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Outdoors one of the more effective strategies to overcome the excessive contrast is put the sun at the backs, expose to keep what the sun hits under clipping to retain detail on skin, hair and white clothing then light the front side with your flash exactly as you would indoors. The net effect is similar to the look of a studio shot with a hair light from behind.
On the front side faces in the shade look at the faces and eyes critically BEFORE adding flash. Most of the time the skylight (which will become your fill when you add flash) will be shaded from the eyes by the brow. The only solution is get the faces up into the light by raising the camera position, which gets light past the brow when the faces look up at the camera.
If you do pose the faces into the skylight like that and were to expose for normal faces without flash you'll wind the faces have very flattering natural soft modeling on then but the backlit sunny parts are 3 stops overexposed.
What you do next is expose instead for the sunny highlights. The faces become underexposed but still have the skylight in the eyes and flattering modeling. Now add the flash. Where do you want to position it? Where it will hit the faces at a downward angle and direction matching the skylight modeling. In terms of role the flash becomes a supplemental "key" light, not "fill" because it will be off axis creating flash highlights over the skylight pattern in the same places. What controls the tone of the shadows and the lighting ratio? The skylight where the flash doesn't hit.
The mistake photographers make with flash outdoors is thinking of it as "fill" and putting it low at eye level or below. What that does is overpower and cancel the modeling of the skylight which most don't even notice or use to advantage in the overall lighting strategy.
The significance of using the flash as "key" vs. "fill" has an impact on modifier choice. Aligned on the same vector relative to the face as the skylight the skylight acts like a huge softbox creating very soft "open shade" modeling on the face. All you need to do with the flash is add brighter highlights on top of that foundation.
If you try the strategy I suggest above starting from a baseline of direct flash, no modiifer, then add progressively larger modifiers the change you see in the character of the lighting on the face will be mostly in the highlights because with one flash the fill controlling the shadow tone comes from the sky. As your modifier gets bigger it's reflection in the eyes (catchlights) will get bigger and more attractive. The "hot spot" specular glare you see on oily damp skin will be reduced as the larger source bounces the light off the round cheek in more directions. You'll see somewhat smoother gradients on the edges of the highlight/shadow borders for the same reason but the core "umbra" part of the shadows the flash doesn't reach will be sky filled and dark. Darker than what looks "normal".
If you use that strategy its best to keep the flashed centered over the camera, which minimizes the shadows seen but the camera. If you want to make the shadows lighter than the shadows appear in the skylight fill alone you need to add a second flash for fill.
The difference between using a modifier indoors vs. outdoors is the spill fill factor off the ceiling. If you switched from a 24" softbox to a 5' umbrella indoors the shadows will wind up looking softer with the umbrella due to all the spill bouncing off the ceiling and walls back into the shadows. Outdoors you don't have any spill fill. That will affect the results a single flash strategy will produce. Indoors you'll typically get so much spill fill with an umbrella you won't need a second flash as fill to make the shadows lighter. Outdoors, even with the sky acting as fill you will because it is necessary to cut ambient exposure to keep the highlights below clipping and retain detail.
Outdoors in backlight with key and fill flashes the concept is similar. First expose for sunny highlights under clipping, next pose the faces up into the sky as you would by a window to get the desired modeling and light in eyes. Then turn on fill at chin level under the camera and raise power, overlapping the skylight, until you see desired detail in black clothing. With just the fill the shot will look flat (because the sky modeling is canceled) and underexposed (because you adjust fill for shadow detail not the faces). Finally turn on the key flash placed at the same angle as the skylight and raise it's power until the front of a white shirt is just slightly darker than the part next to it hit by the sun but exposed for detail. The net effect is similar to a studio shot with key and fill adjusted for a full range of normal detail and hairlight exposed below clipping.
What you will find with this strategy which complements the soft skylight vs fighting the hard sun is you don't need huge modifiers to get flattering results. The results will be more flattering when using two flashes arranged vertically above and below the raised camera lens that with a single flash used above because you'll be able to control the tone of the shadows with the fill. Making the shadows on the faces lighter makes the gradients smoother and the overall lighting seem "softer" perceptually.
The advantage logistically is getting pleasing results with smaller modifiers which are easier to manage in the wind. When I use two speedlights I put the master on my bracket as the key light and place the slave directly below and in front of the ladder I'm shooting from to get the faces into the light. With my studio lights I put the key on the top of the stand and attach the fill to the same stand with a super clamp + extension arm. That way there is only one stand to sandbag in place at it's next to the camera (I rest the lens along the side of the stand between the key and fill.
Try the strategy with direct flash and whatever modifiers you already have and you'll see the cause and effect I describe. It won't take long and will help you to make a more informed decision regardless of what you decide on.