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By January 13, 2019 Read More →

Severe weather conditions expected for mid-week

POTENTIAL SNOW STORM UPDATE: A unique winter storm will likely bring our first snowfall since 2015, but how serious the storm ends up being for Jerusalem is still a big unknown…

From Jerusalem weather forecast:

Hey everyone! So here’s your detailed post about the snowy potential we are facing this week:

(Want to skip all the juicy details and get the bottom line? Scroll down to the TLDR summary.)

As I said quickly last night, the chances are still strong that we’ll finally see our first snowfall in 4 years in Jerusalem sometime on Wednesday this week. As with most snowstorms in Jerusalem though, there is a lot of uncertainty in this range, and a few things too that could go wrong and severely limit just how memorable this storm really ends up being.

Let me first mention just how unique this storm track is for Jerusalem snow. Typically, we receive our best snows when the cold air dives southwest originating first over Russia/Siberia. This enables the cold to go over the Mediterranean just enough that it picks up some moisture, while still preserving the cold on the way down. Whereas, when cold storms originate first over Europe, they typically dive south and start very far west of us, and then spend too much time over the Mediterranean as they travel east, allowing the lower level cold to moderate. These storms bring lots of precipitation and moisture from being over the Mediterranean for so long, but they produce lots of rain and hardly ever produce snow.

The only way storms that originate over Europe can produce snow is if they go the trajectory we are seeing now with this storm. Check out the first 4 pictures I posted and scroll through each one – you will see the cold originates over Central Europe, and instead of diving too far south at first to begin interacting with the Mediterranean sea, it instead dives in a southeasterly trajectory, first over Greece, then Cyprus, and then finally down into Israel. In this path, the storm spends its most time over the Mediterranean only when it passes Cyprus and heads into Israel. This luckily greatly limits just how warm the Mediterranean can moderate the lower level cold. Nonetheless, this path does allow it to pick up plenty of moisture along the way, and since it’s cold stays largely preserved, we can be a lot more hopeful that snow can occur with this storm.

In fact, this trajectory is currently forecast to send in some of the coldest lower level air I’ve seen for many of our past storms. The 850mb values reach as low as -4c in some model runs, while nearly all models are showing 700mb values (the level of the atmosphere where the snow typically begins falling) reaching -14c. These are super cold temps for these levels, great for snow, and quite impressive to say the least.

So where can we go wrong? Right now, that looks to be mainly on the 500mb level of the atmosphere. This level is most responsible for creating strong energetic interactions between the high and low levels of the atmosphere called convection, and convection especially occurs in snow storms when the 500mb temps are able to sink to -25c and below. Convection creates strong thunderstorms and fast moving up/down motions of air between all the relevant atmospheric levels. When this happens, it nearly guarantees that 1) we’ll receive heavy amounts of precipitation and 2) the coldest lower level air will ultimately reach the ground, rapidly dropping the temps we feel below 0c so that the snow created in the atmosphere stays preserved and falls as snow, and better yet, accumulates thereafter.

Remember that miserable cold mostly rainy/icy storm we had last week? That happened because the 500mb temps simply did not get cold enough to create convective precipitation. Snow was created above us, but temps at the surface level stayed way too warm because there was no convective dynamic to send the cold lower level atmospheric air to the surface. This is why all we witnessed then was the melting death of millions of snowflakes on their way down to the ground 😭.

Right now, each model run has been varying to just how south the coldest 500mb temps will dive as the coldest air passes over Israel. The GFS American model has been showing some pretty poor solutions, only bringing 500mb temps down to -23 at best. The European model, on the other hand, dives slightly more south, sending 500mb temps down to -28. In any of these runs, particularly because the lower level temps are as cold as they are, snow should be able to fall and reach the surface. But currently, only the Euro shows 500mb temps levels cold enough to bring accumulating snow.

What is so nerve-wracking about this set up is that the border between extremely cold 500mb level air that reaches levels as low as -31c is very sharp. Any time, 500mb reach -30 and below we can then be most confident of a heavy accumulating snowfall. If this storm could be forecasted by the models to just dive another 50km or so south, Jerusalem would then be in an incredible set up to receive convective precipitation and heavy snow that would no doubt coat the city in a memorable layer of white. In fact, all model runs are showing northern cities in Israel receiving this extremely cold 500mb air (places like Tsfas, and certainly the Hermon) – and it’s nearly certain at this point heavy snow will fall there. But can the storm just drop a tad more south to send that type of cold and snowy precipitation to us? This remains a big question.

It is times like these that is very important to look at ensemble runs, which are when the main models like the GFS and Euro run multiple variations of model runs (with a little less data than their main operational runs) that end up showing the many probabilities of what a storm may actually do. Barry Lynn’s website (a professor for meteorology who I speak with regularly) https://weather-it-is-israel.com shows this detailed data for Jerusalem, and in the last picture I posted here, you can see a screenshot of the latest ensemble forecast. Notice that at the 500mb level several of them bring temps below -30, while the average output of all of them is -26.7. These numbers are much better than what the single GFS operational model has been forecasting and show that there is still some degree of probability that this storm could produce major snow.

But also in that degree of probability, there are clearly some ensemble members that don’t go to very cold 500mb levels, and those members could be the correct forecasts too.

So at this point, we simply can’t be sure whether this storm will amount to much – but the chances are certainly there for something significant.

TLDR SUMMARY:

Truthfully, all I currently feel confident saying is that we’ll likely see a snowfall with light accumulations on Wednesday. The best chance for this snow is currently Wednesday late afternoon into the night which is when the coldest air is forecast to arrive. There is currently a 40% chance the snow will become very heavy, and accumulate to a significant degree. However, there is definitely a chance this storm will only send very cold temperatures, with sparse precipitation of mainly rain and ice in addition to some snow with no accumulations. The next few days will be very telling and will give us the necessary time to iron out all the details.

Stay tuned everyone! If you love snow, pray that the 500mb trough of cold dives just a bit more south than what is currently forecasted, as this will guarantee we are in for an awesome white snowy city Wednesday. ❄️🌨☃️❄️🌨☃️

It could happen! 😀 (But it could not too, grrrrr😠)

Have an awesome week!

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