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Jan 17-18th disco thread--1st big threat of the year


Midlo Snow Maker

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You guys can loop H7 on the NCEP products and see a nice linear line of lift starting after 12z tomorrow. Notice at 18z we have strong lift over central VA. This is due to strong frontogenesis. Look at those wind vectors from west to east over DC and south to north near RIC. This is frontogenesis folks. You are tightening the thermal gradient and as a result, creating a steeper slope to which air moves upwards. Without getting technical, the atmosphere loves to set up a circulation on the warm side of this gradient in order to restore this thermal imbalance we have in a short distance.

Agreed. This is a classic deformation zone with positive frontogenesis. I haven't seen bufkits in the area but if dendritic growth is there where the RH/lift is maximized (looks like it), there will be a solid area of 1-2"/hr type bands sitting over a spot for quite enough time to bust high.

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Bob, if you compare that close up with attention to the cut off with the 12Z run cutoff, this run is much more smoothed with lighter precip heading further n and w

could just be a blip or maybe a sign for 0Z

 

I saw that earlier but wasn't really significant enough to make a point about. But on that note, I do think we will see another move better tonight. I think the lighter qpf on the fringe covers more area to the n&w. This is a dynamic system even though it's small. Give it any room to move north and it will. It comes down to how strong that wall wants to be to our north. That wall already took a major step back since last night on the models. We'll see tonight. I honestly wouldn't be surprised with a move in any direction on the guidance. If I was forced to bet I would go more N&W with both the deform band and lighter qpf further out. 

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Agreed. This is a classic deformation zone with positive frontogenesis. I haven't seen bufkits in the area but if dendritic growth is there where the RH/lift is maximized (looks like it), there will be a solid area of 1-2"/hr type bands sitting over a spot for quite enough time to bust high.

 

It's looks pretty sick with the highest lift located in the snow growth zone and at 95%+ humidity. This is the 18z GFS at 3z tomorrow.

 

post-741-0-73395100-1358379125_thumb.png

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Agreed. This is a classic deformation zone with positive frontogenesis. I haven't seen bufkits in the area but if dendritic growth is there where the RH/lift is maximized (looks like it), there will be a solid area of 1-2"/hr type bands sitting over a spot for quite enough time to bust high.

Agreed. Recall the Jan 30 or 31st event in 2010, the one before Snowmageddon II and III 6-10 days later. Granted, it was a colder event, but one where southern and central VA were targeted for the highest accums. While that was largely thd case, there were bands of 6+ inches that extended well north into CHO, EZF, and DCA, which had every bit to do with the fact that the layer of max fgen lift and moisture, while farther up in the atmosphere compared to 50-100 miles south, just so happened to be in the sweet spot in terms of optimizing dendritic growth (i.e. in the -12 to -18C layer). So, even without as much QPF, the snow ratios were much higher, as portions of northern VA got in on warning-criteria snowfall well north of where the highest liquid equiv pcpn was.

Again though...that was a notably colder event in the BL going in compared to this one...

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It's looks pretty sick with the highest lift located in the snow growth zone and at 95%+ humidity. This is the 18z GFS at 3z tomorrow.

This is the reason that model QPF can really suffer on fringes of deformation zones. At some point, these bands will be made and they will move north and stall. If they reach DC and sit, then the low end stuff isn't going to work out so well. A famous example of this was in the 2009-10 winter where Maryland was on QPF fringe and they ended up with 6-10" ...I'm sure someone can recall this one....

 

The low level temps do suck but nothing is going to stop that dendrite machine at 3z!

 

Edit: See Brian's post above about Jan 2010. That is the event I'm talking about.

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Agreed. Recall the Jan 30 or 31st event in 2010, the one before Snowmageddon II and III 6-10 days later. Granted, it was a colder event, but one where southern and central VA were targeted for the highest accums. While that was largely thd case, there were bands of 6+ inches that extended well north into CHO, EZF, and DCA, which had every bit to do with the fact that the layer of max fgen lift and moisture, while farther up in the atmosphere compared to 50-100 miles south, just so happened to be in the sweet spot in terms of optimizing dendritic growth (i.e. in the -12 to -18C layer). So, even without as much QPF, the snow ratios were much higher, as portions of northern VA got in on warning-criteria snowfall well north of where the highest liquid equiv pcpn was.

Again though...that was a notably colder event in the BL going in compared to this one...

Yes that's the exact storm I was thinking about. Thanks! What happens then if we have similar type banding / dendrite production but it falls into a BL that's warmer? I would imagine if a band stalls over someone, it will cool the BL sufficiently but still warm enough for compacting. Tough call here for accumulation. The low level temps are pretty solid 950-850mb and secondary circulation could really cool some folks even more than modeled.

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Discussion and Forecast

 

Decided not to wait until 11pm.

 

I like what I am seeing on the radar as far as KY/TN and the precip mass along GA/AL border. There is good northward motion even though the overall move is ene-ne.  At this point I think the movement would have to switch to due east in order for DC to miss out and that is not likely.  If anything, we could have more of a temperature issue than a missing out out issue.

Some of the posters have discussed deformation issues and I will leave that out as it's too difficult to predict if, where and when.  This looks like a 2-4" snowmaker for DC area

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Agreed. This is a classic deformation zone with positive frontogenesis. I haven't seen bufkits in the area but if dendritic growth is there where the RH/lift is maximized (looks like it), there will be a solid area of 1-2"/hr type bands sitting over a spot for quite enough time to bust high.

 

True but I can also remember cases when the the gradient was really tight and they got 8 inches in Prince Frederick which is in the middle of my county and I got zilch living in the northern end of the county and I was supposed to get several inches.  Tight gradients can screw you lots of ways.  You can get under the deformation zone and a band that goes stationary and not very far way someone gets nothing.  That's the problem.  The fact that the higher resolution models seem to be holding the precip farther south is a concern at least to me. 

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True but I can also remember cases when the the gradient was really tight and they got 8 inches in Prince Frederick which is in the middle of my county and I got zilch living in the northern end of the county and I was supposed to get several inches.  Tight gradients can screw you lots of ways.  You can get under the deformation zone and a band that goes stationary and not very far way someone gets nothing.  That's the problem.  The fact that the higher resolution models seem to be holding the precip farther south is a concern at least to me. 

 

Oh yeah...the risks are heightened for sure. You obviously know this, but the very strong right bands usually have a subsidence exhaust zone just W or NW of the band.

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not to back pat especially since it's probably not the same overall, but mentioned jan 30 when this first showed up before we went back to ignoring it

http://www.americanwx.com/bb/index.php/topic/38526-january-2013-mid-long-range-disco-thread/?p=1990259

Well, I think it's a "feel good" analog when folks up here in the DC area feel a shaft coming. I will be anxious to see what the 00Z runs show for 21Z-03Z tomorrow evening. It may end up being a small window of opportunity up here, but nevertheless one where 1-3" in the DC metro is still within a 50th percentile (where 10th/20th percentile is nothing and 80-90th is closer to 6+").

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