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Central PA - Winter 2021/2022


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37 minutes ago, MillvilleWx said:

Well, with finally some time tonight, thought I'd throw out some thoughts on the setup for Thursday Night into Friday AM for a nice little snowfall entering the area. There's another pretty decent signal for a marginal to moderate event areawide given all the different features at play. So first off, our culprit is currently peddling out over ID/WY border with sites on the central plains by morning indicative of very progressive flow within the northern region of the jet. This system will be quick mover, but there's some definitive details that make this a chance to a more prolific storm than many of our rapid movers in the past. 

First off, the thermal environment is pretty ripe with the 95 corridor only going a bit above freezing for tomorrow PM before slowly taking a dive after peak diurnal max. West of the fall line should stay near to below freezing, which bodes well for the "stickage" factor. Every single flake outside the beltways of the big cities should stick pretty quickly at the first snowfall. So, one of the factors we have going in the positive is one we often fight in these parts. That should not be a problem this go around!

Now lets take a look at the meat and potatoes of the event with the mid and upper levels that will make this event.

First image below is the 00z NAM Nest from this evening at 500mb


There is a prolific 500mb jet max that will be rounding the base of the trough with the left exit region placed over our area (Inside the box). The divergent depiction out ahead of the main trough axis is a classic for developing qpf fields downstream of the mean trough axis. Also notice the tilt of said 500mb axis going negative as it progresses through the OH Valley. This is also a positive indication for the area as historically, we tend to see a robust response east of the BR with ascent maximized along the Alleghany front out into the Piedmont area. This is no different, and the signals below 500mb are certainly there.

Below is a gif of the same 00z NAM Nest run of the 700mb  horizontal temp advection and frontogenic forcing. 


Something important to note here is the robust warm horz. temp advection signal developing over SoVA, moving NE into the sub-forum. This is also a classic indication of a good moisture signal for the area when it comes to these events. If you take a look at the concurrent 700mb temperatures within that area of + horizontal temp advection, you can see these are very cold temps aloft within the mid-levels, which is very important for snow growth and attendant ratios that could occur where there is precip. -11 to -17C is the sweet spot for the DGZ (Dendritic Growth Zone) as this is where ice crystallization has the best environment for developing its core structure and maintaining a defined appearance upon its descent. But, temps are not the only thing of note here when it comes to the best snowflake formation....lift is also necessary. This is where that + horizontal temp advection comes into play. This provides a great deal of ascent within the 700mb layer, which is important for crystallization and snow structure. It is apparent that there is a 6 hr window where this is a huge positive for much of the area from SW VA up into portions of Southern PA (Mainly up into I-81 corridor near Chambersburg) then east into much of NoVA/Central MD/Northern half of the Eastern Shore. This is where banding is most likely to occur and there WILL be maxima and minima that occur due to the prominent banding features likely within this setup. Someone will get crushed and someone settling outside a band can get squeezed here, so despite being a solid areawide snow potential (forecast later) there will be big winners and losers. 

So, how good is the profile we are talking about for this system? Let's take a look at a few examples of areas that get hit pretty solidly during the event.


Here is an example from JYO that shows a robust area of ascent within that layer b/w 850-600mb, which are the most important layers for snow growth and expected ratios from a storm. This snapshot was taken at the peak of the mid-level ascent caused by the frontogenic forcing at 850-700mb when the jet streak pinpoints the area within the left exit region of the core jet max. With a sounding like this, 12-15:1 are not just a possibility, they are a given. 

Here's an image an hour later downstream near BWI


This is an even more robust signature of mid-level ascent within the DGZ, indicative of a potential for pretty sizeable flakes within a significant band presence. This is what we want to see in a setup like this where the pattern is progressive, but the upper levels are favorable. As long as we can right the path here with the current outlook, this will be a nice storm for a majority of the sub-forum.

So, how much snow we talking? Well, let's take a look at a few things. For me, precip panels will tell a lot of the story because ratios for the storm will be congruent with the banding and overall precip totals throughout the event. Here's a 3hr precip gif for the area leading in till 12z Fri AM


Notice there is a primetime period likely just before midnight until about 5am Friday AM. This is when most of the snowfall will occur, so any totals we have will be in a relatively short period of time. However, considering the banding potential and primed thermal environment, an advisory snowfall for pretty much everyone is well within reason. Someone will likely get the benefit of the deformation band that develops on the western flank of the developing SLP off the Atlantic coast, and that's where the WSW criteria snowfall will have the best chance. Here's a snapshot of the total precip for the event.


The maxima on the eastern shore is a product of the deformation axis being pegged in that location via the NAM Nest, but of course that is not set in stone and will not likely be solved until late tomorrow when the ingredients all come together. 

Here's my current thinking in terms of snowfall for the storm1187745354_0106MASnowMapForecast1.thumb.PNG.cd22b4377150bcdd0a1b5423eaf1bd2e.PNG

This a general 2-6" event for most, but the upslope areas out west are an area I have higher confidence in the biggest totals of the event. Secondary maxima near the Catoctin front and on the Eastern Shore where I believe the deformation axis provides a big punch. The caveat is if the low gets cranking earlier, that could be on the other side of the bay, so keep an eye out for those next 12-18 hrs of model runs to see if any trends are picked up. That's all for now y'all :) 

Edit: Bit of a crude transition across St. Mary's county. Apologies for that look! 



2 minutes ago, paweather said:

Can you share here I don’t want to touch the MA forum thanks 

For your viewing pleasure!

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2 hours ago, MillvilleWx said:


For your viewing pleasure!

That was one thing that really stuck out to me this week regarding this system, the potential for nice ratios with a nice cold column. Your post really details that nicely 

This really has went the classic snowstorm route in terms of forecasting. First in the mid-range everything sees it except the GFS, then the GFS finally sees it and everything goes south for a couple days. And now inside 48 hrs the QPF and northern edge has been coming right back up. 

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Here is some of the CTP discussion today.

Latest ensemble mean qpf, combined with WPC mean snow ratios in
the 12-15/1 range, support snow totals tonight ranging from less
than an inch north of I-80, to around 4 inches over Somerset
and Bedford counties. As for timing, latest hi-res guidance
indicates snow will develop across the southwest part of the
forecast area by dusk, then rapidly overspread the rest of the
area during the evening hours.
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11 hours ago, Atomixwx said:

I know we have our own system coming up, but I'm almost closer to the south towns than LSV at this point. It's a weird drive up 219 from Ridgway to Buffalo, but I may have to consider a chase to Hamburg sometime soon.



According to the NY subforum, this is turning out to be a real snoozer. I'll give you a current conditions from the bus stop outside of Philipsburg. 

At 7:33 AM, the stepkid opened the door to get out and the cold find went straight through my brainless ****ing head and out the other side. 



It feels like winter. Just need a little snow. 


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1 minute ago, Atomixwx said:

According to the NY subforum, this is turning out to be a real snoozer. I'll give you a current conditions from the bus stop outside of Philipsburg. 

At 7:33 AM, the stepkid opened the door to get out and the cold find went straight through my brainless ****ing head and out the other side. 



It feels like winter. Just need a little snow. 


It is coming tonight for us you may be on the lesser side of it this round

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