Education and Questions/Discussion Regarding Weather Features

[EastKnox]

I have started this thread in order to have a place to ask questions about weather phenomena and features we see in models.  In particular, I'll see some discussion happening about an upcoming weather event, but may not understand all of what I'm reading.  I don't want to muck up the main discussion with general questions so I thought that a separate thread could be helpful for us with less knowledge.  Of course, this isn't an atmospheric sciences course condensed into a forum thread, but maybe a few tidbits of info will be shared that we can benefit from.

 

I'll start with one that relates to our current situation.  I hear a lot of about split flow of the polar and subtropical jets.  What is the best way to identify the split flow?  Have I identified it correctly in the 250 mb winds chart below, or would you look elsewhere? (500 mb) and do you look for certain heights (like 540 dm in the 500mb chart) to identify the polar jet?

[jaxjagman]

8 minutes ago, EastKnox said:

I have started this thread in order to have a place to ask questions about weather phenomena and features we see in models.  In particular, I'll see some discussion happening about an upcoming weather event, but may not understand all of what I'm reading.  I don't want to muck up the main discussion with general questions so I thought that a separate thread could be helpful for us with less knowledge.  Of course, this isn't an atmospheric sciences course condensed into a forum thread, but maybe a few tidbits of info will be shared that we can benefit from.

 

I'll start with one that relates to our current situation.  I hear a lot of about split flow of the polar and subtropical jets.  What is the best way to identify the split flow?  Have I identified it correctly in the 250 mb winds chart below, or would you look elsewhere? (500 mb) and do you look for certain heights (like 540 dm in the 500mb chart) to identify the polar jet?

No,stick with the jet stream,like the map you are showing

[Holston_River_Rambler]

There's definitely a split there, but I've found when most people reference the split flow, they are talking about the jet (250-300mb winds) over the Pacific. 

I have tried to show it on this image. (I may be wrong too, but thought I'd take a stab at it). Usually when it is depicted it is shown with very definite, long, and connected flows (split in the mid Pacific and one half going over a big ridge and the other undercutting it), but I think the jet is more variable than that as seen in my image. there are wind maxes, buckles, etc...

[jaxjagman]

I like this thread,it's definite educational of weather aspects.

[Carvers Gap]

If you get a big PNA ridge out west or EPO ridge, sometimes energy will sneak under the ridge.  So, basically you still get cold being sent south by the western ridge and precip cutting under that western ridge.  If we are fortunate, the system(preferably entering at the latitude near Los Angeles) will slide under the ridge and connect w cold air being sent south.  We hope that confluence will happen near the East Coast.  That is what some might call a big dog pattern.

[Carvers Gap]

I posted this recently in the winter thread while discussing how it seemed suspicious that the GEFS and EPS were placing long term BN heights over AK.  To answer your question, if you look at the El Nino graphic, notice how the Pacific jet is working underneath the AN heights in AK and the West Coast.  The cold drives down that ridge and intersects w the jet underneath the ridge, hopefully somewhere in our area.  You can also see something similar during a Nina, but the confluence(joining and area of phasing) of the two jets is over the northern Rockies.  The Rockies during last year's La Nina set snow records in Billings and surround areas.  We like El Ninos because the confluence is over the East Coast.  What we want is split flow where storms are undercutting a ridge over western North America vs a ridge over the Pacific centered just east of Hawaii.  To add to that w like a -NAO over Greenland(high pressure) that causes the jet to buckle right long the EC.  It slows the storms that form and forces them to turn north...also allows for easier confluence because it basically stops up the jet.  

Great thread idea, eastknox.

[BlunderStorm]

Regarding this weekends storm system. I understand the high in the midwest is crucial to CAA resulting in snow as the low supporting precipitation slides under it. However as pressure rises precipitation is less encouraged and is generally lighter. On the 12z GFS the storm was suppressed to the south resulting in no snow at all here. On the other hand for the 18z GFS the high pressure was weakened and as a result I was projected with precipitation except the temperatures were too mild to support snowfall for a majority of the storm. So in a nutshell what I'm saying is how do you get the cold and the moisture together dang it?! 

