By January 20, 2020 Read More →

SNOW – Will it or won’t it?

From Jerusalem Weather Forecasts: A wintery week may be upon us! What we can currently expect and need to hope for to see snow…

SHORT SUMMARY (for those not interested in reading all the details): This coming Tuesday a small surge of cold air will dive southwest from Russia bringing with it a decent chance of a wintery mix of precipitation throughout the day, with rain mixed with ice, and possibly periods of light snow that will not accumulate. This cold air will set the stage for potential even larger dose of cold air sent on Friday from the same Russian location, that in the best-case scenario, could bring Jerusalem its most significant snow in 5 years. However, there is a strong chance this cold will dive too far east from us to produce anything and in this time range, a LOT can change (for the good and bad). With that said, there is reason to hope that the snowier solution will win out and we’ll only begin to start knowing more conclusive details as we get closer to the event.

LONG SUMMARY (with all the fun and interesting details so you can not just know, but also learn why I typed the short summary above)

And here are! Winter may be finally be kicking into high gear this week. After several weeks that have sent Jerusalem more rain than average for this time of year, thanks to an overhead pattern that has been very conducive for wintery weather in this region, we may be finally setting our sights on the stuff this page and its fans hope and dream for: Snow!! 

But before we get too excited and jump into things, I MUST remind all of you that

A) I am NOT a professional meteorologist! I am in frequent contact with professional forecasters and I have made studying weather into a serious passion/hobby of mine for the past decade – but I am nonetheless not a professional meteorologist.
B) My page focuses ONLY on snow potential for Jerusalem. I do not discuss rain or any other weather throughout the winter unless the rain comes with snow and I mostly keep my focus on the Jerusalem region.
C) Snowy weather is extremely hard to predict in Israel, mainly because such weather is quite rare here. Additionally, the weather around the entire world is a very tough science to predict with a high degree of accuracy altogether. But, I strongly believe the weather is also really FUN and interesting to predict, discuss and learn. So my greatest goal here is to teach you all about potential snow events, even if they don’t pan out so much in the end. In other words, sometimes the process is just as fun as the result!

Ok, now onto the potential snow coming up this week!…

It is a long, but I think very interesting and informative read that will help you understand the importance of the jet stream, cold troughs and warm ridges, and the importance of how cold interacts with the Mediterranean Sea to produce snowy precipitation in Jerusalem.

Onto this week (yet again)…

For the past several weeks, we have been in a very ideal overhead pattern with both a positive NAO and AO, which (as you’ll understand in the post I linked above) favors the jet stream to create a very warm ridge over Europe that then often results in cold air troughs further east that dive into our Israel region. You likely all may have noticed this ideal pattern simply by the copious amounts of rain we have received in the past several weeks – rain, which has brought us well over our annual average for this time of year. You can also see the results of this by going north to the Hermon mountain, which has already received tons of snow and started their ski season quite early several weeks ago (a trip up there by yours truly is definitely pending!).

However, all of these cold air troughs that have developed so far in this favorable pattern have first swung well west of us and then traveled east into our area. These sort of troughs are great for rain because the cold air generates a lot of moisture over the Mediterranean Sea, but the sea also warms the lower-level air too much for any of this precipitation to fall as snow in Jerusalem. Significant snow tends to only happen in Jerusalem if the trough of cold air dives perfectly from the Russian cold air source north of us in a slight diagonal tilt (meteorologically called a “positive tilt”) so that that the trajectory goes from the northeast to southwest. Doing so then brings the cold air over the Mediterranean nicely enough so that the cold interacts with the warm waters creating strong convective precipitation. But this orientation also keeps cold over the sea waters short enough so that the lower cold air does not moderate warmer, allowing for the precipitation to stay frozen and fall as heavy snow (it really is quite a fine line to walk on!).

And this sort of cold trough set up is what we may be looking forward to now…

See IMAGE #1, which is a GFS model forecast for Tuesday. This is not by any means an image for a major snowstorm and perhaps any snow at all. But it is important because it shows the favorable pattern we are in now with the jet stream (black line) sending a lot of warm air over Europe (except for a small low of cold air over Spain) and then diving northward from the Russian area with that diagonal/positive tilt. In this case, we indeed get some Russian cold air that will actually bring lower level 850mb air below freezing (which is necessary for keeping any precipitation that falls frozen). But the cold trough is on the weaker end – we would want to see more blues than green over Israel for it to be more ideal, and it, therefore, produces very light precipitation with its influence over the Mediterranean. These sorts of setups commonly send rain to our area with frozen precipitation, mainly ice and sometimes periods of light snow, mixed in. And this is what I anticipate happening over Tuesday. If snow even does fall at that time, I anticipate it will be for very short periods of time with no accumulations.

