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I moved from New Jersey to North Carolina last fall and my migraines jumped from once per 2 months, to once a week, a very dramatic increase. I started taking a supplement which seems to have helped quite a bit (it's been 48 days since my last migraine). My theory is the near constant thunderstorms in the summer in NC, but as with everything else that seems to be a trigger for me, it's never 100% a trigger and therfore perhaps not a trigger at all.
This has worked very well for me. I don't normally buy into hippie shit but there's sound science behind a lot of the ingredients (especially butterbur) and it doesn't have any side effects as far as I can tell other than turning my pee green.
Propranolol is the only other thing that's ever worked, and it worked the best of anything, but that stopped working after about a year at the dose I was on, and the side effects at the higher doses got bad enough that I couldn't handle them.
You have to take it for 4-6 weeks before it starts working. I have a similar headache pattern to you, where I'll pretty much constantly have a mild to moderate headache, but it turns into a migraine that gets worse over 7-21 days before it "breaks" and I finally get some relief. The preventa-migraine keeps me from getting the mild to moderate constant headache and then makes the migraines treatable with OTC medicine (usually a couple rounds of advil).
Ask your doctor about a magnesium supplement as well, if you haven't already tried it. Something like this one.
I live near Raleigh, NC. Normally weather is a big trigger for me, but this preventative has really done the trick; not a single Joaquin related migraine!
I take this and it has seemed to help as well. It took a couple of months but it's working. http://www.amazon.com/gp/product/B00G7VX9S6/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1