Accumulating Snow Still on Track

For meteorologists and anyone who forecasts weather these past weeks have been hell. It is situations like this where I always wish the general public had an idea of why forecasting the weather is so difficult and the process involved in constructing weather forecasts. This is sort of the reason why I construct my posts to be so technical and show so many images. My hope is that the reader gets a little insight into the background behind forecasting.

The past 24 hours...even 36 hours has been an absolute nightmare. Forecast models have been all over the place, wavering back-and-forth between solutions and there has been terrible disagreement between forecast models. Typically when we get into the nowcasting aspect of forecasting (typically that's when you're within about 6 to maybe 12 hours out, things become much more clear because you can see how things will unfold as they're happening...well this still isn't really the case with this system. Why is that?

This system was incredibly complex with multiple pieces of energy moving through the atmosphere. The greater the amount of energies at play, the more complex the situation becomes and it becomes more difficult for forecast models to handle them and how they will evolve. Providing a comparison, think of it as solving a puzzle. When you have a puzzle that has 20...30...40 pieces, the puzzle isn't all that difficult to solve, however, when you have a puzzle that has maybe 1500+ pieces, well it becomes much more difficult to solve the puzzle easily because you have more pieces to work with.

Let's look at some of the inconsistencies the forecast models are still having with this system and what we can expect across Connecticut. We will look at total liquid precipitation accumulation by the NAM forecast model, the GFS forecast model, and the HRRR forecast model through 5:00 AM Thursday morning:

Look at these differences! The HRRR has less than 0.50'' of total precipitation accumulation for the majority of the state while the NAM has generally greater than 0.60'' across the majority of the state and the GFS generally o.30'' to 0.60'' across much of the state with more towards the east. This is just one of the many tools used when forecasting snowfall. On average, 1'' of liquid will equate to about 8-10'' of snowfall. This is an average, however, and can vary based on numerous atmospheric processes including. In the situation with this storm the idea was 1'' of liquid would equate to as much as 10-12'' of snowfall. Seeing these extreme differences in liquid precipitation make this rather challenging to figure out just exactly how much we see. Another issues is the very tight gradient some models, including the European model have in terms of liquid precipitation across the state (with more in the south and a sharp cutoff as you get towards Hartford and points north).

Here is a radar grab from 1:12 PM this afternoon:

 This shows moderate to heavy snows off to our southwest across portions of Pennsylvania, New Jersey and farther south into the mid-Atlantic. While the radar does suggest snow over Connecticut we are dealing with a great deal of dry air within the lower portion of the atmosphere. Until this dry air saturates (becomes more moist) it is going to be very difficult for the snow to reach the ground with the exception of extreme southern Connecticut. The degree of dry air also complicates snowfall forecasts because there are some indications the dry air hangs around for a very long-time across the northern part of the state which would only increase the gradient in snowfall totals from north to south. This bufkit sounding from this morning's run of the NAM forecast model for Windsor Locks, CT at 2:00 PM this afternoon shows the abundance of dry air. Notice how far apart the red line (temperature) and green line (dew point) are? The more spread apart, the drier the air. Forecast models, however, do bring these closer together as we head into the evening indicating moistening of the atmospheric profile:

How is the storm shaping now? The storm is really beginning to take shape and strengthening just off the coast of the mid-Atlantic. This strengthening will continue to occur as the afternoon progresses. This is evident by a tightly closed off circulation developing at 700mb and 500mb just east of Virginia by this evening. As these circulations close off and become more tightly closed (this is indicative by the increased number of "circles" with smaller diameters) this is a tell tale sign of a very strong system:

The big challenge we are still faced with is how far northward can this system push and whether dry air over the northern portion of the state can become more saturated? Forecast models do suggest we will see a band of heavier snow push into the state but not until late afternoon or not even until the evening. From here the question becomes how heavy this band of snow is, how intense the snowfall rates are, how long the band sits, and how far north this band works into Connecticut? So many questions yet to be answered.

Currently the surface low sits east of Delaware where it has been for the past several-plus hours. As the mid-levels further strengthen (which we looked at in the above image) the surface low will continue to strengthen and it will begin to move off to the north and east:

Forecast models still are uncertain with regards to just how much lift we will see in the dendritic snow growth zone and that too complicates snowfall forecasts. Let's again look at Windsor Locks, CT. The 3 km NAM shows a period of extreme upward vertical motion within the dendritic snow growth zone over Windsor Locks, CT later tonight. This would indicate a period of very heavy snowfall. The 12 km NAM is much less robust with the degree of upward vertical motion within the dendritic snow growth zone as is the GFS. Forecast models, however, are more robust across the southern portion of the state:

What should we expect through the rest of the day? Outside of the immediate shoreline we will not see a great deal of snow during the afternoon hours. The air is still very dry and will take some time to saturate and the bulk of the storm is still well off to the south. As we approach late afternoon and early evening, all indications are that the atmospheric column will saturate and as heavier precipitation works into the state we will begin to see snow fall. The heaviest of the snows will occur during the evening hours and lasting into the overnight hours. There is still some uncertainties with regards to how late the snow will persist but there is a possibility it could persist late enough to where the morning commute will be rather messy...messy due to clean-up efforts. Based on the shift with the storm winds should be be as big of an issue as though as we are looking at gusts more in the 35-45 mph range. There could still be some isolated power outages, especially across the immediate shoreline. The worst of this storm will be this evening into overnight. Given everything mentioned above here is the updated snowfall map:

Note: There is a good probability that most locations end up with snowfall totals towards the lower end of these ranges as opposed to the higher end of the ranges. If heavier snowfall works farther northward and persists longer, the higher end of the ranges could be met in more towns and the possibility will exist for some towns to exceed 12'', especially across southern Connecticut: