Photo: Sinjutchupti · CC BY-SA 4.0Midwinter lows in Port Costa stay above those of 89% of U.S. towns
High summer brings afternoon highs near 81°. It is a dry place: less rain and snow fall here than on 89% of U.S. towns. Compared with 1901–2000, the average now sits 1.6° higher. The outdoor season runs from March to November, with highs of 65–82°.
A year in Port Costa at a glance
Each month’s typical high and low, and what it brings.
What stands out about Port Costa’s climate
Compared with towns across the U.S., with Port Costa’s place in California beside it.
What a year in Port Costa feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
A freeze most winters
Frost and day counts come from NOAA’s station at Vallejo, CA, 4 miles from the town’s center, 140 ft higher.
The 2023 USDA map puts Port Costa’s center in zone 9b: the coldest night of a typical winter, averaged over 1991–2020, reaches about 28°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 21 inches of rain a year
January is the wettest month and July the driest. Rain falls on about 68 days a year. Snow is rare: about 0.0" in a normal year at Napa State Hospital, CA, so most winters see none.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Port Costa
71% of the work is heating. Months averaging under 55° run from November through March.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Vallejo, CA.
Living with Port Costa’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
Zone 9b. Frost comes about 7 winters in 10, so tender plants can often stay out through winter.
Months with highs of 62–84° and lows of 40° or more: March, April, May, June, July, August, September, October and November. 0.8 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Port Costa from natural hazards, earthquakes are the largest share (79%); heat waves, cold waves, winter storms and ice storms together are 1%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Port Costa and similar climates
TownRatings has no climate map of the towns around Port Costa yet.
Climates most like Port Costa’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| Port Costa, CA | 40° | 81° | 21" | 0.0" |
| Winston, OR | 37° | 86° | 33" | 0.0" |
| Mercer Island, WA | 37° | 77° | 37" | 3.8" |
| Makawao, HI | 53° | 75° | 23" | 0.0" |
| Alpine, TX | 30° | 90° | 15" | 2.2" |
Temperatures and precipitation come from NOAA’s 5 km grid; snow and day counts from the stations named on this page.
Warmer than it used to be
The last 30 years in Port Costa averaged 1.6° warmer than the 1900s. The biggest change is in January (+2.1°).
A bigger shift than in 63% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How Port Costa compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Port Costa’s place among California’s.
Port Costa weather by month
Highs, lows and precipitation from NOAA’s grid at Port Costa’s center; snow, wet days and snow on the ground from Napa State Hospital, CA. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 57° | 40° | 4.3" | 0.0" | 12 | 0 | 45° – 54° |
| February | 61° | 43° | 4.0" | 0.0" | 11 | 0 | 47° – 57° |
| March | 65° | 44° | 3.1" | 0.0" | 10 | 0 | 49° – 60° |
| April | 68° | 46° | 1.4" | 0.0" | 7 | 0 | 53° – 61° |
| May | 73° | 50° | 0.7" | 0.0" | 4 | 0 | 57° – 68° |
| June | 79° | 54° | 0.1" | 0.0" | 1 | 0 | 63° – 69° |
| July | 81° | 55° | 0.0" | 0.0" | 0 | 0 | 65° – 72° |
| August | 82° | 55° | 0.0" | 0.0" | 0 | 0 | 65° – 73° |
| September | 81° | 54° | 0.1" | 0.0" | 1 | 0 | 64° – 72° |
| October | 75° | 51° | 1.0" | 0.0" | 4 | 0 | 60° – 69° |
| November | 65° | 45° | 2.2" | 0.0" | 8 | 0 | 48° – 59° |
| December | 57° | 40° | 4.1" | 0.0" | 12 | 0 | 45° – 53° |
| Year | 70° | 48° | 21.0" | 0.0" | 68 | 0 |
How much a month can swing
A normal is an average. May swings the most: from 1991 to 2020 its average ranged from 57° to 68°. June is the most reliable, staying within 6°.
Where Port Costa’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Port Costa’s center.
- Snow: Napa State Hospital, CA.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Vallejo, CA, 4 miles from the town’s center, 140 ft higher; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Port Costa’s climate
What is the weather like in Port Costa?
A typical July afternoon reaches 81°, a January night drops to 40°, and snow is almost unheard of. Milder January nights than 89% of U.S. towns; cooler July afternoons than 87% of U.S. towns.
How cold does it get in Port Costa in winter?
January nights average 40°. About 5 nights a year reach 32° or below. In the coldest January of the last 30 years the month averaged 45°.
How much snow does Port Costa get?
About 0.0 inches in a normal winter at Napa State Hospital, CA, so most winters see none.
How hot does it get in Port Costa?
July, with a typical high of 81° and a low of 55°. About 36 days a year reach 90° and 7 reach 100° (Vallejo, CA).
When is the first and last frost in Port Costa?
There is no reliable frost date: a 32° night comes in about 75% of winters at Vallejo, CA, so most years bring a freeze but some pass without one.
What growing zone is Port Costa in?
Zone 9b on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 28°.
What is the best time of year to visit Port Costa?
By one simple rule (highs of 62–84° and lows of 40° or more), March, April, May, June, July, August, September, October and November. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Port Costa getting warmer?
Yes: 1991–2020 averaged 1.6° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 63% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Port Costa.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.