Weekend Storm Disco (Dec 18th & 19th)

[burgertime]

It's the freezing line at 850 mb or around 5,000 ft. (850 mb is the point at which you are below 85% of the atmosphere and above 15% of it). Sea level average is 1013 mb, I think.

Generally, the 850 mb line is a good gauge to use to determine whether you will get rain or snow, but it's possible that there are warm pockets at other levels of the atmosphere, plus the surface may not be cold enough.

That's the problem with this run is the SFC temps, they are staying north of 40 for freezing....however everyone will be happy with this run

[strongwxnc]

Defiantly not OTS

so far

Here's @ 66

[DLI4SCwx]

The freezing line is back much further north on this run compared to the others. Not as much digging. What happened?

[burgertime]

At 72 here comes the cold to chase away our moisture.

[superjames1992]

Could the model be underdoing the QPF? That seems like very little QPF for a low riding up the coast like that.

[CaryWx]

850 is the optimum dendrite formation area, correct? Need the good freezing temps there, right?

[WidreMann]

850 is the optimum dendrite formation area, correct? Need the good freezing temps there, right?

No. Dendrite formation region is -8 to -12 C.

[burgertime]

Any mets care to chime in?

[CaryWx]

No. Dendrite formation region is -8 to -12 C.

Gotcha, thanks

[DLI4SCwx]

Compared to the 0z and 6z runs, the 850 line is much further north. The QPF amounts are diminished too?

[strongwxnc]

is the system getting pushed more into Sat eve Sunday Morning time frame?

[Cold Rain]

This is either going to be a mostly cold rain event or a mostly dry event. There is no mechanism to transport cold in. The only hope I see is that the storm draws cold in. But a weak storm isn't going to do that. And a strong storm too close is going to be too warm for snow. Right now, no model is indicating a strong storm. Maybe it'll trend that way, but it doesn't look very good.

[burgertime]

Compared to the 0z and 6z runs, the 850 line is much further north. The QPF amounts are diminished too?

Compared to 6z for NC the QPF is much better and 850 line was a smidge north but not by that much.

[burgertime]

This is either going to be a mostly cold rain event or a mostly dry event. There is no mechanism to transport cold in. The only hope I see is that the storm draws cold in. But a weak storm isn't going to do that. And a strong storm too close is going to be too warm for snow. Right now, no model is indicating a strong storm. Maybe it'll trend that way, but it doesn't look very good.

Yep that's what I'm thinking. 12z GFS and Euro should be interesting hard to think that we are going to magically get some cold sfc temps though.

[Cold Rain]

Yep that's what I'm thinking. 12z GFS and Euro should be interesting hard to think that we are going to magically get some cold sfc temps though.

Agreed. I'm hoping the storm will phase in some northern energy just perfectly, or the atmosphere is colder than the models realize.

[burgertime]

Agreed. I'm hoping the storm will phase in some northern energy just perfectly, or the atmosphere is colder than the models realize.

[weatherheels]

I'm actually regaining some hope to at least salvage a few flakes out of this system. I'm not expecting anything significant, I'm just encouraged that the Euro has come back somewhat west, and the NAM looks encouraging this morning. Sure the precip shield isn't that expansive and the storm isnt that strong, but the trend is our friend for now.

[Cold Rain]

Haha! Nice!

[Shawn]

The problem I think some people make the mistake of when looking at these models is they just look at the 850 line. Even if you see that 850 line past your area you also need to remember that the moisture behind it was from the previous hours and usually indicates that has happened with the passing front. If you continue to see moisture on later hours, then that would be actual moisture behind the front that could result in something frozen if all the conditions were right.

[DLI4SCwx]

The problem I think some people make the mistake of when looking at these models is they just look at the 850 line. Even if you see that 850 line past your area you also need to remember that the moisture behind it was from the previous hours and usually indicates that has happened with the passing front. If you continue to see moisture on later hours, then that would be actual moisture behind the front that could result in something frozen if all the conditions were right.

The NAM runs last night looked pretty good for us to at least see a lil something frozen. Not looking too good now though, at least from what I can tell.

