Forecast models are a complicated thing: Meteorologists know, when major storms are looming, that they can often provide major hints and clues into how the atmosphere is going to evolve. Unfortunately, many readers or the general public don’t fully understand that models should be used for guidance, and not as fact. Quite possibly, the misunderstanding stems from meteorologists inability to communicate that.
Global models over the past few days have been strongly signaling the potential for a major East Coast storm system later this week. More specifically, many of them have been signaling the potential for a significant winter storm in our area. However, closer inspection of these global models reveals several large differences — and reasons why confidence is very low in any specific storm evolution as we move forward.
We’ll start with last nights GFS model, which shows a significant storm system on the East Coast by Friday. A significant Pacific shortwave digs into the Southern Plains during the middle part of the week. At the same time, it begins interacting with a northern stream disturbance moving east from the Rockies. These shortwaves and jet stream interactions develop a large trough, which forms a closed mid level low over the Southeast United States. As a result, a strong low pressure develops at the surface from the Southeast US toward the Tennessee Valley.
At the same time, confluent flow to the north forces the surface low to redevelop off the East Coast, nearest to the most impressive PVA (positive vorticity advection). The surface low then tracks northward up the Mid Atlantic coast, deepening rapidly underneath the right entrance region of a strong upper level jet.
Meanwhile, high pressure over Quebec settles in to support a cold airmass in place over the Northeast US. As the low tracks north, it begins interacting with this airmass, with an increasing pressure gradient. If this model evolution is correct, the low pressure system will produce snow for the entire area as the storm begins, with increasingly strong northeast winds. As the low tucks in along the coast near Virgina and Delaware and occludes, warmer and drier air starts to intrude aloft. This causes precipitation to mix with or change to sleet along the coast.
Then the the surface and mid-level lows tracks further east of the Mid-Atlantic coast, with colder air rushing back behind the storm changing precipitation back to snow. Despite the changeover, significant snowfall still occurs for much of the I-95 corridor. Interior areas see the most snowfall, with less mixing or no changeover.
The Canadian, or CMC, offers a different solution — albeit the same general evolution of major atmospheric features. Different from the GFS, the CMC is slower to phase the northern stream and southern stream shortwave. More phasing takes place closer to the Mid-Atlantic coast, with the surface low bombing out near the Delmarva. This keeps warm air from intruding into region and thus precipitation is mostly froizen for the I-95 major cities and coastal areas. This results in a significant winter storm for the entire area.
The ECMWF or Euro, is similar in evolution to both aforementioned models, but is slightly slower to phase the two jet streams. The result is a further south and east track with surface and mid-level lows. This keeps even more cold air in place for the entire region. Heaviest snowfalls near to I-95 corridor and coastal areas with blizzard conditions possible.
The 6z GFS model this morning, has shifted towards slower, later phasing that the 0z GGEM and ECMWF favor. Therefore the track is further and south and east. Note also the GFS indicates damaging winds along the New Jersey, NYC, and Long Island shores, in excess of 55mph. These winds combined with the full moon at the end of this week, could also result in major coastal flooding and beach erosion during the high tides on Friday and Saturday.
When looking the ensemble means from each model, on Thursday we see an east-based -NAO and 50/50 low on the Atlantic side, with a deep gulf of Alaska low building a ridge over the West Coast. The southern stream shortwave is notable over the Southern Plains. This is a favorable pattern beforehand for a large winter storm along the East Coast.
However, as we move forward into Friday and Saturday, the flow become much more progressive. The east-based NAO and 50/50 low weakens. The gulf of Alaska low opens up to trough and comes ashore over the West Coast. This causes the ridge shift east and to collapse. The result could support a slower/later phase and further south and east track off Mid-Atlantic coast.
Here’s the bottom line: At this point, it is important to note that the overwhelming majority of global models support the development of a large, powerful storm off the East Coast later this week. However, the exact track and evolution of this storm system remains of critical importance for our area, and all interests along the East Coast.
Exactly how the low pressure evolves at the surface, owing to the evolution and interactions aloft, will determine the intricate details of precipitation timing, type, intensity, and accumulations. The intensity of the track of the surface low will obviously have huge implications moving forward. All major global models differ slightly on the evolution of this storm, with resulting varying impacts in our local area.
Over the next few days, meteorologists and forecasters will continue to monitor trends in both operational and ensemble guidance. These will become important in giving us “hints” as to how the atmospheric pattern will evolve.