HurricaneTracker

Yes. But don't toss the idea of the warm nose being stronger than modeled in the globals

Really no reason not to trust it at the moment. It's been very consistent with this system and has better resolution than the current GFS. And yes, will go operational in January.

Concur. and FV3 GFS (which by the way goes operational in January 2019...so why not use it....) has a MUCH better QPF response accordingly. I'm not dismissing this could be a 4-6" event, but it sure does seem like more is in the offing. We wait for 12Z Euro to confirm.

We may have just gotten NAMed. Don't fall for it yet. Wait to see if GFS/UKMET adjust warmer.

Yes sir! Probability of 12" of snow is 84%. Greater than 24" is 12% chance. BTW, Euro ensemble mean for KAVL is 22", with the upper end being 28" and the lowest being 15". Boone mean snowfall is 17", with a range of 7 to 27". Rutherfordton mean snowfall is 17" with a range of 9 to 27".

Ignore this. Look at QPF instead, then do your own 7:1 ratio.

Some stats from 00Z Euro for KAVL: Probability of 12” of snow = 100% Probability of 24” of snow = 44% Whoa!

Hard not to be stoked. Really seems everything is moving along as projected. Incredible consistency. I know folks in the main thread fear a last minute 180...could happen...but getting a sense we know all the players in the game and their modeled evolution matches fluid dynamics. No? Or are you holding back just a bit?

Don’t need to be skeptical. It’s all there. All the cards are on the table. If anything, we may get constructive interference from the northern stream wave (ala 18Z FV3). Everything looks good to me for the moment. It’s OK to geek out on this one - you may never see anything on the magnitude and scale like this again!

Dude, thanks! I am incredibly humbled that you’re willing to share my long *** write up with your family. I tried to make it light-hearted and witty. You know, share a little fun and excitement for a “generational” snow event. Sure do appreciate your support!

