February/March 2020 Winter's Last Chance Thread

[John1122]

6 minutes ago, Dsty2001 said:

At what point do we start taking the NAM seriously, I know it's performed fairly well this winter but we all have been NAM'd before however it has had a decent consistency with this system for a couple days now 

I won't trust it or really any model until 24 hours or less out. 

[TellicoWx]

5 minutes ago, BNAwx said:

Hoping the NAM isn’t just NAMing us right now...but even the globals have been inching precip more to the NW as we get closer in time.

Normally I would say the NAM is just out to lunch, but you can see in the upper air pattern how the globals are trying to follow it by not squishing the energy. So it adds more weight to what the NAM is showing than usual. GFS trend:

[Chattownsnow]

Lovin that nooga snow hole! 

[TellicoWx]

NAM trend (GFS definitely stepping toward it)

[Carvers Gap]

40 minutes ago, Dsty2001 said:

At what point do we start taking the NAM seriously, I know it's performed fairly well this winter but we all have been NAM'd before however it has had a decent consistency with this system for a couple days now 

I usually like to use it once it is under 48 hours.

[Carvers Gap]

What is the time of day for this event?  Time of day makes a huge difference right now.  In other words...when does it hit Memphis, Nashville, Knoxville, Chattanooga, and TRI?  I think daytime is a no-go if this is marginal.  Nighttime is workable.

[Wurbus]

40 minutes ago, Carvers Gap said:

What is the time of day for this event?  Time of day makes a huge difference right now.  In other words...when does it hit Memphis, Nashville, Knoxville, Chattanooga, and TRI?  I think daytime is a no-go if this is marginal.  Nighttime is workable.

It's looking like it will start in East TN Thursday morning (6-9am-ish), so the majority of it would be during the day on Thursday, but I'm not 100% sure. It is always difficult to determine the timing on the models for me.

[Carvers Gap]

38 minutes ago, Wurbus said:

It's looking like it will start in East TN Thursday morning (6-9am-ish), so the majority of it would be during the day on Thursday, but I'm not 100% sure. It is always difficult to determine the timing on the models for me.

Same here, especially on global models.   I generally find precip to be a few hours ahead of modeling during many events, but not all.  RAP and HRRR is easier for me - I just count the hours to whatever I am looking at.  LOL

On another note, medium and LR modeling continues to be quite cold - as in BN stuff w duration.  The 0z EPS has many BN days and very few AN days.  We will see if that holds.  The EMON is COD city for the MJO.  If so, the inhibitor of the EPO ridge is lessened.  

[Save the itchy algae!]

Has Dr. No come in with the 12z yet? If the trend shows up there in some form then I will start getting excited

[TellicoWx]

15 minutes ago, Carvers Gap said:

Same here, especially on global models.   I generally find precip to be a few hours ahead of modeling during many events, but not all.  RAP and HRRR is easier for me - I just count the hours to whatever I am looking at.  LOL

On another note, medium and LR modeling continues to be quite cold - as in BN stuff w duration.  The 0z EPS has many BN days and very few AN days.  We will see if that holds.  The EMON is COD city for the MJO.  If so, the inhibitor of the EPO ridge is lessened.  

NAM starts bringing th moisture across the southern TN border between the 6z and 9z (closer to 8z = 3am...12z is 7am). GFS is slower closer to 11z-12z (6-7am). Yeah it's a daytime event for TN.

[PowellVolz]

Something we have seen many times is the NAM will lead the way with an aggressive move. What happens next with the globals will give us a good idea which way we are headed.


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[ShawnEastTN]

Daytime events this time of year can be harder, but generally if they start pre-dawn or right around dawn they have a far better chance than if they start later in the day. 6-7am is usually the coldest part of the day if we get the clouds and precip falling during that window it can stall the temps keeping them from rising especially if good precip rates.

 

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[tnweathernut]

1 hour ago, Wurbus said:

It's looking like it will start in East TN Thursday morning (6-9am-ish), so the majority of it would be during the day on Thursday, but I'm not 100% sure. It is always difficult to determine the timing on the models for me.

Still not too excited about this one, but the flow has backed a bit over the last several runs on modeling.  This is probably oversimplified, but IF we can manage a WSW flow vs ESE or even W, there's at least a chance for more generous precip.  In a nutshell I am hoping for the best, preparing for the worst I guess you'd say.

Regarding time, when you see "Z" time, just subtract 5 hours to get to actual time in the EST zone, and subtract 6 hours in the CST zone.

[Holston_River_Rambler]

Euro looks like it will be better than 0z, but maybe not quite as good as 6z. Splitting hairs though. 

[Holston_River_Rambler]

Hvward had a good post earlier this AM in the NC Mountains/ foothills thread about how the NAM might be handling the set up better because it was a "gridded" model, as oppose to the Euro/ UK which are "spherical". Apparently this allows the NAM to do better with small, punchy vorts moving in quickly one after another.

12z Euro:

[Holston_River_Rambler]

15z RAP FWIW:

[Math/Met]

I always try to looks at small details for any event in East TN. Mainly because the terrain can cause things to go unexpectedly wrong or sometimes in our favor. Looking at the NAM, the thing I like about this system is that low level winds over WNC are from a more easterly direction during the start of the system, while winds over the ETN valley are more northerly. This typically happens when we get an inverted trough induced along the mountains, which helps provide some low level lift and often produces more precip than modeled. This current situation is more subtle, but the characteristics are still there. It’s something to watch to see if precip trends upward as we go.

