...WINTER STORM WATCH IN EFFECT FROM THURSDAY AFTERNOON THROUGH THURSDAY EVENING...
Complications in this forecast…how warm do we actually get at the surface tomorrow? I’ll try to address that right off…MOS shows high temps reaching well above freezing tomorrow afternoon with NAM at 35 and climbing to near 40 overnight, and the GFS at 40. Now…as was the case with the last system, it’s kinda foolish to rely too much on MOS for a situation like this, but I like to go ahead and look at it anyway because at the same time, it would be kind of foolish to NOT look at it. Raw guidance is where I will base most of the forecast on once again though. Raw NAM keeps us at or just below freezing all day Thursday and warms us overnight and puts us back above freezing by early Friday morning. Raw GFS has a slightly warmer look and would imply more of a mix of sleet/freezing rain/rain as it kinda bounces around the freezing mark all day, whereas NAM would imply more freezing rain/sleet all day tomorrow and changing to rain overnight Thursday. I personally don’t really see any reason for it to warm much tomorrow with the lack of insolation and any kind of warm air advection at the surface. With that said…my own personal thoughts are that we will be dealing with primarily a freezing rain/sleet event here in Meridian for most of the day tomorrow then gradually mixing in rain overnight and eventually switching over to all rain early Friday morning. However…if I’m wrong, and we do warm above freezing tomorrow, then it will likely be mostly a very cold rain. Either way, though, it looks like it will be a very nasty next few days and potentially hazardous to people on the roads.
So…when will this happen? Models are in decent agreement on this part and it looks like precip could be falling as early as early afternoon, however there is a bit of a dry layer aloft that may delay the precip somewhat.
So…what I’m thinking…tonight we will stay mostly cloudy…lows will drop into the mid-upper 20s (~26) and temps should remain fairly steady around freezing throughout the day Thursday (I personally don’t think we will get above freezing until late tomorrow night). Precipitation will continue across the southeast during the morning hours and will gradually spread northward affecting us here possibly as early as early afternoon…starting as freezing rain/sleet more than likely and unless we do get above freezing, which I don’t think we will, it will remain as freezing rain/sleet throughout the day. We will gradually warm above freezing into overnight Thursday and gradually switchover to all rain that will linger into Friday night. I don’t see ANY snow for us here during this event. Again, this is a very fragile forecast and any small error in the warmer direction in temperature prediction would mean a big difference in precipitation type…basically…if we get above freezing tomorrow, we will likely be dealing with mostly a rain event…
I may update this a bit if 0z model data looks any different...
Wednesday, February 2, 2011
Saturday, January 8, 2011
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...
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...
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