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Back to Sunny Skies

After what has been a very busy weather week thus far, we look to end the remainder of the work week with nothing but clear skies.

Showers and thunderstorms from last night continue to move off to the east. The cold front has passed through our area. Some scattered showers were trying to follow behind it earlier today, but those have since dwindled out. Partly cloudy skies through the remainder of our Wednesday. Behind this aforementioned cold front, we can expect to see cooler temperatures overnight and through the rest of the week. read more

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New Discussion From the Storm Prediction Center

What does this mean for us? Simply put, it is discussing the northern movement of a better severe weather environment. The closer that warm front gets to Davidson/Williamson counties, the more unstable our atmosphere could potentially become tonight. We still think the *best* ingredients for tornado development is ever-so-slightly to our south, but this does not mean we are free from seeing severe weather or possibly a tornado. read more

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The Potential Stormy Stuff This Evening

The Wind

Let’s talk first about the wind…and oh my the wind. We are currently under a wind advisory through midnight tonight.

Sustained winds to start the day will be around 15-20 mph. This will increase to about 20-25 mph by the afternoon. Now these are just sustained winds. We could potentially see wind gusts upwards of 40 mph through the day. This wind is expected long before any stormy weather (which would bring even faster wind speeds). read more

The Wind

A Wind Advisory is in effect Monday from 9 AM to Midnight.

Wind speeds will increase Monday morning. By Monday late afternoon and evening, winds may gust between 40 and 50 MPH.

This will bring down tree limbs and blow around small objects, such as trash cans. read more

Quiet Sunday

South winds will blow slowly, gradually increasing as we get into tonight. Sunny today, high 60°, and dry, with dewpoints in the upper 20°s.

Big changes arrive Monday. There is a lot of meteorology to talk about (low pressures, wind gradients, instability/CAPE, shear, QPF, etc.), but I’m going to leave most of that out because I don’t want to drown out the message.

Rainy Monday

Rain arrives sometime Monday. We may see some light and scattered showers in the morning. After that, a much heavier rain (with questionable storm potential, see below) arrives.

ETAs for the heavier rain (and storm potential), by model:

GFS: 3 PM

EURO: 3 PM or so.

NAM: 6 PM

NAM4: 7-8 PM

How Much? This will be the heaviest rain we have seen in months.

Our last big rainfall was September 17 to 18th, when we got 1.33″. We’re forecast to get that much Monday.

The Euro model has 1.3″, with some areas getting more.  The GFS, below, generally agrees with the Euro:

I have to mention one run of the NAM4 model was showing only 0.5″, so I suppose there is some bust potential for a 1″+ rain on Monday. Most models have us over 1″.

Officially, our NWS is going with 1.19″ in Nashville and 1.26″ in Franklin.

Stormy Monday

We do not expect thunderstorms in the morning. It’s the arrival of a squall line that has potential to cause problems. We expect it to arrive sometime after dark. We aren’t exactly sure when, but we will have a better idea about this tonight and tomorrow morning.

Here’s the NAM4 model’s view of it:

This system has strong storm dynamics. “Forcing” is there to kick things off and make the squall line.

The winds aloft, measured in shear or “helicity,” will be strong, even exceptionally strong. Helicity measures the amount of “spin” in the atmosphere to support the rotation that makes severe thunderstorms, and, if other conditions are right, tornadoes. Check out how high helicity is on the GFS model Monday night at 6 PM:

The reason we are not freaking out about is because of missing of moisture and instability, measured in “CAPE” (Convective Available Potential Energy).

At the same time we will have very high helicity, the CAPE is barely even visible. You may have a hard time seeing it, so I outlined it:

That’s about 100 j/kg of CAPE. It’s just not enough to make a big, bad storm.

HOWEVER, what if the models are wrong? What if CAPE is 300 j/kg? In those cases, and when other things align, we’ve had severe weather, even tornadoes. To be very clear, I do not think we will have any severe weather. There’s not enough CAPE. Some models show an inversion aloft. And the helicity is probably too strong even if we get more CAPE than expected — if you lift up a thin parcel of air and try and make a tornado with it, that helicity is so strong it would probably just shear the top of it smooth off, rather than spin it.

So, we will be watching the CAPE tomorrow. One key element of CAPE is the dewpoint. Right now, the predicted dewpoint is 56°.

If that dewpoint starts creeping toward 60° tomorrow afternoon, our concern will increase.

