Did the manufacturer give you a figure for the beam width - how critical the alignment is?
You should consider using a mesh dish to reduce wind loading.
Any sold flat or concave antenna is going to have to be mounted extremely well or it will move with the wind.
There are very high gain panel antennas too. Typically PCB elements and a reflector. 17db is quite achievable at 2400 MHz in a compact design.
At 5 GHz the loss in the PCB material might be too high.
You should consult your area's historical weather data to find out what kind of winds are seen there during storms.
I'll be using 5 GHz so a mesh dish isn't really that suitable (or available from UBNT). I need the higher gain to work over such a long distance. Remember, these radios don't output a signal any stronger than good quality Wi-Fi gear and the EIRP must remain within the ACMA limits.
Alignment will be fairly narrow being 5 GHz. There are 3 options for antenna that I'm looking at:
AF-5G23-S45 - 37.8cm, 23 dBi dish (10° beamwidth)
AF-5G30-S45 - 65cm, 30 dBi dish (5.8° beamwidth)
AF-5634-S45 - 105cm, 34 dBi dish (3° beamwidth)
If I can get away with using the smaller dishes, that's great, but I have trees to contend with so it will depend on how much of the 1st Fresnel zone is obstructed.
All three antennas have a 45° slant which should assist with noise rejection.
Ubiquiti also make a "Precision Alignment Kit" for fine tuning alignment over long distances. Don't forget though, I'm "only" needing 55 KM range, the Airfibre radio that I'm considering will do 200+ KM with the right antenna and conditions.