The theme for this month so far has been cold, cold, and more cold. And that is expected to continue. Most locations in the NYC area are running around ten degrees below average, which currently places us in the top-5 for coldest Februaries on record.
Tonight, another Arctic blast is on its way, as a large chunk of the Polar Vortex will dive into the US. This is the most impressive Arctic blast yet. Had last year’s Polar Vortex phenomenon not been so strong, this Arctic outbreak would be getting a lot more attention. The Arctic cold will be combined with NW winds at around 20mph, gusting to 30mph, leading to dangerously cold Wind Chills.
The big question among us meteorologists is whether NYC will hit 0 or below tonight. The airmass looks cold enough, as 850mb temperatures (about 4500 feet in elevation) look to drop to -27C at 1:00am, and 925mb temperatures (about 2200 feet in elevation) look to fall to -26C at 7:00am.
When this is combined with 1000-500mb thicknesses around 492dam (extremely low!), the airmass is supportive of temperatures falling below 0 in NYC tonight. But whether the surface responds — or responds enough to fall from the single digits to below 0 — is another question. When the heart of the cold dives to the west of the area, you get somewhat of a return NW flow, which also gets enhanced by a departing storm system in Maine. While a NW wind is a cold wind, it is not necessarily a below 0 in temperatures wind, because westerly component to the winds create downsloping from the Appalachian Mountains. Air gets lifted up the mountain and cools initially, but when it sinks back down the mountain, it warms faster than it initially cooled, creating a net-warming effect. This, combined with 850mb temperatures warming up slightly after 1:00am tonight lead us to believe that NYC itself is unlikely to fall below 0 in temperature.
Instead, we are forecasting lows somewhere between +1 and +3F for NYC. Some suburbs north and northwest of the city have the potential to go below 0, where the downsloping is less. The record for NYC for February 20 is 7F, so we should still easily break that. The odds of NYC hitting 0 or going below are about 25%.
With temperatures falling towards 0 and the strong NW winds, Wind Chills will be approaching -20F throughout the entire overnight and early tomorrow morning.
Highs on Friday will stay in the upper teens, as breezy conditions continue, but winds will gradually be diminishing. The diminishing winds will lead to ideal radiational cooling conditions on Friday night/Saturday morning. This means that most suburbs could go below 0 — perhaps more suburbs than tonight — while the immediate NYC area will probably hold in the upper single digits, since the Urban Heat Island prevents radiational cooling. Fortunately, Wind Chills won’t be nearly as cold, due to the calm winds, but it will still be brutal.
Moving forward to Saturday shows that we have another storm threat.
There are still plenty of details to be ironed out, but a trough will be building in the west, which will allow for warmer air to hit the SE states. When this is combined with our very cold airmass, you get a good clashing of temperatures, and the warm air overruns the cold air. Thus, precipitation can spread from the Tennessee Valley, northeastward into our area.
The cold airmass will allow this to initially start off as snow, and probably accumulate anywhere from an inch to a few inches, before changing over to other precipitation types. The ridging in the SE should allow the surface low to track to our west, which sends building southerly winds. This helps to warm the airmass, particularly at the low-levels. The ground will initially be cold, and with the low-level warmth, precipitation would melt into cold rain near the ground, and freeze when it hits the ground — which is freezing rain. A brief period of freezing rain is possible near the coast, and a few hours of it is possible in the interior, where southerly winds will take a bit longer to warm the ground. Eventually, temperatures should warm for a change over to light rain. Regardless, roads look to be quite slick from late Saturday afternoon through a decent portion of Sunday.
Sometime on Sunday night or Monday, another Arctic front is poised to come through the area. It won’t be as cold as the one today, but temperatures could fall back into the upper single digits and low teens on Monday night.
Our eyes then turn to another storm threat in the Wednesday timeframe.
The above image takes a look at the trough/ridge pattern in the middle of the atmosphere, as well as atmospheric spin, where disturbances are often found. We have a large area of ridging in the western half of North America, particularly in NW Canada, which allows a strong piece of energy in Central Canada to dive towards the United States. There is also energy in the Southwest which tries to eject and phase with the storm system. If all of the pieces come together, we could have a significant Nor’Easter with a lot of snow in our area on Tuesday night and Wednesday.
However, there are also a lot of signals against a snowstorm as well. The ridging out west retrogrades to the Pacific, forcing a trough at some point to try to move into the Western US. This would make it harder for the big piece of energy in Canada to dive far to the south, which is what is ideal for a major snowstorm. Thus, we are not necessarily confident in the ingredients for a snowstorm holding all the way through its development process. The strongest signals, as of now, would be for a snowstorm to hit New England (Boston, again?) while we are on the teetering edge of uncertainty.