4/17/04 FCST:PLAINS

[Shane Adams]

I'll be in Elk City by noon Saturday......stop and say hi if you're in the area.

 

[Amos Magliocco]

Originally posted by Shane Adams
I'll be in Elk City by noon Saturday......stop and say hi if you're in the area.

Shane, was about to PM you and say my target was Clinton and we should meet up out there, but after looking at these model initializations, I have to put the entire weekend back into the "iffy" category. Only because I live 800 miles from Clinton. If I lived in Norman, it would be a no-brainer.

As badly as the model initialized this morning (and apparently has for the last several runs) concering the pool of moisture in the Gulf (which exists only on graphics and nowhere in reality), I have to throw out the rest of the run in terms of moisture and instability progs. It seems to me that this has profound impact on the character of the dryline as well, not to mention the theta-e value of our warm sector boundary layer. While I have much more confidence in the southern plains recovering faster, everything I thought I knew about the weekend was apparently based on some very wacked out moisture progs.

I guess we'll see what really transpires over the next 24 hours. I'll be sticking closely to obs, vapor loops, and soundings.

 

[Austin Ivey]

Here is an interesting article publicly available at the SPC website. Authored by Rich Thompson, this paper documents the process of bringing the GOM back to equilibrium after frontal passages in the cool season (roughly nov.-apr.). It also gives a useful chart for finding what the equilibrium DP might become based on SST observations at a certain buoy.

I thought this would be an interesting read especially given the debate on mositure return for chasing prospects this coming wknd.

http://www.spc.noaa.gov/publications/thomp...n/buoy/buoy.htm

 

[Amos Magliocco]

Excellent link, Austin. I knew Rich Thompson had done a lot of work in this area, but hadn't taken time to find his articles. He's using Buoy 42002 for this analysis, obs for which are here:

http://www.ndbc.noaa.gov/station_page.phtm...l?station=42002

Here's three very important points from this paper, with direct implications for our weekend:

"Surface buoy observations from the NOAA buoy 42002 were the primary data source for this study...This location was considered ideal for investigating the open water characteristics of surface return flow affecting the southern Plains and lower Mississippi Valley. This buoy is located south of the shallow continental shelf waters. The shelf waters experience relatively large seasonal variations in temperature and are not considered relevant when evaluating air-sea equilibrium during cool season return flow episodes (Thompson et al. 1994)."

"Equilibrium thermodynamic principles can be applied successfully to forecasts of deep convection. From operational experience, the majority of southern/central United States outbreaks of strong/severe thunderstorms during the cool season are associated with return flows having greater than equilibrium dewpoint temperatures for much of the Gulf (dewpoints >21 C). Examination of several years of cool season SST data reveals that the area of the Gulf near buoy 42002 typically supports equilibrium dewpoint temperatures ranging from 17-19 C during the late winter and early spring months (late January through mid/late March). Thus, dew point observations at or greater than 21 C (~69 F) suggest region(s) of warmer SSTs (Bay of Campeche, Loop Current, Caribbean Sea) are the likely source for those return flows associated with the strongest severe thunderstorm episodes."

"Super-equilibrium" conditions were almost always associated with cases where return flow duration exceeded offshore flow duration (ratio Rfd/OFd >2), implying ample time for airmass modification and surface trajectories originating over warmer waters of the southern Gulf or Caribbean. "

I am now wondering if perhaps the higher dewpoint readings along those shelf buoys Thompson discounts in his methodology section (along with the saturated Texas coast surface stations?) could be to blame for the ETA's delusion about a pool of 65F dewpoints off the coast?

 

[Robert Dewey]

Originally posted by Amos Magliocco
I am now wondering if perhaps the higher dewpoint readings along those shelf buoys Thompson discounts in his methodology section (along with the saturated Texas coast surface stations?) could be to blame for the ETA's delusion about a pool of 65F dewpoints off the coast?

This is quite possible...And once the ETA initializes the higher dewpoints/instability, which are in error...It basically creates a "chain reaction"... The excessive instability produces storms, which produces quite a bit of precip, which in turn adds more low level moisture, and which in turn cycles back into excessively high Td's...At least thats my theory.

Robert

 

[Bob Schafer]

I thought about starting a new thread for this question, but since it so directly applies to Saturday, I proceed:

Go to CoD's 6Z ETA run from today (4-15), and look at vert velocities at 700mb for 0Z Sunday.

What is that happening in W TX?

I don't know that I've seen (certainly didn't notice) such downward vert vel directly upstream of upward vert vel before.

My first thought was "derecho???", but Mike Geukes provided some info about very strong 500mb winds being a primary key parameter for derechoes, which isn't present in that area 0Z Sunday.

So what is up with that down vert vel?

