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Snow Potential Late December

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Re: Snow Potential Late December

Post by Reb on 2011-12-21, 10:31 am

snowdog wrote:
Reb wrote:GFS coming back around?

If you mean snow, not quite. Knoxville Skew-T 06z GFS at 102hrs.

I said coming back around, not quite there yet. trended colder that run for sure.

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Re: Snow Potential Late December

Post by Toot on 2011-12-21, 10:38 am

Good explanation snowdog...thanks Very Happy

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Re: Snow Potential Late December

Post by Stovepipe on 2011-12-21, 10:40 am

snowdog wrote:
Cyclonicjunkie wrote:
snowdog wrote:
If you mean snow, not quite. Knoxville Skew-T 06z GFS at 102hrs.

Would you mind explaining how that skew T works so people can understand what you're talking about...Not saying its real useful at over hundred hours out or anything, but a little explanation would be nice to go along with the graphic if you dont mind?

I'll give it a shot.

Red Line is Temp.
Green line is Dew Point Tmp.
Blue line is Wet Bulb Temp.

Y axis is your Pressure. 850 corresponds to your 850mb maps. 500 corresponds to 500mb. Etc. Usually the colored lines stop at the ground location. X axis is temperature in Celsius.

So find the 0 on the x axis and that is the freezing mark. Work your way up the diagonal line starting at the 0 mark. On the graph I showed you can see the column doesn't start to hit freezing till 850mb's. Meaning the lower levels from about a mile up in the air down to the ground are too warm to support frozen precip.

Also you can see the colored lines come together at 500mb and stay together. This is indicative of a saturated column. In say a virga storm you may see them together from 500 to 850 then spread apart meaning the whole column isn't saturated thus while it looks like precip on a radar it isn't making it to the ground.

Sleet signature would have warm temps aloft and cold temps from about 850 down. Freezing rain would show warm air aloft and a cold layer right at the surface.

There is also a way to interpret snow growth regions from the skew-t but I'm not very good explaining that as I don't understand that facet of the graph fully.


Thanks snowdog. I'm going to add that to the Weather Resources forum. hurry
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Re: Snow Potential Late December

Post by Stovepipe on 2011-12-21, 1:33 pm

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Re: Snow Potential Late December

Post by jmundie on 2011-12-21, 1:57 pm

Euro holding the ULL back... but it is showing acumulation for the plateau, east tenn, kentucky, arkansas, and a dusting in Nashville between 144-150 per the wunderground snowfall maps

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Re: Snow Potential Late December

Post by Stovepipe on 2011-12-21, 3:38 pm

Apparently some progress is being made on the stratospheric warming front. Euro showing some favorable signs.

Good discussion here: http://www.americanwx.com/bb/index.php/topic/30871-stratospheric-warming-event-on-the-way/page__st__175
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Re: Snow Potential Late December

Post by Toot on 2011-12-21, 6:55 pm

It looks like this threat has all but disappeared as the euro is the only one holding out hope and even it has delayed it until like the 27th. It's getting into the nam's range now and it may be agreeing more with the euro than any other model at this point.. so I wouldnt exactly turn my nose to those solutions...but I would hate to be in Bastardi's shoes right about now as he was painting CHRISTMAS time accums from the southern Apps all the way up into New England yikes

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Re: Snow Potential Late December

Post by Toot on 2011-12-23, 10:00 am

Boy...things are starting to trend colder and more wet again with each passing run of the GFS and NAM...still not buying a threat at this point but if another run trends colder than the past.... a snow threat will again be a possibility. whistle

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Re: Snow Potential Late December

Post by Adam2014 on 2011-12-23, 10:03 am

Cyclonicjunkie wrote:Boy...things are starting to trend colder and more wet again with each passing run of the GFS and NAM...still not buying a threat at this point but if another run trends colder than the past.... a snow threat will again be a possibility. whistle
Yep another ULL I have been watching this.

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Re: Snow Potential Late December

Post by jmundie on 2011-12-23, 10:03 am

Cyclonicjunkie wrote:Boy...things are starting to trend colder and more wet again with each passing run of the GFS and NAM...still not buying a threat at this point but if another run trends colder than the past.... a snow threat will again be a possibility. whistle

Was just about to mention the NAM

Looks like Tues - Wed is not out of the question.

I just have a hard time believing the closed low won't have some positive interaction with the northern stream. The Euro has held pretty steady with this feature (though it was delayed a bit).

