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January 3 CAPE Storm


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URGENT - WINTER WEATHER MESSAGE
National Weather Service Baltimore MD/Washington DC
428 AM EST Sun Jan 2 2022

VAZ036-508-021730-
/O.NEW.KLWX.WS.W.0001.220103T0600Z-220103T1800Z/
Nelson-Central Virginia Blue Ridge-
428 AM EST Sun Jan 2 2022

...WINTER STORM WARNING IN EFFECT FROM 1 AM TO 1 PM EST MONDAY...

* WHAT...Heavy snow with snow accumulations of 4 to 6 inches.

* WHERE...Nelson County, and Central Virginia Blue Ridge.

* WHEN...From 1 AM to 1 PM EST Monday.

* IMPACTS...Travel could be very difficult. The hazardous
  conditions could impact the morning commute.
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This snippet from Mount Holly AFD made me smile for some reason-

Hard to overstate how high the uncertainty remains with this system due to how quickly the forecast is evolving. Looking at some of the initial 02.06z runs, it`s possible areas well north of the watch could also be in play for significant snow, even approaching metro Philly. And it`s possible current forecast maximum amounts could be considerably too low, and they will be if the overnight guidance is correct. But definitely did not want to jerk the forecast all the way in that direction at once; the current forecast already constitutes a major change. Once again, continue to closely monitor the forecast today.

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More from Mount Holly--

Bottom line up front: A major change in the forecast has occurred, at least for southern portions of the area. A potentially significant snowstorm is now expected to impact portions of the region Sunday night into Monday. This is a rapidly evolving forecast and users should monitor the forecast closely through the course of today as additional changes, potentially significant, are possible.

A Winter Storm Watch is now in effect for portions of the Eastern Shore of Maryland, central and southern Delaware, and far southern New Jersey. For days, we have been monitoring the potential for a wave of low pressure behind today`s cold frontal passage to potentially graze the region with precipitation, but most indications have been that it would stay mainly to our south. Starting on yesterday`s model runs, and much more so overnight, there has been a major shift to a more amplified, further north track with that wave of low pressure. Meteorologically, this is tied to a much larger separation between a shortwave trough moving into the southern Plains and the frontal system which has been affecting us this weekend. This shortwave now has the potential to amplify and acquire a negative tilt as it moves through the Southeast US, spurring strong cyclogenesis downstream.

Extreme frontogenetic lift will cause a new precipitation shield to rapidly develop to our southwest by early tonight, eventually overspreading portions of our region. Colder air will be rushing into the region tonight. Temperatures will fall through the night, eventually near to below freezing across the entire area. Steady precipitation should begin to move in during the middle to latter portion of the night, starting to the southwest. Initially, temperatures will probably remain warm enough for it to begin as rain across the southern zones. With time, continued infiltration of cold air and increasing precipitation intensity should support a flip to snow. Several hours of moderate to potentially heavy snow are then likely from early Monday morning into Monday afternoon. Snowfall rates of 1 inch per hour or greater at times will be possible in the Winter Storm Watch area.

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3 minutes ago, Maestrobjwa said:

Wondering how much of this is noise vs. another bump nw...can't imagine it could go too much further? (Of course I'd like it to, lol)

It's not really a bump NW. It is indictive of the deepening low and an expansion of the heavier precip. Note the increased snow amounts on the SE edge of the precip shield as well. The location with the heaviest stripe of snow as depicted didn't change- amounts are just higher in this region on the 6z mean.

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1 minute ago, CAPE said:

It's not really a bump NW. It is indictive of the deepening low and an expansion of the heavier precip. Note the increased snow amounts on the SE edge of the precip shield as well. The location with the heaviest stripe of snow as depicted didn't change- amounts are just higher in this region on the 6z mean.

Ah I gotcha. So deeper low=more precip shield...and any adjustment "north" we see would be because of that.

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21 minutes ago, Maestrobjwa said:

Ah I gotcha. So deeper low=more precip shield...and any adjustment "north" we see would be because of that.

The shortwave is sharper and takes on a bit of a negative tilt while still to our sw, thus the flow out in front isn't as flat as was previously depicted, and the low is able to gain a bit more latitude as it exits the NC coast. N MD will have a good period of snow but it will be of shorter duration than places further south as the low tracks east and the colder/drier air comes in. There will be a tight gradient and a sharp cutoff on the north edge somewhere- looks like southern PA right now.

Moisture depiction at h7 from 6z GFS-

1641200400-WOITWBh7VSw.png

1641211200-0tRGeVkYpGw.png

 

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giphy-downsized-large.gif

 

It's gonna snow!  So what does this storm have going for it that we didn't appreciate 72 hours ago or before?  (except @CAPE).  

1.  The increasing separation between yesterday's warm rain and the cold from that will pass through later today gives this storm room to amplify, while also giving the cold air just enough time to filter in ahead of it.  Watch how the shortwave has gotten distinct from the cold front (indicated by the blue offshore and to our N/NE over the last few days:gfs_z500a_eus_fh30_trend.thumb.gif.5170bb0a4666035238befe14fad74329.gif

 

2.  With this increased separation, we are getting MUCH better dynamics to support a broad expansive precip shield.  Beautiful positioning in the right entrance region of the strong jet streak over New England supports strong lift.  gfs_uv250_eus_5.thumb.png.9eaca6456fc4bcbee9849d84774b8cee.png

The 500mb low is passing in ALMOST the ideal position.  6z GFS tweaked it back south every so slightly, but here is the 0z GFS which shows Position A for DC snowstorms:  with a closed 500mb low passing along the VA/NC border:

gfs_z500a_eus_7.thumb.png.7d643259d10e85847107f0a0198918ae.png

 

3.  Take a look at the strong 700mb (also at 850mb) frontogenesis (in the insert image below).  This is basically a measure of where temperature gradients are strengthening and a proxy for strong vertical lift and thus heavy precipitation. It's focused near DC and just south, where things should be ripping 24 hours from now.  The cross section shows the DGZ (between the dashed red lines) and the very strong upward motion within the DGZ through the region.  Ideally, that DGZ would be a bit deeper, but we've seen far worse before. If the GFS is right, at around 12z tomorrow, we're puking dendrites all across the region.

2104227354_gfs_2022010206_fh30_xsection_39.99N75.42W_36.88N78.96W_FGEN-Theta-e-Omega.thumb.png.fba64ec99c34b44fb9b76ceb84f73ace.png

 

 

Ok, so what's working against this?  

1.  It's moving fast.  Euro is <12 hours of snow in DC.  GFS is a bit longer and GGEM even a little shorter than the Euro.  There's just not that much time to snow. This is what happens in fast flow/unblocked conditions. 

2.  It's going to be 60F today. This is not a problem because 4 inch soil temps or some shit like that are too warm.  No, it's just because we have 30F to cool down from!  That just takes time, even with a strong FROPA like we'll get today. So, it very well may start even as rain briefly in DC and points south and/or it will be snowing with temps in the mid/upper 30s to start and take some time to cool down to freezing where it can accumulate more efficiently.  

But this is a very different situation than the anafrontal wave that this looked like 3-7 days ago.  That's much more of a "cold chasing precip" sort of situation and we are not in the situation now.  Still, freezer doors open tonight wouldn't hurt.  

 

giphy.gif

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