[nrgjeff]

The question is age old in the South. True that textbook HP is fair weather. However we do need a supply of low level CAA for snow. At the same time we need WAA aloft to create the precip. It is a delicate dance anywhere. In the South it's a delicate dance on a tight rope that often results in a fall, lol!

Sometimes 500 mb offers the answer. PVA just south, with surface low farther south (a good tilt) and cold air in place, usually good. PVA at latitude / overhead, risk of ice if cold air in place. PVA north, probably rain.

EDIT for below: I did not realize Knoxville climo has shifted so much. Yuck!

[John1122]

37 minutes ago, BlunderStorm said:

Regarding this weekends storm system. I understand the high in the midwest is crucial to CAA resulting in snow as the low supporting precipitation slides under it. However as pressure rises precipitation is less encouraged and is generally lighter. On the 12z GFS the storm was suppressed to the south resulting in no snow at all here. On the other hand for the 18z GFS the high pressure was weakened and as a result I was projected with precipitation except the temperatures were too mild to support snowfall for a majority of the storm. So in a nutshell what I'm saying is how do you get the cold and the moisture together dang it?! 

It's never easy here. For whatever reason this area is one of the very tougest to put together a winter storm out of the Gulf of Mexico. They used to be our primary source of snow but these days WAA eats them up badly. Thats why snow averages have been cut in half over the last 30 years, especially south of 40. Knoxville had a 14 inch snow average in the 1951-1980 period.  It's down to 6 or so from 1981-2010. 

The perfect storm features over the top blocking with a good cold source  and a potent low. If you have marginal cold or the top you have to hope the low is weaker. 

The current event we are tracking would have a better shot for success if this were after December 20th or so. The deeper into winter the colder our source. 

[BlunderStorm]

So essentially the source for our cold is limited and the low in some runs is shown as being too strong. In other words we have a juggling act. The drop in annual snowfall for Knoxville in the past 40 years was really eye opening, thanks for that tidbit of info. 

[nrgjeff]

I promised some TROWAL explanation on the Dec 8-10 storm thread. Trough of Warm air Aloft stands for TROWAL. It often happens behind an occluded front/system. One can easily find cross section diagrams of an occlusion with a web search. However these two images show what I'm looking for when forecasting. 

Here it is on satellite, an old storm over the Great Plains. Bring it baby!

On an 850 mb chart (also 925/700) one can see WAA from the north, vs usual southeast. Average comma head is associated with the cold conveyor belt, and some split off the warm conveyor belt. Usually the better precip is off the more robust warm conveyor belt. (Conveyor belts are also an easy web search.) TROWAL feature is interesting because it can amp up the normally secondary comma head. WAA is notable into the comma head.

Example below is ECMWF forecast valid Monday. Most of the snow has fallen, but the 850 mb chart shows it better at the end of the event. Note WAA from north to south. Favors I-40 north. CAA south of I-40 shuts off precip.

[BlunderStorm]

10 hours ago, nrgjeff said:

I promised some TROWAL explanation on the Dec 8-10 storm thread. Trough of Warm air Aloft stands for TROWAL. It often happens behind an occluded front/system. One can easily find cross section diagrams of an occlusion with a web search. However these two images show what I'm looking for when forecasting. 

Here it is on satellite, an old storm over the Great Plains. Bring it baby!

On an 850 mb chart (also 925/700) one can see WAA from the north, vs usual southeast. Average comma head is associated with the cold conveyor belt, and some split off the warm conveyor belt. Usually the better precip is off the more robust warm conveyor belt. (Conveyor belts are also an easy web search.) TROWAL feature is interesting because it can amp up the normally secondary comma head. WAA is notable into the comma head.

Example below is ECMWF forecast valid Monday. Most of the snow has fallen, but the 850 mb chart shows it better at the end of the event. Note WAA from north to south. Favors I-40 north. CAA south of I-40 shuts off precip.

Thank you for the explanation. I had a partial understanding before this just picking up on things but this really cleared things up.

