There is an old saying in meteorology and folklore which says “Dry begets dry”. An adaptation of that saying could be true over the next 7 days in our area; as “Snow begets snow”. Monday’s snowstorm, as the story is written, brought 7-10″ to a vast majority of the forecast area. With periods of moderate to heavy snow beginning during the early morning and continuing into the afternoon, the storm was “high impact” by nature. And the 6+” of snowfall set a record for the day at Central Park which was previously set in 1996. Despite the significant snowfall in much of the area on Monday, there is no time for reminiscing — attention will turn to more events this week and into the weekend which could also prove to be of significant nature.
The first winter weather event on the horizon comes on Tuesday night and Wednesday, and the potential combination of snow, sleet, and freezing rain has already prompted the National Weather Service to issue a Winter Storm Watch. Yes, this means we are currently under a Winter Storm Warning for Monday’s storm, and a Winter Storm Watch for a future storm, at the same time. It has been decades (no, really) since the NYC area was under a congruent Winter Storm Warning and Winter Storm Watch for different events. This speaks volumes to the active nature of this weather pattern. The second storm we have to track is for the Sunday and Monday period, which some modeling data has already indicated the potential for a blockbuster. Remember, there is a major difference between potential and hype, and we highly suggest you read our article about it if you have not already.
Why is the weather pattern becoming so active?
The main reason for this is the retrogression of the -EPO ridge. All winter, the pattern has been dominated by large ridging in Alaska, which has yielded record warmth there this winter. This ridge helps to dislodge a lot of the Arctic cold south and eastward, and places a large chunk of the Polar Vortex in SE Canada. Now that this ridge has retrograded a bit to the west, however, the pattern becomes cold, but a tad less cold, yet a lot more stormy.
Previously, this ridge had been located a bit further east, and higher heights had been further east into Canada. This allowed for more of a meridional flow in Central Canada diving down through the Plains, carving out a large trough in the East, and a ridge in the West, leading to lots of bitter cold and some snow chances. However, now that this ridge is further west, so is the meridional flow. Previously, our storm systems had all been northern stream “late bloomers” which would dive down from Central Canada into the United States, and try to gain moisture quickly at the last second before amplifying off the coast.
This time around, however, those same storm systems will be diving down into the country much further west, because of the new position of the ridge. This allows for more troughs to form in the West, which leads to a tad more ridging in the East; particularly the Southeast. This is why temperatures are a bit warmer now than they have been. These storms in the West will be able to dig to the south at a much further west Longitude than our past storms, which will allow for a lot more Gulf of Mexico moisture to be involved this time around, as well as a more consolidated area of energy initially, and the ridging in the East will allow this moisture and energy to creep northward to our area. Lots of times, the description in this paragraph would lead to storms that would “cut” to our west and give us more rain than snow. However, the -EPO ridge still being quite strong may very well serve to prevent this.
Storm #1 – Tuesday Night into Wednesday: In the above image, one can already see the potent shortwave diving down just south of Phoenix, thanks to the strong, retrograded -EPO ridge, with ridging out ahead of the shortwave itself. It is this shortwave that will be scooting to the east and then northeast, and gain lots of Gulf of Mexico moisture as it heads our way.
The EPO ridge initially retrograding is what allowed the storm to dive down to the West, and force some ridging in the East. However, while this is happening, the EPO ridge is going to be reloading to the west again, forcing the initial EPO block to migrate to the east, closer to where it was in January.
This does a few things. It allows the flow to split a bit. Our storm system was initially able to dive southward thanks to the initial westward position of the EPO block, but then instead of the entire western trough phasing into our storm and it cutting well to our west, higher 500mb heights remain in-place in Western Canada. This allows the storm to stay to our south a bit longer, allowing it to gain Gulf of Mexico moisture, and not warm us up all that quickly.
What this also does is force the large piece of the Polar Vortex to be pushed to SE Canada, as evidenced by the closed height contours there. This allows for a “brick wall” of confluent flow to form to our north. Storms cannot simply run into the Polar Vortex, and the due westerly flow rapidly increases there. Since the westerly flow blows from west to east, it would thus follow that a storm trying to approach the vortex will also be pushed to the east. Additionally, the pseudo-split flow pattern allowed for warmth and moisture to gather in the South, as well as the Southeast ridge to strengthen a bit thanks to the initial position of the EPO block. This warmth and moisture will now run into the cold air that is still in-place to the north, via the piece of the Polar Vortex. Thus, once the storm hits the Ohio Valley, it will have copious amounts of moisture running into a cold air source, and then be forced to redevelop to our south and east. This will prevent the storm from being able to warm us up as much as initially forecast, since the storm redeveloping to our south and east will allow our surface winds to stay northerly and northeasterly.
What ultimately results at the surface from the GFS can be seen below, courtesy of weatherbell.com.
