Vol4Life

Great discussion by MRX.. LOVE the Trowal part!! .SHORT TERM...(Tonight through Monday)... Key Messages: 1. Difficult snowfall forecast for low elevations. Definitely a battle of snowfall rates vs. air/ground temperatures. Impacts what amounts people will wake up to Monday morning. 2. High confidence in accumulating snowfall occurring tonight, even in southern parts of the TN valley. 3. High confidence in significant snowfall event for the higher elevations of SW Virginia and the east TN mountains. Discussion: Synoptically speaking, a strong upper trough over the Arklatex region this afternoon will transition to a cutoff low over the next 12-18 hour as it shifts east across Mississippi and Alabama. As this occurs, this will place our forecast area beneath a strong deformation band this evening and overnight, with the potential for high snowfall rates and resulting significant snowfall event. Precip associated with this system will spread north from Alabama later this afternoon and early part of the evening. Surface temperatures and thermal profiles universally support an onset ptype of rainfall. This will quickly change however as a TROWAL develops overhead around and shortly after midnight tonight. Strong lift through the dendritic growth zone accompanied by negative EPV yields the potential for convectively enhanced precip and subsequently banded precip and high precip rates tonight. Model soundings show rapid cooling through the column tonight, suggesting dynamic cooling will contribute to snowfall reaching the ground with surface temperatures that might not otherwise be supportive of much more than a rain/snow mix. Meanwhile, more substantial CAA will be ongoing after 06z to 09z as surface cyclogenesis is in full swing east of the Appalachians and cold continental air begins to spill into the valley, further aiding in potential for accumulating snowfall. Regarding confidence levels, confidence is very high in the presence of snowfall tonight. That does not appear to be in question. The big unknowns are 1) where does the deformation axis and associated heavy, banded snowfall set up, 2) how high will the resulting precip rates be, and finally 3) will these factors be able to overcome warm ground temperatures and at least initially warm surface air temperatures. The last two factors will have huge implications on what people wake up to see in terms of snow depth. Heavy precip rates, on the order of 2"/hour, are expected but will be short lived generally only for 3-6 hours at most. This would be enough to cause travel impacts and overcome the ground temperatures for a short period. For those traveling overnight this could be an issue. The big question is what snow depth will actually be seen by most people. If a valley location gets 4 inches of snow overnight and it ends at 3-4am, will the ground remain warm enough to melt most of that by the time people wake up at 5 to 6am? That remains to be seen. However, all of the available guidance I have at my disposal points to a notable snowfall event for many places, especially at higher elevations, and have adjusted the forecast in that direction. Regarding snowfall amounts, accumulations along/east of the I-75/I- 81 corridor in the valley have been upped a bit and amounts in the TN mountains have been increased significantly. It is quite possible that portions of the Smokies could see a 12-13" snowfall out of this storm. In the valley, expanded the 3-5" accumulations and resulting winter storm warning to include the Knox metro area town to McMinn county along I-75, as well as added eastern Polk, Cherokee and Clay counties in NC, and Lee county in Virginia. Cherokee and Clay counties will mostly be higher elevations though. One big uncertainty I still have is the plateau counties. Those areas will be the furthest remove from deformation precip, but will also see the colder air quicker. Have left them all in the advisory for now but would not be surprised to see some warning level accumulations given how this system is shaping up. On the back side, precip wraps up around or very shortly after daybreak tomorrow. Perhaps some upslope showers continue in the mountains through late morning but believe it will be mostly done by then.

Totally agree. There’s truly never been a greater jinx.

I can confirm that it is 80/20 snow right now in West Knox

Was just out in Oak Ridge about 30 minutes ago. It was starting to sleet there

Without a doubt, that will be what MRX uses for their afternoon update. Lol!

Thank you so much Holston! That was a great explanation!

From a storm evolution standpoint, what needs to happen in order for this to be more of a snow event versus zr? Clearly, there’s some warming aloft. Do we need the HP to be stronger or positioned differently? Or is this more of a storm track issue?

What would this do to the temp profile in the Valley?

