Enchantments (Core zone). 5 days, September 30.

http://www.ncdc.noaa.gov/cdo-web/
For historical weather data

http://www.weather.gov not .com for the weather forecast. What's nice about the .gov website is that there's a topo map where you can select the area in the mountains and are not limited to a city which may be 20+ miles away and 1000s of feet lower in elevation.

Obviously both will just give you a rough idea and are not necessarily exact. Any time I backpack in the mountains here in Washington, 12 months per year, I'm prepared for temperatures to reach freezing regardless of what the forecast reads. Fortunately I can do this with a sub 10lb base weight.

I can't speak for the Enchantments at the end of September but for the Cascades, you may find yourself hiking in shorts and coming home with a sunburn (I did this last year in the Cascades) as likely as you may freeze your arse off.

Plan for lows reaching 30* with mixed snow/rain and you'll probably be fine.

Which way are you planning on going in? Since you've got a Core permit you can camp in any of the zones. I would consider doing it as a thru. Take your time and work your way through the different zones. There is a shuttle available out of Leavenworth that you could arrange for a pickup to take you back to whichever TH you leave your vehicle at… I've hitched down HYOH, YMMV, etc.

I ordered historical weather data from the Upper Wheeler weather station and it just hit my inbox. This is near or in Wenatchee so 1/2 hr away from Leavenworth and a few thousand feet+ lower than the Upper Enchantments so it's likely that it was cooler there (edit "there" being Enchantments). There's probably a weather station closer to Leavenworth but I didn't invest any time researching it.

24hr max/minimum recorded temperatures for those dates in 2013:

Sep 30 40*/32*
Oct 1 43*/32*
Oct 2 39*/34*
Oct 3 43*/33*
Oct 4 46*/34*

Two weeks before this, temps were 70*/49*
Two weeks after this, temps were 58*/44*

As an FYI, when you "order" the data, it's free of charge. Don't worry when you see that the order has been placed in a shopping cart. All you have to do is give them your name and email.

It's possible you'll get a snow storm, but not super winter conditions

Could rain a lot

Likely you'll get nice weather but cool

Look at weather reports as you get closer

Thanks a bunch guys!

Best bet is to go look at historical averages rather than any particular year such as 2013. I often use PRISM's website for that purpose. You can get both temp and precip avg for various time periods for any lat/long and date combination. Very useful. If you are GIS savvy you can download the data and create a temp profile along a route to see how it varies.

Do you have a link for this site mate?

I have gotten snow and night time temps into the upper 20's in August. That late in September is solid shoulder season in the Enchantments. I would take a 20 degree bag and a puffy for sure.

http://www.prism.oregonstate.edu/normals/
http://www.prism.oregonstate.edu/recent/

Thanks a lot for that. Is there a way to zoom in more on specific areas with names etc. I can't really pinpoint exactly where to look on that.. just a rough area.

Try the Geo_explorer here:
http://prismmap.nacse.org/nn/

Thanks again!