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

Webberweather53 said:
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."

View attachment 57455


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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So what you are saying is that the wet bulb has to be below -1C in the mid-levels or it will experience lower ILRs.

Anything colder then that the ILRs are more in the realm of IP?


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RainlessSnowless & Grumpy said:
It always has been a cold rain here. Nothing has changed.
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Plenty of models have ice well into the upstate? And models have trended colder. Only model that is warm is the euro with its over amp idea and trying to drive the LP directly into the CAD and that isn’t going to happen. Even if you had the over amp wave like the euro, it would deviate around the CAD and you’d deal with a stronger warm nose which means nothing here as we were going to be ICE regardless.
 
Comparison. Most strong cad warm noses fail to make it to Wilkes. This event easily should bring it north into Danville Virginia if not further N. Meaning...cold rain more likely in the south at times from Mooresville to Charlotte etc with freezing rain barely hanging on north of there near Elkin NC. Light to moderate event. Not a severe ice storm by any means that I see. I honestly wouldn’t prepare or stock for this event but I would avoid early morning travel if possible from Elkin NC north and elsewhere in the piedmont I would just slow down over bridges before dawn. That’s all.
 
Webberweather53 said:
For a really big ice storm, you usually want to see dews in the 10s or even single digits, low-mid 20s as shown verbatim on the 3km aren't ideal but potentially just enough to get the job done for a low-moderate end icing event.

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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.
 
One thing to note is those DP have been dropping more and more every run. South and western VA are in the teens from the onset with low 20’s even in the upstate. You keep those dropping another 4-8 degrees and this really will be a problem quickly.
 
What we already know....

Hazardous Weather Outlook
Hazardous Weather Outlook
National Weather Service Raleigh NC
411 AM EST Sun Dec 13 2020

NCZ007>010-021>026-038>041-073>076-141000-
Person-Granville-Vance-Warren-Forsyth-Guilford-Alamance-Orange-
Durham-Franklin-Davidson-Randolph-Chatham-Wake-Stanly-Montgomery-
Moore-Lee-
411 AM EST Sun Dec 13 2020

This Hazardous Weather Outlook is for central North Carolina.

.DAY ONE...Today and tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

A soaking rain is expected late tonight through Monday as low
pressure crosses the region. Rainfall amounts up to 2 inches are
expected, and minor flooding is possible.

A second storm system Tuesday night and Wednesday will bring a threat
of wintry weather, particularly over the northern and western
Piedmont of North Carolina.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

$$


And RAH's long range discussion:

.LONG TERM /TUESDAY THROUGH SATURDAY/...
As of 345 AM Sunday...

The chance for wintry weather across the N and W Piedmont Tue night
into Wed is increasing. Chilly temps will persist through Sat.

Tue through Wed night: In contrast to tonight`s/Monday`s Miller A
storm, the midweek system will be a Miller B, centered around a
potent mid level shortwave trough moving from the S Plains through
the Ohio Valley to the Northeast Tue to Wed night, along with a
frigid Canadian high centered over S Quebec nosing southward into
NC. Despite the expected slight weakening of the mid level trough,
it will still approach Tue evening/night with a surge of strong
forcing for ascent, including vertically-aligned 850 mb moisture
convergence, 500 mb DPVA and height falls, and vigorous upper
divergence in the RRQ of a 135+ kts jet streak. The inverted trough
over SW NC/E TN/N GA will weaken/fill as intense cyclogenesis takes
place just off the GA/SC coast, with this low deepening (and
possibly bombing) as it tracks NNE along coastal NC to the Mid
Atlantic/Northeast. This type of cyclogenesis pattern, with a chilly
high centered in a favorable location and a surging warm nose aloft
atop the surface-based cold layer, supports a good chance of icing,
especially across the N and W Piedmont (roughly N/W of a line from
Lake Gaston to the Triangle to Albemarle), although we`re likely to
see light rain mixed with light sleet at the onset. Will trend pops
up to good chance then likely after midnight Tue night through Wed
morning, with mostly freezing rain and a little sleet over the N/W
Piedmont and mostly just rain elsewhere. Will depict a gradual
warming to near and above freezing by early Wed afternoon with the
influence of the growing warm nose aloft and the limitation process
of latent heat release, although any ice accrual could last through
the day. Expect pops to trend down and out SW to NE late Wed
afternoon into early evening as the deep lift and coastal surface
low both exit. Highs 43-52 Tue, lows 28-39 Tue night, highs 36 to 60
(NW to SE) Wed, and lows 26-35 Wed night. Any lingering ice esp in
the Piedmont could produce travel problems into Thu morning. It`s
worth noting that a lot can change over the next few days, so stay
tuned for forecast adjustments as this time frame draws closer.
 
