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Posted by Ballpark Frank (69.178.8.75) on 13:04:36 02/02/16
In Reply to: Lamar posted by Amber
Amber,
What likely occurred last August was a dose of upslope moisture delivered courtesy of the "Arizona Monsoon". This weather phenomenon is the primary engine behind the moisture-laden late summer thunderstorms that form over the Continental Divide each afternoon, and slowly march east. It has to do with a seasonal low pressure system that sets up to the south of Colorado, with counterclockwise circulation picking up moisture from the Gulf of Mexico, sweeping it north, and slamming it up against the east side of the Continental Divide, where convective heating provides the ignition source to produce those thunderstorms. Occasionally, but thankfully, not too often, the low sets up further north, still gathers Gulf moisture, and brings it up to Central and Northern Wyoming. This system can develop at any time in August, but the later you go in the month, the less chance there is of encountering it, particularly the last 7-10 days of the month. On the flip side, the later you go in August, the greater the possibility of running into another relatively rare weather anomaly, i.e. an early winter storm pattern. Over the years, we have seen these Pacific storm systems drop snow on the Northern Rockies in late August and early September. There have been a few famous dumps of snow on Labor Day Weekend that isolated backpackers, campers, and early season hunters temporarily. Thankfully, these are more rare than the Arizona Monsoon.
In my experience, spanning several decades, the last part of August has the most reliably dry weather. It is also the least crowded part of August, in terms of park visitation, thanks to schools starting in various parts of the country. That's the good news. The bad news is that the bison rut starts slowing down toward the end of August. Also, the days are shorter and the temperatures are cooler. The streams and rivers will be a bit lower, which is good for fording them. I'm not sure how it affects the fishing.
As Granite Head mentioned, fire season typically starts producing smoky skies over the park as the summer proceeds. Most years, late August is one of the worst times for smoke. The one saving grace is that occasionally, you get a weather system that produces substantial wind, which will disperse the remnant smoke, and provide a few days respite.
All in all, given what you have shared, I would recommend visiting in late August.
Ballpark
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