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The models the past two days have trended toward a more impactful and snowier scenario in regards to the next nor’easter that we have been keeping an eye out for about a week now. The first nor’easter, which brought damaging winds and days of near-record coastal flooding, arrived on March 2nd. That one cleared out, allowing for a second coastal storm to move in a few days later on March 7th. Now there is a third storm, which didn’t look like it was going to affect really anyone, but every new model run that has come in paints a more grim picture Monday through Tuesday night.
This is all because of the current weather pattern we are in since the start of March. A record strong negative NAO (North Atlantic Oscillation) allowed for the first nor’easter to develop and slam portions of the Northeast. Ever since then, the NAO has remained negative and will likely remain negative through much of March. This will keep the East stormy and cool, something we’re sure most of you aren’t happy about based on the wild weather so far this March.
Believe it or not, the most recent runs of the European (EURO) and American (GFS) models from Saturday afternoon are in remarkable agreement for this far out in time on where this third nor’easter will track. Both of these storms are expected to develop off the Carolina coast on Monday and will rapidly strengthen, moving northeastward somewhere off the US East Coast. It’s the track the is the best determinant of the impacts different locations of the Northeast will feel. When storms move over the 40/70 benchmark, impacts from storms are often maximized. This benchmark is located at 40° N, 70° W, or south of Cape Cod and east of about Atlantic City, NJ, which we stared on the model graphics below. Both the latest runs of the EURO and GFS bring this storm dangerously close to the benchmark, something that needs to be watched this far out in time. If a storm moves inside these coordinates, or closer to the coast, then the concern for rain arises at the coast while north and west of I-95 could get pummeled with heavy snow. If a storm moves outside these coordinates, of further away from the coastline, then much of the Northeast will be scathed from snow while portions of the coast experience a heavy snow.
Agreement is one of the factors forecasters look at when we try to predict how a storm will act several days in advance. Another great tool that we love to use is by taking a look at the ensemble members, which the EURO and GFS models produce. If we take a look at the GFS’s ensemble guidance from the Saturday 12z cycle for Bridgeport, a city that is very much in play with this next storm, it still contains a very large spread among the different ensemble members in terms of snowfall. Each line on the plot below is a different member, showing a different snowfall output from half an inch to a foot. This represents how the forecast is still uncertain, and that it’s still to early to release a forecast with much confidence. If you hear someone saying that a feet of snow is coming to Boston, based on this guidance, that is possible but not likely.
Now if we look at the EURO ensemble guidance, it also shows that spread. Instead of showing it on a city-specific plot like the one above, below you’ll see the different potential low pressure center locations from the 50 ensembles members plus the operational member. Each red ‘L’ on that map represents a different possible position for this coast storm.
Again, there is still a large spread. If the storm is closer to the coast, then there will be greater impacts; if it’s farther than the coast, then impacts will be lessened. This storm may not even impact the Northeast at all! Also notice the pressures many of these members are predicting. Most are expecting this storm to experience pressure drops into the 970s or even 960s (millibars). That would bring the concern for damaging winds and flooding at the coast, something that we don’t want to see again. We also mentioned earlier how this storm will rapidly strengthen. The term, bombogenesis, is used to describe this strengthening process. In order for a storm to become a “bomb cyclone,” or a storm that undergoes bombogenesis and rapidly strengthens, is when the minimum central pressure drops by at least 24 millibars in less than 24 hours.
One last component to this forecast discussion that’s worth mentioning is what the highest resolution of the NAM (North American Model) model is beginning to hone in on. The last couple days, the NAM at a lower resolution was not buying this storm threat or really the development of a storm at all. Now that the highest resolution of the NAM is able to forecast this far out in time (up to 60 hours), we are beginning to gain a better picture and consensus of what both the global models, such as the EURO and GFS, as well as the mesoscale models, like the NAM, are thinking in regards to the future of this next storm.
If we take a sneak peak of the 18z NAM run, it is definitely on the bullish side, by bringing a moderate to heavy snow to all of the I-95 corridor as the lowest pressure tracks toward the 40/70 benchmark. Now while this is possible, again we cannot focus on each model run because the uncertainty among the ensemble guidance remains high. Another uncertainty is the temperatures. It’s mid-March, temperatures begin to warm up and the sun angle increases, both limiting factors for snow. Even if the best potential for heavy snow is at the coast, temperatures will likely be above freezing (see model image below), which can make it difficult for as much accumulation, especially on the roads. We saw it with the last storm where it snowed all afternoon-long in New York City, even with heavy snow at times, but only about three inches managed to accumulate due to the temperatures in the mid to upper 30s.
With all of that said, we crafted this snowfall potential map below, highlighting where the best areas for significant snow (6+ inches), light to moderate snow (less than 6 inches), and solely the chance for snow (both accumulating and non-accumulating).
Stay tuned for a special report, featuring our first forecast on this next storm, Sunday night at 8pm on jacksonsweather.com.