In response to the original question, an alternate is required for B, as the weather at the arrival time is now below alternate minima, and a Hazard Alert is broadcast to assist aircraft to avoid hazardous weather situations (ERSA GEN). It is current until the updated TTF or TAF is issued.
3.4.3 The TTF supersedes the TAF for its validity period and is the
current forecast for pilots of aircraft whose arrival time falls within
the validity period.
The TTF supersedes the TAF. If you have updated weather for your arrival, you have to use it (as described in the ATSB recommendations below).
73.2.10 TTF may have either one visibility or two visibilities included in the
report. Operational requirements will apply when:
a. the sole visibility is less than the alternate minimum, or
b. the higher visibility is less than the alternate minimum.73.2.11 Flights which cannot use TTF will plan the flight on the current TAF
until such time as the destination ETA falls within the validity period
of a TTF.
So, a requirement exists (ie an alternate or holding) when visibility is less than the ALTERNATE MINIMA (not the landing minima as some have suggested!)
2.4 In-flight Information
2.4.1 The in-flight information services are structured to support the
responsibility of pilots to obtain information in-flight on which to
base operational decisions relating to the continuation or diversion
of a flight. The service consists of three elements:
a. ATC Initiated FIS;
b. Automatic Broadcast Services; and
c. an On−Request Service.
As you can see it is the responsibility of pilots to obtain information inflight to decide whether to continue or divert.
Operational information, such as meteorological forecasts and the status of airfields and navigation aids, is critical to flight safety. Knowledge of this information is the basis for many operational decisions made by flight crews, such as whether to continue the flight to the planned destination or to divert to another airfield. Operational information obtained prior to a flight should be updated in flight, where necessary, so that decisions are based on the most current information.
The changes explained in this document have meant that flight crews must be more diligent in obtaining their in-flight information. Instead of being routinely provided with updated weather and NOTAM information by an operational control service, the flight crews themselves must now actively seek this information from Flightwatch. Flight crews who have been slow to adapt to these changes may not have received crucial in-flight information.
Some flight crews trained under the previous system could be labouring under the misapprehension that updated operational information is still automatically passed to them. On those occasions when information has not been automatically passed to a flight crew, it may have been perceived as a failure by ATS to provide an in-flight update of operational information, or that the information was not available.
Shortcomings in flight crews' knowledge of procedures for updating in-flight information may need to be reviewed and addressed through company training and checking programs.
(ATSB SAN19980092)
Finally, a real world example from the ATSB of how if weather deteriorates in flight below the alternate minima then you require an alternate. Note that even though the weather is above the landing minima, it still generated a report because the destination and alternate weather had deteriorated below the alternate minima in flight.
200104228 SA227-DC Metroliner
When about 60 NM inbound to Adelaide, at about 1830 CST, the pilot of the Metroliner was advised by ATS of hazardous weather conditions at the destination airport. The pilot requested current weather conditions for the planned alternate aiports and was advised of the weather by ATS. The actual conditions indicated that those planned alternate airports now had alternate requirements, hence were not suitable for a diversion. The pilot declared a PAN* phase due to insufficient fuel for a suitable alternate.
The pilot was cleared by ATS to conduct an ILS approach, and landed the aircraft safely on runway 23.
* PAN is an urgency phase that is normally broadcast by radio.
ATSB Summary
Following an unforecast weather deterioration at the destination, the aircraft subsequently landed in weather above the destination landing minima.
I hope that everyone now appreciates that, if the weather changes inflight to below the alternate minima, they either need to divert, or have enough fuel for holding or an alternate, and that they are reponsible for getting updated forecasts.