Computer glitch?

scratching bald head........

Bob

 

[Robert Dewey]

Originally posted by Bob Schafer
My first thought was \"derecho???\", but Mike Geukes provided some info about very strong 500mb winds being a primary key parameter for derechoes, which isn't present in that area 0Z Sunday.
Bob

That is correct, derechoes tend to have 500mb winds in excess of 50knts from >260 degree angle (WNW). While a strong 500mb jet is nosing into the western TX region, it is from the SW...That and many other ingredients are missing, so no derecho.

Originally posted by Bob Schafer
So what is up with that down vert vel?

Computer glitch?

scratching bald head........

It appears that the strong positive VV's are in response to the cold front/dryline initiating convection (creating upward motion/positive VVs), while behind the deep convection, very strong subsidence is occuring (negative VVs/downward motion of air). This ETA has the right physics here, but whether or not this is what will really happen remains to be seen. This is just my theory, correct me if I'm wrong.

Robert

 

[Jeff Snyder]

Originally posted by rdewey+--><div class='quotetop'>QUOTE(rdewey)</div>

<!--QuoteBegin-Bob Schafer

My first thought was \"derecho???\", but Mike Geukes provided some info about very strong 500mb winds being a primary key parameter for derechoes, which isn't present in that area 0Z Sunday.
Bob

That is correct, derechoes tend to have 500mb winds in excess of 50knts from >260 degree angle (WNW). While a strong 500mb jet is nosing into the western TX region, it is from the SW...That and many other ingredients are missing, so no derecho.


Robert[/b]

I think it's important to be cautious how you characterize derechoes as occuring in environments with WNW 500mb flow... I seriously doubt the absolute direction has much to do with anything here. I think derechos tend to occur in WNW flow events not because there is something special about WNW winds, but because of boundary orientation. Consider how most derecho's occur in the warm season, and many during the summer months, when instability can be quite extreme. More often than not in derecho cases, a stationary boundary is oriented pretty close to E-W, making is nearly parallel to the 500mb flow. Depending upon the location of the instability axis and many other elements, this orientation allows the complex to ride right along the boundary, similar to good tornadic supercell situations... However, I think the key here is both the strong 500mb flow and the fact that it is parallel to the boundary. With that said, I think there is a very similar chance at having a derecho in SW flow IF the other parameters are in place (strong instability upstream, flow parallel to a stationary front, etc)... Granted, I don't think that's the situation for this weekend, but I just wanted to make that comment....

This is kind of similar to saying "supercells like southwest flow aloft". To this, I'd say "no". In the central US, southwest flow aloft can lead to a supportive synoptic environment (lee troughing, good directional shear, etc), but the fact that the 500mb is SW is not what makes a storm become supercellular. You can shift everything clockwise 90 degrees and have the same favorable environment for supercells (NW 500mb flow, SSW sfc winds, etc etc etc)... So again, it's not the direction of the flow itself that it important, but the fact that the direction of the mid-level flow TENDS (climatologically) to create a at least a semi-favorable synoptic environment...

 

[Scott A. Kampas]

Chris noted most of the things I was going to mention about moisture in another thread. The western GoM has recovered quite well, though most of the Gulf is still horrible (40s T/ds not uncommon). However, there is a nice Cu field going up I35 right now too. 50s T/ds have reached well into E KS. Sfc T/d and theta-e progs continue to look decent, and even depth of moisture looks good for this weekend as far north as the Upper Midwest.

A few more days of a strengthening fetch from an improving source region should bode quite well to raise T/ds and the depth of the moisture. You can see moist Pacific air streaming in aloft, as well, on vis and wv (though the GoM still looks sick). The big event may be the middle of next week, but I'm torn between chasing the S Plains Saturday and/or the Midwest on Sunday. Hopefully Saturday's stuff stays discrete whether or not I chase that day, so it doesn't interfere with the moisture streaming north. I'm relegated to weekends for long chases until school ends, and expect all hell to break loose during finals week as happened last year... so, I'm am ready to go.

I like those H850 winds: http://www.rap.ucar.edu/weather/model/disp...r48hr60hr72hr84

Scott

 

[Tony Laubach]

Car passed the inspection.. now just to figure out where to go...

Been eying the models, but haven't seen them since this morning to comment on.. I briefly went over the forum and caught myself up on the discussion.. finally see the forum split the South and North Plains possibilities.. I'm still thinking south.

I will take stock in mid to upper 50s dewpoints east of the dryline (23Z SFC OBS). Looks like the moisture may try and sneak its way in there by Saturday.. still a bit iffy on the timing of the system.. I can only squeeze one chase day over the weekend.. so gotta pick the best one! I like the chance of discrete storms, though.. would definitely make for easier chasing! Will stay tuned...

