winterstorm

Cold, active winter pattern will continue

Whether your area got hammered or only moderately brushed by the Blizzard of 2015, it is clear that the pattern has finally truly shifted towards a cold and snowy regime. This month has generally featured -3F temperature departures with above average to much above average snowfall for the region. While it is not a lock that a lot more snow is in the immediate future, the cold and active weather will continue. This means that there will be a few chances of snow, but does not guarantee anything.

The main feature that has continually supplied our cold air this month is a large amount of ridging over the Pacific. The location of this ridging has ranged from  Alaska to British Columbia including the west coast/western half of the US. These help to favor troughing downstream, or in the Eastern US. Particularly, when the ridging extends far enough north to poke through the British Columbia and into Alaska, you can somewhat displace the Polar Vortex to the south, which creates a nice cold air source to our north for winter storms. This was part of the pattern that preceded the Blizzard, and this part of the pattern looks to generally continue.

The first threat for snow is a clipper system that will be approaching our area late tonight and tomorrow morning. It will be crossing the Appalachian Mountains, which weakens the moisture somewhat, but the initial punch it will pack combined with trying to grab Atlantic Ocean moisture at the last second could lead to 1-2″ snowfall amounts for our area. This is particularly true away from the immediate coast, as southerly flow ahead of the storm may warm temperatures above freezing on Long Island and cause some mixing with rain. While this won’t be a major event, it will be enough to make the morning rush slick, so it will be wise to leave some extra time tomorrow morning.

Moving forward, however, is when things get more potentially interesting. This is the atmospheric setup on Saturday night pertaining to a more significant snowfall threat on Sunday night into Monday.

Today's GFS model valid for Saturday night shows a favorable pattern in the middle of the atmosphere taking place for a snow event  (tropicaltidbits).

Today’s GFS model valid for Saturday night shows a favorable pattern in the middle of the atmosphere taking place for a snow event (tropicaltidbits).

We can see a large trough over the southwest with plenty of energy and moisture. There is also some energy diving down British Columbia into the US. While a good portion of the trough in the southwest will stay where it is, pieces of energy will be ejecting from it and attempt to phase with the energy in British Columbia. Assuming they phase together, lots of energy and moisture will be added to the system, considering the moist source region of the subtropical jet.

What’s also important is the pattern in the Atlantic. The clipper helps to reinforce cold air behind it and it also phases with the Polar Vortex in Central Canada. This creates an elongated area of confluent flow, which maintains a cold airmass for increased baroclinicity for the storm to work with, but also prevents the storm from cutting north. Additionally, hints of a -NAO are also emerging, which helps to keep the Polar Vortex in place.

The result is a large area of precipitation breaking out in the Plains and spreading eastward towards our cold airmass. It’s a classic setup for overrunning precipitation — where precipitation and moisture “overruns” the cold airmass. There are hints from some guidance that this could be a significant snow event on the order of 6″+…which is certainly possible given the pattern. However, there are still plenty of uncertainties, because how far north the system goes is highly dependent on the degree and location of the initial phasing between the energy in the southwest and the energy diving down British Columbia.

Today's GFS valid for Monday morning shows a storm system skirting the region with plenty of moisture (PSU-eWall).

Today’s GFS valid for Monday morning shows a storm system skirting the region with plenty of moisture (PSU-eWall).

The stronger the ridge is out west, the more due south the British Columbia energy can dive. This would allow for a further west phase, and thus a more amplified storm system that could bring significant snowfall to our area. But any weakening of that ridge, and the pattern becomes a bit too progressive for a very clean phase, and the snowfall would instead be relegated only to the Mid-Atlantic. There is also an in-between scenario where we get “clipped” by the storm, and because of the extremely cold temperatures aloft, high snow-to-liquid ratios could still provide us with a few inches of snow.

The degree of ridging is predicated on the exact orientation of the large upper-level low in the NE Pacific. That helps to pump up ridging downstream, but if that shifts a bit to the east, the ridge would get shunted a bit, and this has happened a few times this winter (and would reduce snowfall potential). This, combined with the fact that the NE Pacific is in an area void of a lot of data leads to a lot of uncertainty with this upcoming event. But the potential is there for 6″+ of fluffy snow on Sunday night into Monday.

Regardless of the ultimate track of the storm, once it passes, lots of cold air will be funneling in behind it, which could continue the cold and active pattern. We’ll have updates on this storm threat as it approaches — including details on the exact track and precipitation as they become more clear.

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