If the gfs starts trending towards the Euro and Nam it will be time to start paying attention again

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Re: Snow Potential Late December

Post by Toot on 2011-12-23, 11:38 am

jmundie wrote:
Was just about to mention the NAM

Looks like Tues - Wed is not out of the question.

I just have a hard time believing the closed low won't have some positive interaction with the northern stream. The Euro has held pretty steady with this feature (though it was delayed a bit).

If the gfs starts trending towards the Euro and Nam it will be time to start paying attention again

The 12zgfs seems to somewhat phase pretty early which pulls the whole system north...im not sure what to make of that run...one thing looks for sure...we're not gonna know the exact outcome of this system anytime soon and its only a few days out. That makes it a great canidate to be a very sneaky system that could lay some snow down somewhere without anyone seeing it coming yikes

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Re: Snow Potential Late December

Post by jmundie on 2011-12-23, 12:15 pm

Canadian looks like a phase of sorts. But I can't read the canuck maps very well.


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Re: Snow Potential Late December

Post by jmundie on 2011-12-23, 12:39 pm

THe phase is a bit too early for us... maybe some backside snow showers/flurries


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Re: Snow Potential Late December

Post by Toot on 2011-12-23, 4:16 pm

jmundie wrote:THe phase is a bit too early for us... maybe some backside snow showers/flurries


I would definately keep an eye on this system... how and where the northern stream interacts with the ULL vort energy in the southern stream will dictate where this ends up....it was just last Christmas when everybody including the NWS had written off a southern stream piece of energy because it wasnt phasing in time to effect the east coast and the models didnt start phasing it correctly until it was RUC time...and I actually remember the HRRR was the first to pick the correct solution...by that time people where already traveling an unaware of the powerful blizzard that was about to take place evl

Not saying this will be a blizzard or anything but I guarantee you the models are not handling it correctly yet and I actually think west Tennessee will have a decent shot at seeing some white stuff but at this point its anybody's guess who ends up with some flakes falling.

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Re: Snow Potential Late December

Post by Adam2014 on 2011-12-23, 5:13 pm

18z GFS has the low further west, the low cuts right through the center of Tennessee. Not cold enough though. Alot of moisture to work with.

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Re: Snow Potential Late December

Post by Math/Met on 2011-12-23, 6:34 pm

If you take the 18z GFS literally, then it shows a fairly significant mountain wave windstorm for the mountains and foothills of East Tennessee on Monday night and Tuesday morning. Obviously a lot could change, but this run would suggest a significant wind event for the favored areas.

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Re: Snow Potential Late December

Post by Toot on 2011-12-23, 10:18 pm

Math/Met wrote:If you take the 18z GFS literally, then it shows a fairly significant mountain wave windstorm for the mountains and foothills of East Tennessee on Monday night and Tuesday morning. Obviously a lot could change, but this run would suggest a significant wind event for the favored areas.

Interesting MM..would you care to explain how one goes about recognizing a MW event here in the southern Apps?



On another note the NAM is smoking some serious stuff tonight ...notice the 850mb freezing line




lmao

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Re: Snow Potential Late December

Post by Stovepipe on 2011-12-23, 10:25 pm

I like the cut of the NAM's jib and would like to subscribe to it's newsletter. If it's showing that tomorrow it'll be time to dust off the Fred Sanford pics.

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Re: Snow Potential Late December

Post by Reb on 2011-12-23, 10:31 pm

holy toot n balls stove you were right!!!!

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Re: Snow Potential Late December

Post by Math/Met on 2011-12-24, 12:39 am

Cyclonicjunkie wrote:
Math/Met wrote:If you take the 18z GFS literally, then it shows a fairly significant mountain wave windstorm for the mountains and foothills of East Tennessee on Monday night and Tuesday morning. Obviously a lot could change, but this run would suggest a significant wind event for the favored areas.

Interesting MM..would you care to explain how one goes about recognizing a MW event here in the southern Apps?


The first thing I look for is a strong Low Level Jet at the 850mb level. You want the direction of this jet to be close to due south or southeasterly. You can actually have a direction that is slightly west of due south at this level, but it at least needs to be close to 180 degrees as it approaches the mountains. In a lot of situations, the strongest wind speeds with the LLJ don’t even need to be directly over the mountain (the strongest winds are often located to the NW of the region). The winds below 850mb need to be southeasterly in order to become perpendicular to the mountains in our region (the terrain will often make the low level winds more southeast than the models indicate). The winds below mountain level should also increase quickly with height.

