16-18 July

Outcome added…

The ensembles are showing a little bulge late next week. There’ll be some weather, but I think not in the league of what we’ve now come to expect from season 2014.

Update (11 July): What’s the defining characteristic of Australian snow weather? Rain. Even at high altitudes our winter precipitation snow:rain ratio only averages about 80% (water equivalent). This event is looking like the first major rain event of the season, with rain showing all day Tuesday (15 July), then showers Wednesday with snow not arriving ’till Wednesday night. The Bureau of Meteorology’s ACCESS-G model has Tuesday’s weak northerly feed looking very cold (thickness ~540*), but is it cold enough? Given the pattern that seems unlikely.

Update 2 (13 July): Tuesday – Wednesday’s weather remains surprisingly marginal for snow (the surface pattern says it really should be too warm). The Bureau of Meteorology’s high-resolution ACCESS-R model has 850 hPa temperatures near 2°C Tuesday night. At that time 850 hPa pressure is at around 1500 m elevation, suggesting a freezing level (which BOM don’t plot) near 1700 m. There may be snow at higher levels Tuesday night.

Update 3 (16 July): Tick the wet Tuesday night snow; resorts are claiming 10-20 cm (add grain of salt). As foreshadowed way back on 8 July what follows isn’t all that powerful, but there’s some “shape” to the modelled flow Thursday night (17 July), which may see a decent fall.

 

Outcome:

Another nice fall. The resorts claimed up to an aggregate 50 cm (Perisher Valley — geez guys, really; let’s see what the Hydro have).

 

 

* In meteorology, thickness conventionally refers to the atmospheric height difference between the 1000 hPa pressure level and the 500 hPa level, expressed as decametres (to avoid confusion with the isobar labels … just knock one zero off the metres). That’s roughly the thickness of the bottom half of the atmosphere by mass — pretty much the bit that does weather, called the lower troposphere. “Thickness” is just volume in the gas equation and that is of course proportional to absolute temperature (PV=nRT), so we can think of thickness as indicating the average temperature of the part of the atmosphere that makes most of our weather. It turns out that “540” (5400 m thickness) is the magic number that usually divides rain and snow at altitudes that matter. Well, err, sort of; there’s other things … actually lots of other things.  (Meteorology amounts to gravitationally bound three phase fluid dynamics on a rotating spheroid, driven by an uneven energy flux; it’s not exactly trivial. BTW, when the surface pressure is less than 1000 hPa the “1000 hPa level” will be below ground level. To compute thickness it is just estimated by extrapolation.)

2 comments to 16-18 July

  • Matt

    I’d love to see a TV weather forecaster one day read his/her 3 minute script with such technical precision… Hardly anyone would understand it but it would let everyone know how scientific the field of meteorology is! I think most people have no idea to be honest.

    • Gerg

      As an outsider I observe a major transition in meteorology over the last couple of decades. At the operational level it used to be an art, plain and simple. Sure you needed plenty of physical understanding, but the primary tasks were to estimate (guess) the current state of the atmosphere from very limited weather station and sounding data, then outright guess what it was likely to do next. Knowledge and experience helped, but “guess” is fair I think, and explains the cliches about forecast accuracy. Until recently old school meteorologists appeared to treat numerical forecasting with quiet derision … with some justification given the woeful performance of the likes of the Bureau’s old GASP model. That has all changed. Better models, much better computers, and particularly better data assimilation from satellite remote sensing see the current best models get pretty close from 7 days out (ECMWF) and rarely miss in any important way from 3 days (BOM ACCESS-R). The general community fails to notice. Retail weather is still all about tomorrow, when much more is available. And yes, few have a clue as to the complex mathematics and physics behind it all. Perhaps if they did they might begin to grasp that blind terror in the eyes of your average climate scientists (who is driving similar tools).

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