Freezing Rain, Sleet, Snow, Rain...Kitchen Sink (12/15-16)

[Ethan80963]

The NAM looks like it’s coming in pretty beefy for western/central NC and the northern Upstate but it is warmer than the GFS suit and CMC suit this might be because the NAM’s hp is 2-3 mb weaker than those models. However it looked like it did increase QPF this run.

 

[iGRXY]

The NAM looks like it’s coming in pretty beefy for western/central NC and the northern Upstate but it is warmer than the GFS suit and CMC suit this might be because the NAM’s hp is 2-3 mb weaker than those models. However it looked like it did increase QPF this run.
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Doubt that. It’s still the very long range NAM so whether it’s showing beefy totals or nothing at all it shouldn’t be taken serious until we get inside 36-48 hours. It had a 1035 HP sitting further south closer to the US border so I think we are sitting pretty as of right now.

 

[Oconeexman]

Ballgame for upstate...trend has been less precip and warm. Only extreme north get anything other than cold ass rain!

 

[iGRXY]

Ballgame for upstate...trend has been less precip and warm. Only extreme north get anything other than cold ass rain!

What trend? Outside of the euro there isn’t a trend.

 

[RainlessSnowless & Grumpy]

Doubt that. It’s still the very long range NAM so whether it’s showing beefy totals or nothing at all it shouldn’t be taken serious until we get inside 36-48 hours. It had a 1035 HP sitting further south closer to the US border so I think we are sitting pretty as of right now.

Dewpoint. Another poster hit the nail on the head. These warm dews we experience anymore will do a number on what would have been a classic CAD even 5 years ago.

 

[Metwannabe]

Probably will not make too much difference but @60 the 3k NAM is 1 mb stronger with the high, meh, but temps are 2-3 degrees colder than the 12k NAM. Subtle differences but for a few a couple degrees can make it interesting

 

[Brick Tamland]

Most of the models have me at 33 and rain for a good portion of the event . But even if that was to drop a degree of two, that still wouldn't give me (or others in similar setup) a big event. As you said, we need the lower dew point to web bulb down into the upper 20s. Ice cycles are pretty but it means your losing a lot to run off.

We are always right on that line between rain and something frozen. I am trying to recall how the last big ice storm we had in Dec 02 was forecasted. I can't remember if that was supposed to be just rain for the Raleigh area or the ice storm that we actually got. I do remember once in 9th grade getting stuck on the bus going to school one morning because of a freezing rain event that was forecasted to be just rain for the Triangle. The roads got icy quick and the bus couldn't make it up a hill. A Durham County deputy sheriff had to take me home. One degree can make all the difference.

 

[tramadoc]

Really cool study from Sanders & Barjenbruch (2016) wrt ice liquid ratios that's worth reading imo.

https://journals.ametsoc.org/view/journals/wefo/31/4/waf-d-15-0118_1.xml?tab_body=fulltext-display


For ice radial accumulations, which are the standard for measuring freezing rain accumulation, they found that for every inch of radial ice accrual about 3-4" of liquid equivalent is measured (via ASOS).

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Thus, when you see maps like this, especially when there's a ton of QPF, it's usually a good idea to take ~25-33% of these 1:1 QPF-ice accumulations.

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Pivotalweather also now has an option for those w/ a subscription where you can select for the Freezing Rain Accumulation Model (FRAM) accumulations described in the aforementioned study & these are considerably more reliable than 1:1 QPF-ice accum model output. The FRAM model uses regression on modeled precipitation rate, wet bulb temperature, and wind speed to estimate ice accrual. It's certainly better than using straight 1:1 QPF-ice output, but I would still urge caution in using a product like this.

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So a 25/75 percent ration of ice to water? I would really like to see charting of the BTU’s given off during the process. I’m sure that there are lab reports that show that. Pretty interesting. Thanks for sharing.

 

[tramadoc]

Yes, it's more like 1" Radial ZR:3-4" QPF according to this study. Stronger winds actually enhance ice accrual efficiency because it increases the rate of evaporative cooling, the rate of sensible heat transfer from cooler surrounding air to slightly warmer adjacent air near a freezing object that's undergoing latent heating & stronger winds increase a horizontal cross section's mass flux (of ice), all serving to increase ice accumulation efficiency at higher wind speeds. The effect is pretty small until wind speeds begin to approach exceed 10-15 knots.

"It is inferred that the increasing wind speed replaces air near the ice surface warmed by latent heat release, alters the droplet collision efficiency, increases the horizontal mass flux, and promotes evaporational cooling. Although, the data show that these processes are not as noticeable in the ILRs until wind speed reaches >15 kt, and may rely on the thermodynamic environment as well. Wind gusts had a similar influence on ILR with an increasing trend in ILR as wind gusts increased."

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As for surface temperatures, your intuition is correct in that there's a strong temperature dependency, with wet bulb temperatures above -1C (~30F) experiencing significantly lower ice liquid ratios (ILRs) than those with temperatures between -4 to -1C. The relationship reverses and lower ILRs are observed for temps lower than about -4C (~25F), as the low-mid levels usually become more supportive of sleet instead of freezing rain, in environments where the surface temperatures are this cold or colder.

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They also found rather unsurprisingly, that ice accumulation efficiency is dependent on precipitation rate with the largest changes in ILRs between rain equivalent rates of a few hundredths to a tenth or so per hour, with it asymptoting towards ILRs of about 1:7.5 or so in heavy precipitation.

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And then I look farther and see what I was questioning. You’re the man!!

 

[BirdManDoomW]

What does the bird man think about Zebulon?

cloudy miserable day. Some cold rain, any ZR or sleet probably last under 2 hours at onset. Patchy mix.

 

[Oconeexman]

What trend? Outside of the euro there isn’t a trend.

2m Temps, dew points have both trended warmer in my area of the upstate on the Nam

 

[Metwannabe]

If I were to draw a map, I'd follow this temp map of NAM for any wintry and about 1 tier of counties NW of that line for potential issues (I'm speaking specifically NC)

 

[SimeonNC]

Doesn't the NAM have the same issue as the Euro of being overamped, especially in the long range?

 

[Myfrotho704_]

Surprisingly the NAM the RGEM look Similar excluding the more crazy ice amounts on the RGEM here

 

[iGRXY]

Doesn't the NAM have the same issue as the Euro of being overamped, especially in the long range?

It does which makes it’s temp profiles in the long range warmer. That’s something that switched on the nam from years past. That’s why the rule of thumb is to really not put much stock into the nam until you get inside 36-48 hrs.

 

[Myfrotho704_]

Basically what I’m thinking

 

[iGRXY]

3km nam is definitely colder with dews and 2m temperatures by 4-6 degrees compared to the 12km

 

[packfan98]

12z GFS definitely looks colder with stronger HP through hr 60.

 

[Myfrotho704_]

More ridging nosing into the GLs with a slightly slower system will likely lead to another icy run of the GFS, maybe colder than 06z

 

[BirdManDoomW]

chances for snowfall accumulations heavily depend on how quick and heavy persistent moisture is (like cmc) . Odds are probably only 30% chance for 1” or so foothills and 60% chance mtns and 10-20% piedmont triad or far western piedmont (Statesville).