
NEMO
Disco 15 Series
Spacious for side sleepers; bulky, temp rating optimistic.

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I also camp on the coast \[PNW\] where it's rarely colder than 35F but it's raw/damp/windy weather. I use a Kelty Cosmic 0 F Down Mummy sleeping bag. I've only had it for this season but it's a well - made quality sleeping bag and warm in these temps. I paid about $175.
While it doesn't get much colder than 32F on the PNW coast, it's damn wet and windy. Last season I was suffering in my 20+ year old Sierra Designs \[synthetic fill\] so after long research I bought a Kelty Cosmic 0 Down sleeping bag for $165 \[before tax\]. It was on sale for Labor Day, but there are still end-of-summer deals out there. I'm just tent camping at parks, so weight was not an issue. I've always preferred down bags and as a retiree I need the extra warmth.
The typical weather in coastal PNW this time of year is lows in the mid 40s and I used to bring a Sierra Designs 20F synthetic bag, but with humidity at 90% and Pacific winds, it isn't comfortable. A fleece liner and extra blankets didn't make a difference so I now just bring a Kelty Cosmic 0 degree down bag.
A Kelty Cosmic 0° is around $200. It’s the often mentioned low cost down bag. It will cut your weight in half.
We already have Kelty Cosmic 0° bags from when we just car camped, but dang, they're huge and 5lbs!
Warmth starts from the ground up. Starting with a good thermal pad. Not all pads are designed for cold temperatures and using the wrong pad is why many people get cold. -15 degree bag should work just fine. That means if you had nothing else but a sleeping bag it would keep you warm at -15 degrees. Adding a good thermal pad will keep you warm. When buying a sleeping bag you will need to consider your height. If you are using a long sleeping bag and you are 5’10” tall, all your heat is going to the empty space below your feet. Getting a bag that fits you perfectly will help keep you warm. I hunt all over the PNW for the past 35+ years. My fall sleeping bag is 15 degrees my winter or late hunt is -15 degrees. Combined with my thermal pad I have never woken up cold. I use a SlumberJack for my 15 degree bag and I use a Marmot. REI Marmot Trestles Elite Eco 0 Sleeping Bag $209 Kelty Cosmic Down 0 Sleeping Bag. Reg $294 on sale $219. I would suggest investing in a better quality thermal pad. Therm-a-Rest NeoLoft Sleeping Pad Or NEMO Tensor All-Season Ultralight Insulated Sleeping Pad **** Best ****
Absolutely seconding this, although I'd probably go with the 0° down cosmic. Late season you very well may get snowed on/have some low temp nights where you'll be at OP.
Not who you asked but I'd consider Kelty to be a lateral move from OP's current Coleman. Both make reasonable bags for car camping, but not anything I'd want to carry. Personally, I use a 32 degree Marmot bag for backpacking and it replaced(and is actually warmer than) a 0 degree bag from Kelty. The UL crowd really likes quilts instead of a traditional sleeping bag so that might also be worth considering. I'd also mention that the sleeping pad is a major factor in warmth so don't forget that. I bought a Neoair when they first came out and that made a huge comfort difference for me compared to the thin foam pads I had before.

NEMO
Disco 15 Series
Spacious for side sleepers; bulky, temp rating optimistic.
Hammock Gear
Classic Burrow 20°F
Budget king; very warm but heavier than ultralight.

NEMO
Disco 30
Spoon shape for side sleepers; prone to zipper issues.

Western Mountaineering
VersaLite
Durable, very warm; premium price, too hot in summer.

Katabatic Gear
Alsek 22°F Quilt
Versatile 3-season quilt; secure pad attachment, high price.

Ranked #1
NEMO - Disco 15 Series

Ranked #1
Kelty - Tru.Comfort Doublewide 20
Ranked #1
Enlightened Equipment - Revelation APEX

Ranked #1
NEMO - Disco 15 Series

Ranked #1
Feathered Friends - Egret UL 20/30

Ranked #1
NEMO - Disco 15 Series