rsdoug1981 wrote:It will all depend on the critical thickness values. Watch for that magical 540 line. Surface freezing temps don't matter for snow to fall...just for snow to stick.
OK...guess it's that time now...to start chiming in on this...
Ihave to disagree with you here. The "magical 540 line" has absolutely NOTHING to do with whether you will get snow or not....neither do ANY thickness values. This is something we discussed last year and I am seeing some make the same mistake again this year.
Thickness values are just that...THICKNESS of an air layer. If you have a really deep 500mb low and a decent high at the sfc...you will have thickness values below 540. However, if your sfc temps are above freezing...and if that freezing layer is greater than 1500 feet thick or if there is another layer aloft that is above freezing...it really doesn't matter what your thickness values are. You could have 534 thickness values and if you have above freezing sfc temps or a warm layer aloft that is greater than 1000'-1500' thick...you will get a cold rain, freezing rain or sleet.
Thickness values are a first look ONLY. When you see the thickness values getting lower...that is your first clue to start looking elsewhere. Most mets...once they see the thickness values getting close...realize that the atmosphere may be getting cold enough for winter precip...then they won't look at thickness again...except at a glance. They look at temps...AND the availability of moisture.
In this case...the 12z run shows (if the 12Z GFS was GOSPEL...which it isn't) it is above freezing over SETX from the sfc to 2000'. There is also a warm layer (above freezing) between 3000' and 8000' on Christmas eve morning at 12Z.
On the morning of the 25th, there is still a warm later aloft...down to 1500' thick and it is still above freezing at the sfc...which is probably wrong since there is no moisture at all and radiational cooling will set in somewhat.
By 18Z...the thickness values are about 534...and the warm slot is still there at 4000'...and temps are above freezing.
The chances for any winter type precip in SETX look very slim...not because of the lack of cold air...but moisture. This is a typical cold shot for us...which has cold/dry air with it.
One more thing to add about the 540 line. The 540 line is only and indication for snow in the plains. IN the Rockies...the value is closer to 552. It changes depending on where you are. IN the Pacific NW...the values you look for are closer to 530...or lower. So remember...it is just a tool to give you a heads up...once you get your heads up...move away from it and start looking at the actual temps in the air column...because THAT is what will determine snow/sleet/freezing rain or just plain old rain....not the 540 line.
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
