Looking at the potential for severe weather as we head through later this evening, I wanted to take a couple of seconds to answer a questions I’ve received a bunch today:
“It is cloudy and cool, is this going to save us from severe weather tonight?”
Not really this time.
Looking at the map above, the 65F air is just off to our south and the only thing keeping it there is the location of the area of low pressure which is currently due west. Once it passes to our north, it will flip our wind from the east to the southeast and eventually to the south. And the warm front, which is to our south now, will lift northward because the trajectory of the area of low pressure to our west is moving from southwest to northeast.
And because it is so windy, the air from farther away that normal will have a chance to make it farther north – just like you can drive farther in an hour going 75mph than 35mph.
The dew point maps above tell the story pretty well. And I’ve laid the. surface features over the top of the maps to help you find the warm front and cold front. As the low moves northeast, the warm front move from our south, to north of us.
Once that happens temperatures will quickly warm from the low to mid 50s into the mid 60s. with dewpoint values around 63F.
And while a 63F dewpoint is barely enough for severe weather in out area, it is still enough.
Especially when the temperature of the air aloft is very cold. The Skew-T data from the HRRR model shows plenty of instability – even with temperatures estimated at 67F – given that temperatures aloft will be around freezing by about 9,000ft.
That sounding produces a potent Karrie Meter number of a 5.66 which should land the area between a Slight Risk and Enhanced Risk from the SPC, and that is exactly where we sit.
But, it also comes with a “TOR LEAN” number of 3.73 which is lower than the Karrie Meter number. And that means the data is suggesting the risk for tornadoes is “lower than it typically is in this kind of a situation.”
Why is that?
I tried my best to explain this during my facebook live during my drive home, but I want to take some time again — this time with a visual, too.
That Skew-T data above is pulled from between Laurel and Hattiesburg. At that same time, the warm front is lifting north and the cold front is moving into the area from the west. It shoudl be in the prime spot for tornadoes, right?
Perhaps not.
In this case, the model data is suggesting that a line of storms will outrun the cold front. And end up in the yellow circled area on the map above. This means their forward momentum would be fueled less by the cold front and more by the cold pool of air behind the storms which has been built up by the rain-cooled air falling out of the storms themselves.
It is a subtle, but very important difference.
Because creating severe weather is all about gradients — the peak of warm air versus that valley of cold air. But in this case, it may be a situation where the warmth of the air being pulled in by the warm front isn’t that much warmer than the rain-cooled air in the cold pool behind the storms. Let’s say the air is 67F behind the warm front, but maybe the air behind the storms is 59F, that is a gradient of only 8F from one side of the storms to the other.
Where the gradient from the air in front of the cold front to behind the cold front has a gradient of almost 20F. Much bigger. And thus a stronger gradient.
And a stronger gradient means more forcing. And more forcing is often what you need when temperatures are a bit cooler like they are in this situation. In particular, you need some sort of ‘umph’ to get the updrafts that are strong enough within a thunderstorm to produce and sustain tornadoes.
And this model guidance is suggesting we may not have that.
So I tend to think we may get a brief lull in storm intensity between 11p and 1a as they pass through parts of southern Mississippi while the storms that outrun the front die off and the cold front then re-catches-back-up-to the warmest air farther east. This wouldn’t mean the risk is zero, but it would limit just how beefy many of the storms will get.
If this holds true – and it is a bit of a big if – then you would have severe storms to the west (orange circle), then a lull (green circle), but then once the cold front catches back up it would be like throwing gasoline on a campfire and things would fire up very quickly and be very intense.
And the placement of the hand-off from severe to lull to intense again would be a bit nebulous but would be in and around the I-59 corridor.
That said, if you are in the green circle above, I wouldn’t let your guard down. Because if any of those circles shift at all as we move through the next few hours, you could find yourself in a much worse situation.
And I say all of that to say that things are going to change pretty dramatically as we move through the next few hours. Hang in there. And a reminder about who will see what….
WHO SEES WHAT DURING THE STORMS?
EVERYONE — Very breezy conditions, brief rain shower with lightning, wind gusts up to 50mph
MOST AREAS — Very breezy conditions, brief heavy rain, lightning, wind gusts up to 55mph
SOME PEOPLE — Very breezy conditions, one extended round of heavy rain, frequent lightning during the rain, wind gusts up to 60mph, small hail
FEW SPOTS — Very breezy conditions, very heavy rain, brief localized flash flooding, frequent lightning during the rain, wind gusts up to 70mph, hail up to the size of quarters, the chance for a tornado (up to EF-3 in strength)
TIMELINE FOR SEVERE RISK
WEST — Now through 1a
CENTRAL — 9p through 3a
EAST — 11p through 5a
Thank you for sharing your expertise with us.
THIS is why you are so missed. We don’t get the in-depth explanation of not just what is happening, but WHY!
Thank you Nick for your in-depth explanations.