December 5-6, 2020 Storm Observations and Nowcast

[Lava Rock]

We just got ours back after about 14 out. Changed the oil and got the genny all put away and just hoping we don’t lose it again.  My connection is outside at the opposite end of the house, so dragging that beast out of the garage last night at 1:00am through the frozen cement and dropping limbs is not the kind of fun I’m looking to repeat. 

Ours back on. I'm leaving genny out for the night just in case. Spectrum cable back on line too

[backedgeapproaching]

DC to Presque Isle, that is one hell of a chase.

[40/70 Benchmark]

DEC

6

 

 

12-5-20 Rain to Snow Verification: A Matter of Lift

12-5-20 Rain to Snow Verification: A Matter of Lift

Friday Warmth Not Overcome.

It is difficult to fault anyone for having been skeptical of an impending large snowfall on Friday afternoon, as temperatures were a spring like 55-60 degrees across the region.

 

 

 

It can and has happened before in the past, however, it goes without saying that everything has to go "right", or wrong, depending on one's perspective. This is because such a warm antecedent airmass leaves virtually no margin for error within the atmosphere. Everything needs to go precisely according to plan, or else the result is the couple inches of slush that many areas saw on Saturday afternoon:

 

 

vs the forecast:

 

This was a respectable forecasting effort in such a challenging system, but obviously as implied above, something did not go according to plan. First of all, the primary critique of this forecast effort is that it did emphasize the importance of elevation enough with respect to snowfall. While topography was considered in a general sense, as the Connecticut river valley had a lighter forecast range of 2-5", the map failed to address just how much snowfall varied by elevation even in Worcester county and northern Connecticut. Secondly, snowfall was over forecast in much of eastern Mass, which had nothing to do with elevation and everything to do subtle nuances in the mid levels of the atmosphere.

 

Proficient Snowflake Production Can Compensate for Marginal Thermal Profile

There is a great deal more to forecasting snowfall than the temperature on the thermometer outside. Snowflakes, or "dendrites" are optimally produced between a temperature of -12 to -18 celcius in the mid levels of the atmosphere. This is referred to as the "snow growth zone". The high quality dendrites that are proficiently produced within this region accumulate very efficiently. In order to get rates that heavy, there is a tremendous amount of "lift", or rising air that is needed to produce the very heavy rates of +1-2" per hour. It is possible to have perfect dendrites falling at only light intensity when lift of paltry, and conversely, ample lift can be occurring outside of the optimal snow growth zone, which is what took place yesterday in eastern Massachusetts. This does not necessarily preclude a heavy snowfall, assuming that low level temperatures are cold, however, yesterday the forecast did not have that margin for error, since we were relying on perfectly formed dendrite falling at very heavy rates owed to well placed lift. We needed everything to go as planned, and it did not. A superior snowfall forecast requires accurately forecasting the temperature at the surface, the temperature at all other levels of the atmosphere, as well as the amount of lift, and where said lift is located relative to that -12C to -18C optimal snow growth region. There is zero margin for error with respect to any of this when temperatures are as marginal as they were yesterday.

 

Forecast Thought Process

The forecasting yesterday was for several inches of snow to the northwest of Boston, however, only about 2-5" actually accumulated. The reason was that Eastern Mass Weather underestimated the importance of the fact that the best lift was located slightly beneath the snow growth region, which greatly limited snowfall since the lower levels of the atmosphere were slightly above freezing.

 

There was no question that sufficient lift would exist, as the developing storm created a strong temperature gradient in the mid levels of the atmosphere, which is called "frontogenesis".

 

 

However, what is evident is that the area in question immediately to the north and west Boston is on the eastern edge of said lift, which further complicated the forecast.

 

Here is the sounding at Lawrence, MA:

 

 

The thought process was that it would be "good enough" for 5-8" of pasty snowfall, but it was not. There was 2-5" of slush. This was the forecasting error. Had the temperature been 1-2 degrees colder, and the lift been slightly higher, then the forecast would have been for 12-18" of snow. An accurate snowfall forecast is a complex endeavor that incorporates the consideration of a myriad of different factors on a continuum, which necessitates a great deal of critical thinking. A far cry from the linear process of simply considering the low level temperature and the amount of water falling from the sky that many people believe it to be.