[Holston_River_Rambler]

Found a really nice interactive website with lessons from NWS that breaks down how to read charts from the surface, up to 200mb. It even explains the cryptic Omega in a pretty easy to understand way. 

https://www.weather.gov/jetstream/constant_intro

The same site has several other categories if you click on the drop down "Topics" menu. 

[Holston_River_Rambler]

Been looking for an example of this for a while:

Mostly using this for an example of what a Rossby wave train/ break is, but the conversation at the link has some interesting discussion of its implications for the SPV if interested. 

[Holston_River_Rambler]

Found a nice discussion of AAM (Atmospheric Angular Momentum, a teleconnection you sometimes see mentioned) and what we want to see:

https://www.33andrain.com/topic/868-teleconnections-a-more-technical-discussion/?page=29&tab=comments#comment-115404 

Maybe not the best description of exactly what constitutes AAM, but if you've ever looked at one of those AAM diagrams and wondered what we want to see for more amplification, this will help. 

 

[Holston_River_Rambler]

Potential Vorticity and water vapor imagery 

http://rammb.cira.colostate.edu/wmovl/vrl/tutorials/satmanu-eumetsat/SatManu/Basic/Parameters/PV.htm 

[Holston_River_Rambler]

 

Finally can see where that band originates, and it is the south flank of the mountains around Frozen Head:

 

 

 

It's like Frozen Head and Crab Orchard mts. create a funnel in the Valley of the Emory River that somehow enhances the precip. as the air flows out into the valley.  Maybe if some air is already banking up on the Smokies in a NW flow, that tube of air rides up it? 

The following image is from the paper “Wind Regimes in Complex Terrain of The Great Valley of East Tennessee“(Birdwell).  The research used surface wind data from several locations to examine how the area’s complex terrain alters the regional wind flow.  The graphic labels a convergence zone resulting from the features you identified earlier today. Near surface winds through the gap (during WNW-NW flow) have more of a westerly component, while winds around the northern part of the barrier have a more northwesterly direction. This convergence on the lee side of the mountain range triggers vertical motion in the valley and could have aided in the formation of the snow band from earlier today.  As you also mentioned, this band is likely enhanced east of Knoxville due to lift from the WNW flow interacting with the terrain in the eastern part of the valley

I've noticed that band several times over the years. It's nice to have a plausible explanation for it. 

 

That's an awesome dissertation. Thank you for that. I didn't realize someone had put so much time into the wind regimes of the Great Valley. 

Based on what you said above, I think I have a speculative explanation for the Bays Mountain band. Forced low level flow funneled between the Smokies and Bays Mt/ Chimney top in Greene county (you can see that on the satellite image), meets NW flow less inhibited coming around the NE flank of Bays Mt. 

Lift provided by the up valley flow, convergence, and overall trajectory of everything towards the higher mountains to the SE. 

You might also be able to argue some of that flow continues on to Boone, through relative gaps between Buffalo and Holston Mountains, the NE flank of Roan, and ends up aimed at the ski resort areas there and Boone. 

 

 

 

 

 

[EastKnox]

Thanks for putting this here, I had wondered if there was an easy way to put that part of the discussion into this thread.

[Holston_River_Rambler]

always wondered what these were called:

 

[Holston_River_Rambler]

@AMZ8990

Sorry for taking a while to get back to you. I'm putting this response in here because I'm still trying to learn about this stuff too and welcome any thoughts. 

As I understand it, AAM (Atmospheric angular momentum) is a measure of the momentum of the atmosphere across the globe at all latitudes at a given time. Sort of like where the most umph is and where it is aimed in latitude and whether or not it is westerly (+), or easterly (-) (yellows and oranges/ blues and greens respectively in the image attached).  The earth is a solid object, surrounded by a fluid (atmosphere). That fluid rotates with the earth, but the sloping surfaces and mountains and just plain old fluxes and ebbs embedded in it can change over time and impact the state of flow in the atmosphere (its momentum) and importantly for us, high latitude blocking. 

For example. this year, from Dec - Feb the + momentum was aimed from the equator toward the pole in the N. hemisphere (orange arrows in the image, latitude markers circled in black on the left). More momentum aimed that way, at least as I think I understand it, would lead to less chances to block things up at high latitudes. It looked like that was changing, but it is now back.  

