I’m dreaming of an Icy Christmas

[Sey-Mour Snow]

Let’s separate some of the talk about post Christmas with some talk only about the icing concerns here. 

[Damage In Tolland]

Ice damage. 

[PowderBeard]

Good day. Tomorrow seems interesting. Currently a couple degrees lower than modeled on the HRRR or NAM.

[CoastalWx]

Looks like a valley day tomorrow in CT for the cold. It starts cold everywhere, but they’ll be last to warm. 

[dendrite]

Nammy is pretty icy here. Be careful in the morning everyone.

[SouthCoastMA]

RGEM looks icy throughout the duration. doesn't even look like it flips to plain rain inland

[Ginx snewx]

25 minutes ago, SouthCoastMA said:

RGEM looks icy throughout the duration. doesn't even look like it flips to plain rain inland

Yikes

[HoarfrostHubb]

Even half of that will cause issues.  Driving certainly.  Power possibly. 
 

not what I want 

[weathafella]

3 minutes ago, HoarfrostHubb said:

Even half of that will cause issues.  Driving certainly.  Power possibly. 
 

not what I want 

Remember that’s not ice accretion but qpf falling as zr.

[HoarfrostHubb]

2 minutes ago, weathafella said:

Remember that’s not ice accretion but qpf falling as zr.

Any quick and dirty ways to estimate accretion vs qpf?  
 

[rimetree]

GFS seems to agree with RGEM to some extent now...seems colder overall.

[ORH_wxman]

2 minutes ago, HoarfrostHubb said:

Any quick and dirty ways to estimate accretion vs qpf?  
 

It varies based on several factors but a rough general rule is that 70% will freeze on flat surfaces and then roughly 40% of THAT will accrete radially…which comes out to about 30% of the QPF. 
 

So in an example where half inch of QPF falls as ice, you may get about 3/8ths inch on flat surface and 1/6th radial accretion. 

[OceanStWx]

4 minutes ago, HoarfrostHubb said:

Any quick and dirty ways to estimate accretion vs qpf?  
 

Depends primarily on rate, wind, and temp. Each ice ratio curve is a little different. Heavier QPF = less ice (runs off too fast), stronger wind = more ice (actually builds up faster), temps near freezing = less ice (obviously). 

For low wind, standard upper 20s temps and typical precip rate you can assume around 80% of QPF turns into flat ice (take 40% of that for radial).

[HoarfrostHubb]

I was reading a bit on FRAM on the NWS site.  Interesting physics

Thanks Will, Chris, Jerry.   

[HoarfrostHubb]

Some FRAM basics

https://training.weather.gov/wdtd/courses/woc/winter/fcst-hzds/ice-storm-accum/presentation_content/external_files/Ice Reference v1.pdf

[WxWatcher007]

Good stuff in here tonight.

[ORH_wxman]

A limiting factor in efficient ice accretion in this event is that 950mb temps are really warm. Tip mentioned this a couple pages back but that is important because you are going to have warm raindrops pretty close to the surface so they have less time to become supercooled. 

In many of our nasty icing events, you’ll see 950 temps below 0C and the freezing line is closer to 900-925. 

[HoarfrostHubb]

A nice dense article about qpf vs accretion here

https://journals.ametsoc.org/view/journals/wefo/31/4/waf-d-15-0118_1.xml

[OceanStWx]

1 minute ago, HoarfrostHubb said:

Some FRAM basics

https://training.weather.gov/wdtd/courses/woc/winter/fcst-hzds/ice-storm-accum/presentation_content/external_files/Ice Reference v1.pdf

It's a pretty good explainer, and most likely what you are seeing from any NWS office forecast. 

It does a pretty good job, mostly missing on the low side with light precip and strong winds or high side with wet bulb temps near freezing.

[OceanStWx]

2 minutes ago, ORH_wxman said:

A limiting factor in efficient ice accretion in this event is that 950mb temps are really warm. Tip mentioned this a couple pages back but that is important because you are going to have warm raindrops pretty close to the surface so they have less time to become supercooled. 

In many of our nasty icing events, you’ll see 950 temps below 0C and the freezing line is closer to 900-925. 