 

Compare this to the sounding to the west in Worcester:

 

 

 

This is why the Worcester county forecast of 8-12" verified nicely, with the exception of the lowest elevations. Low level temperatures were slightly colder with some elevation, and the strongest lift, which was perfectly centered it he snow growth region, directly traversed the area.

Conclusions

This was a solid, albeit flawed forecasting effort that will inform future endeavors this throughout the boreal 2020-2021 winter season. Since this is a la nina season, there will likely be many marginal scenarios that will both hone and challenge forecasting skill.

 

Final Grade: B

 

 

 

 

[40/70 Benchmark]

Should have stuck with First Call back east.

 

[weathafella]

A Tipesque verification blog from Ray.

[Damage In Tolland]

8 minutes ago, weathafella said:

A Tipesque verification blog from Ray.

He had you at hello?

[Heisy]

Tip alluded to this, but something I noticed was when I did flip to snow it stuck to all surfaces practically immediately. In my life in philly I had never seen that during a changeover event. It always took some time for pavement to accumulate. When the fatties were falling it stuck and accumulated pretty much evenly to every surface. This seems kind of odd considering ratios were bad. When that had happened I thought for sure we might bust high or at least do close to expectation. I didn’t notice the melting aspect until the tail end of the deformation zone where I imagine temps might have climbed up a tad. 

[CoastalWx]

Not sure if anybody noticed, but after it cleared on the outer cape, that had some gusts 70-90mph. Mini 05.

[HoarfrostHubb]

ORH set record daily rainfall (1.97”) and snowfall (9.6”)  records yesterday.  Wtf?

[dendrite]

23 minutes ago, HoarfrostHubb said:

ORH set record daily rainfall (1.97”) and snowfall (9.6”)  records yesterday.  Wtf?

Water equiv

[Typhoon Tip]

I looked at the modeled contouring at the traditional sigma levels ...

the 850, 700 and 500 mb lvls

these cyclonic components collocated upon the same axis at nearly the same time and when that happened the wind backed NNW ... right around 21z

1/2 hr later the winds at the surface here, NW of I495, went 350 and flurry mist mixture became the primary fall rate and type. Basically ... our wind shift coincided with vertical stacking and storm loss. 

I have seen this too often in the past and that wind backing tandem. 
 

yeah ... we lost DGZ because the coupled atmosphere stretches when the flow down slopes ... compensating for synoptic UVM forcing.  
 

I feel pretty confident you’ll see this in reanalysis. All that then added to physics of descending motion adding evaporation I think together worked against and is why the back 1/3 to 2/5ths of the event fell apart. 
 

I also like the idea of ‘melt momentum’ and tall column distance.  Didn’t help. 
 

As always the case theres probably a cocktail of reasons

[The 4 Seasons]

Here's the final storm totals for CT. Thanks everyone for all the reports. I figured this one wouldn't work out for southern CT and the valley areas so i wasn't really feelin' it enough to make a map. Also with my schedule i really didnt have the time. Attached is a screenshot from a group chat im in with a few mets from this board. I would have done very well for most of the state but busted too low for NW CT, especially that tight area around Burlington. Nailed E and NE CT especially around the Tolland to Union area.

It was pretty much all rain here except for a brief time around 12-2PM with heavy banding we changed to a 50/50 mix but never fully flipped. When the precip lightened up we changed back over to all rain and ended. Final total was Trace. 

Lets hope we can all sneak one solid event here in CT and the tri-state area before the month is over. It's been almost 3 years since the last warning even during met winter. Too long..

 

[Lava Rock]

Guess we got lucky getting our power back yesterday late afternoon. CMP estimates it'll be Weds night until everyone is restored. They made a comment this was the worst storm/outage of the century. I'm scratching my head on that one. I saw one tree across some lines on our little drive yesterday to scope things out. The lower elevations had no snow on the trees and overall, the damage looked pretty insignificant to me. I though Hurricane Irene (2011) was much worse in terms of damage.

