Alternate selection when the destination forecast is marginal
You plan a 2-hour VFR flight to destination D, arriving at 17:30 local. The TAF for D reads: BKN 1,500, visibility 8 km, temporarily 17:00–19:00 visibility 3,000 m in mist, BKN 800. The TAF for alternate E, 28 NM from D, reads: SCT 2,000, visibility 10 km, no significant change. E has a precision approach and full lighting; D has no approach aid. Sunset is 17:50. Your aircraft has no de-icing and you are not night-current. Fuel allows 45 minutes of holding plus a diversion to E and 30 minutes reserve.
1. Is E a valid alternate for this flight, and does the plan meet the alternate requirement?
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E is 28 NM away with a forecast of SCT 2,000 and 10 km, which comfortably meets the alternate weather requirement (typically ceiling at least 1,000 ft above the alternate minimum and visibility at least 2 km, or the national equivalent). E has a precision approach and lighting, so it is a suitable alternate.
The problem is not the alternate's weather; it is the fuel and the timing. Fuel covers 45 minutes of holding plus the diversion to E plus 30 minutes reserve. That sounds adequate, but the TAF for D shows a temporary deterioration from 17:00 to 19:00, which covers your entire arrival window. You should plan on the temporary condition being the actual condition, not the prevailing one.
If D is at 3,000 m in mist with BKN 800, you may not be able to land there at all. The diversion to E must then be flown, and the 45 minutes of holding is consumed before it. The plan is legal on paper only if you treat the temporary condition as the expected one and confirm the fuel still works.
2. Recompute the decision with the temporary condition assumed, and state your go/no-go.
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Assume D is at 3,000 m and BKN 800 on arrival. That is below VFR minima for a controlled aerodrome in most states (typically 5 km and 1,500 ft ceiling), so a landing at D is unlikely. The flight becomes: 2 hours to D, a missed approach or go-around, then 28 NM to E.
Time budget: 2.0 h to D + 0.2 h to reposition + 28 NM at, say, 110 kt = 0.25 h to E = about 2.5 h. Fuel for 2.5 h plus 30 minutes reserve must be available. If your 45 minutes of holding was counted on top of that, it is not needed if you divert immediately rather than hold - but you cannot count on being able to divert immediately if D is busy.
Go/no-go: no-go as planned. Either (a) delay departure so arrival is after 19:00 when the temporary condition ends, or (b) file E as the destination and treat D as the alternate, or (c) carry enough fuel to divert to E with 45 minutes of holding intact. Option (b) is cleanest because E has an approach and lighting.
3. Sunset is 17:50 and you are not night-current. How does this change the plan?
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Arrival at 17:30 leaves only 20 minutes of daylight. Any delay - a go-around at D, a diversion to E, or holding - puts you airborne after sunset. As a non-night-current pilot you may not legally continue in command after the end of evening civil twilight (or after sunset, depending on the national rule), so the plan has no buffer.
The fix is to move the whole schedule earlier: depart so that you arrive at D (or E) at least 30–45 minutes before sunset, giving time for a diversion in daylight. If that is not possible, the flight should not be conducted as planned. Do not rely on E's lighting to solve the problem: the issue is your currency, not the aerodrome's equipment.