Texas Winter 2011-2012...

[Ntxw]

18z NAM looks to be coming in slightly colder so far through 24hrs.

Soundings from it is saturated below freezing except near the surface at 24hrs. Dynamic cooling would allow a drop from 40 by a few degrees. If it can happen at night maybe that will help!

[iorange55]

Soundings from it is saturated below freezing except near the surface at 24hrs. Dynamic cooling would allow a drop from 40 by a few degrees. If it can happen at night maybe that will help!

It's looking like it'll probably be a night/early morning event! The 18z NAM is looking a little better to me. That ULL looks quite strong on this run in the 500mb levels.

Just needs to move a little further south!

[Ntxw]

NAM goes haywire with this ULL. I wonder what kind of surprises it has in store and why it's doing it, cut off lows yay

[gpsnowman]

Quick question. In order for the North Texas area to get maximum precip with one of these ULL's, where does the dead center of the ULL need to be located? Hope it makes sense.

[iorange55]

Quick question. In order for the North Texas area to get maximum precip with one of these ULL's, where does the dead center of the ULL need to be located? Hope it makes sense.

I am not great at explaining things, but I mean your normally want to be just to the southeast of the ULL, to reap it's full benefits moisture wise. The moisture kind of feeds into it and usually there is some kind of wrap around moisture too on the backside. I hope I got that right...

Btw, the RR model goes up to 18hrs and at hour 18 it shows A LOT of moisture all the way back into new mexico.

[gpsnowman]

Quick question. In order for the North Texas area to get maximum precip with one of these ULL's, where does the dead center of the ULL need to be located? Hope it makes sense.

I am not great at explaining things, but I mean your normally want to be just to the southeast of the ULL, to reap it's full benefits moisture wise. The moisture kind of feeds into it and usually there is some kind of wrap around moisture too on the backside. I hope I got that right...

Btw, the RR model goes up to 18hrs and at hour 18 it shows A LOT of moisture all the way back into new mexico.

Dude, you explain things rather well. As do many others on here. Thanks for the info.

[iorange55]

Looking out west where the storm is creating snow right now. El Paso is under a winter storm warning, they're expecting some heavy snow out there. NWS out there points out how strong this ULL is.

I'd cross your fingers for some surprises.

Dallas/ Fort Worth NWS put out this...

...ACCUMULATING SNOW POSSIBLE ON CHRISTMAS EVE...

AN UPPER LEVEL LOW PRESSURE SYSTEM WILL MOVE ACROSS TEXAS THIS
WEEKEND AND WILL BRING WIDESPREAD PRECIPITATION. FOR MUCH OF THE
AREA...THIS PRECIPITATION WILL FALL AS RAIN. AREAS TO THE WEST AND
NORTH OF A LINE FROM GAINESVILLE..TO WEATHERFORD...TO
LAMPASAS...WILL SEE THE POTENTIAL FOR SNOWFALL ON SATURDAY.

TOTAL SNOWFALL ACCUMULATIONS ARE EXPECTED TO BE LESS THAN AN
INCH...WITH MOST ACCUMULATIONS OCCURRING ON GRASSY OR ELEVATED
SURFACES. TRAVEL IMPACTS WILL LIKELY BE LIMITED TO INTERSTATE
20...MAINLY WEST OF U.S. 287. BRIDGES AND OVERPASSES WILL BE THE
FIRST TO BECOME SLICK.

ELSEWHERE...RAIN COULD BECOME MIXED WITH SNOW DURING THE EVENING
HOURS ON SATURDAY. HOWEVER...SURFACE TEMPERATURES ARE EXPECTED TO
REMAIN ABOVE FREEZING...SO ACCUMULATIONS ARE NOT ANTICIPATED.

EDIT: 18Z GFS has the ULL moving further south and tries to build back some moisture as it passes over, similar to the NAM. Something to look out for...

[iorange55]

Well, folks right now it doesn't look as if a surprise will happen. But, if it does happen, I might look for it to happen after Christmas on Monday when the system passes us.

