Saturday’s Winter Storm has Shifted South for now

We’re getting a nice break from winter for a few days, as a large Northeast Pacific ridge which had given us our Arctic cold has since retrograded, which shifts all the cold air well to our west and north. However, there is still some remnant influence of the North Pacific ridging, as most of Canada is still quite cold. This means that there can still be transient bursts of cold air in an otherwise warm pattern, and that’s what will be happening starting on Friday evening.

The Pacific Jet will be expanding towards the West Coast, but the flow will slow down a bit on the West Coast. This often causes a split flow where some energy digs into the Southwest US, yet due north you get a small ridge in NW Canada. This helps force some of that cold air in Canada further southeast, towards our area and leads to a lot of confluent flow, while the energy in the Southwest US amplifies a Southeast ridge. Thus, we get an extremely strong gradient where a lot of moisture can theoretically ride.

If one were to look at this 500mb map without context, they would assume it’s a very warm pattern and that a storm would cut to our north. There are a lot of red, positive height anomalies over the region, which is too often automatically assumed to be a warm pattern, and thus incorrect conclusions are made from there.

Today's GFS model valid for Saturday morning shows a strong Southeast ridge, but the heights getting compressed by a vortex southwest of Greenland (Tropical Tidbits).

Today’s GFS model valid for Saturday morning shows a strong Southeast ridge, but the heights getting compressed by a vortex southwest of Greenland (Tropical Tidbits).

But notice how the split flow out west forces a trough in SE Canada with a large vortex south of Greenland. On the upstream side of troughs and the downstream side of ridges is where surface high pressures are focused. So upstream of the vortex south of Greenland and downstream of the ridge in W Canada coincides with a large, expansive high pressure in SE Canada, and it can tap into very cold air just to the north. This forces cold air to get sent south into the area.

Today's NAM model valid for the next few days shows us initially with warm temperatures, but an expanding strong high pressure system sends a large swath of cold into the area on Friday night.

Today’s NAM model valid for the next few days shows us initially with warm temperatures, but an expanding strong high pressure system sends a large swath of cold into the area on Friday night.

So it’s clear that the necessary cold air will be sent into the area for snow on Saturday and Saturday night, and there is a good gradient for moisture. However, notice how far south the high pressure is relative to us — that could create a bit too much subsidence and thus dry air and force the system to the south. Additionally, the cutoff low in the Southwest US doesn’t truly tap into the Gulf, so there is not necessarily a latent heat mechanism to rise the heights and send the storm northward.

Just like with the last storm and how there was plenty of jet streak support to have deepening low pressure and precipitation near the coast, we now have plenty of jet streak support for plenty of strong, expansive high pressure just to our north, further indicating that the models are correct with the strength of the high.

Today's GFS valid from Thursday night through Saturday morning shows a strengthening jet streak and upper-level convergence, supporting a strong surface high (Tropical Tidbits).

Today’s GFS valid from Thursday night through Saturday morning shows a strengthening jet streak and upper-level convergence, supporting a strong surface high (Tropical Tidbits).

For low pressure and precipitation, upper-level divergence is what we look for in jet streaks, since that’s a byproduct of upward vertical motion. But for high pressure, we look for the opposite — upper-level convergence — and that’s strongly evidenced to be occurring in this image multiple times in Southeast Canada. Additionally, the jet streak is extremely strong which supports a strong high pressure and an expansive one. The highs are also located in the left entrance and right exit regions of the jet streaks, which is also the opposite of the regions we look for with surface low pressure, and further leads to a stronger high pressure system. Also notice how the east end of the jet streak in Southeast Canada actually bends back to the southeast a bit, indicating that heights are not rising, but in fact, falling. This is why we have thought for several days that this is either a storm that would give us snow, or be suppressed to the south, and there’s actually more evidence of suppression than a hit at this point.

Today's NAM model valid for Saturday afternoon shows a band of light to moderate snow in New Jersey and Long Island, but having a hard time going north of that (Tropical Tidbits).

Today’s NAM model valid for Saturday afternoon shows a band of light to moderate snow in New Jersey and Long Island, but having a hard time going north of that (Tropical Tidbits).

For the most part right now, model guidance has shifted south, though some still brings us on the northern edge of some light snow on Saturday and Saturday night. Since only some of the moisture from the Southwest US cutoff low will be sent eastward, this is not a high-ceiling event, and the surface highs and confluence — while they provide the necessary cold air for snow — may also act to shear out the moisture. But there is still some hope for a north trend to put is back in the game for 2-4″ of snow, since sometimes the models in the medium range underestimate the strength of the Southeast ridge and slowly adjust northward with it in the short range. That may allow the gradient to shift just northward enough for precipitation to make it into the region. And considering the jet streaks are not going away and the confluence is still a very strong feature, it will be nearly impossible for this storm to shift northward enough to make this a rain event, even with the very positive 500mb height anomalies.