 

[Bob Schafer]

Guys, guys, guys....

Anyone have an answer to my question?

What's the deal with the pronounced downward vertical velocity in W Texas? :roll:

Bob

 

[Robert Dewey]

Originally posted by Bob Schafer
Guys, guys, guys....

Anyone have an answer to my question?

What's the deal with the pronounced downward vertical velocity in W Texas? :roll:

Bob

That is just strong subsidence...That usually occurs behind strong cold fronts or drylines...especially when there is convection ahead of them (all that upward motion for convection has to come down somewhere). It could also be enhanced by topography. This is just my theory however...

Hope that helps some, I did reply to this earlier...

Robert

 

[Jeff Snyder]

Oh for those who are interested, I pulled off some ETA Point Forecast Soundings for Saturday late afternoon while at work today... The thermodynamic environment, for the most part, is very nice...

[Broken External Image]:http://www.tornadocentral.com/end21z.gif
END 21Z

[Broken External Image]:http://www.tornadocentral.com/wcok22z.gif
West-central OK 22Z (Notice the lack of much CIN/cap)

[Broken External Image]:http://www.tornadocentral.com/ict23z.gif
ICT 23Z

For the most part, ~3000 CAPE, along with other parameters all appear to be quite nice!

{My apologies for the large size and thus any right-left 'scrolling'}

 

[mrobinson]

Originally posted by Jeff Snyder
Since I can't post images in the Target Area section (... why?...),

I believe pictures are allowed in Target Area and the inconsistancy in the way the rules read has already been pointed out. Its being looked at but in the meantime until Tim gets a chance to review it go ahead and post your images.

 

[Robert Dewey]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

Originally posted by rdewey@
<!--QuoteBegin-Bob Schafer

My first thought was \"derecho???\", but Mike Geukes provided some info about very strong 500mb winds being a primary key parameter for derechoes, which isn't present in that area 0Z Sunday.
Bob

That is correct, derechoes tend to have 500mb winds in excess of 50knts from >260 degree angle (WNW). While a strong 500mb jet is nosing into the western TX region, it is from the SW...That and many other ingredients are missing, so no derecho.


Robert

I think it's important to be cautious how you characterize derechoes as occuring in environments with WNW 500mb flow... I seriously doubt the absolute direction has much to do with anything here. I think derechos tend to occur in WNW flow events not because there is something special about WNW winds, but because of boundary orientation. Consider how most derecho's occur in the warm season, and many during the summer months, when instability can be quite extreme. More often than not in derecho cases, a stationary boundary is oriented pretty close to E-W, making is nearly parallel to the 500mb flow. Depending upon the location of the instability axis and many other elements, this orientation allows the complex to ride right along the boundary, similar to good tornadic supercell situations... However, I think the key here is both the strong 500mb flow and the fact that it is parallel to the boundary. With that said, I think there is a very similar chance at having a derecho in SW flow IF the other parameters are in place (strong instability upstream, flow parallel to a stationary front, etc)... Granted, I don't think that's the situation for this weekend, but I just wanted to make that comment....

This is kind of similar to saying "supercells like southwest flow aloft". To this, I'd say "no". In the central US, southwest flow aloft can lead to a supportive synoptic environment (lee troughing, good directional shear, etc), but the fact that the 500mb is SW is not what makes a storm become supercellular. You can shift everything clockwise 90 degrees and have the same favorable environment for supercells (NW 500mb flow, SSW sfc winds, etc etc etc)... So again, it's not the direction of the flow itself that it important, but the fact that the direction of the mid-level flow TENDS (climatologically) to create a at least a semi-favorable synoptic environment...[/b]

I was going by the standard derecho "checklist" - located at the NWS - http://www.crh.noaa.gov/techpapers/service...rc-dmgwnds.html. It has been noted, that this checklist is usually wrong. Just like the 50knt 500mb wind threshold that Mike noted...I believe that some conditions may compensate for others...Like extreme shear in the 0-3KM layer may override the fact that 500mb winds are only 35-45KNTS.

As far as the northwest flow aloft...I am not sure why that is in the criteria, but I believe it to be that most derechoes occur in the midwest during the mid summer "northwest flow" pattern...So as you said, typical climatology would suggest northwest winds aloft.

As for any derecho (or any MCS for that matter) chances this weekend...They look very slim, but I wouldn't rule it out just totally. ETA is showing the warm front (which could be considered a quasi-stationary boundary) across areas from southern MN eastward into MI with a parallel mid level flow, relatively unstable air along and south of it, as well as decent lifting. Only problem I see is that these conditions don't persist upstream for 250nm, because the jet structure changes. My bet is that storms would be organized into a "cluster" if anything, but with mesoscale evolution, who knows.

Robert