Normally, if the client is flexible, I give them the option of picking a different day if the weather is terrible. Sometimes that is not possible, especially if the portrait is to include out of town family members or rare occasions when all family members are present. I know in my own clan how precious this time can be when we are all under one roof.
In poor lighting situations like these, I bring my own sun in the form of a Paul C. Buff Parabolic Light Modifier (PLM). What the heck is that you ask? It's a huge 5 foot umbrella for my Einstein flashes. On set I call it my moon dish because I am pretty sure that the aliens can see that thing flash from outer space. It literally looks like a gigantic silver radar dish.
The advantage of the PLM is that it can create soft but directional light over a focused area that can look a lot like natural sunlight. I use a warming gel to enhance the effect. Many people would not know that these photos were taken on a day sans sunshine.
I took one dramatic shot where I decided to keep the storm clouds in the photo to prove it was about to rain...
UPDATED: 10/20/2012
Many months later, I found out what happened with the photos... they got turned into a big poster seen below. I think that is cool.
