Saturday, March 1, 2014

Winter Storm Warning posted as March comes in like a lion

Computer model data continues to indicate the likelihood of a potentially significant winter storm late Sunday in the metro. As confidence increases and models remain in good agreement as to the potential, the Winter Storm Watch for most of the metro counties has been upgraded to a Winter Storm Warning.  The warning is issued when the onset of precipitation is within about 24 hours and there is a good chance of "great impacts to society and commerce."

Earlier this morning, the graphic below was issued by the NWS in Memphis indicating the possibility of "significant ice accumulation" while the accompanying text stated the possibility of up to 3/4" of freezing rain and 1/2" of sleet, as well as minor snow accumulation.

As new data has arrived this morning, the threat has changed little and confidence in this storm producing the greatest winter impact of the season for areas along/south of I-40, including Shelby County, remain high.  In fact, as of this morning, NOAA's Weather Prediction Center puts the probability of more than 1/2" of ice by midnight Sunday night at 60% in the metro.

Probability of 1/2" or more of  ice through midnight Sunday night.  The highest probabilities also show where the best chance of significant icing will occur.

Questions You Want Answered:

1.  When will it get cold? And how are we getting ice when it's well into the 60s today?
Temps will remain warm overnight (likely in the 50s), but by mid to late morning will fall as the Arctic front that is bringing the cold air slides through the area. It's not inconceivable that most of the afternoon is spent in the 30s. By late afternoon, areas north of I-40 will fall to freezing with areas in north MS dropping to freezing by early evening. Temps will keep falling, reaching the teens by Monday morning.

Temps at 6pm Sunday according to the mid-day NAM model, showing 31 in Memphis at that time.

As to the second question, I refer you to the day before the 1994 ice storm when temps were over 70.  Yes, warm ground, especially pavement, will initially delay freezing roads, but precipitation will be heavy in the evening. The harder it comes down, the easier it is to accumulate and freeze the ground.  This is especially true with sleet which piles up.

2.What is causing all the precip to fall behind the front? Isn't it usually the other way around here?
Yes, it usually is the other way around, or at least the precip moves out before air sufficiently cold to cause wintry precip arrives. In this case, a strong upper level trough is trailing the front and a warm jet stream of air from the southwest is flowing over the cold front to our south, producing lift necessary to generate precipitation. This is what we call an anafront, which is a cold front with most precip behind it. In fact, there will be enough lift in this storm that I would not be surprised at all to hear some thunder with temps below freezing tomorrow evening. That lift, or convection, serves to enhance precipitation rates, which is bad when it's below 32!

3. So what type of precipitation are we expecting and when?
Precip will start as rain during the day but will become freezing rain as the air temperature hits 32 in the late afternoon (north) or early evening (south).  We're thinking (right now) that this occurs around 6pm in Memphis, give or take an hour. As the evening goes on and colder air filters into the low levels (1000-2000 feet up), we'll see sleet develop, which will mix with the freezing rain. By midnight or so, the warm layer above the surface but in the lower layers will dissipate and we'll likely see a changeover to light snow or freezing drizzle. This entire transition process will take place earlier in Tipton County and points north than in DeSoto County and other parts of north MS, probably by a few hours.

This atmospheric sounding shows that precip falls through warm air as rain, then falls into an area of sub-freezing air, which changes it to sleet or freezing rain.

4. How much are we going to get? No really.
The Winter Storm Warning from the NWS mentions amounts up to 3/4" of ice, 1/2" of sleet, and a small amount of snow.  From our perspective, that is reasonable.  In fact, the latest runs of the 3 major models I looked at this morning all suggest that those numbers are also reasonable.  Total liquid amounts of precip (including melted sleet/snow) between 6pm and the early morning hours could easily be near 1".  The worst case scenario is if all of that falls as freezing rain, which created a glaze of ice on everything that would be 1" thick. The faster we switch to sleet or even snow, the better, as damage to trees and power lines would be much lower (sleet doesn't stick to objects, just accumulates on the roads and ground).

5. What impacts are foreseen with this amount of ice?
Many people attempt to compare ice amounts to 1994, and for good reason. 1994 was pretty much a worse case scenario, with widespread 2"+ amounts and power outages that lasted up to 2 weeks.  However, remember that it doesn't take that amount to cause widespread impacts to commerce, travel, and society. In fact, 1/4"-1/2" of ice will bring down some power lines and tree branches, while a very small amount (less than 0.10") on roadways will make them impassable without treatment. The other issue will be the wind.  With gusts in the 20-30 mph range overnight, power lines with ice on them could easily succumb to the weight of the ice and snap.  In addition, temperatures will remain below freezing until perhaps Tuesday afternoon. Mid-Southerners need to prepare for an extended period (a couple days maybe) of hazardous travel, as well as power outages that could last for a few days or more.  All travel and preparations should be complete by 6pm Sunday night.

6. Any chance this completely passes us by?
Well, technically yes. But there are no indications that we end up with nothing.  The fact that other systems have done so this winter has no bearing on this storm.  No two storms are the same.  In fact, the last major storm that "missed" Shelby County still has Tipton County cleaning up a few miles up the road.  This setup is not the same as those, where temperatures hovered just above freezing for hours then the rain ended. All models indicate that temps will fall below freezing with precip falling and then keep falling throughout the night.  It's always better to be safe than sorry.  I've cancelled a class I was to lead Sunday night due to the likelihood of hazardous weather.

We'll talk about the entire situation more tonight on a Google+ Hangout that you can watch on our Google+ page, the MWN Hangout page, or YouTube at 8:30pm CST.  Follow our social feeds (links below) at that time for the YouTube link. We'll also use our social feeds to bring you the latest on the developing winter storm.  Invite your friends to follow along for the best local weather coverage in Memphis and share this blog with them!

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

No comments: