Winter Storm 2011

...Significant winter weather event on the horizon for Sunday into the early part of Monday...A Winter Storm Warning has been issued from 9 AM Sunday to 6 AM Monday morning...

On water vapor imagery as of about 5:45 PM, you can see the upper level low pushing across New Mexico and into western Texas already streaming moisture across southern Texas and southern Louisiana, and a surface low appears to have already developed across northern Mexico in response to this. This surface low will be the major player in our weather the remainder of this weekend as it tracks along the extreme northern Gulf of Mexico very near to the Gulf coast.

Lets just dive right into the models now. First off, this is a very difficult forecast to make and here's the reason...if models are off by just a few degrees, it will make a ton of difference. It seems to me that right now the best chance for snow here will be during the early afternoon hours of Sunday. The reason I say this is because it looks like we will start out with an area of the atmosphere that is warm and also quite dry so effects of evaporation and melting should act to cool this layer. The "1/3 rule," would suggest that effects from evaporation alone would be enough to cool this, however it looks like warm air advecting into this layer may be too strong to put the entire column at or below freezing...however there looks to be a period of a few hours during the early afternoon that it may cool enough for us to see mostly snow here. This is where the forecast really gets kinda tricky because the column gets so close to freezing, so I think snow/sleet mix is the best call during this time. By later in the afternoon/early evening, we should make a switch over to our icy scenario which will likely start as a freezing rain/sleet mix. We then start to lose moisture in the snow growth region by late evening and the melting layer becomes very pronounced at 4 degrees C and up. Surface temperatures are still forecast to be below freezing during this time so it looks like we will switch over to pretty much all freezing rain by late tomorrow night and looks like it will continue through the early part of Monday. This is all based off the NAM model...if you go by the GFS model then the solution appears much more simple...all freezing rain/sleet with a switchover to rain early Monday. I personally think it be mostly ice here with very little snow. Now...one problem I see is the spread in forecast surface temperatures. MOS ranges from 34 to 37 for highs while raw guidance is near or just below freezing...if surface temps get above freezing then there will likely be rain mixing in as well (WHAT A MESS!!). However it's kinda foolish to lean too much towards MOS numbers for this situation because this is one event where it will really struggle, and the fact is, the overall trend with this has been cooler, so I personally don't think we will get above freezing especially if clouds begin building in early enough. Now...another problem I see with this is going to be the winds. A tightening pressure gradient in response to the approaching surface low will make for some pretty strong winds between about 15 and 20 knots, so if this is mostly an icing event, that could be a real problem as it will make it easy to bring down limbs and could cause some serious problems with power outages. A bit of good news, though, is that Monday should get us back above freezing and whatever ice accumulations there are should melt off.

Bottom line...clouds should start to build in overnight tonight, then by early afternoon tomorrow, we will likely start to see a sleet/snow mix. Later in the day, we should begin to switch over towards more of a sleet/freezing rain mix, then turn to primarily freezing rain overnight and into early Monday morning...accumulations of ice possibly up to a half inch...breezy conditions could put wind chills down into the teens...The further north you go, the more likely you will see mostly all snow, the further south you go, the more likely you are to see more rain...