How much do you value "Cape" in weather watching

[Dave_S]

I understand very little about it other than a quick internet surf.

How quickly does it change.
Can you still have a destructive storm when Cape is low.
Why is Cape low or unaffected in a cyclone.

The image below shows a low in the gulf QLD Australia. There is high Cape in northern WA maybe 2000klms away (it is the season for TS's). The big low in the north Atlantic shows no Cape escalation.

[tkeithlu]

Disclaimer: My meteorology is self-learned and aimed only at protecting a college campus from dangerous windstorms.

CAPE is Convective Available Potential Energy, and potential energy is dangerous.

Just how bad is that front that is coming at you? Thunderstorms that have high CAPE also have high cloud tops, more tendency to spawn tornadoes, and powerful straight-line cells imbedded. Most internet radar sources just show you percipitation and colors from green to red. That could mean 18,000 ft tops, 25 mph cells with no rotation, and 3 Kg/m2 water. It also could mean 38,000 ft tops, 95 mph cells, rotation very near to being a tornado, and 20 Kg/m2 water. Those suckers flip and demast boats without ever being labelled "Tornado." Straight-line cells are the reason airliners no longer land during thunderstorms; they kept slamming planes into the runway.

To me, CAPE is a pretty good indicator of that danger, but here in the US, I use WeatherTap.com, which shows the individual cells and all their characteristics.

[tkeithlu]

Here's a screenshot of the front that is going over west Florida today. The arrows are imbedded cells. Their velocity is pretty low (length of the arrows), under 50 mph, the tops are moderate (25,000 ft) and the water is low except for a few of them. Meso (turning tendency) is substantial. Still, you might want to get your sails down if a cell was coming straight at you.

[dougweibel]

Place similar disclaimer to tkeithlu's here.

If I see a forecast saying we have a high probability of rain during passage but low CAPE then I expect we may see some gail force gust fronts. If I see a forecast for no rain but high CAPE I keep my eyes open and the radar on, and try hard to get out of the way of any cells coming my way, because they could be very nasty. My expectation, right or wrong, is that if there is no rain forecast but high CAPE then storms cells will be more isolated. If I see a forecast for rain and high CAPE I try to be somewhere else entirely.

[EmeraldCoastSailor]

Following as I have often wondered same thing.

[SV_Sweet_Ruca]

We value it enough to download and look at in the gribs when weather routing.

The posts above did a good job defining it. I’ve always looked at it as a measurement of gasoline waiting for a match, and the value tells you how big the explosion (thunderstorm) could be.

I have never looked at it in detail surrounding massive lows such as a hurricane/cyclone as I’ve never had any intention of sailing there. As to “why” my best guess would be that a massive storm is like a bomb that has already gone off, so that CAPE energy or “build up” has already been released.

Following, as it would be interesting to hear a meteorologists take on this.

[JPA Cate]

I'm riding along, too. Good question, Dave_S.

[Palarran]

It is a good question. Having had so many relatively incorrect weather forecasts, it's hard to judge much past windspeeds. I don't even bother with wave heights, rain, small resolutions, etc. I usually just look at the very big - big picture in both ECMFW and GFS.

One thing a sailor showed me quite a few years ago was using Marinetraffic.com to get actual weather readings from freighters. You go to the vessel details and it will show wind speed and direction. That I compare to my weather forecast to see if it's accurate.

[JPA Cate]

There is a low in the gulf of Carpentaria, now, that there is a weather notice about on the Bureau of Meteorology web site, in which the young lady met. person explains a bit of why its development into a cyclone (which they do expect) is difficult to predict as to directions and potential paths. So CAPE is undoubtedly part of that. I liked the WeatherTap image, with the cells and arrows superimposed on the front and the nation drawn in.

Ann

[Alan Mighty]

Originally Posted by Dave_S

Why is Cape low or unaffected in a cyclone.

https://journals.ametsoc.org/view/jo...-11-0254.1.xml

[chrisr]

Originally Posted by Palarran

One thing a sailor showed me quite a few years ago was using Marinetraffic.com to get actual weather readings from freighters. You go to the vessel details and it will show wind speed and direction. That I compare to my weather forecast to see if it's accurate.

are you sure that this is the actual weather as reported by the ship...or not just the weather forecast as portrayed by marinetraffic ?

(you can turn on / off wind arrows as a feature of marinetraffic, but it is just forecast (not actual))

fyi i've recently retired as director of a ship owning co, and i'm not familiar with any AIS setting that allows the ship to transmit real time weather

would be handy tho

cheers,

[wholybee]

The CAPE forecast is related to your chance of lighting. You can have no rain, no wind, and still have lightning. That is what CAPE will show you. So if you are trying to plan a sail a few days out, and see there is good wind, you might check to see if there will or will not be lightning.

I am originally from the west coast (San Francisco) and lightning isn't really a thing there. Earlier this year as I sailed from the Caribbean up the east coast, I looked that the CAPE forecasts and they seemed spot on, allowing me to know a few days out if I would have lightning.

[johnn33]

This is the Voluntary scheme for merchant ships
https://vos.noaa.gov/vos_scheme.shtml
All warships report, usually every 12 hours, and more frequently in extreme weather.
John

[dougweibel]

Originally Posted by wholybee

The CAPE forecast is related to your chance of lighting. You can have no rain, no wind, and still have lightning. That is what CAPE will show you. So if you are trying to plan a sail a few days out, and see there is good wind, you might check to see if there will or will not be lightning.

This is a good rule of thumb but not always true.

The paper below from the National Weather Service says CAPE is useful in predicting lightning, but high CAPE does not mean lightning will occur. They (NWS) produce a Lightning Potential Index calculated using CAPE, Lifted Index, Theta-e lapse rate, 850 mb temp, precipitable water and relative humidity at the -10C level. Apparently a lot more than just CAPE is needed to reliably predict lightning.

Unfortunately the NWS only produces the Lightning Potential Index for limited geographic areas and it is unclear if this model is generally applicable or specific to the south/midwest US.

Lightning Prediction by WFO Grand Junction using Model Data and Graphical Forecast Editor Smart Tools
Paul R. Frisbie*, J.D. Colton, J.R. Pringle, J.A. Daniels, J.D. Ramey Jr. and M.P. Meyers NOAA/NWS, Grand Junction, Colorado

[MartinR]

Originally Posted by Palarran

One thing a sailor showed me quite a few years ago was using Marinetraffic.com to get actual weather readings from freighters. You go to the vessel details and it will show wind speed and direction. That I compare to my weather forecast to see if it's accurate.

Unfortunately these weather reports do not come from the freighters, but Marinetraffic takes the freighters position and overlays it with a forecast for this position, probably GFS. I just checked my present vessel on marinetraffic, and I am positive that we do not transmit weather info. A few vessels do, but this is not send on AIS, but directly to meteorological offices.