Tuesday, May 15, 2007

Heavy snow in May ?

When I woke up the other morning to check the weather at the destination I had to rub my eyes pretty hard to make sure what I was seeing was true. As you can see it is calling for a daytime of high of 16 degrees Celsius but in the current conditions box it is showing Heavy Snow.

Now by just looking at the temperature, winds and visibility for it to be snowing heavily everything doesn't jive. It's 3 degrees, winds are light and visibility is 15 km ( about 10 statue miles). When was the last time you could see 15 km's with heavy snow ?

So I went to check the GFA (graphic area forecast) and it was a big high pressure system sitting over the whole area and not a cloud forecast in the sky. I then proceeded to check the METAR (aviation hourly weather report) and found this....


METAR CYKF 130900Z AUTO 33003KT 9SM CLR M01/M04 A3029 RMK SLP265=
METAR CYKF 131000Z AUTO 29003KT 9SM CLR M00/M03 A3031 RMK SLP270=
METAR CYKF 131100Z AUTO 36003KT 9SM CLR 01/M03 A3033 RMK SLP280=
SPECI CYKF 131125Z AUTO 36004KT 9SM +SN CLR 03/M03 A3034=

TAF CYKF 130939Z 131022 35005KT P6SM SKC
BECMG 1315 07008KT

As you can see the weather has been steady at 9 miles visibility and no clouds for the past 3 hours to all of a sudden snowing heavily. The forecast below is showing for the same time period better then 6 miles visibility (they don't forecast a higher value then 6) and SKC or Sky Clear.

To save money Environment Canada is slowing getting rid of human observers and replacing them with AWOS (automated weather observing system) units. The AWOS units for the most part do a great job at things such as temperature, dewpoint, wind direction, speed and most important altimeter settings. But because they are limited to interpretation and subject to the occasional glitch like reporting heavy snow :) they need to be compared to what is happening around the area to make sure it is reporting what actually exists. With human observers they can also add helpful comments in the METAR's also. Such as fog rolling in quickly or visibilty lower to the north due to thunderstorm and rain. I don't think there is a Weather Observers day, but we sure do appreciate the great service you provide !

I remember once shooting an NDB approach with about a 30 knot crosswind from the west (reported) but having to crab about 10-15 degrees to the east. The whole time I thought something was wrong with my instruments but when I broke out visual I realized the winds were 180 degrees from the reported winds.

What had happened was the wind vane had got frozen while facing the west and no matter the direction of the wind it was reported as always from the west. Another experience was the AWOS was reporting 1/8 of a mile in fog as I was trying to depart, but the actual prevailing conditions were sky clear with a patch of fog near the shore which happened to be were the AWOS was located and with it's limited ability to look straight up where it is reports it as being the visibility around it.

To make a long story short, when using AWOS stations as a determining factor if you should go or not (especially VFR) make sure you compare all your weather information to make sure the picture it is painting is similar to the area around it.

Basically....I trust in God, and everything else I check :)

Safe flying,



Quodlibet said...

I hear ya. I remember going somewhere on a medevac where it was supposed to be nice and the AWOS was saying it was down to 1/4 mile or something like that and then we broke out at 10,000 feet! They're wacky machines. Glad you had no heavy snow in May (one never knows in Calgary though - Calgary is it's own wacky wx machine)!

Flyin Dutchman said...

What gets me is how they can be completely wrong in regards to current conditions and we have to abide by what they say regardless of what we see.

With the new approach ban we can PIREP the visibility but another example was with the same AWOS yesterday. A thunderstorm (to read 50,000 foot CB :) had just rolled through and the AWOS was reporting a mile vis so I couldn't get the visual approach to an airport I saw 15 miles away.

I got the contact of course but still :) Oh well it's going to be the future so may as well deal with it :)

Safe flying