2/28/07 FCST: TX / OK / KS / NE

[Chris C Sanner]

The latest run is very interesting, there is little to no CIN from the red river northward by 00z. Soundings are pretty promising with backed winds especially ADM and PVJ. I was almost surely sitting this one out but if the tds get to 60 and if the cap does in fact erode, we could see some interesting storms for sure. The LCLs look at bit high AoA 1000m, but there are a number of factors that could make those lower. I guess its wait and see time, I'll be very interested to see if the SPC pulls back the slight risk further into Central OK now.

 

[Kenny Drake]

Taking a look at the 00Z model runs here is my analysis. Both GFS and NAM seem fairly consistent in the timing of the 500mb trough ejecting out into the plains. By 6pm Wednesday the trough is progged to be centered over NE Utah. Out ahead of this OK Mesonet readings are showing a frontal boundary draped across northern OK with Tds reaching the upper 40's across southern OK. What is interesting to note is a 54F Td reading at Ardmore (or near Ardmore). IMO, this may be instrumentation problems as all readings nearby are in the mid to upper 40's. 00Z soundings from OUN and FWD show signs of a developing LLJ (but still very weak.)

Both NAM and GFS are progging the dryline associated with the sfc low to be near or just west of I-35 by 6pm Wednesday. Given what happened last Friday, I am expecting this to be the farthest possible extent of the dryline before nightfall (dryline was progged to be across western OK last Fri by nightfall, ended up being about 60 miles west of there.) The first noticeable difference between the NAM and GFS is forecasted Tds. NAM is progging solid 60 Td's almost to the KS/OK border by 00Z. GFS is being a lot more conservative with the 60s holding to southern OK with mid 50s across the rest of the state. Forecast soundings for OUN show the potential for a clear air bust tomorrow. Both models are progging a relatively deep layer of dry air above 850mb. If the dryline ends up being slightly farther west than models are forecasting than this layer of dry air may not be as pronounced across central OK. Also interesting to note is the difference in the wind vectors between the two models. GFS is tending towards a much stronger wind speed solution between 850 and 700mb than the NAM. However, both models show supercell favorable dynamics. For this time of year I would actually prefer the weaker solution (with respects to wind speed) due to the weaker CAPE. NAM is forecasting an enhanced area of CAPE at 00Z from near Ardmore north through OKC and then stretching NNE towards Stillwater and over towards Tulsa. If the dryline ends up being slightly farther west than current model prognosis this area will more than likely shift westward slightly. The majority of the 850mb moisture looks to be well east of the forecasted dryline location, presenting the overall problem. Dry air at 850mb will make initiation difficult for an adiabatically rising parcel. If the air dries up too quickly with height than a rising parcel will never reach condensation before it loses all of its vertical momentum. Along with this LCLs may be too high for those looking for naders before dark.

Regardless, tomorrow looks promising for seeing some Cu billowing over Norman. The main event will surely be after dark, but the chance of at least some isolated cells before dark looks like a possibility. I have class all day until 5:15pm CST (Shapiro's handing back our dynamics 2 exams @4.) For me, the best situation will be getting out of class and seeing a cell right over Norman. Of course, then I just have to figure out how to bike back to my apartment.

 

[Bill Tabor]

The 00z NAM continues to indicate potential for supercell development tomorrow (and in central Oklahoma -- a close chase for me and many others). I won't rehash much, since the two primary issues remain (moisture depth/quality and initiation questions). However, I will say that, if the following verifies, I think we'll see a pretty good potential for a rare late-February tornado event in Oklahoma --> OKC SkewT forecast from NAM valid Wednesday evening. I'm still a little concerned about the moisture quality, since I think the 00z NAM initialized dewpoints a little too high in central Texas (the 55-60+ tds). In addition, the models do not initiate convective precip in the central Oklahoma, so I'm not getting too excited yet.

Jeff why does that skew T not match the forecast sounding off the Storm Machine?: http://weather.admin.niu.edu/machine/fcstsound.html

It shows cape 563 and cinh 76 at 0z on Mar 1.

