Live Blog: 12/17/13 Clipper

3:00pm Update: Precipitation which had changed over to sleet and freezing rain in many areas along the coast and near the city, as a result of a warm layer aloft, will slowly change back to snow from W to E over the next hour. Regional mesoscale analysis shows the potential for one last band of moderate to heavy precipitation as a result of increased lift from the developing coastal system and enhanced support aloft.

This band will shift eastward during the evening commute. We are expected a slippery and wintry commute in many areas, so plan extra time and expect delays.

9:00am Update: Snow has become a bit more spotty in nature throughout the area, so the roads are wet in the city and have likely been treated across the interior. Things are expected to pick up again this afternoon with periods of snow developing and moving northeast from the Mid-Atlantic states into New Jersey and New York..eventually northeastward into Connecticut and Southern New England. Forecast models have been wildly inconsistent with potential development of another coastal low pressure system later this as mentioned a few hours ago, confidence in that is rather low.

That said, seems to be a nowcasting-type situation developing and we’ll be closely monitoring the precipitation, pressure and height trends throughout the day. As of now our forecast remains unchanged with 1-2″ the general consensus throughout the area. All products (brief, discussion, member products) have been updated with the morning package. -JH

HRRR model storm total snowfall forecast through evening on Tuesday.

HRRR model storm total snowfall forecast through evening on Tuesday.

7:45am Update: Steady snow is ending across the area as the initial wave of lift slips off the coast. Snow showers and squalls are moving east behind it, so continued wintry weather through the morning hours but no steady areas of moderate or heavy snow as of now.

An additional area of steady snow is expected to develop later this morning but forecast models have been inconsistent with it’s placement and intensity. So confidence is quite low at this time. Forecast remains unchanged, with 1-2″ expected through the forecast area.

6:50am Update: Light snow throughout much of the forecast area. Snow is accumulating on all surfaces and is expected to continue to fall for another hour or two. Please travel with caution and expect a slower than usual commute. Total accumulations look to be on the lighter end, with the system moving very fast and snow rates not very impressive. Forecast models seem to be overdone with precipitation totals at least in regards to this initial wave of precipitation.

We are expecting additional areas of snow to develop later this afternoon. The HRRR model has been insistent on a second and third batch of snow moving northeast from the Mid-Atlantic in response to the developing surface low. Our forecast of 1-2″ throughout the area remains unchanged, and we have adjusted the forecast to account for periods of snow through the day.


Overnight Update: The evolution of our system has greatly changed in the past 12 hours. What had previously looked like a quick thump of snow from 7:00am through noon with maybe a few snow showers behind it has now become an event where only light snow is expected in the early-morning, followed by a little break, and then much more enhanced precipitation during the mid-afternoon and evening hours.

The reason for this can be seen quite well at the mid and upper levels. We have a vorticity maximum currently swinging through Pennsylvania and is headed in our direction. There airmass is quite cold, especially at the mid and upper levels, which helps to create atmospheric instability and lift. It’s a fast-moving system, so there is not a whole lot of time for it to develop, but it is still expected to throw some moisture our way, especially when it hits the Atlantic Ocean.

What has changed, however, regards a second more powerful mid-level disturbance behind it. Initially, it looked like it was going to lag further behind, which means it would not have been “kicking” our first system out of the way nearly as quickly. This gave the first system a bit more time to develop and potentially blossom for a quick 2-4″ snowfall event. This caused the second piece of energy to not be able to generate enough lift, because the passage of the first vorticity maximum yields negative vorticity advection, as well as mid-level height rises, which helps to prevent lifting in the atmosphere for precipitation. However, the fast flow of the pattern has allowed the second disturbance to fly through much quicker; and given its stronger vorticity maximum, it is taking away some of the thunder from our first system. This causes it to fizzle out after only dropping around an inch of snow or so during the early-morning hours.

Since the first system is now fizzling out, its passage is not nearly as stabilizing to the atmosphere, and there is a bit more room for the second disturbance to blossom and generate its own lift. Very impressive values of positive vorticity advection are associated with the second disturbance, and there are even hints that the trough could go negatively tilted near the Atlantic Ocean, helping to throw back additional moisture to our region. Considering how cold mid and upper level temperatures are, this could create some periods of high-ratio bands of snow — especially away from the immediate coast. The immediate coast may experience some warming at the surface from the Atlantic Ocean, given the close proximity of the surface low track may yield winds being more easterly and southerly at the surface. The details of any changeover to rain, if at all, will be nailed down during our live blog updates.