[STILL N OF PIKE]

10 hours ago, Typhoon Tip said:

I looked at the modeled contouring at the traditional sigma levels ...

the 850, 700 and 500 mb lvls

these cyclonic components collocated upon the same axis at nearly the same time and when that happened the wind backed NNW ... right around 21z

1/2 hr later the winds at the surface here, NW of I495, went 350 and flurry mist mixture became the primary fall rate and type. Basically ... our wind shift coincided with vertical stacking and storm loss. 

I have seen this too often in the past and that wind backing tandem. 
 

yeah ... we lost DGZ because the coupled atmosphere stretches when the flow down slopes ... compensating for synoptic UVM forcing.  
 

I feel pretty confident you’ll see this in reanalysis. All that then added to physics of descending motion adding evaporation I think together worked against and is why the back 1/3 to 2/5ths of the event fell apart. 
 

I also like the idea of ‘melt momentum’ and tall column distance.  Didn’t help. 
 

As always the case theres probably a cocktail of reasons

Thank you 

On 12/5/2020 at 8:41 AM, It's Always Sunny said:

bruiser. 

that was 8am sat HRRR, the model WDRAG said to monitor for now cast

does that not show max lift in DGZ leaving at 20z (4pm) and lift in general going bye bye by 5-530pm (above DGZ) ..isn’t that a big old red flag for a 495 Given what we needed to see 

maybe I’m wrong since this was posted as a reason for a crushing by someone but I’m not confident in reading that but that is my intuition interpretation 

How would that chart be interpreted as a crushing for 5-9pm as was forecast 

[tamarack]

53 minutes ago, Lava Rock said:

Guess we got lucky getting our power back yesterday late afternoon. CMP estimates it'll be Weds night until everyone is restored. They made a comment this was the worst storm/outage of the century. I'm scratching my head on that one. I saw one tree across some lines on our little drive yesterday to scope things out. The lower elevations had no snow on the trees and overall, the damage looked pretty insignificant to me. I though Hurricane Irene (2011) was much worse in terms of damage.

Short memories?  They had over 500k customers out from the Oct. 2017 gale; that's a few thousand more than in 1998, though in '17 most were re-powered within 2-3 days while the ice storm had many thousands dark for 2 weeks +.

[Lava Rock]

Just now, tamarack said:

Short memories?  They had over 500k customers out from the Oct. 2017 gale; that's a few thousand more than in 1998, though in '17 most were re-powered within 2-3 days while the ice storm had many thousands dark for 2 weeks +.

Yeah, I don't know where or why they made that statement, but it didn't seem accurate to me.

[JC-CT]

[40/70 Benchmark]

I didn't do well Saturday, but it was apparent that I did better than Boston. There are only patches in the Everett/Chelsea area. I have fill coverage at home in Methuen.....and that 2" of concrete will not cede ground easily.

[Damage In Tolland]

19 minutes ago, 40/70 Benchmark said:

I didn't do well Saturday, but it was apparent that I did better than Boston. There are only patches in the Everett/Chelsea area. I have fill coverage at home in Methuen.....and that 2" of concrete will not cede ground easily.

Oh it sure will on Sunday 

[40/70 Benchmark]

9 minutes ago, Damage In Tolland said:

Oh it sure will on Sunday 

Obviously...I just mean that it will melt slowly relative to your average 2" snow pack.

[Typhoon Tip]

2 hours ago, STILL N OF PIKE said:

Thank you 

that was 8am sat HRRR, the model WDRAG said to monitor for now cast

does that not show max lift in DGZ leaving at 20z (4pm) and lift in general going bye bye by 5-530pm (above DGZ) ..isn’t that a big old red flag for a 495 Given what we needed to see 

maybe I’m wrong since this was posted as a reason for a crushing by someone but I’m not confident in reading that but that is my intuition interpretation 

How would that chart be interpreted as a crushing for 5-9pm as was forecast 

Yeah that does appear to indicate a 21 to 22z abatement of UVM, sure.  The deeper analysis or understanding of the HRRR's parameterization/physical make-up may reveal exactly "why" it showed that. It could be both synoptics and oreographical reasons combined. 