 

[Jeff Snyder]

Jeff why does that skew T not match the forecast sounding off the Storm Machine?: http://weather.admin.niu.edu/machine/fcstsound.html

It shows cape 563 and cinh 76 at 0z on Mar 1.

It looks like the difference in the calculation of CAPE. It looks like Storm Machine uses 100mb mixed- or mean-layer CAPE, while it looks like wxcaster uses surface-based CAPE (i.e. tracing a surface parcel). The near-surface temperature profiles looks a little different on each site as well, with the wxcaster site showing nearly well-mixed from the sfc to ~825mb (with dry adiabatic temperature profile and constant-mixing-ratio moisture profile) and the Storm Machine site showing less steep near-surface lapse rates and shallower moisture.

EDIT: To me, it looks like the Storm Machine Skew-T only plots T and Td every 50mb, while the WxCaster Skew-T plots T and Td every 25mb.That said, the Td profile in the low-levels is much drier on the StormMachine skewt than on the wxcaster SkewT. For example, note how there is almost saturation at 825mb on wxcaster, while it is very much unsaturated on Storm Machine. If Wxcaster does indeed use more data (in the vertical), we could see significant differences, especially as in this case, where the moisture extends to 815-825mb, something that is not captured if Storm Machine only plots 850mb and 800mb data. Just a guess. If Storm Machine uses MLCAPE, such a difference could be very important, since it would mix in much drier air (between 825-800mb) in the calculation of the ML parcel. This would explain the lower CAPE and higher CINH (since it results in lower Theta-e of the updraft/parcel).

 

[Kevin Polston]

Guys....could it be that it is a different model? On the storm machine site it says it is the WRF.....while what Jeff posted was the NAM output. I looked at the NAM output on NSHARP and the profile is the same as off of the wxcaster site. A slight difference in CAPE calculations (~1700 off of wxcaster and 1543 off of NSHARP). I also agree that it looks like the storm machine output is not as high resolution as wxcaster or what I am looking at on NSHARP.

 

[Kenny Drake]

I was using StormMachine quite heavily last Friday and I noticed some really crazy CINH/CAPE numbers, such as something like 500/50 in an environment where those numbers should have been almost opposite (50 CIN, 500 CAPE). Is there possibily a third party forecast sounding site I can take a look at because if wxcaster's forecast sounding for KOKC @ 00Z verifies, WoW! The StormMachine sounding is far from impressive with much drier air near the LCL height from wxcaster. And of course, it depends on what type of CAPE each of the two sites are calculating... surface based and mean layer can make a huge difference in dry layers like Jeff mentioned.

Kevin: ETA/NAM should be the same model. I guess its possible that either site may have confused the names.

 

[Jeff Snyder]

Guys....could it be that it is a different model? On the storm machine site it says it is the WRF.....while what Jeff posted was the NAM output. I looked at the NAM output on NSHARP and the profile is the same as off of the wxcaster site. A slight difference in CAPE calculations (~1700 off of wxcaster and 1543 off of NSHARP). I also agree that it looks like the storm machine output is not as high resolution as wxcaster or what I am looking at on NSHARP.

The NAM used to run on the Eta, and it now runs on the WRF (i.e. the WRF model runs in the NAM timeslot, having replaced the Eta in that same timeslot last year). I assume any difference are merely typographical (i.e. the person who sets up the script to create the graphics hasn't changed it from Eta to NAM or WRF). There is a very, very outside chance that Storm Machine actually locally runs the Eta, but I can't imagine that'd be the case. So, as Kenny said, I think both sites are using the same model output.

EDIT: http://wwwt.emc.ncep.noaa.gov/mmb/mmbpll/cent4km/v2/ The latest 4.5km WRF with explicit convection shows a strong, isolated storm developing just northwest of OKC and moving northeastward at and after 00z (use Total column condensate product). That run also indicates that the storm is the only storm to develop and sustain itself (more than an hour) in Oklahoma tomorrow and tomorrow night (though convection does develop near 12z in extreme ne OK).