So, my concern level is low. There are other reasons to discount a storm threat, but we won’t go into those. Just know that the helicity — rotation or spin the further up you go in the atmosphere — will be high, and there will be some rain around, so we will need to watch this tomorrow very closely. My gut says it’ll just be rainy and really windy, but stay tuned. If my gut is wrong, it’ll get messy around here.

Wendy Monday

@WendyMonday has tweeted to @NashSevereWx for years, so every time we’re going to get a Wind Advisory on a Monday….we say it’s a Wend….so yeah, you get it.

Monday will be Wendy Monday for sure. NWS has already said it will issue a Wind Advisory.

20-30 MPH sustained winds, gusting to 40 MPH. For those of you who live at a higher elevation, expect 40-50 MPH winds.

Use today to secure any inflatables or stuff that can be blown around.

Also, if you’re out in the rain, wear rain-tolerant shoes, and a rain jacket, because the wind will destroy your umbrella if you aren’t careful.

Also, secure your God’s Frisbee.

Tuesday: Dry and Mostly Sunny

That’s all I have to say about that.

More Rain Wednesday

This is running long, so I’ll leave you to read the excellent forecast discussion from NWS-Nashville this morning:

It`s this second wave [Wednesday] that could bring another 1 to 2+ inches of rainfall to our southern and eastern counties. All totaled, between Monday`s and Wednesday`s rain, all of Middle Tennessee could see 2 to 3 inches, with some higher totals for those southern and eastern counties -- all of which need it, desperately. What we will have to watch closely is any of it piling up in a short period of time. The ground is so hard and dry, run-off from this heavy rainfall could create some flooding issues, so please keep this in mind. read more

Weekend Plans Stay Dry

Overall, a nice weekend is ahead if you need to finish any yard work or holiday decorating.

Sunday is the pick day with highs in the low 60s. Why so warm? A high pressure system currently over the Ohio/Tennessee Valley slides east and we end up on the backside, where warm southerly flow begins to work in. read more

Remainder of Black Friday

If you’re out tonight to catch any last minute deals, temperatures will be in the 40s. Bundle up!

Calm Weekend, Little Warmer

Under a dome of high pressure, we stay dry and find some more sunshine by Saturday and Sunday. Notice the afternoon high on Sunday in the low 60s: this will be a warm up influenced by a switch to southerly winds, also leading to an increase in atmospheric moisture. read more

Great Holiday Weekend Ahead

Patchy fog is possible very early Friday morning, It’ll mix out quickly, and leave us ample sunshine.

A bit colder in the mornings this weekend. Mid 30°s Saturday morning will turn into near/below freezing temps before dawn Sunday morning.

The Big Story: A Dynamic, Windy, Rainy System Monday & Tuesday

This morning, we described the set-up (you can click back to that if you want). Basically, south winds Sunday will transport moisture into Middle Tennessee, effectively eliminating the Dry Air Monster.

Monday and Tuesday, a robust low pressure center will move across Missouri and toward the Great Lakes, that is, if you believe the latest run of the GFS model (12z):

This should make it rain here Tuesday.

Each run of the GFS tells a different story. Some runs send us a lot of rain; others, not so much. But each run consistently says yes, it will rain, and it should be more than we’ve seen in a long, long time.

The Euro model, which we aren’t allowed to show you, is encouraging. It has 1″+ of rain, pretty consistently.

Our NWS still thinks between 1″ to 2″ total, with more rain to our west than east. The Weather Prediction Center has us in the 1.25″ to 1.50″ category:

Thunderstorms or Severe Weather? Certainly not Monday. But Tuesday? Probably not, but it’s worth talking about.

To get severe weather, you need a lot of things to come together. Two main ingredients for severe weather are shear (Tuesday we will have more than enough of that) and instability (what we think we’ll be missing Tuesday).

Instability is measured many ways. Unstable environments make strong storms. To have an unstable environment, you need some “stuff” to make the storm, measured in joules per kilogram (j/kg) as CAPE: Convective Available Potential Energy. It’s kinda like making an omelette — you can have a chef, a stove, a skillet, and some ham and cheese, but if you have no egg, you won’t make an omelette.

Generally speaking, you need at least around 500 j/kg of CAPE to make storms, maybe a bit less than that in the high shear environment we will see Tuesday. Most model runs say we will have no CAPE, as in zero CAPE, nothing, but other model runs toss us 100 to 200 j/kg (like the GFS model below), which isn’t much, but will need to be closely watched.

