Hurricane Hilary (Pacific) and TD 6 (Atlantic): August 19, Update A

As I sit here, sipping my tea, in the skies overhead there are clouds circling and a helicopter circling. One of these is normal for LA. :-)

Hurricane Hilary is at 24.4N, 114.2W, heading NNW at a fast 17mph! They have moved the track forward in time, so if you were expecting her to pass by on Monday, that was yesterday's plan. Now her center will clip Baja tonight... 

Make landfall tomorrow afternoon (Sunday, 12pm = noon), and have zoomed by the LA region by midnight. This doesn't mean that the effects of the storm will be over by midnight on Sunday - those I expect will continue into Monday (you will see why below). 

She's decreased in intensity as we expected and is now a weak-mid level cat 2 storm with winds of 100mph (cat 2 range: 96-110mph), on target to hopefully be a weak cat 2 or strong cat 1 as she clips Baja. Her convection (and circulation) has really decreased quite a lot:

And if you look at the track and where the main convection is - you will see that it's mostly on the east side and south of the storm. This means that the bulk of the rainy sort of weather will be inland California - San Bernardino etc. - if it continues to have heavy convection by the time it reaches that far north.

But even as the center passes by, the storm isn't over because you can see that there is some heavy convection on the southern side of the storm as well - and if that persists, there will be also be a lot more 'weather' in the wake of the eye.

Although she hasn't even got half way up the Baja peninsula, because of wind shear, the clouds (some with rain) are already streaming over Southern California and north - even over Nevada, Oregon, Idaho, with the very outer edge reaching Canada:

That's quite a lot of rain - but I suspect it is needed. 

I was asked how I knew yesterday that the storm was weakening, even though the plane was flying to gather more data:

1. The cloud tops in the infrared satellite imagery were getting warmer (less of the deeper clouds indicated by the red areas compared to the day before).

2. I knew she was heading into an area with dry air to her north and west (as seen in the water vapour imagery - she was less 'circular' and a bit more ragged on her western side).

3. She was moving north and over cooler water - not just the sea surface, but the depth of water that was warmer than 26 deg C had also almost vanished, so anything she churned up to gain energy wasn't going to be enough to sustain her.

4. The wind shear had picked up - and we see that today as well because the clouds are streaming off to the north.

The one reason I thought she may take a while to get off that cat 3/4 ledge is because her circulation (vorticity - science!) was still strong throughout the troposphere (science!), which means she had still got some oomph (technical term ;-)) to her. 

And that brings me to a <Science Alert!>.  Woohoo! :-)

<Science Alert!> The troposphere. Our atmosphere is divided into layers – like a delicious trifle or seven-layer dip or lasagna (depending on what country you are from). In each layer the air temperature either increases with height or decreases with height. The troposphere is one of these layers. It is the lowest section of our atmosphere and extends up from the earth (ground zero if you like) to about 15-16km in the equatorial regions, and to about 8km in the polar regions of the planet. This is the layer of the atmosphere we live in, this is the layer we breathe. All our 'weather' essentially occurs in the troposphere. The troposphere is defined by decreasing air temperature with increasing height. You would know this if you climbed a mountain. Or the easier option, of course, is to just look at pictures of mountains and see the snow at the top (known as the ‘Flat Florida Option’). ;-) So, looking at the infrared imagery, it measures the tops of the cloud temperatures and so colder they are, the stronger the storm. 

The top of the troposphere is called the tropopause. Strong tropical storms have clouds that reach as high as the tropopause - and in a few very strong cases, they can extend even higher into the next layer up - into the stratosphere, which is the layer of the atmosphere above the troposphere. The stratosphere is defined by air temperature increasing with height. The ozone hole is in the stratosphere. The top of the stratosphere is around 50km height and is marked by the stratopause. And the layer above that is mesosphere, where air temperature decreases with height again... and so we go on until we get to space...<End Science Alert!>

Some last minute notes:

- Please do heed your local emergency managers. 
- If you get a lot of rain and wind, and go out after it has passed, be very wary of downed power lines in water. 

- If you are in a flood zone, evacuate. If you are not in a flood zone, then find a place to shelter away from windows if the wind picks up. Water is the biggest cause of loss of life in Tropical Storms, not the wind. 

- Don't go surfing... although I know that's a silly thing to say to the surfers I know! :-) 

- Be safe, enjoy the ice cream!

That's it for the Pacific... in the Atlantic, I see they have Tropical Depression Six... it looks like it will fizzle out in the Atlantic, so I'm not going to bother. I'll keep an eye on the crayon artwork that Mother Nature has produced over there though just in case... 

Toodle pip until tomorrow!
J. 

--------------------------------------

DISCLAIMER:

These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and local weather service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.