12/10/2023 Accumulating Snow Event NW of I95

 Guidance has converged on an anafrontal system with a secondary low that provides the opportunity to see some snow, as moisture is thrown from the secondary low into the cold sector. Thus, this gives us to potential to see snow falling, with (light) accumulation prospects NW of 95.

SYNOPSIS:

A shortwave, which has already moved through the country and continues to do so has a cold front ahead of it and northern piece energy. The trough axis is delayed behind the front, so we set up an anafrontal event(not typical).

 This system overall is very dynamic and should have good PVA in front of it which induces cyclogenesis(intensification of the storm) and moisture advection. Temp gradient is also quite large, with the airmass behind the cold front being Canadian and the airmass in front of it being subtropical.

We also have a dual jet setup associated with this which pops a secondary low when the trough becomes negatively tilted, in the Carolinas. This directs winds and moisture return NW.

The large temp gradient should allow for frontogensis forcing, which will make precip rates extremely high. CAA driven snow will occur in the overnight hours after midnight for most people as temps drop.

One thing to note about this system is that surface temps are extremely marginal, but the rest of the atmosphere is <0C, allowing for it to snow. These heavy precip rates will allow for greater dynamic cooling and a saturated column.

    

OBSERVATIONS:
We are now less than 12 hours away from snow, and the use of models is useless. As of 23z(6P EST), the trough is netural-slightly negatively tilted over MS/AL.

This is ahead of schedule, as it was supposed to be this way at 7-8 PM. This will have better implications downstream as a earlier negative tilt will allow for cold air to fill in faster during the snow, and the secondary low to pop more/quicker in a favorable location.

The cold front was also ahead of schedule, and stronger than modeled, passing the BR by 515-530 PM. 

Temps are dropping quite quickly behind the front, with some station reporting degree drops every 4-5 minutes. Although it is worth noting that sfc temps are warmer than on NAM, they are colder than on HRRR(behind the front), and it just initalized at 22z

The rate temps are dropping behind the CF, catching up to NAMs temps wouldnt take too long either. It's running too warm for some parts of nova at 0z anyways. We'll see though, eitherway temp wise we look good.

And now we look at the radar, we are starting to see the backside precip fill in and amplify. It looks very good with convection off the Carolinas. This system is absolutely juiced, and is exceeding expectatations. 

So overall, it might be safe to say that this event is going slightly better than expectations. Observations look great so far and is expected to continue into the event. 

Overall:

Overall, despite uptrends, the general picture is the same: accumulating snow for NW of 95, mainly in higher elevations. Temps will be very marginal(32-36), but we might push colder during heavy bands due to CAA. That being said, we need heavy snow rates to stick. Otherwise, it will not stick; moderate or below will not do it. Mesoscale banding features are too hard to resolve at this range, so there is no really saying where it will be, but if I had to guess it would be somewhere around here(NW of the fall line): 

As for snowfall, there will be an extremely sharp cutoff on I95. I have trouble thinking anyone inside or east of the beltway will see more than just white rain, with little to no accumulation. Elevation is just too bad there to get anything substantial. Temps will be too marginal there for anything. Even further NW, temps will remain 32-36 for most of the storm, with the exception of lower temps 1000'+ or in the bullseye areas. The reason I'm even going this bullish on accumulations is because of the puking snow rates that will set up somewhere that will stick to the ground despite temps. The rates cannot be moderate or light, they have to be heavy, but we know it will set up somewhere,, and that area should see nice accumulations. Having heavy wind should also help blow the snow around and even help with stickage as wind removes latent heat release; it would help us more than hurt us. This, along w/ heavy rates will make for low visibillity conditions. 

As of N of the fall line, I gave the highest amounts to areas 700'+ elevation and west of I81. The bullseye for this storm overall will be the Blue Ridge and the Apps, as expected. Locally, the bullseye is probably the Parrs Ridge if I'm betting.  In the Parrs ridge, there could be a spot of over 3"+ if everything goes right. I went extra bullish based on new data we have, but this could very well bust. I feel like this is a boom/bust type setup; either we do really well with banding and get puking snow for hours accumulating to over an inch, or we generally stay mostly rain with no acccumulation even if it snows. Ratios will be very low; likely 4:1-7:1 depending on your location.

And as for school impacts, models have trended earlier with the event, generally wrapping snow 5-7A through the region, but before that it will be absolutely puking, along with strong winds as the low intensifies and moves NE. The snow will melt extremely fast afterwards, but I still think theres a chance we could pull off a delay. I'll go 2 snowflakes(50% delay) for this event, if band is a bit later then we'll probably delay. If we can get a WWA in place we probably delay, but it can go either way. Accumulations will mostly be on grassy surfaces, but depending on rates, could be on some side roads and sidewalks. Overall, this event should lock in Dec measurable snow for most areas, and should be really nice to see.