Our NWS is not alarmed:

Models continue to show relatively weak CAPEs for Tuesday. However, with a howling low level jet of 50+ kts, we`ll have to keep an eye on things for next Tuesday just in case our atmospheric instability and thunderstorm chance increases. read more

Happy Thanksgiving! Here’s a PSA:

As you sit around a table with your family today, remember: observe any Airing of Grievances on Festivus, not Thanksgiving.

Save it for December 23. Save it for Festivus, you guys.

Boring Weather Today, Friday, Saturday, and Sunday

Clouds today, maybe some fog very early Friday morning. I would not be surprised to some pre-dawn fog early Saturday because temps will be 37° and dewpoints 34°. This is a 3° dewpoint depression — look for those when looking for foggy conditions. I mention that only because it’s hunting season.

Mid 30°s Saturday morning will turn into near/below freezing temps before dawn Sunday morning. NWS won’t issue more Freeze Warnings for the rest of the winter, so you’re on your own.

The Big Story: A Dynamic, Windy, Rainy System Monday & Tuesday

Let’s make it rain. All images from the GFS model run this morning.

Step 1: Saturday’s north winds will blow in dry air. We can’t do anything with that. Sunday, we switch those to south winds, which blow in moist air.

That’s not exactly a juicy atmosphere, but it’s not dry. The Dry Air Monster cannot survive in it. He won’t be around to eat our rain like he did yesterday.

Our atmosphere is very dry, both at the surface and aloft. System to the west toting its own moisture, but our D.A.M. will eat much of it. pic.twitter.com/ONAFNltNfY

— NashSevereWx (@NashSevereWx) November 23, 2016

Step 2: How about a strong (991 mb) low pressure center passing to our northwest by mid-day Tuesday? Check.

So, those are two main — but not the only two — ingredients we need for active weather. I will now stop nerding and start translating.

Windy Monday & Tuesday

We think 20+ MPH sustained winds, perhaps gusting over 30+. This will depend on the exact track of the low. Some of the models yesterday were going crazy with wind speeds. The GFS had winds at 5,000 feet moving around 60knots, which is crazy high and unlikely to happen, but even close to that is a lot of wind. We may need a Wind Advisory Monday and/or Tuesday.

Rain. Yes. Rain. Maybe Even Lots of Rain

There’s still uncertainty about how everything will shake out. It’s pretty complicated.

When? Right now I don’t think we will see much Monday. West Tennessee will see some maybe Monday night, but I don’t think it will get to us until Tuesday, probably in the morning. ETA is very uncertain.

How Much? Check out the uncertainty in the GFS model.

First, the 0z run of the GFS model this morning really screws us out of the rain with a point-three, but that would be three times the amount of rain we’ve seen from each of our past 2 events.

Then there is the most recent, but less reliable, run of the GFS — we hit jackpot between 1.2″ and 1.8″:

So, no real clear answer from the GFS, which has been going back and forth like this for a few days.

The Euro model, which we aren’t allowed to show you, is encouraging. It has 1″+ of rain, pretty consistently.

Our NWS thinks 1″ to 2″ total.

The Weather Prediction Center has us in the 1.25″ to 1.50″ category:

Thunderstorms or Severe Weather? Certainly not Monday. But Tuesday? Probably not, but it’s worth talking about.

Shear, both speed shear and winds turning with height, is very high, and we would be freaking out about it if there was going to be even a little convective available potential energy (CAPE). You can have severe thunderstorms and tornadoes in low CAPE, high shear environments, but you can’t if there is no CAPE, and we don’t think there will be any CAPE on Tuesday when the best storm dynamics arrive. However, a few model runs have suggested we’ll see around 200-300 j/kg CAPE (for example, this is what the 12z Euro said yesterday), possibly in response to a secondary low pressure center it thinks will form along the front. We usually want around 500 j/kg CAPE to start worrying. Note, however, the GFS does not have any CAPE at all, except for one model run yesterday where there was 3 j/kg, and the Euro runs also have shown no CAPE.

Our NWS is not alarmed:

Models continue to show relatively weak CAPEs for Tuesday. However, with a howling low level jet of 50+ kts, we`ll have to keep an eye on things for next Tuesday just in case our atmospheric instability and thunderstorm chance increases. read more

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Fun Fact of the Day

We’ve seen a lot of records set this year, but this may be the cherry on top. With our freeze this past Sunday, this is Nashville’s longest growing season on record. Nashville went 259 days (Mar. 5-Nov. 20) between freezes according to our local National Weather Service Office. read more