With that said, it is VERY IMPORTANT to note that just a few days ago, this first trough of air was being forecast to miss Israel entirely to our east! Before last Friday, it was hardly on any forecasters mind that snow could even be a remote possibility with this coming event on Tuesday. In fact, it was mostly assumed that Tuesday could be partly cloudy with no precipitation falling at all. And I am being careful to point this out as it relates to the next coming trough of cold air forecast to arrive at the end of the week Friday…

That brings us to IMAGE #2: This is the GFS forecast from last night Saturday that started getting everyone quite excited, because as you’ll see, the jet stream again curves upwards this time more over central Europe, and then dives down in the most ideal northwest to southeast fashion from Russia into Israel, and this time, it’s sending an incredible amount of cold air into our area. What made this image so exciting was that the more reliable European weather model was also forecasting this to be the case earlier on Friday. When we saw both the American GFS and European model forecast the same general idea for this type of cold air trough set up, it significantly spiked the hype and hopes that something big could be on its way. If the forecast from Image #2 were to verify, Jerusalem would likely receive its most significant snowstorm since our last big snow 5 years ago in Feb 2015 (which gave us between 6-8 inches of snow across the city).

But then, in a matter of just a few model runs, the American GFS model this morning forecasted this cold air trough to swing much further east of us instead, which if verified, would bring mostly nothing to our area on Friday. Additionally, the European model from this morning still showed the trough coming west enough to produce some snow in our area, but when comparing it to its previous runs, the Euro showed the cold trough forecast to dive more east than before, which is not a great sign.

It is at this point, that we must mention the importance of not getting too caught up on the details of a single operational model run, which on both the American and more reliable European model, always forecasts these sort of significant shifts in this 4-7 day time range that we are still in for the Friday storm. It is also crucially important to rely more on the Ensemble model runs, which are better for longer range forecasts than the single operational runs like I have posted in image #2. Operational runs use more data to create their forecasts but are effectively one single idea of the computer model run. Ensembles, however, use a little bit less data but are comprised of multiple ideas or solutions per run. These many ensemble members use slightly different algorithms to calculate what could happen, and they then produce a mean map that combines all of this data together, which is a lot more helpful toward building an idea of what could happen further in the future than the single operational run. The American GFS model has 20 ensemble members, while the Euro has an impressive 50 ensembles!

In IMAGE #3, you can see the latest Euro ensemble mean map, and indeed, there still is a very strong storm signal for Friday. And yes, the cold trough is a bit too far east than ideal, but it is certainly better positioned than some of the latest single operational model runs from the American GFS model today. Due to the Euro’s overall better statistical accuracy both with its operational and ensemble runs, and its more consistent forecast for the position of the trough the past few days, I am going to favor it more for the time being. However, the shifts east that we are seeing today certainly keep the Friday storm as a big MAYBE when it comes to snow.

Therefore, I am by NO MEANS confident it will snow on Friday, and it very well could just be a very cold sunny day with literally nothing falling from the sky.

However, with that negativity now out of the way, I do think there are some good reasons to be optimistic that the snow solution could win out in the end! And that brings me back to the closer Tuesday no big deal storm and the reminder that this storm until just a few days ago was also being forecast to be well east of Israel, and sure enough, as we got closer to the event, the models shifted west. The same thing could easily happen with the Friday storm. Secondly, the presence of this storm coming first does a few helpful things: firstly, it cools the atmosphere in advance ahead of the storm following it, which will mean much less cold will be required to cool down the levels of the atmosphere if the second storm does indeed come here. Secondly (and this point may need some refining later), storms are areas of low pressure that sink the air in the atmosphere, and due to the laws of basic physics, two low-pressure systems attract each other to one another. If the Tuesday storm indeed comes further west as predicted, it may be easier for the next trough to get pulled toward the westward trajectory. Finally, the overall winter trend with our cold troughs so far, as you recall above, has been to dive west, and I do believe there is a case to be made that the overall winter pattern can be something forecasters lean on regarding where things will go.

Alrighty, so there you have it! As you can see the potential for snow this week is certainly present, but just like with any storm system 5-6 days away, uncertainty remains and no one should be making any conclusions at this point.

If you want snow, pray the weather models begin to shift the Friday trough less east and more west (but not too far west, of course, cause then we’d just get rain, but again, too far east isn’t good either, because then we get nothing – whew! see how hard it is to just get some snow here!!?)

I will do my best to provide updates as we get closer, until then, have an awesome week

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