Weather Forecasting – Art or science?

If you live in NZ, chances are you are obsessed with the weather.  My husband (who is from Utah – where the weather is continentally stable) reckons that weather and sport are New Zealand’s biggest obsessions.  And rightly so.  Before the internet, I tried to never miss the weather on channel one at 6.55pm.  I’m happy I’m no longer so restricted to the 6pm news but I still try to see the news weather if I can.

As a backcountry ski guide who skis 5-7 days a week, weather forecasting is essential for planning my activities.  Last Friday I began one of my Craigieburn Haute Route trips at Craigieburn Valley and travelled to Cheeseman the same day. On Saturday the plan would be to head across to Mt Olympus. For weeks now, we’ve enjoyed fine, stable weather during midweek only to see it crap out for the weekend.  True to form, last week’s forecast predicted deteriorating weather heading into the weekend.  The good news was that the low would cruise right across the south island, hopefully bringing some easterly snow at the tail end of it’s passage.  The cruel news as Friday approached, was that the storm would begin right as we did.  The tricky part of forecasting was the call on wind and rain/snow levels.  Too windy and wet and conditions would be too dangerous to travel with a big group (there being less of a margin for error with a larger group size in rain and wind).

Sometimes I wish I was better at physics and had better skills for weather forecasting but when it came to Friday, the best thing was to get up early and look out the window.  Hazy stars were visible and it was calm with no frost.  I sat down at my computer and ran through all of my favourite weather forecasts in order to get an idea of the rain/snow line and wind speeds.  All forecasts agreed that winds would be calm but the freezing levels differed per forecast.  My co-guide and I decided that we’d need to stay high – above 1500m to avoid the risk of getting into the rain zone. No wind made it good enough.

I recently asked some local experts where they got their weather forecasts.  My neighbour, author of the guidebook to New Zealand Backcountry skiing, James Broadbent recommended WindyTY.  This site gives a broad overview of Southern Hemisphere wind patterns and direction and you can select temperature and rain/snow as well as wind speed.  The format is similar to that of Google Earth so it is handy for general pattern identification.  I often use this for the general picture.

Doug McCabe, head of snow safety at Broken River and a year round patrol leader in both NZ and the US kindly sent me his process.  A man of restraint, Doug avoids the temptation of going straight for forecast models like or the computer generated ski field models on Metservice.  He uses a combination of weather station data and the simple act of being out at the weather station, on the mountain, to establish a baseline or “nowcast”.  Like me, he checks out weather radar for the South Island to get a sense of precipitation location, intensity and movement.  Both of us have taken the same Mountain Weather Forecasting class in the US and from this experience, we seek out the pressure charts at the 700millibar level (approximately 3000 metres) for wind speed and direction at ridge top and above.  I’m happy to report (since these are the forecasts I mostly use) that Doug believes that both Metservice and MetVuw do a good job at providing this information.  Doug additionally recommends for the big picture.  This site was unfamiliar to me but it’s actually very similar to WindyTY.
My next port of call was Luke Armstrong.  Ops Manager at Porters, Luke was a mine of information.  The week before he’d shown me a weather site that was completely new to me.  It’s called FNMOC and is put out by the US Navy and Luke uses it for the big picture.  Unfortunately my server did not allow me access to it as it is a US government site.  However, I did read the FNMOC guide to forecasts and analyses.  Although very technical, the usefulness of this model was in it’s ability to predict rain/snow levels using the 540mb line.  You’ll need some time to work this one out!

Luke did place credibility on the Metservice “Three-day rain forecast model” and this is one that I frequently refer to also.  Three days out seems like a reasonable prediction in most situations.  The Metservice model is automated and is thus less confusing to look at the the bright pinks and blues and tight isobars of MetVuw (which I still refer to anyway).  We were both dismayed last Friday by the Metservice computer generated forecast for the Craigieburn fields that gave a prediction of snow to 800 metres but a 1700 metre Free Air Freezing Level (FAFL).  Basically this created the distinct impression that Metservice, like the rest of us, were uncertain about snow levels as the low crossed the South Island.  It could snow to low levels or it could rain to high levels.

As it turned out on Friday, we made it from Craigieburn to Mt Cheeseman, thanks to the hospitality of lunch at Broken River’s Palmer lodge and the friendly patrol who let us ride the BR main tow for faster progress.  We hedged our bets on the FAFL and  never descended below 1500-1600 metres unless passing through a ski field and we enjoyed a calm, snowy but wet day with limited visibility and great skiing up high.  Various forecasts had predicted anywhere from 15-25 cm of new snow and we ended up with 15cm new for the rest of the trip across the range.  My studious attention to the forecast in the preceding week had paid off – but not as much as advice from Ski Guide Trev Street, who wrote and told me that his favourite weather forecast would always be  Why? “It never crashes, it’s constantly updated, 100% accurate , and it works off line….”

Get up early and look outside.  If you are not in the mountains, ring someone who is.  Or go direct to the ski field’s website and trust that they are telling the truth.  They’ll provide a 6am nowcast.  Bear in mind that it may change by the time you get to the field – and that’s where a solid weather forecast comes into play.  Look at several of them and get the average prediction for temperature, wind, FAFL and cloud cover.  Good luck!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s