FORECASTS: 1/28/10 - Southern/Central Plains Winter Storm

[John Massura]

The OK mesonet sensors south of I-40 and western okla are now going off line. Power?

Pretty sure that the sensors are all frozen over with ice. Fairly common situation.

 

[Tim Vasquez]

(OKC-centric warning)

Even though it's a winter storm, radar velocity can be quite useful... for example:

Here this tells us that the polar air mass is pretty homogeneous up through 1200 ft AGL with winds straight northeast, then it turns more easterly and increases to 45 kt at 1900 ft AGL. Above 8000-9000 ft AGL they transition from southeast to southwest. For the noobs, the way you can tell wind direction is that anyplace the winds appear "calm" on this product (on a day like this) it's because the wind is blowing perpendicular to the radar. By following the zero line away from the radar (i.e. with increasing height) you can figure wind direction.

So it's safe to say we have about 1200' of pure cold air before hitting the transition zone. The general rule of thumb for discriminating ZR vs. IP is that 800 ft of surface-based cold air depth is needed for IP. We're well above that.

This compares well with the 1200Z morning sounding for Norman which showed a 1600 ft depth for the cold air mass. In spite of these depths, we indeed did have ZR this morning (saw it myself), and presumably that's because the precip layer is very warm (5-7 deg C) and the drops are taking longer than usual to freeze. I'm not sure when we will get a changeover from ZR to IP but it ought to be pretty soon unless there's a detail I'm missing -- or if the rain drops have enough sensible heat to resist freezing (maybe the models will help here).

It would be great to get an 18Z launch. This stuff is fascinating.

OG&E not showing any outages yet except down in Healdton (sorry, Shane!) I hope the grid can hold up today... I'm not looking forward to an evening of candles and blankets.

Tim

 

[Chris Vagasky]

Tim,

OUN released a balloon around 15z, and I have heard they are doing hourly launches today.

 

[Mike Hollingshead]

It would be great to get an 18Z launch. This stuff is fascinating.


Tim

OUN

[FONT=lucida sans typewriter, lucida console, courier]WE ARE PLANNING TO LAUNCH 3-HOURLY SOUNDINGS TODAY TO ASSESS THE
THERMODYNAMICS OF THE AIRMASS. PARTIAL DATA FROM THE 15Z SOUNDING
HAS ALREADY BEEN SENT. .26.
[/FONT]

 

[Charles Kuster]

This compares well with the 1200Z morning sounding for Norman which showed a 1600 ft depth for the cold air mass. In spite of these depths, we indeed did have ZR this morning (saw it myself), and presumably that's because the precip layer is very warm (5-7 deg C) and the drops are taking longer than usual to freeze.

I completely agree Tim! Looking at some of the NAM forecast soundings for Norman, the cold layer (i.e. below freezing) generally extends up to about 900mb and slowly gets higher throughout the day. The warm layer (i.e. above freezing) stays quite warm (~5C at 850mb) through 0Z and is generally about 150mb thick. I would generally expect a mix of freezing rain and sleet, based on these conditions with sleet becoming more and more prevalent as the day goes on. Stay safe out there!

Here is the 21Z forecast sounding: http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=009&STATIONID=koun

Edit: I just got back in from another hourly check on conditions. There is more sleet now then there was at 10:00am CST, but it is still mostly freezing rain (80/20). Ice accumulations are about .1" on the trees now.

 

[Tim Vasquez]

Thanks Chris & Mike! Went ahead and got the 15Z sounding:

I was surprised to see that the TLX velocity was dead on for assessing layer thickness. That does it... I'm setting up a 50-frame velocity loop on GRLevel3 to follow the trends in cold air depth. (EDIT: make that 20 frames... it doesn't allow more than that) Looking at this sounding it's a wonder that anything is hitting the ground as liquid.