MRX seems skeptical (and rightfully so after the flip flopping) on the Sat night/Sun morning storm. Here’s the portion of you their disco that pertains to this storm: AREA FORECAST DISCUSSION National Weather Service Morristown TN 400 PM EST Thu Feb 4 2021 .LONG TERM...(Friday Night through Thursday)... Key Messages: 1. Snow chances have increased Sat night and Sun morning, but confidence remains very low with how this system will evolve. 2. Dry with near to slightly below normal temps Sun afternoon warming above normal Mon. 3. Active weather next week with difficult to time precip chances, but milder temps early in the week cool below normal mid/late week. Discussion: Friday Night and Saturday... Fast, progressive upper level flow will remain across the TN Valley during this time at the base of broad troughing covering much of the northern tier of the country. Weak surface high pressure quickly moving across the Midwest and OH Valley will provide dry conditions and temps just below normal. Lows Fri night will dip into the 25-30 degree range with highs Sat in the upper 40`s to near 50. Saturday Night and Sunday... Well, this period has once again become more interesting as the 12Z NAM and deterministic ECMWF have trended toward the deterministic GFS and GEFS ensemble means in phasing northern and southern stream energy which gives the possibility of accumulating snow Sat night into Sun morning. However, the ECMWF ensemble mean, deterministic CMC, and CMC ensemble means still keep this energy separate leading to a drier forecast. Regarding the synoptic details, a fast moving northern stream mid/upper shortwave will progress across the Midwest states Sat night while a southern stream shortwave moves along the Gulf coast states with a surface response near the FL Panhandle. The N stream shortwave further deepens the broader longwave trough and sends a piece of true arctic air across the Midwest and Great Lakes in its wake, but the different solutions among the guidance stem from whether or not the N and S stream shortwaves can phase before all of the energy moves to our E. Consensus is for 850 mb temps to be -3 to -4 deg C Sat night with surface temps several degrees above freezing early in the night as the right entrance region of a 130- 140 kt H3 jet associated with the northern shortwave induces strengthening warm, moist advection and isentropic lift. The strength and duration of moisture advection and overall forcing will be determined by whether or not the features actually phase, so stayed with NBM slight chance PoPs Sat night/Sun morning, but blended WPC QPF with the GFS/ECMWF QPF to take into account some of the higher QPF probabilities. In terms of snowfall amounts, the aforementioned surface temps above freezing at the onset will lead to a rain/snow mix, but expect a quick transition to snow by late evening as the lower levels cool, possibly aided by dynamic cooling and wet bulbing. The temp profile and WPC/GFS/ECMWF blended QPF yields 0.5 to 2 inches of snow, which is a reasonable starting point based on the flip flopping guidance. This system will quickly exit NE Sun afternoon with dry conditions and highs only in the low/mid 40`s behind the associated cold front.

Lock it in!

If you have the whole run, that would be great! Thanks Holston! I’ve never really paid attention to the Para until the last several days.

Does anyone have the accumulation map for Para-GFS? I’m having a hard time getting it to load.

Man, they really are! Wow!

So it is still snowing at that point?!?

Absolutely pouring snow right now in West Knoxville

How does the Euro show snow in Mississippi and then it turns to rain in TN?

Living in West Knox, the thing that worries me about this storm is the sharp cutoff. On the latest 3k NAM, it shows around 4 inches where I live, but 30 minutes west, it’s basically showing a trace.

We may have received the kiss of death when Jeff got fully on board earlier. The forum area seems to do much better with “pessimistic Jeff”. Haha!

Ironic because in their overnight forecast discussion, they seemed to be coming around to the idea that Knox would see some snow.

It appears that the NW expansion of the precipitation shield that we were seeing in runs yesterday and earlier today has ceased. Hoping that trend will start back up so that our friends to the north and west get in on the action.

I know the NAM has a tendency to over amplify systems, but does it handle thermal profiles pretty well?

Remind me...what model are those plumes based off of?

Very good point. Also, a little OT, but I really enjoy your input on VQ

Thank you Tellico!

Wow...Does anyone have a map of the latest NAM? I’m in a spot where I don’t have access to it.