The next thing you need is a southwesterly wind flow above the mountain level. The 700mb level can be used for this. A southwesterly wind flow for our region is parallel to the mountain level. This establishes a ‘critical level’ in the atmosphere, which basically prevents the flow of air over the mountains from continuing upward too far and deflects the wind downward.

Stability plays a huge role in the formation of mountain waves. You need the boundary layer to be stable. There is actually a balance that must be reached which is related to the Froude Number. This basically means that if the winds are too weak in a stable atmosphere, the wind will become blocked and only weak mountain waves will form. If the winds are very strong in an unstable atmosphere, then the amplitude of the wave won’t be sufficient to reach the surface. In this case, a “cavity” of weak winds forms near the mountains. When you get a nice balance between high wind speed and stability, then the air can resonate which causes strong waves. So, you need to look at a skew-t to check for the amount of stability in boundary layer as well as wind speeds/directions at different levels of the atmosphere.

A lot of times, there will be a slight cold air damming signature to the east of the mountains because of the location of the cool high pressure that is needed for the stable atmosphere near the surface while the LLJ provides warm air advection near the 850mb level.

No models that I know of have the resolution necessary to accurately predict the wind speeds in these events. The models that I look at always underestimate the windspeeds, especially for the Camp Creek area that I focus on.

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Re: Snow Potential Late December

Post by Stovepipe on 2011-12-24, 12:41 am

Math/Met wrote:
Cyclonicjunkie wrote:
Math/Met wrote:If you take the 18z GFS literally, then it shows a fairly significant mountain wave windstorm for the mountains and foothills of East Tennessee on Monday night and Tuesday morning. Obviously a lot could change, but this run would suggest a significant wind event for the favored areas.

Interesting MM..would you care to explain how one goes about recognizing a MW event here in the southern Apps?


The first thing I look for is a strong Low Level Jet at the 850mb level. You want the direction of this jet to be close to due south or southeasterly. You can actually have a direction that is slightly west of due south at this level, but it at least needs to be close to 180 degrees as it approaches the mountains. In a lot of situations, the strongest wind speeds with the LLJ don’t even need to be directly over the mountain (the strongest winds are often located to the NW of the region). The winds below 850mb need to be southeasterly in order to become perpendicular to the mountains in our region (the terrain will often make the low level winds more southeast than the models indicate). The winds below mountain level should also increase quickly with height.

The next thing you need is a southwesterly wind flow above the mountain level. The 700mb level can be used for this. A southwesterly wind flow for our region is parallel to the mountain level. This establishes a ‘critical level’ in the atmosphere, which basically prevents the flow of air over the mountains from continuing upward too far and deflects the wind downward.

Stability plays a huge role in the formation of mountain waves. You need the boundary layer to be stable. There is actually a balance that must be reached which is related to the Froude Number. This basically means that if the winds are too weak in a stable atmosphere, the wind will become blocked and only weak mountain waves will form. If the winds are very strong in an unstable atmosphere, then the amplitude of the wave won’t be sufficient to reach the surface. In this case, a “cavity” of weak winds forms near the mountains. When you get a nice balance between high wind speed and stability, then the air can resonate which causes strong waves. So, you need to look at a skew-t to check for the amount of stability in boundary layer as well as wind speeds/directions at different levels of the atmosphere.

A lot of times, there will be a slight cold air damming signature to the east of the mountains because of the location of the cool high pressure that is needed for the stable atmosphere near the surface while the LLJ provides warm air advection near the 850mb level.

No models that I know of have the resolution necessary to accurately predict the wind speeds in these events. The models that I look at always underestimate the windspeeds, especially for the Camp Creek area that I focus on.

Fascinating. You are a great asset to these forums. rock on
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Re: Snow Potential Late December

Post by Math/Met on 2011-12-24, 12:52 am

Thanks. Mountain waves have always been my favorite weather event (snow is a very close second). I spent over 10 years living near an area (Camp Creek) that is about as windy as any low elevation location that you’ll find. That's why I decided to spend so much time learning everything I could about them.

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Re: Snow Potential Late December

Post by Toot on 2011-12-24, 12:55 am

Math/Met wrote:

The first thing I look for is a strong Low Level Jet at the 850mb level. You want the direction of this jet to be close to due south or southeasterly. You can actually have a direction that is slightly west of due south at this level, but it at least needs to be close to 180 degrees as it approaches the mountains. In a lot of situations, the strongest wind speeds with the LLJ don’t even need to be directly over the mountain (the strongest winds are often located to the NW of the region). The winds below 850mb need to be southeasterly in order to become perpendicular to the mountains in our region (the terrain will often make the low level winds more southeast than the models indicate). The winds below mountain level should also increase quickly with height.

