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Powerful ocean storm likely this week, impacts remain uncertain

Good afternoon! Forecast models have been honing in on the development of a major coastal storm during the week ahead for several days now, and over the past few days in particular have gotten more intense in regards to its formation. A myriad of factors will lead to a heightened chance of rapid strengthening as the storm emerges from the Bahamas and tracks northward off the US East Coast. But the track of the system, and its resulting impacts, remain highly uncertain.

As is the case with a large majority of storms that develop off the East Coast, the system’s development is the result of very intricate interactions in all levels of the atmosphere. In this case, two distinct disturbances – one emanating from the Pacific Ocean and the other meandering over the Plains and eventually Southern United States – will interact in the Southeast States. The result of the interaction will aid in the development of the mentioned low pressure system, but exactly how and when it occurs will determine where the storm tracks and how intense it becomes.

The first disturbance responsible for the storm’s development is situated over the North-Central United States right now, meandering southward from the Northern Plains. The disturbance will continue to shift southward into the Plains states, seemingly harmlessly, in what would otherwise likely be a non-eventful evolution. But lurking back to the west-northwest is a disturbance from the Pacific Ocean, which will drop southward into Alberta, Canada over the top of a large ridge in the Western United States.

The NAM at 10:00am this morning showed the initial two disturbances very clearly (Tropical Tidbits).

The NAM at 10:00am this morning showed the initial two disturbances very clearly (Tropical Tidbits).

That disturbance will then be directed southward into Plains, amplifying southward and eventually interacting with the original disturbance located over the Southeast. As the two begin to interact, lift in the atmosphere will increase. A northern stream jet streak will approach, driving southward into the Tennessee Valley, and as latent heat release and impressive jet dynamics expand off the Southeast Coast and push northward, the development of a significant low pressure system is expected.

Forecast model and ensemble guidance suggests that rapid deepening and strengthening of an ocean storm is highly likely, with the storm becoming extremely strong by Thursday as it moves northward off the coast to a position South or Southeast of Cape Cod. But the western extent of impacts remains the primary uncertainty, and will have a significant impact on the forecast for many people from the Mid Atlantic to New England Coast.

A more westerly track, with lower pressures near the coast, could result in a more significant event. A further east track, less interaction between disturbances aloft, and higher pressures along the coast, would result in a “scrape” with light snow along the coast and the majority of moderate or severe impacts remaining well offshore. Forecast model guidance has been jumping around a bit with the system’s track and evolution, which is adding complexity to the forecast.

A comparison of the NAM and GFS with its evolution at 500mb off the Carolina coast. The NAM shows a more consolidated disturbance that is oriented up the coast, while the GFS is more sheared out (Tropical Tidbits).

A comparison of the NAM and GFS with its evolution at 500mb off the Carolina coast. The NAM shows a more consolidated disturbance that is oriented up the coast, while the GFS is more sheared out (Tropical Tidbits).

So what, exactly, is causing these differences amongst forecast models? It all has to do with the interaction between the two disturbances we mentioned earlier, and the way which the atmospheric flow aligns itself along the coast. The GFS, ECMWF and other global models suggest the southern stream shortwave will almost “slip” east before turning north, which takes the system far enough away to avoid major impacts along the coast.

The NAM, RGEM, and some other high resolution modeling suggest the disturbances will interact more directly, which will align the flow more in a south-to-north fashion along the East Coast. The system is then “tugged” ever so slightly westward, and it tracks closer to the East Coast — and in this case, even if the system doesn’t track directly near the coast, lower pressures extend near the coast, which will create lift for precipitation.

That leaves us wtih the all important question: Which models are correct? Truthfully, the answer probably lies somewhere in between. As meteorologists, it is important to use these situations to understand the atmospheric process, and not rely solely on forecast models. For example, we can utilize knowledge of the atmosphere to tell ourselves that the NAM model suite is likely over-amplified by latent heat release and convection that it cannot handle from the Bahamas to the Southeast US Coast.

The last 6 runs of the NAM showing the trend in the 250mb jet (focus north of Maine). It's showing the jet being stronger and more buckled to the west, which aids in expanding precipitation westward. This indicates that the very dry GFS is probably wrong (Tropical Tidbits).

The last 6 runs of the NAM showing the trend in the 250mb jet (focus north of Maine). It’s showing the jet being stronger and more buckled to the west, which aids in expanding precipitation westward. This indicates that the very dry GFS is probably wrong (Tropical Tidbits).

We also can use our understanding of the atmospheric processes at work to weight into the forecast the idea that the GFS is likely at least marginally too far east and progressive, based on its evolution of the storm system and trends over the past 48 hours. The biggest trend in the broad scale has been with the 250mb jet — it has consistently trended much stronger and more buckled to the west as it goes north of Maine and hooks westward. This leads to a more expansive area of upper-level divergence, aiding in more lift for precipitation to expand westward into our area. This is a famous precursor to models trending to showing more precipitation in the short term, as the models often catch up the effects of the jet orientation.

This leaves us somewhere in between, with a solution near the ECMWF/ECMWF EPS, resulting in a light snowfall near the coast, and a moderate snowfall closer to Eastern Long Island and parts of Eastern New England.

So stay tuned for more updates through tonight and Wednesday as we try to nail down more details, including snowfall totals from this storm.

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