[Weather Czar]

12Z RGEM. Only goes to 48.

http://www.weatheroffice.gc.ca/data/model_forecast/colour_images/12_054_R1_north@america_I_4PAN_CLASSIC@012_048.jpg

[Brick Tamland]

Not sure how it will end up Saturday/Sunday, but I am loving this pattern. Allan Huffman said in his blog today that there is something brewing for early next week and Christmas, too.

[WidreMann]

The problem I think some people make the mistake of when looking at these models is they just look at the 850 line. Even if you see that 850 line past your area you also need to remember that the moisture behind it was from the previous hours and usually indicates that has happened with the passing front. If you continue to see moisture on later hours, then that would be actual moisture behind the front that could result in something frozen if all the conditions were right.

There is no passing front here. The moisture is moving in from the south and later from the east. If you see moisture over you and you are north of the 850 line, that means that it's below zero and cold enough for snow and there is precip. Not too hard to see why this would be the case if you do an ounce of critical thinking.

[FoothillsNC]

The flow looks a little too fast and flat, probably not going to be much if anything for western NC or n. Ga region. Futher east has a slight shot but the overall flow is pretty fast and flat, so I'm not expecting much around most of the central Carolinas and points west out of the Sat. night system. Now would be a good time for the southeast ridge to flex its muscle to help out. Temps too marginal anyway, and since its only 48 to 60 hours to start time for some, theres not much room for a huge change. As new energy catchs the wave toward eastern NC or VA, the GFS amplifies it quickly, so this still looks like a chance for eastern NC, enough to keep watching.

[strongwxnc]

12 GFS has a LP over ILM at 66hrs...

[FoothillsNC]

I just looked at 84 hours and see that the added energy is enough to sharpen it and close to giving the Midatlantic a substantial snowstorm. This would be the ultimate slap to n. Ga and the western Carolinas to miss a southern stream only to have it sharpen up enough to give DC another event.

[snowlover91]

There is no passing front here. The moisture is moving in from the south and later from the east. If you see moisture over you and you are north of the 850 line, that means that it's below zero and cold enough for snow and there is precip. Not too hard to see why this would be the case if you do an ounce of critical thinking.

Right and wrong. Yes if the 850s are below 0C it's cold enough for snow, ALOFT, but not necessarily at the surface. The atmosphere isn't one dimensional, you have to look at all the layers. The 850 is a rough estimate but is not indicative of the rain/snow line. If the surface temps are too warm or you have a warm layer aloft it won't be snow. Shawn was right in what he said regarding the moisture, and barely any falls to the N of the 850 0C line. Not to mention the NAM showed surface temps in the upper 30s during that time in the areas cold enough at 850mb, so it would be rain.

[Poimen]

The GFS looks like a decent event from RDU and points east.

[Weather Czar]

I just looked at 84 hours and see that the added energy is enough to sharpen it and close to giving the Midatlantic a substantial snowstorm. This would be the ultimate slap to n. Ga and the western Carolinas to miss a southern stream only to have it sharpen up enough to give DC another event.

The GFS did just come about 500 miles north and west since 6 hours ago. Nice to see the rain over perform today.

[FoothillsNC]

The GFS did just come about 500 miles north and west since 6 hours ago. Nice to see the rain over perform today.

how much precip did you manage? the models did pretty good on qpf here and the Upstate/85 corridor , with trace to .05" and less. I got ..01", which is about what I was expecting. For the weekend event, the s/w amplifies a little too late at 5H that look is still similar really, I'm not paying attention to the surface reflection, but you'd expect a north Florida or coastal Southeast development with that look and since the flow is amplifying then, it could hook up eastern NC and VA , which again was always the most likely scenario. If the flow were to amplify further west then enough moisture could get drawn up earlier that would get the 85 corridor. You could get some accum. just from the moisture getting wrung out crossing the mountains Saturday, but that wouldn't do anything here. Looking at the long zonal-esque flow east to west, theres room for more amplfi. further west than any models show though, so I guess its possible. Just all the trends still focus the biggest chace being eastern third of Carolinas and southeast Va.