What Icing Looks Like
March 2, 2006
With the recent spate of accidents due to icing, I thought it was worth revisiting the subject of how to make intelligent go/no go decisions.
I had an Angel Flight scheduled for this morning, from northern Vermont to eastern Long Island, then returning to White Plains, NY.
I started my flight review two days ago, at which time the forecasters were already calling for snow today. I checked each locality on www.weather.com, and found that only New York City was slated to actually get snow. I then looked at the Skew-T plots (my favorite site is Plymouth State Weather Center, available through the COPA weather page) for three locations: KBTV (Burlington, VT), KHTO (East Hampton, NY), and KHPN (White Plains, NY) for a variety of time slots on Thursday. These showed no significant weather all the way through the 48 hour forecast, so I kept my fingers crossed that I could complete the mission and still return to NYC before the snow storm arrived.
By yesterday morning, the picture had changed: although the Skew-T plots continued to show no significant weather in Vermont or eastern Long Island, the NY City area was now showing a “wall of icing” (discussed below) starting after 1 pm and continuing through the night. Which meant that I would be cutting it very close to the wire if I began the mission at 8 am and hoped to return back to New York City (around noon) before the storm. Based on that data, I saw a 50/50 chance that I would be stuck on eastern Long Island for the duration of the storm, with lots of time to contemplate why I had chosen to begin a three-legged mission that could not be realistically finished.
I don’t care for 50/50 odds when flying, so I decided to cancel the Angel Flight mission. I also considered the fact that it was unlikely that the pilot with the other half of the mission was going to be able to fly in that weather, so there was no point in leaving the patient stranded far from home. Instead, I decided to go flying locally, to practice instrument approaches with an instructor.
Later that day, as the TAFs were revised, it was clear that the arrival time of the storm was moving up: rather than 1 pm, HPN was calling for light snow by 10 am. Based on that information, I scrapped the training flight as well.
This morning, I went back to look at how accurate the forecasts were. Its just after 10 am as I write this, and it is definitely snowing outside; and the TAFs for the various locations confirm the data in the Skew-Ts from one and two days ago (KISP is Islip, roughly 50 miles west of East Hampton and the nearest locale with a TAF):
Burlington weather is still VFR, with no snow activity until late tonight:
KBTV 021134Z 021212 18008KT P6SM SCT050 SCT250
TEMPO 1214 BKN050
FM1700 VRB03KT P6SM SCT045 BKN250
FM2100 36008KT P6SM OVC040
TEMPO 0206 1SM -SHSN BR BKN020
FM0600 34015G22KT P6SM BKN040
White Plains weather is snow, snow, snow: all day and into the night. While the airport might officially be just at or above the ILS minimums, for all practical purposes it will be closed for single engine GA operations:
KHPN 021415Z 021412 01005KT 1SM -SN BR OVC010
TEMPO 1417 1/2SM -SN FG VV002
FM1700 06011KT 1SM -SN BR OVC004
FM1900 06014KT 1/2SM -SN FG VV002
FM2200 02012G22KT 2SM -SN BR OVC004
FM0200 34012G22KT 2SM -SN BKN007 OVC015
FM0600 32013G21KT P6SM SCT015 BKN060 OVC100
Islip has already begun to get snow, so the storm moved in earlier than originally forecast; this confirms the decision to not cut it close to the wire:
KISP 021431Z 00000KT 2 1/2SM -SN OVC013 M01/M06 A2972 RMK AO2 P0000
KISP 021144Z 021212 VRB03KT P6SM SCT070 BKN100
FM1500 09005KT P6SM -SN OVC050
FM1800 08007KT 2SM -SN BR OVC007
FM2000 06010KT 1SM -SNPL BR OVC004
FM2200 02010G20KT 2SM -SN BR OVC004
FM0300 36012G22KT 4SM -SN BKN009 OVC015
FM0600 32013G21KT P6SM SCT015 BKN060 OVC100
And, finally, the East Hampton METAR shows it as being below minimums for the GPS approaches:
KHTO 021438Z 00000KT 3SM -SN OVC050 01/M07 A2974
Now to discuss how the use of Skew-Ts are vital to understanding winter weather conditions.
For the uninitiated, the red line is temperature; the black dotted line is dew point. If they get real close to each other (or meet up), there will be clouds. If that intersection occurs to the left of the pale green, rightward-tilting line emanating from zero on the bottom scale, then its indicative of clouds with high likelihood of icing (unless its to the left of the –20 Celsius line). Altitude is shown in meters in the black letters on the left (millibars are to the far left, for those who want to correlate this data with the relative humidity products at various altitudes).
First, the Burlington, VT Skew-T at 7 am this morning:
This plot shows a chance of clouds at 1,773 meters (not coincidentally, very similar to the scattered clouds at 5,000 feet shown in the TAF above). But at all other altitudes there are is no visible precipitation.
Next is the 10 am Skew-T for East Hampton:
From 1,367 meters to 4,04 meters (roughly 4,000 to 7,000 feet) the dew point and temperature lines intersect, and that range is to the left of the zero degree Celsius base plot: so one should expect 3,000 feet of icing conditions. Pretty much just at the margin of “safe” (but not legal) flight in an SR22 with TKS.
Below 1,367 meters, the East Hampton plot is showing no clouds. So in theory there would not be a problem doing this approach once the plane was beneath the clouds (and certain icing). Note, however, that the METAR above tells a different story as to actual conditions: 500 foot overcast ceiling, which is below minimus. So the bad weather ended up moved in faster than the Skew-T model had predicted three hours ago.
And now for White Plains Skew-T:
Ugly is the only word to describe this. The overlapping red and dotted black lines indicates solid icing from 480 meters (1,200 feet) to 4,101 meters (over 12,000 feet). There is no way that this condition is even remotely “safe” for an SR22. It is literally a wall of icing that is best appreciated from the warmth of one’s home or in a turbine engine aircraft with two pilots who do this sort of flying for a living.
In sum, there is a wealth of weather products available online (and very easily through the COPA weather links page). IMHO, the Skew-T plots can provide the incredibly valuable information on where and when to expect clouds and icing in the winter.