The above image is valid for 7:00am Wednesday morning. Moderate to heavy snow has fallen throughout the latter part of the overnight, and is still falling for most of the area — even Long Island. There is lots of mixed precipitation to the southwest, as often times in these setups, the warm air advection at the mid levels of the atmosphere occurs faster than it does at the surface, due to less friction aloft promoting stronger winds. This warm air aloft will try to move into the area as the day goes on during Wednesday. However, notice the “heart-shaped” isobars. This is an indicator that as the low pressure system is approaching the Ohio Valley, the “brick wall” the storm runs into is pressing down, so to speak, and forcing the storm to find a new area of least resistance, which will be off the coast of the Delmarva. This will help to put a lid on the warm air advection — especially at the surface, which yields interesting results during the early-afternoon.
For areas well north and northwest of the city, moderate to heavy snow continues. For areas in and around the city, there is a lot of sleet and freezing rain falling, due to the secondary low pressure system being able to wrap in cold air at the surface. We still do think the storm will gain enough latitude for most of the area to change over to sleet and freezing rain, but we are becoming increasingly skeptical of anyone north of Central Jersey or not on Eastern Long Island seeing any plain rain at all. Most of Long Island may change over to some rain towards the end of the storm, but even that seems relatively insignificant in comparison to the amount of frozen precipitation that will fall. Trends indicate 4-8″ of snow, followed by a period of sleet and freezing rain, for most of the area. Some models show even as much as 6-10″ of snow, and it would not surprise us if we had to increase our forecast totals.
Storm #2 – Sunday into Monday: While this storm is almost a week away, it actually has the highest ceiling of potential, due to many of the classic indicators showing up at 500mb of many major snowstorms in the past. We mentioned previously how this huge -EPO block is reloading to the West, which is illustrated below.
The major -EPO ridge has reloaded, and is now a large area of 576 decameters, as opposed to the current block which is only 558 decameters. Generally speaking, the stronger the blocking in the pattern, the more significant effects it will have downstream. This huge ridge still forces some sort of trough in the West, where more storms can originate. What this block also does is that it becomes so strong, that it wraps some negative height anomalies close to it, which favors higher heights once again downstream in Western and Central Canada. Additionally, the strong block becomes so poleward, that some of the higher heights get pinched off throughout the Arctic and towards Greenland and Scandanavia, forcing even more high-latitude blocking to develop. This blocking helps to keep the chunk of the Polar Vortex in SE Canada — right where it needs to be for major snowfall. This is what helps to make Sunday’s potential so large.
Moving forward, let’s switch to today’s European model, valid for Saturday evening.
We can still see the huge -EPO block on the top-left part of the image. Additionally, the cold air source remains in-place thanks to the Polar Vortex in SE Canada. It does seem to be retreating just a tad, but this is actually a good thing in combination with the relatively high heights and ridging we see southwest of Greenland. This forces the energy to stay to the south, underneath the blocking, and slow down a bit. This allows it to strengthen and gain Gulf of Mexico moisture, and amplify the pattern out ahead of it. The Polar Vortex is retreating a bit, but is still far enough south to prevent the storm from cutting to our west, allowing the height rises out ahead of our storm to be the perfect, gradual compromise where the storm strengthens, but our cold air does not erode. Additionally, the blocking in Canada forces some pieces of energy to also dive down below it, which may ultimately phase and interact with our storm, helping it to become even stronger.
Moving forward, our storm system gains plenty of strength and slows down a bit as it climbs our coastline. Additionally, it is interacting with an Upper Level Low in the Great Lakes, which has formed to the south of the Canadian blocking. This helps to tug the storm a bit to the west, allowing it to slow down instead of racing out to sea. This helps lead to a relatively long-duration, heavy snow event, which hits from DC to Boston with 12-18″ of snow.
The ultimate results of this run is a 988mb low off of our coastline, hitting the northeast with heavy snow and strong winds.
Now, one has to remember that this storm is still 6 days away, which is an eternity in the world of computer models. Several nuances of the pattern that are not being detected now can change things dramatically. Thus, it would be highly irresponsible for us to forecast a major snowstorm at this juncture.
That being said, the fact that several pieces of model guidance, including ensemble data, has been extremely persistent in showing this strong storm signal is quite alarming. The -EPO ridge pinching off some blocking in Central Canada, combined with a slowly-escaping Polar Vortex, in the midst of a potent storm system gathering near the Gulf screams plenty of potential. One caveat is that the blocking near the Atlantic side is not what we could call 100% ideal for a major snowstorm, but it still pretty strong and located in a good position. The lack of perfect Atlantic blocking could lead to a bit of messy interaction between the Northern and Southern streams, and perhaps limit the ceiling of potential somewhat. That being said, if everything comes together, this storm could easily be the most significant snowstorm we’ve seen all winter.