It's interesting that the timing there coincides with the backing wind. 

I'm not sure the HRRR has discrete surface topographic/oreographic BL parameterizations - which is interesting if it does. I mean it could be ending lift from synoptics for other reasons.  Then, if/when having the backing wind at all levels, from 500 to the sfc would only mitigate the event further.  The problem in assessing 'what when wrong' is that the downslope factor cannot be precluded.   Rain shadowing is a very real.  It is a physically reproducible, empirically observable phenomenon that does dictate regional fall biases and ...this isn't open to Trumpian alternative fact notions of reality... the wind backing from 500 to the surface to roughly a 330 deg direct DID play a part.  sorry - not debatable.  It's a matter of how much -  

I did not admittedly look at the HRRR model and frankly ...don't that often?  Nothing against the HRRR - that's not why...  If you must know I grow increasingly disenchanted at a world mired down - the unimaginable speed of transmission and computing power slowed by clogged tedium of penny-profit schemes... People need to feel pain and anguish to the tolerance of man again - learn some humility and virtuosity ... Instead of trying extort breaths for money.  It's embarrassing really... I find myself equally mired down by wishing on cancer diagnosis ... It's no way to live.. eh hm...

So, I avoid the effort and rely on increasingly granular products that no one cares about - until petty greed attempts to capture mere pennies there too ...at which point I'll do the world a favor and just stop being involved altogether - lol.   ...not being entirely serious here ...

[ORH_wxman]

I think the lift location is more important than “shadowing” which implies a drying out of the atmosphere. 

The precipitation was pretty prolific still looking at LWM on the upper 495 belt pretty quickly. I see hourly bucket tips of 0.16, 0.18, 0.12 after 5pm. 

That is heavy stuff. They just couldn’t accumulate it. It was white rain. 

[40/70 Benchmark]

9 minutes ago, ORH_wxman said:

I think the lift location is more important than “shadowing” which implies a drying out of the atmosphere. 

The precipitation was pretty prolific still looking at LWM on the upper 495 belt pretty quickly. I see hourly bucket tips of 0.16, 0.18, 0.12 after 5pm. 

That is heavy stuff. They just couldn’t accumulate it. It was white rain. 

Yea, my take is that the lift being JUST below the SGZ was fatal given how marginal the low level thermal envt. was.....we needed optimal dendrite production in order to offset that and we did not get it. The heavy fall rates in and of themselves were not enough...we also needed perfect growth.

[Typhoon Tip]

31 minutes ago, ORH_wxman said:

I think the lift location is more important than “shadowing” which implies a drying out of the atmosphere. 

The precipitation was pretty prolific still looking at LWM on the upper 495 belt pretty quickly. I see hourly bucket tips of 0.16, 0.18, 0.12 after 5pm. 

That is heavy stuff. They just couldn’t accumulate it. It was white rain. 

Depends what you mean:  are you talking about "snow" - or - the precipitation distribution and output from the storm in general? 

I'm considering the latter.  Altho, I argue that if fall rates did not weaken it would have continued to snow more - so.. it may also be hard to separate the two.   

It is not just a compressional drying question.  It's also a fluid mechanics issue. If/when the wind backed at all levels, 500 clear to the surface such that it is coming down from BTV/RUT VT... That stretches the column and offsets the UVM by "pulling" downward... that's a geophysically clad -.  

No argument that the lift abated - trying to get to why. So the shredding radar, and lack of ground truth/ in bucket. I'm just trying to figure out why the NAM and even the global models had 4 to 6 additional hours of wrap-around deep QPF that failed to realize from the night before.   

HRRR seemed to pick up on that Saturday morning ...agreed with the previous poster.  interesting..

