The NAO blocking has arrived, and much of the area saw periods of snow during the day on Saturday, enough to remind us that winter is still holding its grip on us, despite the calendar turning towards Mid March. We are also following the potential for two storm systems; one for Monday evening through Tuesday morning, and another one as we approach the March 24 period. Monday evening through Tuesday morning looks more rainy and messy than snowy, though further northwest of the city might be able to hold onto enough cold for minor snowfall accumulations along with some ice. However, March 24 is a period that needs to be watched for more serious snowfall potential, albeit it is quite far away, so caution should still be urged.
A stronger storm system will be developing to our west, as energy enters the United States from the Pacific Ocean. Considering we have a decently cold airmass in place right now, the precipitation that moves into our area could start off as snow, as warm, moist air overruns the cold air in place. If enough solid precipitation can break out to the east of the storm system, then we might see precipitation before the warm air advection would settle in. The RGEM model illustrates this well, as it shows a several hour period of snow for the NYC area, and then turning over to sleet.
The problem with this scenario is that the faster the precipitation moves in, the closer we will be to peak insulation hours, meaning the mid March sun angle would make accumulations tough to come by. Additionally, although there is an area of surface high pressure in Canada, it is a broad area that also extends to the Atlantic Ocean, which helps to yield an easterly flow in the low levels of the atmosphere. This makes me skeptical of accumulating snow in the NYC area and immediate suburbs. I do expect the precipitation to start off as snow, however.
Once you get a bit further west of NYC, an easterly flow does not have as much of a warming effect, since you are further away from the ocean. Thus, the surface temperatures might be a bit more stubborn to warm up. Additionally, with the low pressure system tracking well to our northwest, this will yield strong southerly mid-level warm air advection. Thus, with a colder surface and warmer mid levels, the potential for a prolonged period of sleet to the west and northwest of NYC exists, with a more brief period of sleet elsewhere. The exact timing and duration of sleet will be fine-tuned during the day tomorrow. But the 00z NAM sounding valid for 8 p.m. on Monday night, taken in Central NJ, illustrates the sleet potential quite well.
Eventually, the winds will shift southerly at the surface later on Monday night, so all precipitation, even well NW of NYC, will turn over to rain. The coldest areas will be rain by 11 p.m. at the latest. Rain will be heavy at times through Tuesday morning, before tapering off to just clouds and showers for Tuesday afternoon.
Behind this storm system, however, temperatures will become much colder, with highs struggling to reach 40 on some days, with 850mb temperatures nearing -10C on some days. This is an airmass that can strongly support snowstorms, and we might have to watch for the potential for one as we head to the March 24-26 period.
First of all, why didn’t the NAO block stop this past system from becoming a rainstorm? This is because when it is late March, heights are inherently higher, as more heating occurs in the southern latitudes. Thus, you need blocking further south than what is typical to truly suppress stronger storms. Having a block located in Greenland will not be able to force a Polar Vortex (PV) far enough south to prevent storms from cutting during this time of year. Weaker storm systems, like yesterday’s will just tend to follow the original orientation of the height contours, since this storm was not yielding any height rises out ahead of it. Stronger storm systems can still cut to our west. However, once the blocking shifts southward, things will change.
Notice how the PV is pretty elongated and far to the north, despite the Greenland block. This allows height rises ahead of our storm to actually get into southern Canada, which is not a suppressed look at all. The confluent flow to our immediate north will help for a quick wintry onset, but it will not be enough to prevent this storm from cutting to our west. The PV is not far enough south to force the storm south to begin with, meaning it gains too much latitude before it can even run into the more suppressive confluence closer to our longitude, rendering that almost irrelevant.
Let’s fast forward a few days. The 00z GFS at day 6 shows a much better look. The Greenland ridging has shifted to the south into Central Canada, which helps to greatly suppress the height rises out ahead of our storm system. Additionally, we have a PNA ridge that shifts eastward, and combined with the block, we have energy that is forced to dive further southward into the US; it gets wedged between the PNA ridge and the block. We also have a pseud0-split flow: a polar airmass in SE Canada, as we are under the influence of a slowly departing ULL into the 50/50 low position, and more moist, Pacific flow to our west and southwest. We can also see a piece of the departing ULL partially phase into our storm system, which will allow for height rises down the road. Additionally, the block in Central Canada is more broad and not too overwhelming, meaning it proves a nice, elongated area of resistance preventing the storm from cutting to our west, but is not strong and focused enough to really allow our storm to truly cut off and mature too early. That was a problem we ran into with the March 8 event. Since the circulation is also bit more broad, it can also tap into some Gulf of Mexico moisture. Gulf of Mexico moisture a great source for latent heat release, further yielding height rises; something we did not have with the March 8 coastal storm. Eventually, we see a very favorable surface reflection, with a major snowstorm climbing up the east coast.
Now, what can go wrong? As we alluded to in our previous blog post, there was a wave of energy that actually enters the country a couple of days before the one that the GFS blows up. This one never has a chance to amplify, because the upper level low does not move out of the way in time, so it gets shredded. Although the upper level low will be in a much better position this time around, it still has a chance to be in a position too far southwest to allow for major amplification near our coastline.
The European model that comes out handles the storm a bit differently. It splits the energy into two pieces. The first piece, which is the one that the GFS had being more organized and a major snowstorm, gets shredded by the upper level low. However, the energy that hangs back phases with an additional piece of energy diving down from the PNA ridge, and at that point, the upper level low has moved out of the way, yielding room for a snowstorm for March 26 instead of March 24. So we’ll have to watch the evolution of the upper level low over the next few days. It’s moving out of the way, which is a good sign for snow lovers, and we actually want the moving out of the way to be a slow process, so that cold air can remain in tact. But there is the chance that it does not move out of the way fast enough, and we’d have to rely on a European model solution, or be left high and dry. We’ll also have to watch to see if any piece of the upper level low breaks away and phases into our storm, which the GFS shows happening. We might not need a phase if the core of the ULL is in the right position, but it certainly would not hurt the cause for a snowstorm.