February 3, 2011

Another Winter Storm Coming Very Soon

Filed under: Mid-latitude cyclones — Erin @ 10:16 am

Here in Mississippi, we are on the verge of getting yet another round of wintry weather. We missed the worst parts of the Groundhog Day Blizzard and even the severe weather was, for the most part, a whimper rather than a roar. However, our luck is about to run out. A system from the Gulf of Mexico is entering the area, and after vacillating for days, the forecasters (both human and computer) have reached a certain amount of consensus about what type of precipitation it will produce and for where.

The writing was actually on the wall last night, in my opinion, when both the 00Z NAM and the 00Z GFS came in with very similar QPF forecasts, both in amount, in type, and in geographic location. What they predicted was not good: up to 3/4 inch of freezing rain, based on the soundings, plus snow, sleet, and rain mixed in (what they call a “wintry mix”). I am not completely sold on 3/4 of an inch of freezing rain now, because it is a bit colder today than expected. This observation ties in with my general philosophy for using the computer models. I don’t pay that much attention to what the big flagship models say for short-term forecasts less than a day out, because they do not accommodate actual observations in their forecasts. Models exist that do, and if you are going to use any models for “nowcasting,” these should be the ones rather than models like the GFS and NAM. So whatever those two are saying today, I’m not paying attention to it. Instead, I am looking at what is happening, and what is happening is that there is more precipitation showing up on radar than the models “believed,” and the temperatures are colder. Not noticeably colder, mind, but just cold enough that it could, I hope, mitigate the amount of freezing rain by changing over to sleet and snow instead.

I think that areas north of I-20 are all going to get something that is frozen. Unlike the last snow event that we had in the South, the one of January 9-11, I do not expect that north Mississippi—the areas that are in the Memphis National Weather Service office’s region—will hit the jackpot. Instead, all indications are that QPF and therefore accumulated precipitation will drop off sharply north of Highway 82 (perhaps Highway 78). The belt of maximum precipitation with this system should be a bit farther south than the January storm.

There will be freezing rain for pretty much every area that gets anything frozen. Fortunately, I don’t think it’s going to be as bad as it looked last night, and it should be mixed in with sleet and snow. The atmosphere is hovering around freezing at several levels, and this will continue until it warms up Friday morning and the precipitation shifts to plain rain. The rain may help to melt whatever falls tonight and early Friday morning. Most of what happens on Friday will be a tapering off, especially since the observed QPF with this system is coming in stronger and a bit earlier than the computers predicted.

There is actually the possibility of yet another round of something in the middle of next week, after a brief “warm” spell this weekend (don’t hold your breath waiting for springlike conditions like what we had last weekend, however; the warmest high will be about 52 on Sunday). It’s too early to say just what this will be, especially since the safe thing to do with respect to such forecasts in the South is to be conservative and forecast cold rain at first, and then modify the forecast as the event approaches—if conditions warrant. Right now, a high of 38F on Wednesday does not warrant modifying the long-term forecast, as utterly miserable as rain would be in that. We’ll have to wait a bit longer to see how next week’s system shakes out, so let’s take this one winter storm at a time for now.

Tags: , ,

January 8, 2011

Winter Storm Expectations

Filed under: Mid-latitude cyclones,Severe — Erin @ 6:07 pm

I have been very hesitant to make any kind of forecast regarding this system, because it has been a wretchedly difficult storm to predict. Winter storms in the Deep South usually are. And it really doesn’t help when the computers, supposed objective arbiters of the atmospheric data, can’t make up their “minds” either. (Models are supposed to be guidance for the forecasters, a way of looking at possibilities in a straightforward and easy-to-grasp way. They are NOT supposed to replace thinking about one’s own forecast and understanding meteorology.) Just one model, the GFS, has shown nothing, a rain event, a mega snowstorm (in Deep South terms), a crippling ice storm, and wintry slop. And that’s just over the past week. Fortunately, two of those options seem to have been ruled out. We’re going to get frozen precipitation. That’s a given.