I've noticed this since first becoming privy to weather modeling back in the 1990s ... that routinely, when the wind backs NW... models tend to hold onto QPF too long ... This smacks enough to bloody a nose as having some of that modeling tendency with this thing.   I think it is nice explanation frankly...  Deep layer structure create a parallel flow that down slopes ... offsets the backside CCB and that is why the rad shredded and ground truth dwindled from NW to SE prematurely over the course of the evening, and because the fall rates were being stemmed ..that stopped a marginal situation from snowing as prolifically...  

Either way, the fall rates were not there...  I don't think LWM is much refutation on that, because that area was not part of the initial back wind field in the deep layer and can be explained by proximity to the closing surface aloft easily enough -

 

 

[OceanStWx]

17 minutes ago, 40/70 Benchmark said:

Yea, my take is that the lift being JUST below the SGZ was fatal given how marginal the low level thermal envt. was.....we needed optimal dendrite production in order to offset that and we did not get it. The heavy fall rates in and of themselves were not enough...we also needed perfect growth.

Right. Kind of had a partial pupu platter and could never complete the order. 

We had rates, but not the dendrites or low level cold. A little low level cold (elevation) went a long way in this event to help save things. Valleys needed one or the other (dendrites) to come through and neither did. 

[Typhoon Tip]

14 minutes ago, OceanStWx said:

Right. Kind of had a partial pupu platter and could never complete the order. 

We had rates, but not the dendrites or low level cold. A little low level cold (elevation) went a long way in this event to help save things. Valleys needed one or the other (dendrites) to come through and neither did. 

Maybe up your way ...? Down here, radar around 5 or 6 pm flashed less and began shredding all over SE VT/ S NH and N Mass... ahead of guidance frankly.  I recall some theory being floated regarding signal source attenuation but... mm, I was here, in that area, and what was happening out of doors precisely matched that attenuation - I'd even argue that some of the 'green' banding there was also being undercut/theft a bit, too when the llvs probably (subgeostrophic argument) was backed even more..

Obviously you have ample access to elucidate this shit already but ... here, run this: you can see precisely when this abatement phenomenon swept through and it was real. I was here and observed flurries and mist immediately take over, while this radar was transmitting:

https://weather.rap.ucar.edu/radar/displayRad.php?icao=KBOX&prod=bref1&bkgr=gray&endDate=20201205&endTime=23&duration=4

The only thing I can see that really offers that kind of quick larger scale physical forcing was that if we look at the deep layer, the winds backed at all levels - I could be off on that timing...admittedly...but it appeared to be the case.

I don't know - fuggit... I'm done  

 

[40/70 Benchmark]

16 minutes ago, Typhoon Tip said:

Depends what you mean:  are you talking about "snow" - or - the precipitation distribution and output from the storm in general? 

I'm considering the latter.  Altho, I argue that if fall rates did not weaken it would have continued to snow more - so.. it may also be hard to separate the two.   

It is not just a compressional drying question.  It's also a fluid mechanics issue. If/when the wind backed at all levels, 500 clear to the surface such that it is coming down from BTV/RUT VT... That stretches the column and offsets the UVM by "pulling" downward... that's a geophysically clad -.  

No argument that the lift abated - trying to get to why. So the shredding radar, and lack of ground truth/ in bucket. I'm just trying to figure out why the NAM and even the global models had 4 to 6 additional hours of wrap-around deep QPF that failed to realize from the night before.   

HRRR seemed to pick up on that Saturday morning ...agreed with the previous poster.  interesting..

I've noticed this since first becoming privy to weather modeling back in the 1990s ... that routinely, when the wind backs NW... models tend to hold onto QPF too long ... This smacks enough to bloody a nose as having some of that modeling tendency with this thing.   I think it is nice explanation frankly...  Deep layer structure create a parallel flow that down slopes ... offsets the backside CCB and that is why the rad shredded and ground truth dwindled from NW to SE prematurely over the course of the evening, and because the fall rates were being stemmed ..that stopped a marginal situation from snowing as prolifically...  

