Monday, December 28, 2015

Weather It Is (Wind/Cold/Rain/Snow?)

Good Afternoon:

We've been talking about the weather for the last week or so, hinting at what might be, and now the hint has become more than a hint.  It's 12 Celsius, 41% humidity, and the winds are gusting to 35 km/h.  It feels like winter and strangely the weather map agrees.

We've been discussing a number of global atmospheric indices that have a possible influence on our weather.  Here's another (The East Pacific Oscillation -- thank you to Jonathan Hoffman for pointing this one out).

This index is forecast to go negative about the same time our next storm should arrive.  This index is associated with greater waviness in the atmospheric circulation pattern, and indeed it seems to be having an effect (perhaps with other global indices like the North Atlantic Oscillation which is suppose to return to positive in a few days as well) The interesting thing about our next storm -- or rather the forecasts concerning our next storm -- is that the latest 00 GMT Global Forecast Ensemble now shows much more variability (on the cold side) than before.  Along with the cold come several very cold ensemble members that drop down to close to -30 Celcius at 500 mb.  Combined with a 60% chance that temperatures at 850 mb will fall below freezing (and several predicting -2 to -3 C), we're getting indications that Friday and Shabbat could be snowy in the  upper elevations.

To put a number on it: the probability is now 25%, with a near 100% chance of measurable rain (although the ensembles are not indicating more than 50 mm yet, on average).

Moreover, with the cold should come very strong winds, even damaging winds.

It's actually warming a bit at 850 mb until the end of the day, but temperatures will head down thereafter until early next week.

Barry Lynn

P.S: It is necessary to consider that our weather exists within the global circulation patterns and hence these indices may be in part a reflection of our weather, rather than the cause. On the other hand, it necessary to send a block (ridge) northward into Europe  to allow cold air to dive southward and for a coupling of a southern stream storm with the northern stream Jet and strong baroclinic zone.

P.P.S: From  On days when the NAO Index is high, there are strong winds, bringing mild and wet conditions over western Europe. This is usually associated with colder weather here, as the block extends further north than usual.

P.P.P.S. Jonathan created composite maps of past snowstorms of the 500 mb height and height anomalies.

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