Wednesday, January 27, 2010

Wednesday thoughts on winter weather

The upcoming winter storm is the headline-grabber of the day for the Mid-South. A Winter Storm Watch will go into effect tomorrow at 6pm, though it appears right now that wintry weather in the metro area will hold off until at least midnight or later (early Friday morning).

The computer model runs of the past 24 hours or so are indicative of why forecasting winter weather in this part of the world is so difficult. Precip onset, amount, and types have changed (at least slightly) with nearly every set of model data in one way or another. However, one thing has not changed, and that is that it doesn't appear we will dodge this storm completely. Based on what I am seeing this morning, my thinking has changed very little from yesterday at this time, as I have yet to see any model trends or convincing enough data to change my mind.

So, what am I thinking?? The long version...
I expect scattered rain showers to start breaking out Thursday afternoon and continue in a more-or-less scattered fashion through the evening hours. After midnight, those showers will likely become steadier as low pressure moves into southern Louisiana and "overrunning" takes place (in which moisture is transported from the south over a frontal system or cooler air at or near the surface). In addition, northeast wind will pick up on the back side of a front that will, by that time, be to our south. The northeast wind will help to bring colder air in the lowest levels of the atmosphere (lowest couple thousand feet) and surface temps will fall. Meanwhile, thanks to southerly air aloft, warmer air will be in place on top of the cold air, setting up an icing scenario (see the schematic below that diagrams the precipitation types). All of this to say that, as the cold air moves in at the surface, temps continue to fall and rain turns to freezing rain, possibly mixing with sleet at times. I believe this will occur in the wee hours of Friday morning (between midnight and 6am). That timing is one of the question marks that still exists in my book.

After 6am, through about noon, is likely when the heaviest precipitation will fall. During this time, models are having a little more difficulty in this morning's run pinpointing precipitation type. The general consensus is freezing rain, or glaze. One reputable model actually introduces colder air into the warm layer during the morning, possibly changing the metro area over to sleet and/or some snow. During the afternoon, there again is some consensus that the cold air begins to push into the warm layer aloft more completely, changing the precip to snow at some point during the afternoon before tapering off to flurries Friday night.

So, what am I thinking?? The short version...
Scattered showers Thursday evening mixing with sleet, then changing to freezing rain/sleet in the pre-dawn hours Friday. Freezing rain, maybe mixed with sleet, Friday morning, then changing to snow showers in the afternoon and tapering to flurries Friday night.

The other details:
Temps - will drop near freezing after midnight Friday and stay in the 30-32 range through Friday morning, then drop into the upper 20s Friday afternoon. Wind of 15-20 mph gusting to 25-30 mph will put wind chills in the teens.

Accumulations - I knew you'd ask! Best guess at this point is for 1/4" to as much as 1/2" of ice and 1-2" of sleet/snow in the metro. Less ice, more snow to the north. Less snow to the south. Some areas of west Tennessee north of the southern tier or two of counties could get mostly snow and end up with 4-6". For more on the rest of west TN, I encourage you to check out West Tennessee Weather Online and my good friend Eddie Holmes.

This storm is one to actively take preparations for. Though there is still a chance that it may not be as bad as we're painting it right now, you should prepare for hazardous road conditions, possibly treacherous on bridges and overpasses and scattered power outages due to ice on power lines and falling tree branches onto power lines (the wind gusts will not help this situation). Following the storm, very cold air will sit over the region through the weekend. It could be Sunday before the sun comes out, and it could be Monday before temperatures climb enough to really complete the melting process. Lows in the teens are expected in the city Sunday morning, so it will be frigid. Check out our Cold Weather Tips. will have this storm covered from all angles. Check out the resources below, set your bookmarks, and thanks for relying on MWN as your local weather source!

MWN resources to use this week:

MWN Blog - for updated forecast discussions, school closings, and road conditions

WXLIVE! - realtime weather conditions from Bartlett, TN

WXLIVE! Severe Weather - Weather alert notificatioan system to receive weather alerts by e-mail or Twitter

Mid-South Watches and Warnings Map - clickable map to see what alerts are in effect

StormView Interactive Radar - with winter mode, to see whether it's snow, ice, or rain!

MWN on Facebook and Twitter - for more frequent updates and breaking weather information

Get the latest weather conditions and much more by checking out on Facebook and Twitter!

1 comment:

angie128 said...

Thanks for the good info.