This system is one of those where we humans have had to sort through things ourselves and make a judgment call. That’s good for learning, but I’m quite glad I am not in a position where my forecast (or input for a forecast) will have any discernible effect upon the local economy and opening/closing decisions. The question really is whether we will get a foully unpleasant, persistent, cold mix of slop for a day and a half, which makes things pretty miserable but doesn’t require anything to be shut down, or whether it will be a crippling winter storm. Oh, and if it is a crippling winter storm, what type that will be. Mega snowstorm or destructive ice storm?

Wouldn’t want that judgment call either, would you?

Unfortunately, I began leaning toward a worst-case scenario a couple of days ago, though I was not confident enough in it to post such a forecast on the Internet. That’s good; I was prepared to call for an ice storm for everywhere from I-20 to Highway 78. Things began pointing in a different, snowier direction yesterday.

OK, I’m going to put this up first and then go into a bit of meteorological explanation for it. This is not an official forecast from any weather agency, public or private-sector. This is MY forecast.

The system is going to be formed from the mixing of a cold arctic air mass coming down from the north and a moist tropical-type low in the Gulf of Mexico. This type of system is very good at bringing significant winter weather conditions to the Southeast, because it gets all the ingredients in the right place and all that becomes important is the amount of each type of ingredient, whether warm air, cold air, or moisture. Usually, systems from the north that are cold enough to bear snow are too dry; warm air intrusion often kills the chances of frozen precipitation for those that are not too dry. It’s a balancing act.

The precipitation is going to begin as snow across much of the state. I think an exception will be areas south of the I-20 corridor, who will probably get sleet first. This is because the cold air will have arrived and the system from the Gulf is just getting started. In areas where the precipitation type will not remain snow, I doubt there will be much snow accumulation from this part of the storm, because high temperatures have been in the 40s and 50s ever since the New Year’s Day tornado event.

As the Gulf low cranks up, warm air advection from the south will increase. The surface temperatures will still be around or below freezing, but air aloft will be warmed by this intensification of the Gulf low. This will result in a changeover from all snow to a mix of sleet and snow from Starkville-Columbus down to perhaps De Kalb. Areas south of that will probably see a changeover to freezing rain. I have been very concerned about the possibility of a major ice storm for the I-20 corridor for several days, as I watched round after round of forecast Skew-Ts come in with a freezing rain signature. This has all the hallmarks of being just that. The National Weather Service is saying up to half an inch of freezing rain (this is a solid coating of ice); I would put the maximum at 3/4 of an inch. Meridian and Jackson are both going to be clobbered by this part of the storm.

The precipitation should continue in these forms through Sunday night. I think that in areas where there is a snow/sleet mix, what percentage of each type you will see will depend on your latitude. I am inclined to put the belt of maximum snowfall a bit north of Highway 82.

After balking at it, I have come around to agreeing that somebody will get a major snow accumulation on the order of 8 inches. There may be isolated areas where a foot of snow falls. As disruptive as that can be, and as risky as it indeed is for power, I think the greater threat exists in the ice storm corridor. Please, please, please, if you are in this area, make sure you will be able to stay warm!

Everyone from that area remember the ice storm of February 1996? I don’t blame you if you’ve put it aside, but this may refresh your memories.

Freezing rain occurred over all the above counties causing widespread damage to trees and power lines. Accumulations of one-half to one inch of ice were common over this area. Over one hundred thousand customers were without power during the event. Most roads and bridges were impassable, and some of the roads had to be closed.

Though I do not think ice storm conditions will envelop the exact same area as before, a repeat of this event is very possible on Sunday for the I-20 corridor!

Stay warm, stay safe, and if you are in line for snowfall, enjoy it!

Powered by WordPress. This theme is a heavy modification of the WordPress Classic theme planned to match the layout of ErinThead.com. Because of its very specific and personalized nature, it is not available for public download. Content copyright ©2005-2015.