 

[Kevin Polston]

I've been looking at the model output this evening and I don't know...maybe it's just me....but I am impressed by the severe potential towards 06Z in southeast KS/southwest MO/and northeast OK. I agree that there could be some storms develop back towards central OK along the dryline before sunset. However...parameters really become focused (according to the NAM) over the se KS/ swMO/ ne OK areas by 06Z. The surface low has deepened to 986 near CNU by then....which has brought the 60 degree dewpoint line into far se KS and sw MO. I think the difference between the NAM and the GFS is that the NAM starts deepening the low a little sooner...allowing for the quicker return of moisture. The GFS has a 990 low near Tulsa at 06Z then deepens it to a 982 low between MKC and COU by 12Z. What impressed me however was the combination of parameters focusing in on this area. VGP values of .4 to .5, 0-1KM EHI of 2-3, LCL's below 1000m, (LFC's are also quite low....less than 2000m in se KS dropping to 800m in sw MO), 0-1km SRH is in the 300-350 range and the nose of the mid level jet - with 60kt winds - is punching into this area as well. So with the tightening dewpoint gradient along the dryline and the upper support it seems things look favorable for supercells and tornadoes into and possibly through the night. Looking at point forecast soundings....it is remarkable to see the changes in the profile over a few hours. The lapse rates steepen, the cap erodes, and the shear is impressive. Oh...also 0-3km CAPE values increase during the night...suggesting storms can get rooted close to the surface. So...that's what I am thinking. I am curious to hear other opinions on this.....as it is based off the NAM and maybe its not correct. Nevertheless...if it is close to accurate then it appears to me that some "stuff" is going to happen.

 

[Kevin Polston]

Here is a somwhat crude looking forecast sounding from EMC.....but it shows the same sounding as what was seen on wxcaster.

http://www.emc.ncep.noaa.gov/mmb/soundings.nam/stations/723570.html

The CAPE value is not as high as wxcaster (I've noticed over the years some of the values seem a little high) but it is ~1200 which is a far better number than 500! Also...not much CIN showing up....somaybe this can help validate the wxcaster model sounding. Besides....I always like to say choose the model which shows the better numbers!

Here is one other site you can check out. Unfortunately it looks like the page is slow in updating. Check out http://www.nssl.noaa.gov/wrf/ and then click on the OUN and HUN sounding areas. You may already know about this site in which case just disregard this.

 

[Jeff Snyder]

One last tidbit before I go to bed...

I tried to do a similar crude dewpoint analysis as was done by Jeffrey Miller before the 2-23 event. As you can see, moisture recovery is much further ahead currently than with the previous event.

0640z 2/28 Td analysis
0440z 2/23 Td analysis

Note that is doesn't speak to the depth of the moisture, however! An 850mb Td analysis would show considerably more dry air compared to the last event. Fortunately, 850mb is quite a ways higher off the ground in central and eastern OK than it is in the western TX panhandle, so a 1 to 1 comparison may be slightly misleading.

 

[Brandon Lawson]

What is interesting to note is a 54F Td reading at Ardmore (or near Ardmore). IMO, this may be instrumentation problems

It might not be an instrumentation problem. There is a narrow band of upper fifties dewpoints streaming northward out of Texas. However if you want to look at instrumentation problems, check out the current winds observed at Clovis Cannon AFB, New Mexico. 253 mph!!! LOL
http://www.wunderground.com/cgi-bin/findweather/getForecast?query=cvs

 

[Jim Bishop]

What a very delicate situation. The exact timing of the upper system in conjuction with the right mesoscale forcing along the dryline in North Central Oklahoma could allow for convective initiation before 00z, but it's iffy with some problems...

First I wanted to point out the 12z WRF. It has the left exit region of the 250mb jet streak nosing its way into Northwestern Oklahoma and Southwest/South Central Kansas at 00z. Also, weak vorticity advection should be occuring at that time over that same region. Based on that I'd say convection has a decent shot at firing at or just before 00z.

The problem is the 12 RUC is slower with the timing of the upper jet streak. However it fires convection by 00z. I'm pretty skeptical about that convection...

