Weather Blog

'Summer school': Learn how to forecast summer weather

'Summer school': Learn how to forecast summer weather

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

For you armchair meteorologists, I thought I'd give a little "summer school" reference here so you can play along at home to see how the forecast is shaping up, and show what to look for on forecasting models. 

First, a quick primer: Air flows from higher pressure to lower pressure. Thus, the main trick in the summer time is to see where the pressure field sets up. If you've got higher pressure offshore and lower pressure inland, that creates a west wind that comes off the ocean, and is typically a cooler, cloudier day. On the flip side, a thermal trough can cause lower pressure offshore and bring a warming east wind.  Varying degrees of west, east, or no wind are what drives the summer around here.

How To Read The Forecast Models:

On this version, the isobars (lines of constant pressure) are the black lines. (The red and blue dotted lines are "thickness lines" that help denote temperature.) The green and blue colored blobs are predicted areas of precipitation.

Strong Marine Breeze:

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.

Look for tight packing of isobars strong high pressure to the west and lower pressure inland. Each isobar on this map denotes 4 milibars of pressure and when we see roughly 4 milibars difference between the coast and Seattle, that is a very strong push.

Moderate Marine Breeze

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.

Here, we still have a good west-to-east wind, but not as strong. Look for maybe about 1-2 milibars difference between the coast and Seattle.

Light Marine Push

If we have a light marine breeze, that usually means just brief to no morning clouds, with sun in the afternoon, but a sea-breeze developing in the afternoon due to inland heating drawing in air off the water. 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. This is probably the most pleasant pattern the region can find itself in.

Here, we are talking maybe 1 milibar of onshore flow.:

The "No Breeze" Pattern

If the pressure difference is zero or close to it, the hot summer sun takes over and it's a full sunny day, and temps will make it into the 80-86 degree range everywhere. We can still get a light seabreeze but without much of any kind of westerly push to help it along, the breezes are light and confined right to the shoreline, so there is not so much temperature difference between those who live near water and those who live farther inland. The coast usually gets into the low-mid 70s.

Note how there are hardly any isobars around, meaning no wind at all. The differences here are more subtle versus the light push version I wrote about before, and other factors come into play such as the warmth of the air mass (Looking at this example, this is probably a good 85-87 degree day in Seattle due to how high the thickness values are) but even a light flow versus no flow can be the difference between a high of 80 and 86 in Seattle.

Offshore Flow: The Summer Cooker

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.

The usual culprit here is a thermal trough, also known as a "heat low" which is a very hot fetch of air that moves up from the south. Since hot air rises, it creates its own lower pressure on the surface where the air has "gone missing", so to speak.  So air races in from the outlying areas to replenish this missing air.

When the trough is to our west, that means we get an east wind -- a double whammy for heat. First of all, that air is coming from Eastern Washington, which means if it's summer, they're usually hotter than we are, so that air being blown our way is quite toasty to begin with. Second, as the air rises and sinks down the western slopes of the Cascades, it sinks and compresses, which causes the air to heat further.  This also occurs on the western Olympic Mountain slopes and is why Forks really bakes on these kind of days, despite being right on the ocean.

Anyway, what to look for here is what I call the "upside-down horseshoe" off our coast. Here are two examples:

Here, now you can see things are flipped -- higher pressure is to our northeast/east and lower pressure is offshore. What I do is draw a line through the apex of the horseshoes -- this is where the thermal trough exists, and air flows toward the line. For example, here is a blue line I drew for the first example:

As long as that line is to our west, the wind blows toward it -- from the east here -- and it's a hot day. It stays hot until that line moves inland and to our east. That usually triggers a moderate or strong marine push as the cooler air rushes in behind it.

The Kitchen Sink

And then there's everything else, when we get rain systems and such which are their own weather beasts and it'd probably turn this blog entry into a full textbook to explain them all :)

Do It Yourself Forecasting

Now that you have a handle on how to forecast the wind differences, here is how to track what they are right now. 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/sew/dients.php

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 around +1.0. A moderate push (Scenario 2) would be +1.5 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. This is good for tracking winter windstorms -- when that number gets over +10, that's noticably windy. Historical wind storms get to be around +17 or greater with the record being +23.2 in the Haunkah Eve Storm. 
  • SEA-EAT is Seattle to Wenatchee -- a good indicator of cross-Cascade winds. Positive would be west wind, negative is east wind.