(Premium) Comparing This Arctic Blast to 2014 and 2015

The big debate among weather enthusiasts and meteorologists alike is whether NYC (Central Park) will hit zero or below tonight. This has not happened since 1994, and common opinion is that climate change along with the Urban Heat Island Effect would prevent NYC from ever going below zero again. While that thought process is not necessarily without merit, tonight is definitely the best shot NYC has had to go zero or below since 1994 — and perhaps should be even colder than the Arctic outbreaks from the past two seasons.

On February 20th, 2015, NYC got down to +2F, and on January 7th, 2014, NYC got down to +4F. There are certain criteria we look for to see how cold it can get in NYC: 1) 850mb temperatures, as they are a good indicator of the airmass, 2) Timing and degree of cold air advection — NYC due to the Urban Heat Island Effect does not cool off very much from radiational cooling, so they are very reliant on cold air advection, 3) Wind direction — generally a more northerly flow can more efficiently draw trapped colder surface air from the valleys, whereas a more westerly component has the tendency to downslope off the Appalachians and thus moderate somewhat when it hits us, and 4) Degree of mixing — if the surface air “mixes” with the airmass aloft, not as much cold air will be trapped at the surface, making it hard to truly go below zero.

When comparing this Arctic outbreak with the ones from the past two winters, most of the criteria above is either just as favorable, if not more favorable for cold than the previous two Arctic outbreaks. This is why we ultimately favor low temperatures in NYC between -2F and +2F, since it should at least get down to what we had last year, with a good shot to get even colder.

A comparison of the airmass at 1:00am during the morning of the cold via NAM analysis shows that this year should have 850mb temperatures at least as cold as last year’s, if not 1-2C colder than last year. It looks like right around -26 to -27C last year, and -27 to -29C this year. The airmass as a whole that’s coming into the Northeast is certainly colder this year with much more widespread -30C readings and they come very close to NYC, whereas last year did not have the same -30C readings. Since we got down to 2F last year, and 850mb temperatures valid at 1:00am look at least as cold — if not colder — then this year should get down to 2F or colder. Also notice the wind direction at 850mb — more westerly last year and more northwesterly this year. While both promote some downsloping, this year having a more northerly component should favor less downsloping this year than last. This further promotes a very realistic chance for NYC to hit zero.

While the airmass from 2014 was extremely cold, the timing of it was off. The coldest 850mb temperatures were still well to the west at 4:00am, and the strongest cold air advection occurred slightly after sunrise as opposed to during the overnight. At 4:00am, 850mb temperatures were still only around -24C (and even warmer at 1:00am), and the flow aloft was generally westerly. 2014 only got down to 4F, and it’s clear from the above analysis that tonight should easily get colder than 4F.

Going a bit further into the wind direction argument, let’s move to the surface. Below is the NAM’s surface forecast valid for 1:00am: it shows a clear NW wind direction and very cold temperatures. While NW still isn’t ideal and could be a reason to be hesitant on forecasting below zero temperatures, there is still a decent northerly component with that wind direction.

This NW surface wind is expected to persist for the rest of the evening through tomorrow morning. However, when comparing this to the observations from February 20th, 2015 as well as the observations from January 7th, 2014, it is clear that those two years had a much stronger westerly component to the wind, with many surface observations having wind directions west-north-westerly, or even due westerly. This promotes more downsloping than a Northwest wind. This means that not only does tonight have a slightly colder airmass, but it also has a more favorable surface wind direction for cold than 2015 and 2014, and 2015 got down to 2F. This is further evidence that NYC should get down to less than 2F, making zero or below a very realistic proposition.

Now, what may end up preventing NYC from falling below zero is the amount of mixing that occurs in the atmosphere, which is something that is still not yet determined. Sometimes when winds are very strong, the entire airmass “mixes” together, as distinct layers cannot form because atmospheric eddies are “spinning” the air together. This means that less cold air would get trapped at the surface and put a lid on how far we can fall tonight. It is for this reason that we are ultimately forecasting a low temperature between -2F and +2F, with a very realistic (but not a guaranteed) chance that NYC hits zero or below tonight. If the surface air can truly be a trapped layer, then NYC stands a very good chance to hit zero or below.

Overall, the factors discussed promote more cold than the past two years, which were already very cold in themselves. People often like to argue about the snow pack not being substantial this year, which would mean less cold. However, snow pack is only a factor when the atmosphere is radiationally cooling, as snow has a high albedo and thus radiates warm air back into space. But tonight’s cold is not from radiational cooling — it is pretty much all from cold air advection. Plus, NYC has too much Urban concrete to radiate much, anyway. Thus, snow cover is not much of a factor.

It will certainly be interesting to see whether NYC can get down to zero or below tonight. Whether it happens or not, this is the best chance for them to do it since 1994, when it last happened.