For Use Time Outside of Valid Time for Winds Aloft

1 Answers

1. 
   

   
   

   Skyfox on Nov 15, 2016

   I love obscure questions like this! This is a good question that I’m having a hard time finding an answer for. Maybe we can figure it out by going through some sample info. From the Pilot’s Handbook of Aeronautical Knowledge [FAA-H-8083-25A] figure 12-13:
   ~~~~~
   FD KWBC 151640
   BASED ON 151200Z DATA
   VALID 151800Z FOR USE 1700-2100Z
   TEMPS NEGATIVE ABV 24000

   Explanation of Figure 12-13:
   The heading indicates that this FD was transmitted on the 15th of the month at 1640Z and is based on the 1200Z radiosonde. The valid time is 1800Z on the same day and should be used for the period between 1700Z and 2100Z. The heading also indicates that the temperatures above 24,000 feet MSL are negative. Since the temperatures above 24,000 feet are negative, the minus sign is omitted.
   ~~~~~

   Based on this and information from the Aviation Weather Services publications [AC 00-45E and AC 00-45G], observations are taken twice a day at 0000Z and 1200Z. Forecasts are built from models based on those observations and issued four times a day at 0000Z, 0600Z, 1200Z, and 1800Z with forecasts for 6 hours out, 12 hours out, and 24 hours out. I’d like to copy and paste a table of forecast issuance and valid times from AC 00-45G but the formatting really doesn’t work out here and I don’t know the formatting codes to make it look all pretty. So please take a look at AC 00-45G, page 7-45, table 7-11. (It seems like the for-use time can vary.)

   As it says in the above example, that particular FD forecast was issued at 1640Z based on data from 1200Z. It’s valid starting at 1800Z and is for use between 1700Z and 2100Z. However, this does bring up the question: if it starts being valid at 1800Z, what if you want to take a flight at 1700Z? Do you use this forecast in your flight planning even if you haven’t reached the valid time yet? Or do you use the previous forecast from 1200Z assuming it overlaps past 1700Z?

   Let’s take a look at an actual forecast I pulled from Duats. I told it to plan on a 23 hour 59 minute flight so it would need the 6 hour, 12 hour, and 24 hour forecasts. Here are the headers from each:
   ~~~
   DATA BASED ON 160000Z
   VALID 160600Z FOR USE 0200-0900Z. TEMPS NEG ABV 24000

   DATA BASED ON 160000Z
   VALID 161200Z FOR USE 0900-1800Z. TEMPS NEG ABV 24000

   DATA BASED ON 160000Z
   VALID 170000Z FOR USE 1800-0600Z. TEMPS NEG ABV 24000
   ~~~

   Neither one lists an issuance time like the example from FAA-H-8083-25A does; maybe that’s just how Duats displays the data. Anyway, in all 3 cases they use the same observation/model data from 0000Z. The first, 6-hour forecast becomes valid at 0600Z, 6 hours after the observation it’s based on, and is good for a 7-hour span until 0900Z. The second, 12-hour forecast becomes valid at 1200Z (12 hours after the observation) and is good for a 9 hour span until 1800Z. And the third, 24-hour forecast becomes valid at 0000Z the next day, 24 hours after the observation, and is good for a 12 hour span until 0600Z.

   And this returns to the question: if we want to take a flight at 0500Z, do we use the forecast for 0600Z which isn’t valid yet or do we need the forecast from 0000Z? And if we want to take a flight at 0900Z do we use the one that became valid at 0600Z and really doesn’t forecast beyond our departure time or do we use the forecast that starts at 1200Z and won’t be valid for 3 hours after we depart?

   Note that the 0600Z forecast begins its “for use” time at 0200Z, which is 2 hours after the observations. According to one flight instructor’s discussion on the topic, that time “is when the data should be available. Thus the forecast should be usable as soon as it is available,” and “Note also that the For Use time indicates that the data was for use as soon as released.” [http://www.flyingactivity.com/mod/fileman/files/FBDoc.PDF] This means that we can use a forecast for a flight that begins during the for-use time but before the valid time of that forecast.

   And from another source at the NOAA: “EFFECTIVE TUESDAY… SEPTEMBER 27 2005 AT 1200 COORDINATED UNIVERSAL TIME /Z/…THE FOR USE TIMES SPECIFIED IN THE UPPER WINDS AND TEMPERATURES ALOFT 6 AND 12 HOUR FORECAST BULLETINS WILL BE CHANGED TO COVER THE GAP BETWEEN THE ISSUANCE TIME OF THE BULLETINS AND THE STARTING TIME OF THE INITIAL FOR USE PERIOD. THE VALID TIMES OF THESE BULLETINS WILL NOT BE CHANGED. THERE WILL BE NO CHANGES TO THE UPPER WINDS AND TEMPERATURES ALOFT 24 HOUR FORECAST BULLETINS.” [they love their all-caps; http://www.nws.noaa.gov/os/notification/old/tin05-54For_Use_Times.txt%5D

   In practice, I would use the earliest forecast that comes up in the briefing and overlaps my flight. For 0500Z I’d use the one that’s valid at 0600Z because that’s the earliest one available. For 0900Z I’d plan more heavily on the 1200Z forecast but I’d keep the 0600Z forecast in mind in case my navigation wasn’t working out based on the winds I was expecting.

   I hope that answers your question because honestly that’s the clearest answer I could find. It appears they have standard valid times every 6 hours, and then forecast for a block of time around that valid time.

   +1 Votes 1 Votes  0 Votes