What does this terminal forecast mean to you?

[Piloto]

31 minutes ago, scottd said:

That's not what this forecast that I posted is suggesting, however.  The phenomenon I posted is called nonconvective LLWS and is quite easy to forecast.  

What the forecast indicates conditions favorable for windshear. Only a  control tower with WX radar access will indicate windshear activity and where in the field.

José

[Scott Dennstaedt, PhD]

2 hours ago, Piloto said:

What the forecast indicates conditions favorable for windshear. Only a  control tower with WX radar access will indicate windshear activity and where in the field.

José

José,

This is called nonconvective LLWS.  It's a form of vertical speed shear and has nothing to do with convection or thunderstorms or the horizontal shear you may be referring to.  This is not something that the Terminal Doppler Weather Radar (TDWR) can track like it can with wind shear due to thunderstorms, microbursts, gust fronts and outflow boundaries.  I helped develop some of the software for these Doppler radars decades ago in association with Lincoln Labs.  

The catalyst is a temperature inversion like you see below on this Skew-T log (p) diagram for KLAW.  Notice the winds (shown on the right) rapidly increase with height (that's the vertical speed shear component).  It's also called a low-level jet maximum.  Essentially what happens is that the clear skies and relatively calm winds at the surface will allow radiative cooling to occur.  This cools the air near the surface of the earth creating a nocturnal temperature inversion (in this case).  That inversion causes the winds to decouple from surface friction and allows them to freely accelerate just above the surface creating the maximum winds you see shown here (very easy to forecast).  Given the strong inversion, this prevents atmospheric mixing from occurring creating smooth (non-turbulent) conditions.  

This is something that isn't taught in primary training and most pilots see it and think of it as a forecast for turbulence.  It is not.  

[Danb]

Scott giving the cape is zero and the cin is low wouldn’t this depict smooth conditions with little to no lifting making one thing there would be no turbulence, my initial impression would be blank or nothing to prevent my departure or for that matter my approach to landing?

[Scott Dennstaedt, PhD]

1 hour ago, Danb said:

Scott given the cape is zero and the cin is low wouldn’t this depict smooth conditions with little to no lifting making one thing there would be no turbulence, my initial impression would be blank or nothing to prevent my departure or for that matter my approach to landing?

Dan,

CAPE only tells you if there's sufficient moisture and instability and says nothing about whether air is moving up or down.  It's just "potential energy" at rest in the atmosphere.  You can have an enormous amount of CAPE with no chance of tapping into that potential energy when there's a high degree of convective inhibition also present.  So, even with high CAPE, mixing can be unlikely.  Usually a zero CAPE value (there are several variants of CAPE) the chance of convective mixing is low except near the surface where the lapse rate is often large (3 degrees/1000 ft).  Once the level of free convection (LFC) is penetrated, then that high CAPE can turn into dangerous convective turbulence.

The key to this sounding analysis I attached is that the wind shear layer (from surface to 1000 ft AGL) is very, very stable.  Stable air won't mix (up or down).  Without mixing, there can't be turbulence.  So, in the original TAF I posted, the forecast for nonconvective LLWS with a broken cirrus deck tells me to expect very smooth conditions...not turbulence...and that's what I want to see when I fly.  

[Piloto]

[Scott Dennstaedt, PhD]

Yep, that's CONVECTIVE wind shear.  The TAF I posted was forecasting NONconvective wind shear.

[Scott Dennstaedt, PhD]

Check out this 36 minute YouTube video I just released that discusses this phenomenon in a bit more detail.  And, if you haven't done so, please subscribe to the channel to stay connected to more educational videos like this one.

What the FAA does not want YOU to know about weather 

[Piloto]

I use www.aviationweather.gov for all my trips planning including those outside the US and found it to be pretty accurate. I also use the Garmin Pilot WX which is pretty good. I plan the trip routing based on the winds aloft and weather motion so I will avoid trending weather. Early morning departure is the best time to avoid the nasty builds up in the afternoon. Stormscope is a must here in Florida, specially in the afternoon.

José