First snow. Here is what it does to new grass, open ground, and an irrigation system you did not winterize.
Snow on the valley floor this morning. Here is the honest answer to what it does and does not do, because most of what gets repeated about first snow is wrong in both directions.
Your established lawn is fine
Better than fine. Snow is an insulator. A consistent snow cover holds the ground at a more stable temperature than bare exposed turf, which goes through repeated freeze and thaw and is harder on a plant than being covered and left alone.
The thing that damages dormant turf is not cold. It is traffic. Walking, parking, and particularly driving on frozen or snow covered grass crushes crowns that cannot repair until spring, and the tracks show up in April looking exactly like the path somebody took in October.
New seed depends entirely on where it was in the process
Seed that germinated and put down even modest root is in reasonable shape. It will go dormant and resume in spring, likely thinner than it would have been, and will benefit from overseeding once conditions return.
Seed broadcast recently that had not germinated is a different story. It sits on or near the surface through winter, some of it moves with melt water, some is taken by birds, and germination rates in spring are unpredictable. That is not a disaster and it is not a finished lawn either. Plan on assessing it rather than assuming it.
Recent plantings are the real question mark
Anything in the ground long enough to establish roots will handle this. Anything planted in the last couple of weeks went into cooling soil with limited time to anchor, and the risk is not the cold so much as frost heave, where repeated freezing and thawing physically lifts the root ball out of contact with the soil.
Mulch helps, because it moderates soil temperature swings. Checking in a few weeks to see whether anything has lifted and pressing it back down helps more. It is unglamorous and it saves plants.
If your irrigation system still has water in it
This is the one with a cost attached. A single night near freezing may not be enough to split a line, but it is the first of many and the warning has now been delivered. Above grade components are the first to go, and the backflow preventer is both the most exposed and the most expensive to replace.
If the system has not been blown out, get it scheduled this week rather than after the next cold snap, because the valley is about to be doing this all at once and availability tightens quickly.
The thing nobody mentions
First snow usually melts within a few days and is followed by a stretch of workable weather. People read the snow as the end of the season and stop, which means a week or more of usable time gets thrown away every year. Watch what actually happens over the next ten days rather than what the first snowfall seems to announce.
Snow does not damage established dormant turf and generally protects it by insulating the crown against repeated freeze and thaw. The common sources of winter lawn damage are foot and vehicle traffic on frozen or snow covered turf, and snow mold where long grass or leaf litter traps moisture.
Ungerminated seed near the surface can move with melt water, be taken by birds, or be lost to inconsistent conditions, so spring germination is unpredictable. Seed that germinated and rooted before the freeze generally goes dormant and resumes growth in spring.
Avoid it where possible. Traffic on frozen or snow covered turf crushes crowns that cannot recover until spring, and the resulting damage usually appears as visible paths once the snow clears.
A single night near freezing may not split a line, but it is a warning rather than an all clear. Above grade components, particularly the backflow preventer, are most at risk, and damage accumulates over repeated freezes.