iGRXY said:
Plenty of models have ice well into the upstate? And models have trended colder. Only model that is warm is the euro with its over amp idea and trying to drive the LP directly into the CAD and that isn’t going to happen. Even if you had the over amp wave like the euro, it would deviate around the CAD and you’d deal with a stronger warm nose which means nothing here as we were going to be ICE regardless.
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We'll see. And I'll gladly come on here and eat crow if I have to. I'm just not feeling this one at all. We'll see where it goes in the next 24 hrs. For some reason even with a strong high in decent position the cold air push isn't what it should be. Even the NAM is very borderline which is telling to me. Anywhere south of 74 in NC is on the outside looking in if you ask me.
 
I did not like 6Z NAM 00Z Euro. If that is the beginning of a trend to warmer drier, this will not be much of a storm. We had a bad run 18z Friday I think from several models and that reversed. But closer to go time now so reversals seem less likely.
 
RainlessSnowless & Grumpy said:
We'll see. And I'll gladly come on here and eat crow if I have to. I'm just not feeling this one at all. We'll see where it goes in the next 24 hrs. For some reason even with a strong high in decent position the cold air push isn't what it should be. Even the NAM is very borderline which is telling to me. Anywhere south of 74 in NC is on the outside looking in if you ask me.
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The NAM isn’t even in its wheelhouse yet. We are still dealing with the Hr69 to Hr84 nam. Still have at least 24 hours minimum before we even get into the NAMs range.
 
Sorry if this doesn't belong here but something I don't get is how TWC and other met forecasts have highs in the lower 40s for the CLT metro area when even the warm Euro has us at the upper 30s at the warmest and the GFS keeps the area in the mid 30s.

Are they anticipating a weaker CAD?
 
SimeonNC said:
Sorry if this doesn't belong here but something I don't get is how TWC and other met forecasts have highs in the lower 40s for the CLT metro area when even the warm Euro has us at the upper 30s at the warmest and the GFS keeps the area in the mid 30s.
Are they anticipating a weaker CAD?
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Probably anticipating the lull in precip (as seen in a lot of guidance) could easily send temps up a few degrees. Doesn’t help that overall qpf is going down either.
 
BHS1975 said:
These elevated dews we've been having the last few years is gonna make this another cold rain for RDU.


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Right now I would have to agree with you. Still time for the models to trend a little better. Still a few wild cards out there; like snow cover up over the western sections of the MA. Maybe this could help. RAH currently has slightly colder temps forecasted over the Triangle area. They're usually conservative (but very good) so they may be thinking a slightly colder solutions will win out. But I would say a win for us would be a few hours of ice in the trees on Wednesday morning. Here's my grid forecast:
Tuesday Night
A chance of freezing rain and sleet between 1am and 3am, then a chance of rain or freezing rain after 3am. Mostly cloudy, with a low around 31. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.
Wednesday
Rain or freezing rain, becoming all rain after 8am. High near 43. Chance of precipitation is 80%.
 
If there’s any weenies out there that want to see onset sleet I bet it occurs very far south (North half of Georgia, upstate South Carolina, Charlotte, NC) before the warm nose takes over to liquid. But may have to stay up overnight. I’m not sure about Raleigh as it’s so Far East and less favorable timing.
 
BirdManDoomW said:
If there’s any weenies out there that want to see onset sleet I bet it occurs very far south (North half of Georgia, upstate South Carolina, Charlotte, NC) before the warm nose takes over to liquid. But may have to stay up overnight. I’m not sure about Raleigh as it’s so Far East and less favorable timing.
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What does the bird man think about Zebulon?
 
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