Question about Airport's Vicinity (VC) Usage
(last updated 8/3/2005)
From a DLAC student:
The Writing Effective TAFs module states, "Important Note: VC has less stringent operational impacts on users than PROB30 groups." I agree with the statement (as it stands), but does this imply that NWS aviation forecasters should use VCTS instead of PROB30 -TSRA when we expect isolated or maybe widely scattered thunderstorms? I could see using VC within an hour or two of a TAF issuances (in lieu of a TEMPO group) with precipitation associated with a well-defined front, or maybe in later periods if there is some topographic feature (mountains) within 5-10 miles of the airport that a forecaster is reasonably sure will help trigger convection. I know some forecasters use VC as a hedge to mention diurnal thunderstorms (in this case) in the TAF but not having the same impact as a PROB30 or TEMPO group. I've never liked that practice for our area where topography is not a significant factor in thunderstorm development.
Dan Cobb's response:
Yes, for two reasons:
- A PROB30 group infers that it will likely not happen, but is translated by the FAA as PROB100!
- The above is compounded by the fact that, on average, when a PROB30 TS is forecast, it only occurs between 5 and 10 percent of the time.
So, in terms of our customers, we tend to over-forecast the occurrence of thunder, followed by a re-interpretation to 100% via FAA rules. The effect is an extreme FAR that costs airlines around $100 million in carrying and expending extra fuel.
Vicinity is not re-interpreted as meaning a 100% chance; it requires no explicit action by the pilot or airline. It does allow you to distribute information about a low probability event to the customer without penalty. When storms form and move toward your terminal, you would ideally update to categorical T about two hours (if possible) before they affect the terminal.
Follow-on Discussion:
Student: Thanks for the response. I guess we will have to agree to disagree. I don't think this is what was envisioned when the TAF rules were formulated/developed, but forecasters found a loophole in the system to hedge against low-probability chances. IMO, the vicinity remark should be just what it states, a weather phenomenon within 5-10 miles of the airfield.
Don't get me wrong, given the state of verification of thunderstorms and PROB30, I can see why the vicinity remark is used. I just think it has evolved for the wrong reason. I would think VCTS would be useful when convection near mountains is expected. I was a forecaster in southern California in the Air Force way back when, and we used TAF remarks such as TS VCNTY OMTNS NE-SE (or something like that) which is not an option today.
Dan's Response: This is really my point of view too! In a political context, I (as well as NWSHQ) recommend VC usage over PROB with the assumption that many forecasters are often strongly compelled, principally by past practice, to mention the threat in some way. Alternatively, one could also use "FEW060CB" in an afternoon prevailing group to communicate the presence of isolated "unfocused" (nice way to state it) TS.
Student: If PROB30 TS verification is so low (not surprising since NWS TAFS are generally pessimistic), I wonder what the verification of VCTS is? I discourage the use of both here for unfocused diurnal convection since it is a low probability and generally short-duration phenomenon.
Dan's Response: That is a good question. The donut area of VC is exactly 3 times the size of the area of the aerodrome. So subjectively, it would be a lot easier to hit the VC area versus terminal area. Also, as a personal observation, ALDARS is very generous at our TAF sites; CAR, PQI, HUL, and BGR often indicate VCTS in the METAR when AWIPS is indicating lightning is 15 to 20 miles away.
Student: Maybe the PROB30 TS usage should only be allowed 12 to 24 hours out (instead of the present 9 hours) so it will have even less impact operationally.
Dan's Response: Always looks good on paper, but this good intention has oft backfired. One paper, soon to be published in WAF, indicates that TEMPO groups are often simply substituted for PROB groups in the prohibited periods! Ultimately, it is in the hands of the FAA to routinely review their weather-related operating directives with respect to increasing forecast skill in meeting the demands for safety and economic benefit. (When will a PROB30 mean PROB30?)
Student: Incidentally, our PROB30 TS verification is probably even worse than most since our SGF tower "observers"/ASOS augmenters rarely augment thunder into the observation, even though they are supposed to do so (a constant exercise in diplomacy).
