December 16: Wild Weekend Weather, Daily Links

WEATHER

We got off to a very cold start this morning!
friday-observed-lows

Our weather is going to be on quite the roller coaster ride over the next 48 hours.
paul-timeline

We’ll warm up to the mid 40s by this afternoon, with a more clouds than sunshine overhead:
friday-highs
Temperatures will actually rise overnight — we’ll start in the low 50s Saturday morning…
saturday-lows
…then shoot all the way up to 70° by late Saturday:
saturday-highs

Off-and-on showers will develop tonight and continue during the day Saturday — a scattershot hit-and-miss pattern, so I can’t promise you rain/no-rain at any one specific time or place:
rpm-10p-fri rpm-2a-sat rpm-6a-sat rpm-10a-sat rpm-2p-sat

As the next shot of cold air advances toward us, it will really put the squeeze on the atmosphere and give us a good chance of thunderstorms Saturday evening.  Look at how strong the front is!  You rarely see a nearly-40-degree temperature gradient on such a small scale.
paul-rpm-4km-temperature
The storms will rapidly intensify in the unseasonably warm air that will be in place, and they’ll move west-to-east across the Midstate:
rpm-6p-sat rpm-8p-sat rpm-10p-sat rpm-12a-sun rpm-2a-sun
The limiting factor this time of year is instability — yes, temperatures will be warm, but in mid-December it takes near-record high temperatures to really get the atmosphere fired up.  One measure of instability that meteorologists use is called “CAPE” — that stands for Convective Available Potential Energy.  Sounds like a word salad, but it basically just means “storm fuel”…CAPE levels will be highest to our southwest:
nam4km-mlcape-us_ov-2016121606-loop
We’ll have a TON of wind energy overhead though, so even the limited instability could yield a damaging wind threat and possibly an isolated tornado risk.  The Storm Prediction Center has outlined a “Slight Risk” area (level 2 of 5) to our southwest, with the majority of the Midstate in a “Marginal Risk” (level 1 of 5)…for now.
paul-severe-risk-region
The “analog forecast” method (comparing this pattern with similar historical patterns) shows the greatest severe potential off to our southwest as well:
prallc01_nam212f048
However, the SPC’s short-range ensemble forecast model (SREF) shows a 50-70% chance that severe weather ingredients will be in place over the Midstate Saturday evening:
sref_prob_combined_supercell__f042-2
And the “Significant Tornado Parameter” isn’t zero in that time frame…far from it:
sref_sigtor_median_mxmn__f042
So everything hinges on the instability…IF temperatures warm up enough, and IF enough humidity accompanies the warm air, the storms could be off to the races.  Two huge “IF”s.That doesn’t happen often this time of year, so I’m viewing the SREF’s output as a worst-case scenario.  Regardless, I’ll be here keeping an eye on it all day Saturday.
tumblr_lsqoefm9eq1qlzkw5

The storms will be followed by a huge drop in temperatures by early Sunday morning, with some very light icy (freezing rain/sleet/snow) showers:
rpm-4a-sun rpm-6a-sun rpm-8a-sun rpm-10a-sun rpm-12p-sun
No accumulations expected on the roads, but elevated surfaces (rooftops, road signs, deck railings) could pick up a thin layer of ice or sleet.  Futurecast is showing the potential for some minor accumulations of sleet on the ground in the higher terrain of southeastern Middle Tennessee:
paul-rpm-4km-ice-accum

The rest of Sunday will just be cold and windy, with temperatures hovering around 30°, and wind chills in the teens.
sunday-temps sunday-chill
Lovely!  After another cold (but not as windy) day Monday, we’ll slowly warm up through mid-week, with the next chance of rain showers approaching Wednesday night:
wsmv-4-plus-4-am

 

LINKS

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About paulheggen33

Morning meteorologist for WSMV-TV in Nashville.
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