Monday, December 12, 2016

Weather It Is (One Storm And Then A Jerusalem Snow Storm?)

Good Afternoon:

We're waiting for forecast updates.

However, our upcoming severe wind and heavy rain storm may be followed by extreme cold and heavy snow (in the mountainous areas, and even down to a few hundred meters of elevation) early next week.

Barry Lynn

December 12th, 2016

Today’s note is based on some e-mail correspondence that I received.  One of the correspondents prefers to remain anonymous, while the other (Jonathan Hoffman) contributed much of the material on the AO and NAO that follows below.

Basically, we often look for atmospheric “connections” between large-scale patterns we can measure, and the weather that eventually impacts our region – in this case with a potential large snowstorm.

For example: the following shows the forecast GEFS ensemble 850 mb temperatures for the next two weeks.  The ensemble of temperatures indicates that there is a high probability that it will be very cold, and even extremely cold.  If you combine this information with other information from the same forecast, you can infer that we may have a significant snow storm in about a week's time.

Jame Gleick is credited with the following:
Big whorls have little whorls
Which feed on their velocity,
And little whorls have lesser whorls
And so on to viscosity.”

In between the large atmospheric whirls, there are smaller wave patterns, which contain within them the stormy weather systems that impact our weather, and may bring our first significant snow in two years.

In this vein, I received an e-mail from an anonymous writer who wrote:

“A huge uncertainty in the teleconnections forecasts, especially after the 16th has been gradually resolved, it seems, in favor of a strongly rising AO  (Figure 1).  The NAO forecast is less certain, as forecast divergence still exists after 16th.

“The stratospheric PV (Fig. 3; Polar Vortex) is forecast to make a strong recovery. A lobe of it may pass close by, around the 24th, according to GEFS (Figure 4).

“What does this portend for the weather in our area? And, of course, everyone wants to know – ‘how does that affect our snow chances?’"

As noted by Mr. Hoffman: “Statistically speaking, Israel is more likely to receive cold air troughs with the polar jet when the NAO and/or AO are positive.” Such polar outbreaks increase our chances for snow. He correctly, I think, attributes this to what might be called an “open highway” in the streams of air that move across our globe.  “The polar jet is more easily able to flow north across the [Eastern United States and Atlantic Ocean] because it is not ramming into circular domes of air in the North Atlantic and Arctic regions.” One important factor, in other words,  is that storms that start on the northeast USA coast should have a clear path across the Atlantic ocean until they often come to “rest” in the area of Greenland, which then encourages a ridge (a bend to the north in the river of air) to form over the UK and western Europe, with a corresponding trough (bend to the south) of cold Russian air extending southwestward into the eastern Mediterranean. This, he notes “is more likely to happen with a +NAO and/or +AO.”

In contrast, if the AO and NAO were negative, storms would be less likely to move into Greenland.  The Jet Stream (streamers of high winds in the upper atmosphere) and storm within it would forced to stay south of that region thanks to the “dome”of cold air that has moved southward off the poles in association with the measured   negative NAO and/or -AO. Instead, storms would likely pass over the Atlantic Ocean, under the dome, and enter western Europe in a way that would start the European ridge further east such that it might even go over Israel. This would then send abnormally warm air into our region, killing the chances for snow.

Or, the trough would form over central Europe, directing typically wet, but not particularly cold air masses into our region.

Mr. Hoffman provides a couple of examples from recent forecasts for this week (starting December 11th, 2016). He notes the position of the polar and southern jet streams in Fig. 5 and the temperature anomalies associated with them. One notes how the polar jet bends northward over Europe, creating a ridge, and then comes downward with a trough close to Israel. This causes relative warming over Europe and relative cooling over Israel (and nearby countries). Note, also the southern Jet stream depicted here, which can funnel storms moving eastward across the Mediterranean into the trough, spinning up a potent storm in the base of the polar trough (just to our northwest).

Figure 6 shows more clearly the influence of a low pressure system and the NAO for creating this overall pattern. He points out that the low-pressure storm system is placed in the north Atlantic, a bit west of the UK, close to Greenland, and that a ridge forms over western Europe before dipping down as a trough over the eastern Mediterranean.

A similar storm situation, but stronger and better defined, may bring us our first significant snow in two years.

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