What is Seattle's usual summertime pattern?

The neat part about forecasting weather in the Puget Sound area in summertime is that it basically falls into six different patterns, which are all dependent on the prevailing winds.

  1. If you have a strong westerly/marine breeze, we usually get stuck under morning clouds for most of the day (perhaps some drizzle) and highs in the mid-upper 60s.

  2. If we have a moderate westerly marine breeze -- which is the most common day-to-day scenario around here -- that's our morning clouds, afternoon sunshine, with highs in the 70-75 range -- a tad warmer away from the water.

  3. If we have a light marine breeze, that usually means just brief morning clouds, with sun in the afternoon, but a sea-breeze developing in the afternoon. That will usually cap our temperatures in the 74-80 degree range in Seattle -- depending on the strength of the sea breeze -- and there is a noticeable difference in temps near Puget Sound as opposed to inland areas (which can get into the mid 80s). Yet the Coast is still in the mid 60s.

  4. If we have pretty much no breeze, it's a full sunny day, and temps will make it into the 80-86 degree range everywhere, but not so much a gradient between water and inland temps. Coast usually gets into the low-mid 70s.

  5. If we have an easterly/offshore breeze, it's a full sunny day, and temps can range anywhere from 85 to 99, depending on how strong the breeze is (stronger the breeze, the hotter it gets.) Eastern foothill areas are usually 5-10 degrees warmer than Seattle, and it even gets into the 80s/90s on the Coast.

  6. And then there's everything else, when we get rain systems and such which are their own weather beasts.

Do It Yourself Forecasting

There are a lot of different factors in forecasting summer weather winds , but as a general "guesstimate", check out this link:

http://www.wrh.noaa.gov/Seattle/products/DIENTS

This will give you a measurement of how strong the differences in pressure are between two places. This is fairly cryptic, but here's a quick lesson in decoding this product.

The DD/HH is the day and hour of the observation. The top observation is the most current. It is listed in UTC time (the old Greenwich Mean Time), which is 8 hours ahead of Seattle on standard time, and +7 on Daylight Savings Time. The units are in millibars of pressure.

One of the best columns to look at is HQM-SEA. That's Hoquiam to Seattle. If the number is positive, that means the pressure is higher in Hoquiam than in Seattle, so air would flow "onshore". A negative number would be offshore flow.

Again, this isn't a slam-dunk forecasting tool, but take it as a "rule of thumb" type forecasting tool.

A light marine push (Scenario 3 above) is probable when the HQM-SEA number is between +1.0 and +2.0. A moderate push (Scenario 2) would be +2.0 to +3.5, and strong push (Scenario 1) would be greater than +3.5. If it's between
-1.0 and +1.0, that's a weak flow day (scenario 4), which would be sunny and warm . A strong negative number (less than -1.0) would mean a strong offshore flow day (scenario 5), and very warm temperatures.

Some others to track which way the wind is going:

  • OTH-SEA is North Bend, Oregon to Seattle.
  • UIL-BLI is Forks to Bellingham (good indicator of winds down the Strait of Juan de Fuca -- positive numbers mean west wind.)
  • PDX-BLI is Portland, OR to Bellingham (positive number is southerly wind through I-5 corridor. Negative is north wind)
  • SEA-EAT is Seattle to Wenatchee -- a good indicator of cross-Cascade winds. Positive would be west wind, negative is east wind.