The models have been trending further south and they have been trying to back build some moisture. If the trend continues, then we could get a surprise one day too late.

I'm just not sure if we'll see anything more than a few flakes tomorrow night/Christmas morning.

[TrekkerCC]

I just want to make a quick post; I can certainly make a more detailed post later if anyone wishes for me to elaborate. Actual temperatures were one two degree lower than NWS forecast high temperatures today at the major airport sites in Dallas Ft. Worth area. Not too terribly off, but off on the warm side. I compared NAM/GFS forecast soundings from 12z for 00z verification time with the actual FWD sounding. Generally, both GFS and NAM did ok forecasting temperature; but GFS and NAM had too warm low level temperatures (from 925mb up to 850mb). I thought the GFS had a better handle on temperatures of the atmospheric column (disregarding the too warm low level temperatures) than the NAM did. See here (University of Wyoming Upper Air Sounding website) for more details: http://weather.uwyo.edu/upperair/sounding.html .

While not strictly a forecast post - I think it is a good practice anyway:

The above post is NOT an official forecast and should not be used as such. It is just the opinion of the poster and may or may not be backed by sound meteorological data. It is NOT endorsed by any professional institution including storm2k.org. For official information, please refer to NWS products.

edit: To clarify my second sentence

[iorange55]

That is good information TrekkerCC! Kind of tells you how the models are handling this system.

Some of the short range models seem to disagree quite a bit on when the precip arrives.

[GRAYSONCO.WX]

That is good information TrekkerCC! Kind of tells you how the models are handling this system.

Some of the short range models seem to disagree quite a bit on when the precip arrives.

What exactly does does TrekkerCC's post mean in regards to tomorrow's precip type?

[Longhornmaniac8]

That is good information TrekkerCC! Kind of tells you how the models are handling this system.

Some of the short range models seem to disagree quite a bit on when the precip arrives.

What exactly does does TrekkerCC's post mean in regards to tomorrow's precip type?

I'm not an expert by any stretch, but the way I interpreted it is that it's going to be really close for the Metroplex folks.

The chart that wxman57 posted earlier showed that the temperatures in the air column were right on the borderline for rain vs. snow, especially at lower altitudes. If the models were off on the warm side, it could mean that it actually will be cold enough for snow.

[iorange55]

That is good information TrekkerCC! Kind of tells you how the models are handling this system.

Some of the short range models seem to disagree quite a bit on when the precip arrives.

What exactly does does TrekkerCC's post mean in regards to tomorrow's precip type?

Well, nothing major. However, it could mean that it might be a little bit colder tomorrow. Will they be off enough to produce some snow? Probably not.

You never know, though. Just have to watch temps/dew points tomorrow morning. See if any cooling happens thanks to any evaporation we might have, before everything moistens up,

[GRAYSONCO.WX]

Thank you

[TrekkerCC]

What exactly does does TrekkerCC's post mean in regards to tomorrow's precip type?

Well, I'm just noting trends at the moment; through, it can have an effect for the future. Models have errors in them; some of them are inherent to the model themselves - the way a model solves the complex equations to generate a guess on how the atmosphere will evolve, chaos in a non-linear atmosphere will intrude, and the models don't have observations for every point in the atmosphere and has to "guess" the atmospheric state, especially upper-air, where the observations are spaced out by in some cases hundreds or thousands of miles. It is good to note if the models are handling track of the upper low (given it might change how much precipitation we will get and how much dynamic cooling occurs) and other data, since the models might be error prone due to all these reasons. If the forecasted temperature is too warm, then it might mean something in marginal events.

Short answer: If the models are forecasting too warm temperatures during the event, the rain will transition to snow quicker than projected and it might totally change over in places (such as DFW metroplex) that are currently forecast to have a rain/snow mix. Usually, very slight changes in atmospheric conditions can be the difference between a snow or rain event. Right now, the models from 12 hours ago slightly over-estimated the low level temperatures observed, if this continues, it might mean that the atmosphere might change over to snow quicker than what the models are forecasting. Now, like iorange55 said, it might not make a difference, but in these marginal events, even slight differences in the atmosphere can cause a event to go from rain to snow.