850mb moisture will be a problem. 12z 850mb chart shows the air at that level over Oklahoma coming from the Texas Panhandle - dry! By 18z we're advecting air from Midland - dry! By 21z...ok now 850mb backs to the south-southwest in advance of the upper trough...we're advecting in decent 850mb moisture. It's rather shallow, but not too bad, although it will be in a narrow corridor. The question is how mixed out that moisture becomes? Considering the southwest flow over Western North Texas and Southwest Oklahoma, probably pretty mixed out initially. But moisture should be much better after dark. Surface moisture, on the other hand, look fine with upper 50s to even low 60s probably by 00z.

So initiation seems possible, but not very likely, between 22z & 00z over North Central Oklahoma. But the moisture won't be very deep and likely very mixed out. Therefore storm mode wouild be LP supercells with a low tornado threat.(The northeast/southwest orientation of the moist line [not true dyline, just northern extent of moisture]over North Central Oklahoma does not bode well for supercell mode, btw)

However within a couple hours after 00z the moisture depth should increase and the really good upper forcing should be in place. Thus initiation is likely after 00z. Plus the 850mb jet should kick in, increasing the low level shear - tornado threat will increase over Eastern Oklahoma/Southeast Kansas.

If I lived in Central Oklahoma I'd be out for this one to maybe see a lp supercell. But for anyone who has to travel a great distance it seems like a waste. Just my opinion anyways...

 

[Jeff Snyder]

12z 850mb chart shows the air at that level over Oklahoma coming from the Texas Panhandle - dry!

Worthy to note... The 12z FWD sounding shows the top of the appreciable moisture just below the 850mb surface (likely near ~880mb), with what looks like to be ~90mb deep moisture, which isn't too bad. The 12z OUN sounding is more ankle-deep in regards to moisture, likely owing to the WSW winds just above 850mb that are acting to usher in very dry air aloft.

This complicates things, for me at least. I like the area in northcentral Oklahoma, but I have doubts that the ~850mb flow will back enough to advect northward the better moisture. Near I40 and south, we're much closer to the 80-90mb deep moisture, which gives that area a better chance of seeing an acceptable depth of moisure come late afternoon. In all honesty, it doesn't look like a bad idea to stick it out in OUN until we have a better idea of the moisture come afternoon. The latest 12z NAM shows backing in the 850mb between 18z to 00z (westerly to south-southwesterly), which I think will be required if we hope to see initiation around the area. It sure would (will?) be nice to see OUN and FWD 18z soundings...

 

[Donald Giuliano]

I usually completely ignore situations where there is a 35-40kt west wind at 500m AGL in Purcell, as there is this morning. Typically these situations lead to intense capping, but in February we might be able to get away with it. Of course the key is that, as others have mentioned, the 850mb winds must back around to the south during the afternoon, otherwise forget it. I am also concerned about the very deep nature of the boundary layer on the 12z NAM, this could indicate that the BL will be mixed out, leading to very high LCLs along the dryline, and even more critically, mixing out the BL shear.

However, I've seen plenty of cases where the NAM appropriately mixed out the BL shear and moisture, only to see both rapidly increase as the sun goes down and moisture advection continues (last Friday, for example). So while I am not particularly fond of seeing dry adiabatic lapse rates to 700mb in Norman at 00z, I don't think we can rule out some activity if we can get storms to fire. This last bit in my opinion is the least of our concerns, as the dryline is forecast to be very strong by the NAM, with strong vertical motion along it.

 

[Tim Marshall]

TARGET: OKLAHOMA CITY TIME OF DEPARTURE: NOON Got the afternoon off work. Weather situation is coming together for severe storms this afternoon across NC-NE Oklahoma. PROS: I like the bombing surface low forecasted by the RUC to form in western Oklahoma, excellent jet aloft, and the dryline to sharpen up along I-35 later this afternoon. Low level moisture is better than the 24th but we are at lower elevations here. Strato-cu just broke here in Dallas at 11 AM, yeah. CONS: Elevated mixed layer has capped the region --for now. Looks like convection is more likely in northern OK than southern. I am also concerned about seeding storms along the cold front in NE OK. Hope to pick the tail end charlie storm at OKC and follow it up I-44 to Tulsa. TIM M.