There are also two ways to measure it: relative and total. Notice the total in this image is currently off the charts. Also notice that total was relatively low back when we got our Dec storm. 

 I don't understand it much more than that and used it in my speculation earlier only in the sense that it was aimed back toward the equator at that time. Its certainly just on piece of the puzzle and I really don't understand how to use it right now more than to say "oh look, it's changing and "X" expert said that's good" so still a ways to go for me. 

Here is a nice .pdf I found, but the mathematical side is currently over my head. But then again I've never seen anyone show their work, so to speak, when using AAM charts to make a pattern point, so I suspect there is just more meteorological depth to using these charts than I currently understand. 

https://uol.de/fileadmin/user_upload/icbm/ag/physoz/download/cl_dyn/ang_mom.pdf 

 

[Holston_River_Rambler]

Some additional thoughts about AAM from Isotherm (this isn't really a description, but the way he is using the features at least is helping me to try to understand it better):

"I'd say it's fairly incontrovertible that the Pacific structure will be ameliorating, finally, as a function of the ewd propagation tropical convection and realignment of W/C forcing in more canonical orientation. Phase 8/1 entrance in a few days + 7-8 day lag, should elicit vicissitudes in the PNA domain by the end of February.

 

The more intriguing and challenging inquiry for me, is can we get the NAO to play ball, at all? We have discussed this heretofore, and it's something @Snowy Hibbo noted I believe; namely, that much of the positive angular momentum has been concentrated in the southern hemisphere. Further, I noted that the equator-pole distribution of the angular momentum has been "wrong" and antithetical to the genesis of sub-tropical troughs, which accelerate jet flow and tend to initiate decelerating polar jets immediately to the north. Those conditions remain unpropitious through the next 7-10 days; however, if the CFS is correct, there may be a window within which a legitimate NAO *could* develop and actually retrograde a bit toward Greenland. This window of opportunity is approximately March 1/2-March 10th +/- a couple days. Besides the phase 1 lagged effects, forecasted equator-pole zonal winds are suggestive of an alteration from this winter's base state in the March 1-10th period by the CFS. Now, we continue to have persistence and descending wly QBO modulation countervailing, but if other variables rearrange, that shouldn't totally obviate the chance for at least a transient blocking feature. We'll have to monitor it, but I like what I see on the CFS zonal wind proggs in the first 10 days of March. Momentum diminutions in the SHEM, and reorientation of +AAM in more conducive places. We'll see. But, the Pacific alone will make things colder for the first half of March.

 

The question is, are we talking about a decent pattern (which would be much better than what we've had = ghastly, hideous, and diabolically impudent), or, could it be "better" than decent? That's contingent upon whether we can induce a transient blocking period. As said, there's a chance. But it also may not happen.

Right, James. The tropical forcing and extratropical progression, i.e., MJO, GWO, etc., would certainly augment the argument for a retrogressive HLB feature. But the -AAM in the sub-tropics needs to decrease to permit it." 

[Holston_River_Rambler]

Always wondered what Rossby waves looked like;

 

Kind of glad those suckers formed since it looks like they may be sucking up some of the moisture that may have been aimed at us otherwise. 

[Holston_River_Rambler]

 

[nrgjeff]

Rare and exciting when the Atmosphere actually looks like the textbook. Neat stuff!

[Holston_River_Rambler]

https://ams.confex.com/ams/WAFNWP34BC/techprogram/paper_94815.htm 

 

[EastKnox]

Found this gem from WFO Louisville for all of you severe weather trackers.

SvrWx_Fcstg_TipSheet.pdf

[Holston_River_Rambler]

I found (I think) the paper that Tellico has been talking about and have screengrabbed some images from it (diagrams and conclusions).

Still a little under the weather and not sure I could figure this out without more time than I have even if I was felling better, but thought we might like to see it as the MJO moves forward.

NPST = North Pacific Storm Track

Source: Wang, Jiiabao, et al. "Modulation of the MJO and the North Pacific Storm Track Relationship by the QBO," Journal of Geophysical Research: Atmospheres, (2018), 3976 - 3992.