But to illustrate how critical the factors in marginal events can be: In 2007 or 2008, we had an ice event, where Ft. Worth got ice accumulations of 1/4"+ (ice storm criteria) from heavy freezing rain, but in Dallas, the heavy precipitation brought down the warmth from above to prevent surface temperatures from getting below freezing until the end of the event - in Dallas, we ended up with light ice accumulations. The models didn't see the full extent of the warmer air being brought down by downdrafts during heavy precipitation.

edit: slight clarification

[GRAYSONCO.WX]

What exactly does does TrekkerCC's post mean in regards to tomorrow's precip type?

Well, I'm just noting trends at the moment; through, it can have an effect for the future. Models have errors in them; some of them are inherent to the model themselves - the way a model solves the complex equations to generate a guess on how the atmosphere will evolve, chaos in a non-linear atmosphere will intrude, and the models don't have observations for every point in the atmosphere and has to "guess" the atmospheric state, especially upper-air, where the observations are spaced out by in some cases hundreds or thousands of miles. It is good to note if the models are handling track of the upper low (given it might change how much precipitation we will get and how much dynamic cooling occurs) and other data, since the models might be error prone due to all these reasons. If the forecasted temperature is too warm, then it might mean something in marginal events.

Short answer: If the models are forecasting too warm temperatures during the event, the rain will transition to snow quicker than projected and it might totally change over in places (such as DFW metroplex) that are currently forecast to have a rain/snow mix. Usually, very slight changes in atmospheric conditions can be the difference between a snow or rain event. Right now, the models from 12 hours ago slightly over-estimated the low level temperatures observed, if this continues, it might mean that the atmosphere might change over to snow quicker than what the models are forecasting. Now, like iorange55 said, it might not make a difference, but in these marginal events, even slight differences in the atmosphere can cause a event to go from rain to snow.

But to illustrate how critical the factors in marginal events can be: In 2007 or 2008, we had an ice event, where Ft. Worth got ice accumulations of 1/4"+ (ice storm criteria) from heavy freezing rain, but in Dallas, the heavy precipitation brought down the warmth from above to prevent surface temperatures from getting below freezing until the end of the event - in Dallas, we ended up with light ice accumulations. The models didn't see the full extent of the warmer air being brought down by downdrafts during heavy precipitation.

edit: slight clarification

Thank you so much for the detailed discussion!

[TrekkerCC]

Thank you so much for the detailed discussion!

You're welcome. BTW, I have nailed down the date for the ice event; it occurred during January 2007. NWS Ft. Worth actually wrote up a great description of this event in their Thunderbolt newsletter. It shows how shallow some of these arctic air intrusions that we periodically get in North Texas are: http://www.srh.noaa.gov/images/fwd/pdf/Spring2007.pdf .

[SouthernMet]

18z NAM 18 hours out : http://raleighwx.americanwx.com/models/ ... nty018.gif

[Ntxw]

This thing just does what it wants. The reason snow for DFW chances have lessened thus far by the models is because the ULL the past day or so has trended into a tightly wounded stalled system once it curves into NW TX. There was NO guidance or forecasters who could've said this would happen just two days ago (it was supposed to be near the atlantic coast). If that is the case warm air will be pumped ahead of the system aloft which decreases the chances at least until the strengthening vorticity passes by which who knows when at this rate lol. Our best chance is when the ULL passes by overhead if it ever does.

[iorange55]

I'd still watch out for something late sunday or into monday. I know the backside is pretty dry right now, but like I have said the models have been hinting at more moisture, perhaps. It is a long shot, but anything can happen with these strong systems.

EDIT: ^^^ I completely agree with you NTXW. I'm starting to think we might get something when it (finally) does decide to go over head. The models didn't expect what it's doing now, so why not? Plus, they have been ever so slightly trending that way.