The next thing you need is a southwesterly wind flow above the mountain level. The 700mb level can be used for this. A southwesterly wind flow for our region is parallel to the mountain level. This establishes a ‘critical level’ in the atmosphere, which basically prevents the flow of air over the mountains from continuing upward too far and deflects the wind downward.

Stability plays a huge role in the formation of mountain waves. You need the boundary layer to be stable. There is actually a balance that must be reached which is related to the Froude Number. This basically means that if the winds are too weak in a stable atmosphere, the wind will become blocked and only weak mountain waves will form. If the winds are very strong in an unstable atmosphere, then the amplitude of the wave won’t be sufficient to reach the surface. In this case, a “cavity” of weak winds forms near the mountains. When you get a nice balance between high wind speed and stability, then the air can resonate which causes strong waves. So, you need to look at a skew-t to check for the amount of stability in boundary layer as well as wind speeds/directions at different levels of the atmosphere.

A lot of times, there will be a slight cold air damming signature to the east of the mountains because of the location of the cool high pressure that is needed for the stable atmosphere near the surface while the LLJ provides warm air advection near the 850mb level.

No models that I know of have the resolution necessary to accurately predict the wind speeds in these events. The models that I look at always underestimate the windspeeds, especially for the Camp Creek area that I focus on.

Thanks M/M....sounds like a pretty complicated process but very interesting...im assuming in addition to the southerly flow of an approaching LPS you always have to have the flow of a high pressure system for a tighter gradient (in addition to its stabling effect) somewhere east of the Mountains for an event to take place?

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Re: Snow Potential Late December

Post by Math/Met on 2011-12-24, 1:07 am

Cyclonicjunkie wrote:
Math/Met wrote:

The first thing I look for is a strong Low Level Jet at the 850mb level. You want the direction of this jet to be close to due south or southeasterly. You can actually have a direction that is slightly west of due south at this level, but it at least needs to be close to 180 degrees as it approaches the mountains. In a lot of situations, the strongest wind speeds with the LLJ don’t even need to be directly over the mountain (the strongest winds are often located to the NW of the region). The winds below 850mb need to be southeasterly in order to become perpendicular to the mountains in our region (the terrain will often make the low level winds more southeast than the models indicate). The winds below mountain level should also increase quickly with height.

The next thing you need is a southwesterly wind flow above the mountain level. The 700mb level can be used for this. A southwesterly wind flow for our region is parallel to the mountain level. This establishes a ‘critical level’ in the atmosphere, which basically prevents the flow of air over the mountains from continuing upward too far and deflects the wind downward.

Stability plays a huge role in the formation of mountain waves. You need the boundary layer to be stable. There is actually a balance that must be reached which is related to the Froude Number. This basically means that if the winds are too weak in a stable atmosphere, the wind will become blocked and only weak mountain waves will form. If the winds are very strong in an unstable atmosphere, then the amplitude of the wave won’t be sufficient to reach the surface. In this case, a “cavity” of weak winds forms near the mountains. When you get a nice balance between high wind speed and stability, then the air can resonate which causes strong waves. So, you need to look at a skew-t to check for the amount of stability in boundary layer as well as wind speeds/directions at different levels of the atmosphere.

A lot of times, there will be a slight cold air damming signature to the east of the mountains because of the location of the cool high pressure that is needed for the stable atmosphere near the surface while the LLJ provides warm air advection near the 850mb level.

No models that I know of have the resolution necessary to accurately predict the wind speeds in these events. The models that I look at always underestimate the windspeeds, especially for the Camp Creek area that I focus on.

Thanks M/M....sounds like a pretty complicated process but very interesting...im assuming in addition to the southerly flow of an approaching LPS you always have to have the flow of a high pressure system for a tighter gradient (in addition to its stabling effect) somewhere east of the Mountains for an event to take place?

That is correct. You need the tight pressure gradient between the low to the west and the high pressure to the east. You also need the pressure gradient to align so that the wind has a southeasterly direction in the low levels, and not a southwest direction. That has been a problem several times this year, the low level flow had too much of a westerly component with most of the systems this year. Slight changes in the wind flow and stability can make a big difference.

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Re: Snow Potential Late December

Post by Toot on 2011-12-24, 1:36 pm

Looking like highs today is about 5 degrees colder than expected in the MRX CWA...things that make you go hmmm study

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Re: Snow Potential Late December

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