Details on a roller coaster weather weekend

If you’ve ever ridden a roller coaster you know that there is typically a long steady climb then, just after peaking, a plummet that results in your stomach relocating to somewhere around your neck. Well hang on to your hats grandma, because our weather pattern (at least the temperature) is about to do the same thing! I’ll break it down day by day.

An overview of hourly conditions through early Sunday from the NWS. (weather.gov)

By Sunday, most of us may need our Dramamine (or at least something for the headache!)
Arctic cold air in animation, shown hourly from 3pm Saturday to 6am Sunday. (PivotalWx)

Friday night

A surge of warm, Gulf-moisture laden air is moving north into the Mid-South on southerly wind which will really pick up tonight as the low level jet cranks up. The temperature has risen from the upper 20s this morning to the upper 40s this afternoon, even with cloud cover as wind has shifted south and that warm air moves north. Typically temperatures fall at night, but not tonight! With southerly wind increasing to 20-30 mph, the mercury will continue to rise overnight (just like the big hill on the roller coaster) as scattered showers also break out. If you’re headed out this evening take the umbrella as you could get caught by a couple. When you wake up in the morning, temperatures will likely already be near 60°. Rainfall tonight will likely be a tenth to no more than quarter of an inch.

Saturday

Strong southerlies continue Saturday with gusts to near 30 mph continuing. We’ll still see scattered showers, but there are some signs that there could be a lull from mid-morning through the early afternoon. If you have last minute Christmas shopping, an umbrella is a good idea, but you may not need it much until mid to late afternoon. By then, an Arctic cold front will be drawing closer and more showers are expected to pop up. However, ahead of the front, temperatures may approach the record high of 76° by late afternoon! The roller coaster continues to climb… In fact, with abnormally strong wind at all levels of the atmosphere and that very warm air in place, instability and wind energy will combine to create a setup suitable for scattered thunderstorms by late afternoon. Most storms should remain below severe limits during this time.

At 3pm Saturday afternoon, instability (as measured by forecast Convective Available Potential Energy, or CAPE, from the American NAM model) is forecast to be near 1000 J/kg, which this time of year is plenty to sustain thunderstorm updrafts. The warmer we get, the more unstable we should get. (PivotalWx)

In addition to instability, the forecast wind shear in the atmosphere at 3pm Saturday (again, as forecast by the NAM model), will be excessive, approaching 70 knots. Shear is the ingredient that supports rotating thunderstorms, when they are present. (PivotalWx)

Saturday evening

Just ahead of the front, strong lift provided by the front is added to the instability and wind shear already in place – ingredients that could be sufficient for the strengthening of some of the storms. A few severe storms are possible in the early evening hours in and near the metro and especially points east and south of us. The primary threat with any storms that form will be damaging straight-line wind, but a tornado or two cannot be ruled out, especially in north MS and southeast of the metro. This shouldn’t come as a big surprise when it gets into the mid 70s the week before Christmas and you know the temperature crash is coming…

The forecast temperatures at 6pm Saturday show you the imminent “crash” of temperatures as the Arctic front advances. While still near 70° in Memphis, temperatures are below freezing just over 100 miles northwest of here and negative values blanket a large portion of the north-central portion of the country. (PivotalWx)

At 6pm Saturday, just ahead of the cold front, the NAM model is forecasting Significant Tornado Parameter (STP) values to be between 1-2 east of the Mississippi River. Values over 1 are sufficient to support some tornadic potential. These values are not “off the charts” by any means and just mean that if supercells form, a tornado threat is present due to turning  of the wind in the lowest levels of the atmosphere. (PivotalWx)

As storms will be basically building in the vicinity of the Mississippi River as they encounter air conducive to their formation, they will be gaining strength and likely coalescing into a squall line just to our east. We recommend any Saturday evening plans be indoors and that everyone should keep one eye on the weather situation, just in case warnings are issued. (We recommend setting StormWatch+ in your MemphisWeather.net app, then going about your business. It’ll let you know if you are in harm’s way, plus you can check our Twitter feed for updates!) The chance of severe weather is low, but something to certainly keep in mind.


Saturday overnight

So you know what the roller coaster does when it reaches the top right? Yeah, it comes back down! Once the front moves through in the evening hours, temperatures plummet. Quickly. It is highly likely we’ll be in the mid 30s by midnight. That’s a roughly 40° drop in six hours or less! I have a headache just thinking about it… So what happens after midnight? MOST precipitation is gone by then, but some lingering moisture is expected and since temperatures are dropping so fast, there’s a chance-worth-mentioning that we could see a bit of a wintry mix in the wee hours of Sunday morning. Precipitation totals are expected to be relatively minor (less than 0.10″) with the most likely precip type being either sleet or freezing rain. It depends on how fast the cold air in the lower few thousand feet arrives.

An explainer graphic on how various precipitation types form. (NWS)

Anything we get should be gone by sunrise (none falling and probably nothing on the ground) but temperatures will be in the upper 20s with wind chills in the teens as you wake up Sunday. The other concern, with rain in the evening and plummeting temperatures is freezing on the roads. I don’t think that will be a major concern. The 70s on Saturday will help some in that regard, but the strong wind will help even more. Even a cold blow drier is effective at drying your hair. With north wind gusting to 30 mph Saturday night, the roads should dry after midnight with little concern for travel. Because of the rain prior to freezing though, roads will NOT be treated in advance, so keep that in mind if you have to be up very early Sunday on the flyovers or elevated roadways.

Forecast wind chills at 8am Sunday according to the NAM model. With temperatures in the upper 20s and wind at 10-15 mph, wind chills will be in the teens locally. Sub-zero readings are not that far upstream! Fortunately, they stay there. (PivotalWx)


Sunday

C.O.L.D. Clouds will be slow to clear but even with some afternoon sun, the strong cold air advection (movement of cold air into the area) will keep temperatures from rising. After a low in the upper 20s, we probably won’t get to freezing for the high. Couple that with a relatively stiff breeze and wind chills will remain in the teens. BRRRR!!! Dress in layers if you have to be out, even though you had your shorts and flip flops on Saturday basking in the mid 70s!


Sunday night/Monday

The coldest night of the year is expected Sunday night with temperatures well down into the teens and wind chills in the single digits. At least the kids don’t have to be at the bus stop! Monday will see a good deal of sunshine but temperatures don’t recover much, if any, above freezing.

Looking ahead

Beyond Monday, a temperature recovery takes place as highs climb back into the 40s and 50s and lows moderate back into the 30s. There will still be a couple of cold mornings early in the week though. Very little, if any, precipitation is forecast for the week before Christmas and an early look at Christmas Day indicates mild temperatures. Things don’t look good for a white Christmas this year! Get the complete 7-day forecast from us in our mobile app or website linked below.

Erik Proseus
MWN Meteorologist

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

MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Recent Posts

Subscribe
Notify of
guest
0 Comments
Inline Feedbacks
View all comments