Is it possible that you can accrete airframe ice at a temperature of -30°C at the end of February in southern Idaho? Although it is not typical, it is still possible, especially if the icing is convective. Yes, convective. Let's say you were planning to make a flight at 17,000 feet on February 27th in the late afternoon through southern Idaho. You took a peek at the Forecast Icing Product (FIP) to choose the best altitude. It would be easy to conclude that using this 6-hr forecast for icing severity (below) that the risk of icing at 17,000 over a very large area is very low. In fact, if you consulted FIP at15,000 feet and 16,000 feet as well as all altitudes above 17,000 feet, they all looked pretty similar. Seems pretty much like there's no risk of icing and that 17,000 feet would make a good cruise altitude through this area. Well, that would be a bad assumption.
Turns out the forecast was not entirely correct. In fact, a pilot flew through this area at 17,000 feet and accreted ice at a temperature approaching -30°C. It's not likely that most pilots would pick up on this, but understanding the risk is all rooted in the big picture. Let's take a look at the big picture to understand why.
First, the official forecast for icing from the G-AIRMETs issued at 2045Z on February 27th was that widespread moderate icing would extend as high as 14,000 feet MSL in the southern Idaho area. That would likely be the case for a majority of the region, but the icing at altitudes above 14,000 feet could still be very spotty (not widespread).
A weak cold front moved through the southern Idaho area the night before as can be seen on the surface analysis valid at 00Z February 27th.
The cold front quickly moved well to the east into central Wyoming by 06Z February 27.
And by the early afternoon of February 27th, it was located well east of the Rocky Mountains leaving behind just a broad area of surface high pressure.
But that's not the entire story. The surface front just provides you with a very limited understanding of the atmosphere. The 500 mb (~18,000 feet) constant pressure chart told a different story. With the passage of the cold front, the upper level weather system was just passing overhead during the afternoon on the 27th. Here you can see that the area was under a broad upper level trough. As is often the case, this upper level trough often lags behind the surface features. A trough represents a cold pool of upper level air.
While there was some cloud cover throughout the area, there were locations where the sun was shining as evidenced by the red and orange areas in the color-enhanced IR satellite indicating some clear skies in southern Idaho.
With the heating of the day near the surface and the upper level cold pool of air, this creates the opportunity for upper level support and generates instability. Looking at a nearby radiosonde observation (RAOB) at Boise, Idaho valid at 00Z February 28, the dewpoint depression at 17,000 feet MSL is 21°C. That's very dry and it's unlikely to contain clouds, much less supercooled liquid water. However, this is just one observation at one location at one time.
When you add a parcel lapse rate (magenta) line to the sounding (below), it depicts some important aspects of this environment that must be considered.
Going back to the earlier discussion of the upper level trough, there is ample cold air aloft. With strong insolation, this produces instability. This is depicted in the convective available potential energy (CAPE) that is apparent from the lifted parcel. Also notice the capping inversion that starts ~15,000 feet and extends up to ~17,500 feet. This means that rising air can rise all the way up to the parcel's equilibrium level at ~16,000 feet. In fact, if you release a basketball at the bottom of the pool it will rise and "pop up" and overshoot the surface of the water a bit since it has momentum as it rises in the pool. That's what can happen here as well as air will likely overshoot well into 17,000 feet. Some intense convection can overshoot by 10,000 feet or more.
In fact, that's exactly what happened and you can see a very cellular structure to the clouds evidenced by the blue speckled areas in south-central Idaho in the IR satellite image valid at 00Z February 28. Likely the tops of these clouds were higher than 17,000 feet in some areas given the cloud top temperatures. It is one of these tops that the pilot mentioned earlier flew through and accreted ice at 17,000 feet.
In fact, while the icing forecast did not capture the threat at 17,000, the forecast at lower altitudes provided some clues. Notice the cellular structure at 12,000 feet that is very much under the upper level trough shown earlier. This implies there is a convective process in place as seen in the Skew-T diagram above.
Lastly, take note of the forecasts for precipitation at the surface. Showery precipitation in the TAF is also a sign of a convective process as you can see from the TAF located in southeastern Idaho showers in the vicinity (VCSH) tells you to expect deep, moist convection in the area.
KPIH 271720Z 2718/2818 25013KT P6SM VCSH SCT025 BKN060
TEMPO 2718/2722 BKN025
FM280200 21008KT P6SM SCT030
FM280800 VRB03KT P6SM VCFG SCT005
FM281600 VRB03KT P6SM SCT015=
This form of convection is not going to generate lightning given the low-topped nature, but it can produce icing down to much colder temperatures as see in this example.
Most pilots are weatherwise, but some are otherwise™
Scott Dennstaedt, PhD
Weather Systems Engineer
CFI & former NWS meteorologist