Either way, the fall rates were not there...  I don't think LWM is much refutation on that, because that area was not part of the initial back wind field in the deep layer and can be explained by proximity to the closing surface aloft easily enough -

 

 

Yes....hence the "backlash" member of the fraud five. Bruce Showegler never did manage to wrap his mind around this over the course of a very esteemed 33 year career as an OCM.

[40/70 Benchmark]

2 minutes ago, Typhoon Tip said:

Maybe up your way ...? Down here, radar around 5 or 6 pm flashed less and began shredding all over SE VT/ S NH and N Mass... ahead of guidance frankly.  I recall some theory being floated regarding signal source attenuation but... mm, I was here, in that area, and what was happening out of doors precisely matched that attenuation - I'd even argue that some of the 'green' banding there was also being undercut/theft a bit, too when the llvs probably (subgeostrophic argument) was backed even more..

Obviously you have ample access to elucidate this shit already but ... here, run this: you can see precisely when this abatement phenomenon swept through and it was real. I was here and observed flurries and mist immediately take over, while this radar was transmitting:

https://weather.rap.ucar.edu/radar/displayRad.php?icao=KBOX&prod=bref1&bkgr=gray&endDate=20201205&endTime=23&duration=4

The only thing I can see that really offers that kind of quick larger scale physical forcing was that if we look at the deep layer, the winds backed at all levels - I could be off on that timing...admittedly...but it appeared to be the case.

I don't know - fuggit... I'm done  

 

It was both.....sure, precip cutoff early in the evening, but it had been accumulating like shit all afternoon, despite heavy fall rates.

[OceanStWx]

6 minutes ago, Typhoon Tip said:

Maybe up your way ...? Down here, radar around 5 or 6 pm flashed less and began shredding all over SE VT/ S NH and N Mass... ahead of guidance frankly.  I recall some theory being floated regarding signal source attenuation but... mm, I was here, in that area, and what was happening out of doors precisely matched that attenuation - I'd even argue that some of the 'green' banding there was also being undercut/theft a bit, too when the llvs probably (subgeostrophic argument) was backed even more..

Obviously you have ample access to elucidate this shit already but ... here, run this: you can see precisely when this abatement phenomenon swept through and it was real. I was here and observed flurries and mist immediately take over, while this radar was transmitting:

https://weather.rap.ucar.edu/radar/displayRad.php?icao=KBOX&prod=bref1&bkgr=gray&endDate=20201205&endTime=23&duration=4

The only thing I can see that really offers that kind of quick larger scale physical forcing was that if we look at the deep layer, the winds backed at all levels - I could be off on that timing...admittedly...but it appeared to be the case.

I don't know - fuggit... I'm done  

There was definitely beam attenuation going on with BOX. Winds were starting to turn west of north (which is why the dome was attenuated in that direction), so the storm was going to begin to wind down. Like Will mentioned though LWM still had 0.34" between the hours of 5 and 7, so it's not like it was spitting out. It just was shitty snowflakes and a shitty environment to keep them frozen for very long.

[Typhoon Tip]

Just now, 40/70 Benchmark said:

It was both.....sure, precip cutoff early in the evening, but it had been accumulating like shit all afternoon, despite heavy fall rates.

Not sure I understand this... the 5/6 pm attenuation of radar ( I just provided ) was not in the afternoon ?

My idea here is that a critical backing/timing therein ...around that 5 to 6 pm time frame, coincided with a rad attenuation as well as a ground truth abatement/weakening of the event - and trying to figure out exactly why. 

Now, the shadowing and downsloping is a geophysical truism -.  Regional air motion going from elevation to lower elevation, pulls the atmosphere down and offsets UVM, but also ..compressional drying do to PV=NRT of the entire mass... The total phenomenon causes what is referred to as 'rain shadowing'  ... 

When this thing closed off at all those levels and the flow was paralleling ( more so than less...) through the deeper troposphere to surface, ...I think it's bit too cutely coinciding with rad/ground truth to not implicate the models as yet again, over doing that back shit.

I like that fraud thing ..funny... Yeah, I remember Bruce was big on that back-lash.