Hi all. I hear some folks have asked for my take on the current winter storm. Happy to help and participate here when I can and when I think my posts will be informative (rather than just banter or MBY endless posts). So here goes. AND do note - this is for the mountains of WNC only. Sorry other readers. I think most, if not all, of you have the general idea that a big winter storm is on tap. That storm system is currently entering Southern California (which is a great thing for them!). They are in serious drought out that way and every storm system helps. The southern position of the upper level low moving onshore in associated with a strong subtropical jet are two very common characteristics of El Niño. So you will see some folks in the main thread refer to this as an El Niño wave. In El Niño years, the subtropical jet (STJ) flows fast from west to east from SoCal through southern Texas and into the Gulf Coast states. That is exactly what we have here. By Saturday, this jet buckles tremendously as our low gets carved out, with wind speeds greater than 180 knots (almost 200 mph)! This is very intense and only seen with the biggest storm systems in the mid-latitudes. The southeastern states will be under the right entrance region of the jet (a proxy for upward vertical motion). Then we have the full wave at 500mb moving in across the ArkLaTex region at 18Z Saturday. Meanwhile, a strong northern stream wave is forecast to be located over central North Dakota. Over time, the GFS model does not ever phase (I.e., merge) these two waves into one. Instead, the primary wave that comes into California scoots across the Deep South and induces a surface low pressure system near Houston, TX basically first thing Saturday morning. This low draws upon rich and deep gulf moisture (characterized by precipitable water values in excess of 1.40” - incredibly deep for December and sourced from the STJ and eastern Pacific! Wow! By Saturday night and into Sunday morning, this deep moisture is pointed straight at the southern Apps, much like a fire hose but with deep tropical connections. Meanwhile, as the surface cyclone moves into southern Georgia, we have southwest winds aloft but locally we have southeast winds circulating around our low. This in turn brings in a secondary layer of moisture at the surface, combined with deep tropical moisture aloft. I just cannot emphasize how big a deal this tropical fire hose is. Even in spring this would be a big deal (for severe weather/flooding reasons). The surface low however has other things to work with. A seasonally-strong cold cP (continental-polar) airmass, will be building into the Midwest on Friday and Saturday. In winter, anything over 1040mb is a very cold airmass. This high pressure is basically squeezed east as the STJ buckles and mass evacuation (I.e., air parcels being ejected from the center of the low pressure in Texas), act to strengthen the squeeze (confluence) in the Midwest. Let’s not forget this high will be moving over some snow cover, which acts like a refrigerator. Some models handle the intensity of such highs differently. Regardless, all models take the high into New England and then the cold oozes southwest, hugging the entire Apps Chain all the way to western NC. Enter the wedge. This wedge is sourced from cold air in New England. Wedging is reinforced (I.e., positive feedback loop) by dynamical cooling from precipitation and ageostophic adjustment due to barrier winds. We have all those ingredients here. Southeast winds around the low act to also serve as an “upslope” mechanism, inducing upward vertical motion and enhancing any existing synoptic-scale vertical motions. Oof. Score one for the NAM here. GFS has been very bad so far this year with wedging air masses (too warm). It has always played catch up while the CMC and RGEM greatly (and I do mean greatly!) overdoes the cold wedge. NAM seems to be the sweet spot (did I just type that?). It has a 1038mb high moving into position during the day Saturday. So, the question is, how cold? Ignoring surface temperatures for the time being, the 850mb temperatures (~5000’) stay below freezing the entire time! The cold is REINFORCED by precipitation and 35 knot southeasterly winds above the surface. Surface temperatures are also below freezing, as a stready wind vector from the NE funnels cool and dry cP air into the storm system, and the forecast soundings (thermal profiles) remain ALL SNOW on the GFS model. Take a moment and examine the thermal profile I shared here for 7pm Saturday night. Note that very deep isothermal layer (layer of equal temperature...below freezing) from the surface to 700mb. This is a classic WNC heavy duty, paper machete, snow sounding. And I can tell you our thermal profiles stay this way for a while. Oof. So, let’s review. We have 1) cold high pressure wedging down...check. 2) source of upward vertical motion (double check). 3) moisture? Oh heck yea. CHECK. And this is just the southern stream wave! The northern stream eventually comes down to reinforce the growing trough over the east coast giving us our second punch in the gut. OK.. now here is where I lay down my personal thoughts having digested and explained it all here. <takes off the gloves> I want to impart a few things on you guys. Stop looking at the snow maps. They are assuming 10:1 ratios....and just by looking at those soundings, you can absolutely tell it won’t be 10:1 (I am thinking 7:1, based solely on experience). You should be looking at trends in the QPF. And you should NOT look at the QPF from the GFS - why? Because it has been wrong by a factor of 2, and even 3, most of the fall season in western North Carolina! (Not saying it could be right elsewhere, but for now, WNC has been missed badly by the GFS). Here the Euro and NAM models have performed very well in this department. And you gotta think...BIG source of tropical moisture. Plus Atlantic moisture. Plus vertical motion. Plus jet dynamics. When the dynamics are there the moisture will find a way. We are not lacking dynamics. Euro was around 3.00” of liquid equivalent. NAM isn’t out this far yet. GFS was 1.40” (multiply by a factor of 2 and you get 2.80”). Dang! That’s a LOT of liquid. Thermal profiles support snow, and maybe some sleet far southern escarpment as the warm nose tries to invade (briefly). 10:1 ratio gives you 28-30” of snow. Bwa ha ha ha. I can’t do it. Just can’t do it. What did GSP say, “generational” event? You bet. 7:1 ratio still gives 15-18” of cement snow. Oof. Who in their right mind feels good about issuing a forecast like that around here? It’s unheard of. Can it happen? It could. Would I go with these values at day 4 (looking at you WYFF)....no. Why not? Because the atmosphere is always in motion. Things flow like saltwater taffy being pulled from two ends. And we still have time to watch this. BUT BUT BUT...we have been watching this since Saturday, Dec 1. It is equally mind-boggling to me to think that the large scale pattern and such has been so incredibly consistent for so long! The Weather Prediction Center now has the mountains 70-90% chance of a major winter storm at DAY 4! Wow. The Euro has a 96% chance of 6” or more from this system. The GEFS mean is creeping up (on QPF...more snow, less sleet/ZR as the wedge is better reflected). Oh and start time? BANK ON EARLIER rather than later. I have seen this time and time again. These systems break out precipitation along the warm fronts and into the cold dome earlier than most systems. GFS might have this right. Another trend to watch next 2-3 days. Alright, enough rambling. Hope it’s helpful. TL;DR - get ready. Prepare now for a big snow event with potential for prolonged power outages. This is as big a deal since December 2009. if you live anywhere in the cold air damming favored areas of WNC (sorry Franklin), this storm has your number.

