While waiting to board my Alaska Airlines flight from Juneau to Seattle April 9, the all-to-common announcement came over the PA about a likely weather delay. As I happened to have my laptop handy with Internet access available, I quickly checked the Juneau airport weather observations. I did not see any weather issue that would keep the 737 on the ground. The ceiling and visibility were way above minimums. The wind was strong, but pretty well aligned with the runway…not too bad. Here are the observations in METAR format (click here for help in reading them).
PAJN 091353Z 12021G36KT 10SM -RA FEW018 BKN036 OVC050 06/02 A2918 RMK AO2
PK WND 12036/1349 PRESFR SLP880 P0001 T00560022
PAJN 091453Z 13028G37KT 7SM -RA FEW013 BKN032 OVC045 05/03 A2914 RMK AO2
PK WND 13041/1431 SLP867 VIS LWR S-SW P0000 60004 T00500028 58048
PAJN 091553Z 12022G33KT 3SM -RA FEW013 BKN032 OVC045 05/03 A2912 RMK AO2
PK WND 12036/1501 SLP859 P0004 T00500033
What was I missing? Before I could dig deeper, one of the pilots got on the PA and re-educated me, and the whole crowd–mostly seasoned Alaskan flyers with way more weather and aviation savvy than you’d find at a typical airport down south.
The problem was with wind, wind shear, and the resultant turbulence along the departure path rather than at the airport. There are not many options for departing Juneau ( a fact Juneau residents are acutely aware of), and with the full load we had, none that could bypass the areas of channeled winds coming out of the mountain valleys.
The surface chart shows the big picture. A strong low was making its way toward Yakutat, keeping off the coast, but ready to run its front over the panhandle. The map is for about 3 hours before the weather situation we were facing, so the front approaching the coast was probably right on our doorstep. The strong wind with a low like is made very turbulent as it passed between peaks, over ridges and accelerated down channels of the panhandle.
Surface chart for 12z 9 April 2013
A few years ago it would have taken a very careful look at many weather factors and a good amount of local knowledge to call this situation correctly and keep a plane from flying into the jaws of extreme turbulence, without grounding all flights. Now we have the JAWS of life for pilots and passengers: The Juneau Airport Wind System. This system uses an array of wind observations to predict the likelihood of moderate or severe turbulence as would be experienced by a Boeing 737 aircraft. Some of the wind obs are from the airport and surrounding ground-based anemometers, several from ridge and mountaintop anemometers, and a wealth of data streams in from three ground-based profilers, which detect the wind from the surface up through several thousand feet. Computer algorithms based on research aircraft flights flown in various weather situations grind out real time assessments of current turbulence conditions. Here’s what it looked like on the morning of April 9. [At the time I thought it a strong storm for April, but today, May 1 there is similar storm, somewhat weaker, but very vigorous for May, and with severe turbulence again flashing up on the JAWS page! Find the current conditions at http://pajk.arh.noaa.gov/jaws/jaws.php]
Juneau Airport Wind System screen capture for 15z 09 April 2013
Note the “bottom line” block in the lower left showing any alerts: in this case four of the six areas had severe turbulence expected. The data on this page is well labeled, and there is much more info in the help page, and a good background of the system linked from the help page.
Shortly after our Alaska Airlines crew briefed us on various options, one of which involving lightening the load and using a different departure path, they said conditions had improved and they wanted to load up and take off as soon as possible. No doubt they had a constant eye on the JAWS data, and we climbed above the jagged peaks with only light turbulence. We will never know if the severe turbulence was there a short while earlier. I’m not volunteering to challenge this system. The research has already been done, and from what I know, done well. This is truly a valuable system and a wise investment by the government which has doubtless saved much inconvenience, discomfort and possibly lives.
There’s more. On the descent into Ketchikan, we endured quite a bit of turbulence, easily in the moderate range for much of the time. People applauded after we were safely on the ground. While Ketchikan does not have the mountains around it quite like Juneau, just the strength of the wind over the lower terrain was enough in this case. Here are the METAR obs surrounding the time. Note the 50 kt gusts from the roof of the FAA facility.
SPECI PAKT 091405Z 14022G35KT 6SM -RA BR OVC021 07/07 A2954 RMK AO2 PK WND
13043/1354 ROOF WIND 12035G50KT P0000 $
PAKT 091453Z 14020G32KT 10SM BKN019 OVC024 07/07 A2949 RMK AO2 PK WND
13043/1354 RAE09 PRESFR SLP986 ROOF WIND 12035G45KT P0000 60022
T00670067 58025 $
PAKT 091553Z 14020G28KT 4SM -RA BR OVC021 08/07 A2949 RMK PK WND 13034/1524
RAB36 SLP985 ROOF WIND 12035G50KT P0001 T00780067 $ VIA AUTODIAL
Here also is the morning balloon sounding from Annette Island, a short distance to the south. The 65 kt (33 m/s) winds matched the winds from the Juneau profilers, but at a much lower altitude!
balloon sounding for 12z 09 April 2013 at Annette Island, AK
Now after the concern for turbulence on coming out of Juneau, then hitting a lot if it (without any prior talk) going into Ketchikan, the crew made sure to warn newly-boarded passengers about turbulence on the continuing southbound takeoff. It was then quite smooth, of course. That’s the weather business for you.