Tim

 

[Andrew Francis]

The two precip shields have filled in nicely. This looks like a possible catastrophic ice storm for OK. Still getting light FRZA here in Edmond, I occasionally see a few ice pellets mixed in, but it is mainly rain. Alot of puddles on the roads. The ground temps are still allowing for not much freezing. But you can see the glaze on all other surfaces.

I could even see a nice dumping of snow here in Central OK on the wrap around late tonight into tomorow morning.

 

[Jay McCoy]

Amarillo is getting pounded right now. Almost whiteout conditions at times and thundersnow. I-40 is about to be shut down from both state lines. Multiple wrecks on I-40 just west of the city. So far 4-5" in the city and its really just getting started. My Emergecny Service group has just been fully paged out so time to go play in the snow.

Here is a link to our local TxDot cameras for a personal viewing..lol

http://amaits.dot.state.tx.us/AMA-ITS/default.htm

 

[Tim Vasquez]

For OKC I noticed the 15Z RUC actually warms central Oklahoma surface temperatures by a couple of degrees. On its precip type forecast it has IP at 18-21Z and converts it over to ZR at 00Z. I hope this eroding/thinning scenario does not happen... that would be bad news indeed.

Tim

 

[Tim Vasquez]

Looking at the RUC some more, it appears that the 15Z RUC is already out to lunch on the surface fields in Texas.... it appears the low in the Abilene-Eastland area is outrunning the models or further east than forecast...... which would be good for OKC as far as getting the CAA going again.

I don't envy the OUN office... with this wx situation you come away with more questions than you start with.

Tim

 

[Tim Vasquez]

Just some data here to highlight the model problems.

I went ahead and hand-analyzed the 18Z data as shown to get a handle on the actual observed mass fields and boundaries. You can see the fronts, the baroclinic low near Eastland/Cisco, and an inverted trough. I'm not sure what the cause of the trough is.... the most likely possibility is it represents the axis of deepest elevated warm air aloft (favoring freezing rain) or it could signify some other discontinuity within the polar air layer itself. I really don't know.

Comparison of 15Z RUC for 18Z. It's clear that the RUC does not have a good grasp on this system, including the basic composition of the MCS (granted these are 3-hour precip totals but they are still too far west).

And the 18Z OUN sounding just came out, indicating a slight deepening of the cold air from 15Z. Temperature is as low as 21F at 1600 ft AGL, but outside we are getting ZR- to ZR--.

Tim

 

[Mike Hollingshead]

http://radar.weather.gov/ridge/radar.php?rid=tlx&product=N0R&overlay=11101111&loop=no

Click on storm total.

If I was in that general area, I'd be sure to check out that max once the roads are a little better. That area has to have gotten slammed given as much of it has been reported as freezing rain. I noticed Hobart report a 44 knot gust earlier too, though that was a bit above everyone else...but I don't know their elevation or anything either.

http://www.spc.ncep.noaa.gov/exper/mesoanalysis/s3/850mb.gif

That is quite the waa pump se 60 knots, e 50 knots.

 

[Greg Blumberg]

Not sure if anyone has mentioned this yet...but there's a Tornado Warning down in Texas (expired now) west of Stephenville. GR2AE has pegged some nice TVS signatures regarding the storm. The velocity data is nothing like I've ever seen before in my life. Just east of Eastland the velocity is spazzing out along the convergence zone of the squall line. Check it out from the KDYX radar.

Any ideas for what's going on?

 

[John Farley]

Several have commented (some on the REPORTS thread) about the dry-slotting coming into the AMA area. That is true, but if you look at the national radar loop you can also see that there is a lot of precipitation out to the southwest associated with the trailing disturbances. This covers a good deal of NM and even well down into Mexico, so I would think that as this progresses eastward/northeastward that with time some of that dry slotting will fill in again. Maybe I'm missing something, but I would not call this storm over just yet for the panhandle.

 

[Tim Vasquez]

Beware of the bright banding... melting level is at 6500-8000 ft MSL and this is distorting some of the TLX and FDR radar products.

Tim