Outflow and Katabatic Conspire

Looking out the window and doing my daily weather review for paragliding and realizing that, as expected, it has started again: with days getting shorter, solar delivery is for a shorter time, and it is also less intense.

Just one of the many cycles we enjoy in paragliding, during the seasonal patterns of change, as we progress in our loop around the sun.

One great way for Vancouver paraglider pilots to save a drive out to the valley that would result in para-waiting, something we have managed to avoid almost completely for many years, is to take the time to make some observations.

Vancouver paraglider pilots are in an optimum location in that they are central to sites in the Vancouver, Pemberton, Chilliwack, Bellingham and Seattle sites and so can choose the best site to account for daily and seasonal wind and weather changes.   

With solar delivery shorter, the land-breeze (out-flow) persists longer and Katabatic (downhill) flows also persist longer.  These two conspire to amplify each other to produce big out-flow that generally gets progressively longer and stronger as we go through September. It is as if a "big switch" goes off, usually in the last week of August to the first week of September, and the Fraser Valley starts to "exhale" longer than it "inhales". So, often even sunny days result in outflow and un-flyable lee-side conditions.

A good hint to watch for outflow is that if the visibility is exceptional, and from Vancouver you can easily and clearly see distant mountains on the horizon, including Baker and others: it is because the smog has been pushed out of the valley to the sea by the land-breeze.  A very blue and clear sky is a good clue that it will be beautiful to do other things, but not paragliding in Chilliwack, due to east outflow yielding lee-side conditions at most of the west facing launch sites found there.

Here is a more typical in-flow condition in the Fraser Valley earlier in the season. Note the brownish haze/smog in the valley pushed in from the sea-breeze. 

If you use the iParaglide Paragliding Webcams and Paragliding Weather Tool widgets in the lower right of each screen page, an outflow condition will be easy to predict, by clicking from top down, in order. 

In this case, the 3000 ft winds aloft forecast for YVR were east (080) to 26 km/hr (14 knots) until 14:00 (21 Zulu) and then easing to light and variable.

Similarly for Seattle, the 3000ft winds aloft were east (100) to 23 km/hr (14 mph) until 13:00. 

Using the real time ground based wind measurements Vancouver>Abbotsford>Chilliwack>Hope: progressively further east and into the narrowing Fraser Valley gives the ability to quickly picture the status of outflow or inflow at various points.

Here we see Chilliwack producing some gusty outflow in the morning: 

Further up the valley in Hope it is also outflow, as expected:

Note that outflow manifests as a North wind in Pemberton, Squamish, Bellingham and Seattle sea level areas. 

 

The good news is the start of outflow in the Fraser Valley also signals the time to head to other great locations to go paragliding! 

Today will most probably be epic at Pemberton, which generally has the opposite problem.  When inflow (south) occurs it gets amplified by anabatic flow and further amplified by venturi effect caused by the narrow and tall valley.  This results in the "Whistler Express":  too strong a south wind.  By contrast, with a light outflow condition, this tends to cancel out the other effects, and you end up with glorious light and variable flows, which means mellow and beautiful anabatic lift on most sides of the mountains! What would you like to fly? A triangle? Square? A giant double sided "Y", enjoying lift on all sides following the intersecting valley? Go... 

A detailed weather check each morning helps one learn these daily and seasonal patterns and see how they change.  

In turn, paragliding pilots can flock to the right mountains and enjoy more flying...

Comments

[MarkC]

My 'on the ground' sign has been the flag at the firehall between Mission and Deroche. Just before 11, it was nearly limp meaning the outflow was subsiding and the inflow starting to take hold - even though the winds in Agassiz were still showing 10 to 40 from the NE.

At the mountain around 1130, it was blowing downhill a plenty - the windsock doing the dreaded salmon swimming upstream impression. By noon it died off and was dead calm. Early afternoon, reverse launch-able cycles. By mid afternoon, the thermals were tame and wide enough for a gaggle of a half dozen to share a single one.

If one has the time and gas to burn, it doesn't hurt to come and wait and observe.

[Dion Vuk]

Agreed there are windows to fly once outflow subsides.

Sep 6, as predicted, outflow till 12:00. We had a pod kiting in the strong east down at Harvest Market and getting couple of flights in later on when the switch to inflow came. We also had a pod headed up first thing in the morning to Pemberton and were rewarded with an extended flying window and flights to 8000 ft! :)

Sep. 7 the outflow was lighter and it was flyable earlier in most Valleys.

Since then Degas has been reporting some pretty nasty gusty outflow in Chilliwack-Agassiz most days until 15:00.

In general, the predictability of when and or if the outflow will end decreases as we progress further into the year. It becomes a bit of a crap shoot: sometimes the outflow will subside, others it will blow right through the day.

This time of year we tend to head to other valleys where outflow does not last as long and we can get more flights, with safer conditions avoiding lee-side, and often better performance as well.

Occasionally, the outflow doesn't nuke in the morning in Chilliwack and we head there on select days. :)

Cheers