FRAM documentation sort of covers that too, with warm rain drops being less efficient but not captured by the algorithm. 

One of those situations where a stronger wind (like 2008) actually helps you by transporting the latent heat away from the drops as they freeze.

[ORH_wxman]

12 minutes ago, OceanStWx said:

Depends primarily on rate, wind, and temp. Each ice ratio curve is a little different. Heavier QPF = less ice (runs off too fast), stronger wind = more ice (actually builds up faster), temps near freezing = less ice (obviously). 

For low wind, standard upper 20s temps and typical precip rate you can assume around 80% of QPF turns into flat ice (take 40% of that for radial).

The 2008 storm was cool because it had offsetting factors working which kind of made the forecast a really tough call. 
 

On one hand, you had marginal temps barely below freezing and very heavy QPF (I should really say rates instead of QPF)….those would limit ice accretion. But on the other hand, you had like a 15-20 knot wind out of the NE…advecting in upper 20s dewpoints the whole time into N MA and down to about ORH. 

[ORH_wxman]

Ha we both mentioned 2008 at the same time…nice. 
 

Obviously the storm was huge but it was a greaT case study on the different variables working. 

[ORH_wxman]

There was also a little bit of terrain effects in that storm too. The west Side of ORH hills had drastically less ice at the same elevation as the eastern side that was catastrophic accretion. 

[HoarfrostHubb]

Just now, ORH_wxman said:

There was also a little bit of terrain effects in that storm too. The west Side of ORH hills had drastically less ice at the same elevation as the eastern side that was catastrophic accretion. 

Was the west side somewhat sheltered by the winds in that case?  

[ORH_wxman]

2 minutes ago, HoarfrostHubb said:

Was the west side somewhat sheltered by the winds in that case?  

My best educated guess was that the terrain blocked some of the dewpoint advection and it also likely had a mild warming effect of downsloping (and the east side had the opposite effect of mild upslope cooling). In a marginal temp profile, stuff like that will matter more than usual. 

[OceanStWx]

10 minutes ago, ORH_wxman said:

Ha we both mentioned 2008 at the same time…nice. 
 

Obviously the storm was huge but it was a greaT case study on the different variables working. 

I feel like a ton of ice research has come out since then proving that stronger winds are in fact a net benefit to ice accretion.

8 minutes ago, ORH_wxman said:

There was also a little bit of terrain effects in that storm too. The west Side of ORH hills had drastically less ice at the same elevation as the eastern side that was catastrophic accretion. 

 

7 minutes ago, HoarfrostHubb said:

Was the west side somewhat sheltered by the winds in that case?  

I think sheltered from winds, but there is also some subtle cooling on the east side as air ascends over the terrain and even a wet bulb temp change of 0.5 degrees can matter a lot. 

[40/70 Benchmark]

1 minute ago, ORH_wxman said:

My best educated guess was that the terrain blocked some of the dewpoint advection and it also likely had a mild warming effect of downsloping (and the east side had the opposite effect of mild upslope cooling). In a marginal temp profile, stuff like that will matter more than usual. 

I was gonna say, the cold tuck piles up against the east slope of the ORH hills...not so much on the other side of the spine.

[OceanStWx]

Big triple bun ice energy in here tonight.

[ORH_wxman]

1 minute ago, 40/70 Benchmark said:

I was gonna say, the cold tuck piles up against the east slope of the ORH hills...not so much on the other side of the spine.

Yeah and I’ve seen it before in a lot of smaller dinky events but in the 2008 event it happened when there was 3” of QPF so it made it that much more noticeable. Like Leominster at 300 feet had catastrophic ice but then if you went to Orange MA on that other side of the spine on route 2 it was almost nothing despite Orange actually being a couple hundred feet higher. 

[40/70 Benchmark]

7 minutes ago, ORH_wxman said:

Yeah and I’ve seen it before in a lot of smaller dinky events but in the 2008 event it happened when there was 3” of QPF so it made it that much more noticeable. Like Leominster at 300 feet had catastrophic ice but then if you went to Orange MA on that other side of the spine on route 2 it was almost nothing despite Orange actually being a couple hundred feet higher. 

Same reason NYS and W VT torch alot easier than most of NNE....CAD