I (and many others) have oft repeated the fact that the monsoon starts at Philmont around mid-July, so treks in mid- to late-July and in August tend to be wetter. I suspect a lot of folks don't understand why (that is, what the heck are we talking about?)
To most Americans, "monsoon" implies torrential rains for months on end in southern and southeastern Asia. In weather terms, however, monsoon is not an phenomenon that is specific to southern and southeastern Asia, but rather a general term, to whit: a seasonal change in the prevailing wind currents over a region (any region). In India and the rest of southern/southeastern Asia, the monsoon is a 3 month change where the wind blows from the southwest. This entrains huge amounts of tropical moisture off the Indian Ocean (one heck of a humidifier!) over the continental land mass - and yeah, it rains buckets as a result.
At Philmont, the prevailing wind flow for about 9 months of the year is from the northwest. This air has already been "scrubbed" of most of its moisture in its travels over numerous mountain ranges from the Pacific Northwest to New Mexico. So, generally drier conditions.
In early to mid-July, however, the monsoonal flow over New Mexico changes from northwesterly to south and southwesterly. This means that the primary moisture source becomes the Gulf of Mexico and the southern Baja coast of Mexico - like the Indian Ocean two hot, moisture-rich areas. So, you start seeing a dramatic increase in rain and truly impressive afternoon thunderstorms as this tropical moisture gets dragged over the mountain ranges throughout the desert southwest. This is easily viewed on the Weather Channels national radar images - you can actually see the thunderstorms in New Mexico heading south to north across the Ranch.
Is this a hard and fast rule? Of course not. There is a constant battle between the northwesterly flow and the monsoonal flow all year long; the monsoonal flow is just a bit stronger for 3 months. A particularly strong high pressure system in the desert southwest can reverse the trend (back to northwesterly) for a week or more. In addition, conditions in the Gulf of Mexico and off the southern Baja coast are not always favorable for pumping up a lot of moisture - there's plenty of times when you can see on the satellite imagery that both of these areas are 100% cloud-free. Translation: drier weather at Philmont for the next few days. This is why people can go to Philmont in late July and August and report pretty dry weather conditions for their trek, or sporadic thunderstorms that missed them. Conversely, people that go in June and early July can have very heavy and nearly constant rain (e.g., Wally Feurtado, the co-author of Cooper Wright's Advisor's Guide, once had a June trek where it rained heavily nearly every day. And on one of my mid-90's treks, it rained very heavily for 6 out of 7 days, then the next 5 days were "perfect." Go figure.)
However, I agree with those who have stated that the weather is just another part of Philmont. We have an expression in my Troop for all camping outings which I think you'll appreciate: "Weather or Not." Got lemons? - make lemonade.
- Dr. Bob
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