is the synopsis from Los Angeles intl. [LAX SYNS] valid on day 19 from 12Z to 03Z the next day [191203]. An upper level ridge is along the west coast with a northerly flow aloft [UPR RDG ALG W CST WITH NLY FLO ALF]. Surface high pressure continues to build over the Great Basin, for continuing Santa Ana conditions [SFC HI CONTS TO BLD OVER GT BASIN FOR CONTG SANTA ANA CONDS]. Note that there was no route number in the synopsis, the conditions may affect a number of routes.

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