Friday's Storm

Afternoon Update: A late morning/midday dry slot killed some of my forecast ideas, as we picked up a quick 1 to 2" this morning and then had only periodic light snow from late morning into early afternoon - a window of time in which I expected steady snowfall. As such, accumulations will likely fall short of my expectations. Probably topping out between 2 and 4", with perhaps a spot 5" total near the canal.

Friday Morning Update: The initial wall of snow arrived earlier than expected and as such is even more intense as surface temperatures are only in the low to mid 20s this morning. Snow will have no trouble accumulating and will fall at 1"/hr. rates at times through the morning and into the afternoon hours. As time passes, I am becoming increasingly confident that the majority of this storm will fall in the form of snow here on the Cape, especially from the Hyannis area westward. Temperatures are wet bulbing into the low 20s on the Upper Cape, which means a slower and longer climb towards freezing. The 4 to 6" snow fall forecast issued Thursday morning should verify, though totals could end up higher than forecast on the Upper Cape. Updates to follow...

Thursday Update: A series of low pressure centers will slide off the Mid-Atlantic and pass southeast of New England Friday night and Saturday. While these systems will be relatively weak in terms of central pressure, they will have a good deal of moisture with them and should be decent precipitation producers over the Mid-Atlantic and Southern New England. While some of this precip. is certain to fall in the form of snow, questions still remain about just how much warming will occur during the event, and this has obvious implications on final snowfall accumulations.

Looking at everything available, I think the most important aspect to the forecast for us here on Cape Cod in terms of snowfall impact is the nature of the existing air mass over New England. At the outset of the storm, the lower levels of the atmosphere will be very cold and dry thanks to the chunk of arctic air that moved into the Northeast Wednesday night. Temperatures will drop into the 10s tonight and dew points will remain in the single numbers through much of the night. This type of cold is difficult to dislodge, regardless of where a storm ultimately tracks. As such, even if mild air eventually wins out with the advancing storm center, the ingredients are in place for several hours of steady accumulating snow.

With mild air advancing up and into New England ahead of the storm system, the atmosphere will be primed to produce bands of moderate to even heavy snow over Southern New England Friday afternoon. I expect snow to develop around lunchtime or shortly before that, and I'm looking for a quick accumulation of 2 to 4 inches during the afternoon. Snowfall rates of 1"/hr. are possible. This should make for a tough drive home Friday evening.

Now, it's during the evening that the forecast becomes far more tricky. While the existing air mass is cold and dry, the high pressure center responsible for that cold will be moving eastward away from New England at the same time the advancing storm center approaches the area. That is not a prime set-up for holding in cold air. In fact, that configuration favors an east or even southeast wind for a time and climatology would tell us that such a direction would mean warmer air gets involved at some point in the storm. The difficulty in the forecast is determining when this happens and to what extent.

While a multitude of factors shape this process, much of it has to do with the strength of the storm center and the related mechanisms driving the precipitation. Sometimes this process transpires late in the game, after the strongest, most organized shield of precipitation has passed, and results in a change from snow to light sleet and eventually light rain or drizzle. In such a circumstance, we pick up a healthy dose of snow before the changeover and are left with several inches of heavy, wet, compact snow when all is set and done. This is often how things transpire in a light to moderate warm advection snowfall. In other cases, the changeover occurs earlier in the game, and bands of heavy snow change over to moderate to heavy rain - ultimately washing away most of what accumulated. Such a case is more typical of a traditional nor'easter.

So where do we end up this go around? Well based on the strength and track of the storm and the dynamics driving this event, I believe the set up is more in line with the earlier scenario. We have to remember that much of the precipitation that is coming will be the result of the initial surge of mild air pushing into Southern New England. There is no strong consolidated energy center driving this snow event and inducing a rapidly strengthening low center. In fact, nearly all of this is the result of warmer air attacking a slowly retreating cold air mass. Computer guidance shows the strongest lift occurring during the late afternoon/early evening, when conditions should still be favorable for snowfall. Cold air masses are often poorly handled by computer guidance, and we have seen that today with model initializations near the storm. And while I've seen some of the latest guidance and the milder trends, I'm not necessarily sold on half of the total precipitation falling as rain.

So, this leads me to believe that we should see several hours of steady and sometimes heavy snow prior to any changeover. This is the basic reasoning behind the snowfall forecast issued, as I feel we can reach at least 3 or 4", perhaps a bit more, with the first wall of snow Friday afternoon. This is quite similar to what we've seen a hand full of times this winter - except this go around the storm is a colder one overall. The best chance of reaching higher totals would be up near the canal where it will take longest for the southeast wind to erode the cold air.

I will try and update things later today when new information becomes available.