[ShawnEastTN]

I always try to looks at small details for any event in East TN. Mainly because the terrain can cause things to go unexpectedly wrong or sometimes in our favor. Looking at the NAM, the thing I like about this system is that low level winds over WNC are from a more easterly direction during the start of the system, while winds over the ETN valley are more northerly. This typically happens when we get an inverted trough induced along the mountains, which helps provide some low level lift and often produces more precip than modeled. This current situation is more subtle, but the characteristics are still there. It’s something to watch to see if precip trends upward as we go.

Would be awesome if we can get that NNE wind down the valley during this event, those have been life savers for these scenarios in the past.

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[snowmaker]

Next Nam run coming up soon should be interesting. Hope we can keep that northern trend into Tennessee going on this one. At least it something to follow, maybe someone in the south will get a surprise.

[nrgjeff]

Great post Math/Met. Inverted trough is the only way Chattanooga can get snow when surface temps are above freezing; otherwise, it's straight downslope hell. Indeed the NAM/GFS hint at a slight inverted trough in wind fields. It does not stand out as well with isobars. Hvward also makes a good point about the Spectral vs Gridded models this set-up (Mountains, SE sub) which was valid with the Chattanooga surprise as well.

No major concerns/changes with the 12Z data. American gridded NWP sticks with sticking snow. Euro does not have much. Appears the Euro is missing subtle wave strength. This is not post-frontal junk. We have a true short-wave with isentropic lift coming through Thursday. Euro might not resolve the best early to midday isentropic lift. I get at that indirectly at 700 mb. Euro is CAA. Others are neutral with a frame or two slight WAA. That's going to also be isentropic lift.  Rose/snow colored glasses? Maybe, but I'm going with American guidance.

Possible the Euro is picking up on more strength in the dry northern stream. However pattern recognition favors the wave producing the upglide/isentropic lift. Pattern recognition favors the American guidance Thursday.

[TellicoWx]

The handling of the 925 energy is key...12z NAM has the piece slightly quicker and the furthest north on guidance (along central AL/MS line)..12z Euro is furthest south (along the central LA coastline). GFS is the middle ground, really dont won't that piece going any further north on the NAM..especially anyone south of 40. 12z NAM/Euro:

[PowellVolz]

Not that it really matters storm to storm but the last system that did well in SE Tennessee, only the NAM for several days had snow into Knoxville and thats what happened. Also the Euro didn’t have the precip field far enough NW a few weeks back for the system that effected SC/NC.


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[Vol4Life]

1 minute ago, PowellVolz said:

Not that it really matters storm to storm but the last system that did well in SE Tennessee, only the NAM for several days had snow into Knoxville and thats what happened. Also the Euro didn’t have the precip field far enough NW a few weeks back for the system that effected SC/NC.


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

[PowellVolz]

18z NAM out to 57 looks on track.


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[TellicoWx]

18z NAM slightly shifted the 925 track south (12z thru Montgomery AL...18z 30mi south of Montgomery) which shifted the heaviest totals south of 40. Good run and probably best track along southern TN border (not too far north to mess with temp profile, but still keep the better dynamics across TN. Also deepens the energy quicker as it transfers off the SE coast (great for NC folks and possibly NE TN if it can wrap the moisture over the mountains).

[TellicoWx]

18z NAM

[nrgjeff]

PowellVolz is right about the ECMWF trouble two Saturdays ago. Actually the systems do have some similarities. Both follow a first wave that pulls in cold. Both are greater than post-frontal junk, yet not WAA monsters. Really they are the pattern recognition I seek.

NAM is probably too amped but I think the Euro is missing the wave and some isentropic lift. See my previous post. I'm guessing the 18Z GFS stays similar to its previous runs, somewhere in the middle. Look for trends on the 18Z or overnight ICON and Euro, hopefully less bearish even if staying on the light side.

Finally, I had to put Darth Vader in Banter again. Cheers!

[PowellVolz]

I always try to looks at small details for any event in East TN. Mainly because the terrain can cause things to go unexpectedly wrong or sometimes in our favor. Looking at the NAM, the thing I like about this system is that low level winds over WNC are from a more easterly direction during the start of the system, while winds over the ETN valley are more northerly. This typically happens when we get an inverted trough induced along the mountains, which helps provide some low level lift and often produces more precip than modeled. This current situation is more subtle, but the characteristics are still there. It’s something to watch to see if precip trends upward as we go.

Just thinking out loud here but with marginal temps the valley needs a possible inverted trough to stay over the mountains. If it lines up in the valley wouldn’t downsloping become a problem?


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[TellicoWx]

18z GFS looks very similar as far as track with the 925 energy vs 18z NAM (just slightly weaker)...not quite buying the Chatt snowhole with that setup.

[Math/Met]

1 hour ago, PowellVolz said:

 

Just thinking out loud here but with marginal temps the valley needs a possible inverted trough to stay over the mountains. If it lines up in the valley wouldn’t downsloping become a problem?


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In these cases, the inverted trough is induced by the terrain. The air flow is different on either side of the mountains, so you can basically get a "pinching" (not a very technical way of describing it) between the two flows. Neither side has to deal with much downslope because it creates rising air in the low levels. It is a fixed feature because the mountains are a permanent feature. Hope that makes sense.