2013 Blizzard on Agripas Street by Hanay [CC BY-SA 3.0] via Wikimedia
Are the "stars aligning”? Many atmospheric indicators — the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and the Madden Julian Oscillation (MJO) — are all suggesting that large scale atmospheric conditions are favorable for building a deep trough in the eastern Mediterranean. Such a trough is forecast to affect our area from Wednesday night into Friday morning, and should bring a good possibility of snow to Jerusalem and surrounding higher elevations.
We've spoken previously about the AO and NAO. As these indicators turn positive, cold weather is often found over Greenland (NAO). The relatively cold air over Greenland tends to build a ridge of warmer air over western Europe. Downstream of this ridge is often a trough of colder air that bends its way southward towards the eastern Mediterranean.
In contrast to the AO and NAO (which are based on static atmospheric parameters), the MJO transverses the tropics, often returning to its starting point in 30 to 60 days. As it transverses the globe it can bring severe weather to the eastern U.S., and when aligned with the other two indices it can enhance trough development in our area. As pointed out by a colleague at Jerusalem Weather Forecasts, the MJO is forecast to be in Phases 7 and 8, which are the phases of the MJO often associated with stormier weather in our area as well.
And, so it will be. A chilly winter storm brought rain from last Shabbat into today, with snow on the Hermon. It should then remain chilly until late Tuesday, when temperatures will start to climb ahead of our approaching storm. Temperatures should then fall off sharply Wednesday night and heavy rain in the central mountains should transition to rain and snow — and possibly all snow — into late Thursday or Friday morning.
For snow-lovers, the probability of snow in Jerusalem later this week remains up in the air. Yesterday, the forecast ensembles reverted to their previous forecast positions with a low forming closer to Crete than Cyprus. However, the latest update (12z) has intensified the low further east and southward. Moreover, wind speeds have increased to gale force, and precipitation amounts have increased by about 30% to relatively high amounts for typical winter storms which are generally less than 50 mm on average.
In any case, the resulting storm will be the interaction of a low over the eastern Atlantic, which is forecast to move southward of England, and another low moving southward from Scandinavia. Both storms are originating in areas with relatively low atmospheric sampling — especially the storm over the eastern Atlantic.
Weather — like politics — is local. We won't know the details of the storm until really only Monday (three days before) when we know how these two storms will interact and affect our weather. In any case, it will get winter-cold for several days. Just how much, though, is unknown.
Image credit: The Jerusalem Herald
Dr. Lynn is a lecturer at The Hebrew University of Jerusalem, Earth Sciences Department. He is also CEO of Weather It Is, LTD, a company that specializes in reducing weather risk. Click here to read more of this writer’s work in The Jerusalem Herald.