Using the tools to make good weather decisions

[Scott Dennstaedt, PhD]

Was doing a one-on-one session with a customer yesterday who was planning a flight from Florida northbound to Asheville, NC (KAVL). His plan was to depart this morning around 9 am for a 2.5 hour flight.  Mostly VFR conditions (green) expected along this route until reaching the Georgia where IFR (red) and low IFR (magenta) could be possible.

The flight was planned well ahead of a major weather system moving in from the west, so no issues there. However, with cold air in place near the surface over the Piedmont of NC and SC, the chances of some freezing precipitation were expected ahead of the primary weather system as warm air from the south was overrunning the cold air at the surface. This was expected to give rise to a temperature inversion and multiple freezing levels as shown below (screenshot taken yesterday during the online session).

Notice above that about 09Z, multiple freezing levels exist in this meteogram view for Asheville and then by 12Z, the warm air takes over aloft and creates a single freezing level after 12Z. The precipitation chances are shown as ramping up around 09Z as well with low IFR conditions (300 ft, 1SM) at Asheville at 12Z. This sets the stage for a freezing rain (or freezing drizzle) event with LIFR conditions at Asheville.  So this certainly solidified the plan to wait a couple hours after 12Z to arrive in Asheville.  Note this aircraft has a TKS certified ice protection system.  

Below, the Asheville TAF, on the other hand, has some freezing rain probabilities from 01Z through 12Z with MVFR ceilings and vis through the period. So, the forecasts above and the TAF agree that the freezing rain is expected to end at 12Z which is nicely represented in the route profile view above.

Now that we can look at the observations, let's see how well the forecast above did versus the TAF. Here are the surface observations for KAVL from 01Z (at the bottom) onward. I've removed some of the remarks to keep the coded observations short.

KAVL 231454Z 17003KT 3/4SM BR OVC003 01/M01 A3028 
KAVL 231449Z 17003KT 3/4SM BR OVC003 01/M01 A3028 
KAVL 231419Z 00000KT 1 1/4SM BR OVC004 01/M01 A3028 
KAVL 231354Z 00000KT 1 1/2SM BR OVC004 01/M01 A3029 
KAVL 231349Z 00000KT 1 1/2SM BR OVC003 00/M01 A3028 R
KAVL 231341Z 00000KT 1 1/4SM -FZRA BR OVC003 00/M01 A3028 
KAVL 231331Z 00000KT 1 1/4SM -FZRA BR OVC002 00/M01 A3028 
KAVL 231305Z 00000KT 1 1/4SM -FZRA BR OVC003 00/M01 A3028 
KAVL 231254Z 16003KT 2SM -FZRA BR OVC003 00/M01 A3028 RMK AO2 UPE40FZRAB40 
KAVL 231246Z 18003KT 2 1/2SM -FZRA BR OVC004 00/M01 A3029 RMK AO2 UPE40FZRAB40 CIG 002V007 
KAVL 231239Z 18003KT 2SM UP BR OVC004 M01/M01 A3029 RMK AO2 CIG 002V008 
KAVL 231224Z 18004KT 1 3/4SM UP BR BKN003 OVC007 M01/M01 A3029 RMK AO2 CIG 002V005 
KAVL 231220Z 18004KT 2SM UP BR OVC005 M01/M02 A3029 RMK AO2 CIG 002V008 
KAVL 231154Z 00000KT 3SM UP BR OVC008 M01/M02 A3029 RMK AO2 CIG 003V009 
KAVL 231054Z AUTO 22003KT 3SM UP BR OVC007 M01/M02 A3032 
KAVL 231020Z AUTO 00000KT 3SM UP BR OVC009 M01/M02 A3032 RMK AO2 CIG 004V010 
KAVL 230954Z AUTO 00000KT 5SM UP BR OVC010 M01/M02 A3034 
KAVL 230943Z AUTO 00000KT 4SM UP BR OVC009 M01/M03 A3035 
KAVL 230854Z AUTO 00000KT 9SM OVC010 M01/M03 A3036 RMK AO2 UPB08E44 
KAVL 230754Z AUTO 11003KT 10SM OVC014 M01/M06 A3040 RMK AO2 UPE42 
KAVL 230654Z AUTO 14004KT 10SM UP OVC016 M01/M09 A3041 RMK AO2 UPB20 
KAVL 230554Z AUTO 14004KT 10SM OVC018 00/M12 A3043 
KAVL 230454Z AUTO 16004KT 10SM OVC021 01/M13 A3046 
KAVL 230354Z AUTO 14007KT 10SM OVC023 01/M13 A3047 
KAVL 230254Z 14004KT 10SM OVC026 01/M14 A3048 
KAVL 230208Z VRB03KT 10SM OVC029 01/M14 A3048 
KAVL 230154Z 16004KT 10SM OVC030 01/M14 A3048 
KAVL 230112Z 14003KT 10SM OVC031 01/M13 A3048 
KAVL 230054Z 16004KT 10SM OVC029 01/M14 A3048 

Using the observations, the first indication of any precipitation wasn't until 07Z with UP (unknown precipitation type). Likely freezing drizzle occurring (an ASOS can't automatically report freezing drizzle and likely reports it as UP). Then at 0943Z, more UP.  The  TAF only showed a 30% probability of freezing rain through 08Z whereas the forecast we used above showed a 0% chance up to 09Z. Then, notice the freezing rain actually was first reported at 1246Z and last reported at 1341Z as the temperature warmed to above freezing by the 14Z observation. 

The biggest difference was in the ceiling forecast. The forecast we used above had the flight category in the IFR range by 09Z and then LIFR at 12Z. The actual ceilings dropped from VFR at 01Z to IFR by 0945Z. Then by 1224Z, KAVL reported a ceiling of 300 ft. The forecast we used picked up on this trend really well and was much better overall guidance than the TAF.  At this point in time KAVL is reporting very LIFR conditions and the pilot smartly diverted to Greenville Downtown (GMU) which was an expected outcome due to the potential of LIFR when we looked at alternates yesterday. 

KAVL 231601Z 16008KT 1/2SM FG VV002 01/00 A3022

Overall, the tools we used did a pretty nice job showing the overall adverse weather hazard trends and allowed the pilot to plan a solid alternate accordingly.