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Nor’Easter to bring major travel impacts, several inches of snow

Two strong mid level disturbances moving through the Southeast United States will interact later this week, and in doing so will help force the development of a low pressure system off the East Coast. Previously inconsistent forecast models have come into better agreement on the track and intensity of a significant Nor’Easter which will impact the area on Wednesday. With a low pressure system forecast to develop just off the Southeast Coast of the Carolinas and head northward to a position just west of the 40/70 Benchmark, significant impacts are expected in and around our area.

Still, models are wavering slightly with the exact track of the storm system and its intensity. And, for our purposes, even a slight wobble or change in trajectory could have major implications. In short, a wobble to the west could draw warmer air in near the coast, while a wobble to the east could mean colder air throughout the area during the height of the storm. These intricate details won’t be ironed out for another 12-24 hours, but the developing consensus has helped us to formulate some ideas moving forward in regards to the upcoming storm.

What exactly has changed since the last update?

The forecast models have been playing their usual games. Some which were further west and much warmer/rainier for even interior sections have shifted to an east and colder track, bringing the possibiliy of accumulating snow back into the forecast for New York City and even the coastal plain. Other data which was further east and weaker has shifted west  — leading to a developing consensus. This is not surprising, considering there are features in the weather pattern that were going to prevent the storm from riding the coastline, but also features to the pattern that were going to prevent the storm from escaping out to sea completely

What does this mean as far as impacts are concerned? 

Travel impacts are still going to be quite high in the entire area, including the I-95 corridor, from around 7:00am Wednesday morning through midnight Thursday morning. Precipitation will initially start off as light rain, with the thermal profiles not yet cooled to support the production of snowflakes. During the afternoon on Wednesday, however, increased lift for heavy precipitation will cool the atmospheric column in many areas away from the coast — allowing for precipitation to flip to heavy snow. Areas closer to the immediate coast will have a later changeover to snow. The consensus as of now is that places to the north and west of I-95 are the most likely to see several inches of snow, with a few inches expected along the I-95 corridor, and 1-3″ for immediate coastal regions — with eastern Suffolk County not getting much snow at all.

Storm Total Snowfall Forecast issued by our meteorologists at 11:00pm on November 24th, 2014. The forecast is a rough estimate as to where snowfall will accumulate in the upcoming storm.

Storm Total Snowfall Forecast issued by our meteorologists at 11:00pm on November 24th, 2014. The forecast is a rough estimate as to where snowfall will accumulate in the upcoming storm.

Although there is a decent model consensus beginning to form, there are still some uncertainties to iron out. Models are still varying in regards to the strength and exact location of the storm, — which could lead to a several-hour difference in any changeover to snow for places who are currently in the battle-ground. The fact though that the warmest and most amplified solutions have trended a bit east and colder does indicate to us that our initial thoughts about the pattern being too progressive for a wrapped-up inland storm were correct. The biggest uncertainties lie along the I-95 corridor and points eastward. One potential saving grace for the main roads is the fact that surface temperatures may stay just above freezing even when snow is falling heavily — making it difficult at times for snow to accumulate.

Regardless, traveling on Wednesday will be an absolute mess, and the evening rush hour could have heavy wet snow in the I-95 corridor.

In what ways can the forecast change? 

Although there was a notable convergence amongst the outlier solutions on forecast models today (i.e the model farthest west shifted east, and the model farthest east shifted west), the more pronounced trend in today’s data was to make temperatures a bit colder throughout the thermal profile. Much of this can be attributed to the forecast models regressing with their idea that a strong southeasterly flow would develop in the mid levels of the atmosphere during the height of the storm system. Such a feature would cause mid level warmth to not only hamper the development of snowflakes, but change precipitation to a cold rain. The intrusion of colder air into the storm system on todays forecast models has increased confidence that plain rain should be relegated to the coastal plain during this storm system — and that areas just a few miles from the shore will see at least some snowfall accumulating.

In obvious terms, also, the track of the storm system can force major changes to the forecast. A slight wobble to the west or east, in our situation, would have major implications on the sensible weather in our area.

What is causing these potential changes? 

More background on the formation and variables of the storm can be found in an article we posted yesterday, which may give a context to understand the recent changes.

NAM model showing snow overspreading the region on Wednesday afternoon.

NAM model showing snow overspreading the region on Wednesday afternoon.

But the forecast models adjustments are not just fantasy. There is a culprit: And it lies hundreds and hundreds of miles to our west — over the West Coast, actually. We have a strong jet streak of 150 + knots on the East Coast, which helps to create lower surface pressures just off the shore. This could, theoretically aid in the westward track of the developing low pressure system and intrusion of warm air into the area. However, there is also a strong west-to-east flow crashing into the Pacific Northwest, which somewhat hinders the amplification of the pattern and forces a tug eastward of the important weather features. In the US itself, the flow is still very fast from NW to SE, and not a pure N to S direction — and the easterly component to the jet stream will act to keep the storm system from tucking inland near the coast.

NAM model showing partial interaction between two disturbances on Wednesday.

NAM model showing partial interaction between two disturbances on Wednesday.

The flow in the Atlantic is also very fast, so even though the jet streak in the Atlantic is oriented in a way that allows the storm to track close to the coast, the flow is fast enough for the initial disturbance to run out ahead of the trailing disturbance over the Plains States. This leads to only  partial interaction — not a full phase — between the two features when they pass near the East Coast.

What can still throw a wrench into the forecast? 

Although the trends over the past day or so have been colder, snowier, and farther east with the track of the storm — that does not mean there cannot be a few wobbles than change the location of the rain/snow line.

One potential caveat is the track of the 700mb low (a mid level low center which will pass by with the surface low center). Although the surface low, and centers in the lower levels of the atmosphere at 925 and 850mb are taking textbook tracks for heavy precipitation/snow in our area, the 700mb low will actually be tracking overhead. This creates some issues for our snowfall foreacst. First, it introduces slight warm air advection in the nearby levels of the atmosphere as it passes. This could create a thin warm layer near 700mb that could reduce the effeciency of snowflake generation and lead to snow mixing with sleet or rain.

NAM model showing the 700mb low (black circle) passing directly overhead during the storm system. The filled colors show the temperature (C) at that level.

NAM model showing the 700mb low (black circle) passing directly overhead during the storm system. The filled colors show the temperature (C) at that level.

However, that same warming can also help to increase frontogenesis at that level, due to what we call a TROWAL (Trough of Warm Air Advection), which increases lift for precipitation. Thus, there is a very fine line between the amount of warm air advection that helps to generate heavy snowfall rates, but also enough warm air advection to make it too warm for snow. The latest guidance is showing an almost ideal balance from a snow-lover’s perspective where the warm air advection greatly enhances lift in the atmosphere, but isn’t overwhelming enough to change locations over to sleet and rain. But a subtle increase in the warm air advection could still greatly cut snow totals down.

The 700mb low tracking over us may also lead to a dry-slot trying to work its way in as well, but the TROWAL tends to increase precipitation right near a dry-slot, so it’s going to be an interesting battle between these features. We think any potential dry-slot should be overcome by this TROWAL and the lifting caused by the 150 + knot jet streak.

All in all, over the next day or so we recommend keeping a close eye on the website and our social media accounts. Although we aren’t anticipating significant changes to the forecast, we will be continually posting new data/information and updates from our meteorologists. If you’re headed out on Wednesday, leave extra time and prepare for hazardous travel. An update to our snowfall products is scheduled for 10:00am Tuesday.

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