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EZTip No. 31 - Nonconvective LLWS on the Skew-T diagram

Surface-based temperature inversions can have a significant impact on the weather you might face. This includes the potential for radiation-fog with IFR ceilings and reduced visibility. But surface-based inversions also enables the potential for what is called nonconvective low-level wind shear (LLWS). Let's take a look at a recent event.

In the TAF below for the Adirondack Regional Airport (KSLK) in Saranac Lake, New York found in the EZWxBrief progressive web app EZAirport view, notice the three specific forecasts for nonconvective LLWS. Is this a forecast for severe turbulence? Although most pilots will say "yes," it's not a forecast for turbulence. If we look at the forecast shown in the red rectangle below, it is specifically telling you that the wind at 2,000 feet AGL (020) will be at a wind direction of 160 degrees relative to true north and at a wind speed at 55 knots. This forecast implies, however, that the winds will increase rapidly with height within that 2,000-foot wind shear layer.

One may believe that faster flowing air over slower flowing air may introduce a rolling action a lot like a tumbleweed moving across the ground with a strong wind. However, this phenomenon is all courtesy of a very stable situation thanks to that surface-based temperature inversion. Inversions are very stable layers and inhibit vertical mixing and thus inhibit turbulence.

Let's take a look at the synoptic weather chart to get a sense of the big picture. At 00Z, notice that there is a fairly significant area of occluded low pressure centered over northern Lake Huron. An occluded front extends from this low pressure south into the western New York and Pennsylvania. With the triple point in western Maryland, there's a warm front that cuts through the mid-Atlantic. Adirondack Regional Airport is located in Upstate New York and the surface winds are generally out of the southeast which is a cool and moist flow. This synoptic weather pattern often creates the potential for nonconvective LLWS.

In fact, the Aviation Weather Center issued a G-AIRMET for nonconvective LLWS over this region as shown on the EZWxBrief EZMap view below. This covers most of the area just to the north of the warm front and east of the occluded front...a common setup for this kind of LLWS event.

Shown below is the GFS 850 mb analysis (~5,000 feet MSL) found in the EZImagery and depicts strong southerly winds of 45 - 60 knots over a good portion of New York and Pennsylvania. With light southeasterly winds at the surface and nearly 60 knot southerly winds aloft, this sets the stage for a nonconvective LLWS event.

But the true visualization comes from looking at the Skew-T log (p) diagram. The 3-hour forecast from the Rapid Refresh (RAP) model below clearly shows a temperature inversion from near the surface to roughly 7,000 feet MSL. This inversion allows the southerly winds within the wind shear layer to decouple from the surface friction to freely accelerate to a maximum of 60 knots. This produces a low-level jet that can be seen on the wind graph of the Skew-T below.

Again, you might believe this would cause significant turbulence near the surface. There indeed may be some isolated locations where low-level wind shear may be reported especially in the vicinity of higher terrain, but it's more likely than not conditions were rather smooth in the wind shear layer. In fact, as shown below in EZWxBrief most of the turbulence reports were in the flight levels around this time.

In general, most nocturnal nonconvective LLWS events are benign. It can get more interesting when these events are coupled to a large scale weather system as it was in this example. Moreover, if there are moderate or heavy rain showers along with a nonconvective LLWS event, you may have some likelihood of wet microbursts as the momentum of those strong winds aloft are transferred down to the surface. The key is to look at that temperature inversion aloft. The more stable the atmosphere, the less likely you will see turbulence. When the stability is weak, there is a risk that some mixing may occur and turbulence may be problematic.

If you want to learn more about the Skew-T diagram, you can order your copy of The Skew-T log (p) and Me: A Primer for Pilots that is available in both softcover and eBook format.

Most pilots are weatherwise, but some are otherwise™

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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