Afternoon Convection and a Single-Runway Destination
It is 1400 local in July. You plan a 2-hour VFR leg in a normally aspirated single. Current observations along the route:
KABC 151753Z 20012G20KT 10SM SCT045 BKN060 29/19 A2992
KXYZ 151755Z 21008KT 8SM VCTS SCT035CB BKN050 31/21 A2989
The destination TAF reads: KXYZ 151720Z 1518/1624 21014G22KT P6SM VCTS SCT040CB BKN070 TEMPO 1519/1522 3SM TSRA BR BKN025CB FM152300 32006KT P6SM SCT060.
Sunset at the destination is 2035 local. Your personal minimums require 5 SM visibility and a 3,000 ft ceiling. The destination has one runway, no instrument approach, and limited services. You have 3.5 hours of fuel aboard and no alternate currently planned.
1. Decode what VCTS and the TEMPO group actually tell you, and identify the window of highest risk for your arrival.
Read the worked answer
VCTS means thunderstorms in the vicinity - between 5 and 10 statute miles of the reporting station, not over the field. It is a warning that convective cells are already producing, not a forecast.
The field is currently 8 SM with a 3,500 ft CB layer and a 5,000 ft broken deck - marginal against your 5 SM/3,000 ft personal minimums even before the TEMPO window.
TEMPO 1519/1522 is a temporary fluctuation valid 1900Z to 2200Z, meaning conditions are expected to intermittently drop to 3 SM in thunderstorms and rain, mist, with a 2,500 ft CB ceiling. "TEMPO" is not a guess: it means conditions are expected to occur for less than an hour at a time during that window.
Convert to local: 1900Z–2200Z is roughly 1500–1800 local. Departing now puts you over the destination at about 1600 local - squarely inside the TEMPO window, at the climatological peak of afternoon convection, and only about 4.5 hours before sunset on a field with no approach and no alternate.
2. Make the go/no-go decision and defend it with the evidence in the briefing.
Read the worked answer
No-go as planned. Reasoning, not instinct:
- Conflict with personal minimums. You required 5 SM and 3,000 ft. The forecast explicitly allows 3 SM and 2,500 ft during your arrival window, and the current observation is already 8 SM with CBs.
- Timing is wrong. The TEMPO group brackets your ETA. Even a 30-minute delay does not clear it; the improvement is not forecast until FM152300 (1900 local), well past a safe arrival with daylight to spare.
- No escape route. One runway, no approach, no alternate planned, and terrain/darkness closing in after 2035. A diverting airplane needs somewhere to divert to.
- Gust front risk. The surface wind is already 210 at 8 gusting to 20 at a station 30+ nm away. Outflow from collapsing cells can push 30–40 kt gusts and a wind shift well ahead of the visible rain.
Acceptable alternatives: depart so as to arrive after 2300Z (1900 local) with a daylight margin and a confirmed alternate; or choose a different destination outside the convective area; or wait until the next morning. The unacceptable choice is to launch and "take a look" - that converts a forecast problem into an in-flight emergency over a field with no options.
3. If a dispatch decides to launch anyway, what specific in-flight decision points and fuel planning should exist before takeoff?
Read the worked answer
Build decision gates before you need them:
- A hard abort point 30 nm out - if the destination METAR shows a CB within 10 nm or a ceiling below 3,000 ft, you divert; you do not continue to "check it out."
- Pre-selected alternate with weather at or above your minimums and a published approach or at least a wide, lit runway. Fuel must cover destination, missed approach, alternate, and 45 minutes at normal cruise - compute it in gallons, not "about three hours."
- Escape headings for each segment, away from the higher terrain and toward lower minimums.
- Lightning and radar awareness - any question about a cell's location means a 20 nm standoff, because hail and severe turbulence can extend well beyond the visible precipitation core.
- Sunset discipline: your last arrival must leave enough daylight to see the runway environment and any obstruction on a single-runway strip; a 2035 sunset means an arrival no later than about 2000 local if you want a visual margin.
Note that the gust spread (8G20) at a station 30 nm away is already evidence of outflow - treat it as a warning, not trivia.