Understanding Weather Forecasts

[Admin]

Hi All

 

I am nearly finished developing the new weather page and want to include a brief "Easy Guide to Understanding Weather Forecasts" but have run out of time - the new weather page has taken two days so far, so if anyone wants to helps I would be greatly appreciative - PLEASE :big_grin:

 

This is what I have done so far but may not even be the right way to go about it - your thoughts:

 

- Understanding Weather Forecasts -

 

Understanding weather forecasts isn't really that hard once you know the abbreviations and the structure that is used. There are some software applications etc that can present the weather forecast in a "Common English" format but we can never be assured that they will be available or working when we need them. This is why we are trained to read weather forecast in their native format and it is this standardised format that is used all around the world.

 

CAVOK - When you see this term used in a forecast it means that "Ceiling and Visibility OK" - visibility of 10km or more, no cloud below 5,000ft and no precipitation (rain), thunderstorms, shallow fog, low drifting snow or dust devils.

 

Thunderstorms - Thunderstorms are associated with cumulonimbus (Cb) clouds and the amount of Cb clouds in an area is indicated by:

 

• ISOL - isolated, for individual Cbs
   
   
  
• OCNL - occasional, for well separated Cbs
   
   
  
• FREQ - frequent, for Cbs with little or no separation
   
  

 

Cloud Heights - Cloud cover is measured on a scale out of 8 using the term oktas yet a forecast uses different abbreviations for each measurement of oktas:

 

• SKC - sky clear - 0 oktas
   
   
  
• FEW - few - 1 to 2 oktas
   
   
  
• SCT - scattered - 3 to 4 oktas
   
   
  
• BKN - broken - 5 to 7 oktas
   
   
  
• OVC - overcast - 8 oktas
   
  

 

FM - This abbreviation literally mean "From" and is usually associated with a time e.g. Fm 0830 simply means from the time of 08:30

 

TEMPO and INTER - These indicate that there will be temporary variation in the forecast

 

• TEMPO - indicates a change in the conditions that will last for a period of between 30 and 60 minutes
   
   
  
• INTER - indicates a change is expected to occur frequently for a period of up to 30mins duration
   
  

 

ARFOR - Area Forecast is a statement of meteorological conditions that is issued every 6hrs for a specific area and whilst they usually have a duration of 12hrs they can be updated at any time to reflect any changing conditions.

 

TAF - Terminal Area Forecast is also a statement of meteorological conditions but only refers to within a radius of 8kms of a specific aerodrome.

 

PROB - This abbreviation stands for Probability and is displayed with a number such as 40 so PROB40 means there is a 40% probability.

 

Other things that you need to know are:

 

Time is always displayed as a 6 figure date-time group in GMT time (which is the same as UTC time) and symbolised with a "Z" for Zulu - the GMT time zone

 

FOG simply means that there is fog

 

HAZE advises that there is a lot of haze that will impact visibility

 

Wind is always measured in KT (knots)

 

Altitude is always measured in feet

 

Temperature is displayed simply by the letter T

 

QNH is the air pressure at MSL (Mean Sea Level) for the forecast area and is displayed by the letter Q

 

Now that we know what abbreviations we will find in a weather forecast let's have a look at an example of an ARFOR (Area Forecast) AREA32 (32)

 

ARFOR AMEND AREA FORECAST 120340 TO 121700 AREAS 30/32

 

AMD OVERVIEW:

 

FRONT TO THE SW, FORECAST WEBS/YKII AT 11Z AND YNRC/CHOMP AT 17Z.

 

SCATTERED SHOWERS S OF YBOR/YMNG/YSCB, BECOMING WIDESPREAD NEAR

 

FRONT. ISOLATED THUNDERSTORMS/SMALL HAIL OVER SEA/COAST NEAR AND SW

 

OF FRONT. SNOW SHOWERS ABOVE 3500FT. LOW CLOUD PATCHES IN

 

PRECIPITATION. FOG PATCHES DEVELOPING NE OF YNAR/YMNG/ELW/YCOM AFTER 12Z.