Will do. Hopefully this evening after dinner time

Frankly looks very close to current operational GFS.

It’s internally inconsistent. 300 mile shift northward in one model run. Plows the low from New Orleans to northern Alabama and right into the southern Apps. Looks suspicious to me. Lows don’t move into wedges. CAD wedge is excellent on this run and 1036 high sits over Philly on Saturday. Would agree it’s an outlier run for now as it disagrees with GFS and NAM from this mornings runs.

Absolutely. Though it is the lowest on QPF...but has been dry on every event this season....sometimes by a factor of 2 or 3. So look at thermal profiles and just how STOUT that wedge has become on the GFS. This is a big deal folks. Be prepared.

A big point for discussion here is that much of this water falls between 12Z Saturday and 12Z Sunday. Some places will see 2 inches of liquid in just 24 hours. Some of that will fall as ice/snow. Yowza.

Um...this latest GFS run was incredible. And VERY VERY wet!! CAD wedge was intense. Throw out the snow maps because they are useless at this range...but focus on the QPF. MANY AREAS ARE WELL OVER 2.00" of liquid and there is a good portion of the mountains nearing 3.00" of liquid. You do the math... If this continues to be the forecast...y'all better start preparing. Get your winter kits set up and stocked. And your chainsaws ready.

Blue line is the operational (control) run of the GFS.

Just for fun, here is the snow plume diagram for KAVL from the 12/04 06Z GFS. The mean is 12.01" of snow...low of zero, max > 30" (HA!). But just another tool to consider. http://www.emc.ncep.noaa.gov/gc_wmb/tdorian/gefs/EMCGEFSplumes.html

This particular storm we are watching does not seem to favor a dry slot (at the moment). It is a classic overrunning situation with warm moist air being thrown back into the CAD wedge (which is very likely underdone on GFS). Then as the the low gets going off the coast we get the 2nd punch. We really need to be focusing on the QPF from each model at this point rather than if it is showing snow/ice/sleet, etc. Those details will work themselves out in due course. But...needless to say the liquid equivalent of whatever falls is unlike anything we have seen since December 2009 event that crushed the mountains (and I personally had no electricity for 5 consecutive days). Perhaps a bit too early to discuss impacts as everyone is wowed with the clown maps, but our infrastructure simply cannot handle 20-30" of paste snow. More to come on that later this week. For now, watch the QPF and watch the signals in the SKEW-T's for a deep isothermal layer between the surface and 850mb. That's our classic heavy wet paste snow signal around here.

I’ll just leave this here.

No. Soundings support sleet and freezing rain. Elevation dependent which affects the depth of the cold layer. My experience is the HRRR does poorly in these setups. Just look at its runs today - all over the place from one run to another.

Oh and the 12Z NAM just came in a ton warmer at the surface. It has a very narrow window of sub-freezing surface temperatures between 1am and 4am before rapidly warming as the precip comes to an end. Overall, the system looks sped up to me. Less QPF and much less ice (even by NAM standards). 850 temps are above freezing from AVL southwest (and just barely below 0 northeast). NAM has consequently greatly scaled back the ZR ice accumulation forecast. Just about a non-event at this point. Interesting trend.

To add to this, there are some "negative" factors working against ice accumulations. First is that the scenario is rain changing to sleet/freezing rain. It is very hard to accrete ice when the transition is warm to cold (whereas ice is easier to accrete starting out cold and then warming to a rain). There is just residual latent heat in the rain, even on tree branches, that must be overcome before freezing rain can even begin. Then there are the very heavy precip rates which are expected tonight during the peak window of maximum CAD. Heavy rates offset cooling and can actually lead to less ice accretion. Yes NAM and RGEM are cold. GFS is not. It has zero (!) freezing rain. NAM always seems to portray doomsday scenarios for ZR when reality is closer to GFS. Will there be pockets of freezing rain and ice accumulations? Sure. Widespread? Probably not.

100%! Track made all the difference. Upslope will turn from easterly to southeast through the day and southerly overnight. This is why I said NAM was an outlier and QPF was too low from HRRR and other models. We don’t even maximize our precipitable water until 8pm this evening.