 

WIND:

 

2000 5000 7000 10000 14000 18500

 

210/20 200/20 200/20 200/20 MS07 200/25 MS14 200/30 MS24

 

REMARK:

 

WINDS TENDING 230 DEGREES AHEAD OF FRONT, AND INCREASING BY 10/20

 

KNOTS NEAR AND SW OF FRONT.

 

AMD CLOUD:

 

ISOL CB 3000/25000 SEA/COAST NEAR AND SW OF FRONT. PATCHES BKN ST

 

1000/2500 IN PRECIPITATION, MAINLY NEAR RANGES/FRONT. BKN CU/SC

 

2500/8000 S OF YBOR/YMNG/YSCB. BKN CU/SC 3500/6000 IN THE N, TENDING

 

SCT AFTER 11Z. CU TOPS TO 16000 WITH SHRA/HAIL NEAR/SW OF FRONT. BKN

 

AC/AS ABV 8000 NEAR FRONT.

 

WEATHER:

 

SHSN, SHRA, HAIL, FG, TSRA.

 

VISIBILITY:

 

0500M SHSN/FG, 3000M SHRA/TSRA/HAIL.

 

FREEZING LEVEL:

 

4500

 

ICING:

 

MOD IN CU/SC TOPS AND AC/AS.

 

TURBULENCE:

 

MOD IN CU/AC. OCNL MOD BELOW 7000FT LEE OF RANGES, POSSIBLY SEV

 

NEAR/LEE RANGES NEAR FRONT.

 

AMD CRITICAL LOCATION: HEIGHTS ABOVE MSL

 

KILMORE GAP: 9999 -SHRA SCT ST 2000 BKN CU/SC 3000

 

INTER 0308 5000 SHRA BKN ST 1500

 

TEMPO 1417 5000 SHRA BKN ST 1500

 

Area 32

 

- ARFORs begin with the Area Code that the forecast is for - this one is for Area 32

 

ARFOR AMEND AREA FORECAST 120340 TO 121700 AREAS 30/32

 

- This tells us that the forecast is an AMEND (amended) area forecast and is current on the 12th

 

 

[Ben Longden]

What about adding an English translation of that..... its always usefull in refreshing the grey matter when getting the met before planning a flight.

 

Ben

 

 

[facthunter]

weather forecasts.

 

That seems to cover it ,Ian, except for a list of meanings for the weather abbreviations, which might help if included. Don't take this as a criticism. I really do appreciate what you have done in this area lately. Nev

 

 

[Ben Longden]

I could not agree more.

 

When I need to check Area 30 Wx, the first place I go to is the forum. very easy and quick.

 

Ben

 

 

[Guest Fred Bear]

Ian, as I posted in the other thread here is a good link:

 

http://www.bom.gov.au/general/reg/aviation_ehelp/

 

Will save one alot of writing and I am sure you can link straight to that off there.Hope it helps.

 

 

[Guest Fred Bear]

Username:bomw0007

 

Password:aviation

 

 

[Admin]

Thanks Darren - I will look into it tomorrow

 

 

[bushcaddy105]

Ian, I have often puzzled over waypoint locations issued with Arfors, particularly here in Sth Oz where they are often used in conjunction with airstrip identifier codes. Some locations used do not appear in Ersa's list of either IFR or VFR waypoints. An approach direct to the BoM produced the "I don't know where you'd find out" answer!

 

Can anyone shed any light on where to to get a full list of locations used in Arfors?

 

 

[facthunter]

deciphering.

 

bushcaddy 105, I don't think you are alone in SA., with that problem, and the answer is not good enough really. The trouble is that all these little "empires" operate in their own little pond, constrained by their own rules and practices. No one looks at the end result. The user (us ) has to put up with it. CASA has spent a lot of time and effort recently, with their weather-related loss of control roadshow, where the latest weather information (or lack of it) plays a prominent part in the bad outcome. There's a quote around that we might well adopt. It goes something like " USER PAYS,USER SAYS " Why can't we get the information in a form that can be readily understood. If you went to the corner store and asked for an ice cream, would you be happy if they gave you sugar, powdered milk, Gelatin & water, instead? N.....