Thursday, June 16, 2016

The Seattle Rain Shadow

During the past few weeks it has been strangely dry around Seattle and central Puget Sound.  Rain comes in off the coast, hits the south Sound and north Sound.  Hits the Cascades to our east.  But leaves Seattle and vicinity weirdly dry.  Several of you have commented about it on this blog or sent me emails.

Has Seattle really been so dry?  And if so why?

Well, it is true.  It has arid within sight of the Space Needle.

Plot the precipitation for the 24h period ending at 6 AM Wed. morning and you can see the pattern...a few hundredths of an inch or nothing in the central Sound, but lots more to the north, south and east.

The radar-based totals from Seattle RainWatch for the same period shows  a similar pattern.

The NOAA/NWS River Forecast Center rainfall total for Tuesday shows the precipitation hole as well.

Impressively, the high-resolution (1.3 km grid spacing) UW WRF model even predicted the :Seattle dry hole" a day before (the map below shows the 24h total precipitation ending 5 AM Wednesday).

And this crazy pattern has been very persistent!  Here is the 14-day precipitation totals:  the central Puget Sound area has been superdry!  My poor plants really needed some serious watering and my grass is turning yellow.

But why so dry around Seattle?  Blame it on the Olympics---we have been in the Olympic rain shadow.

I have talked about rain shadows a lot in the blog and particularly how air dries out as it descends on the lee side of barrier.

During the winter, the winds approaching the Olympics are typically from the south to southwest, producing a rain shadow over the Sequim or Port Townsend.    But this time of the year the coastal winds often rotate into a more westerly direction as high pressure builds over the eastern Pacific.  Westerly winds (from the west) cause the downslope flow and drying to rotate to over central Puget Sound.

To illustrate the current situation, here is a weather map for 850 hPa  (around 5000 ft) on Tuesday at 2 PM, showing winds and heights.   Winds are nearly westerly.

This time of the year rain shadows over Seattle and central Puget Sound are not unusual.  June can be cloudy and cool, but it is rarely very wet around the Seattle Metro area and to its west.  The lawn knows:


KR said...

For the last two days our weather has come from the south -- not west over the Olympics -- and yet the rain shadow persists. How do you explain that?

Unknown said...

We experienced the rain shadow a lot this winter up by the Canadian border. While Seattle was near record rainfall, we were barely at average up here (below average some months).

Organic Farmer said...

Wet wet wet at the Admiralty inlet. Over 2 inches in the rain gauge. Record wet winter too. Storms from the west, not South West.

Ready for Junuary cold and rain to end it destroys crops, lots of lost strawberries.

drysider said...

Think of it as a microcosm of the rain shadow cast over Eastern Washington by the Cascades or, my favorite, the rain shadow of Mouna Loa and Mouna Kea over the leeward side of the Big Island.

Seriously, areas downslope of mountain ranges present some unique forecasting challenges. Congratulations on your model nailing it, Cliff!

John Marshall said...

Welcome to the shadow, Seattlites.

We suffer all winter from this dry, sunny phenomena on the NE side of the Olympics. Winter storms love coming from the SW, crashing into the Olympics to partially exhaust themselves before skirting around, drawn to Olympia and then the western Cascades like a magnet, dousing greater Seattle along the way. But nary a drop for the rainshadow afflicted.

Now it's your turn. Just when you need water the most for your garden, the weather denies it and heartbreakingly sends it to someone else. You can start growing cactus, or live with drip irrigation and brown lawns like we do in Sequim. We're enjoying receiving some of your missing moisture.

tracksdc89 said...

Thank you 1000 times over, Cliff! It feels great to know that my observations of the odd Seattle dryness were not just my imagination. I really appreciate this explanation ... you're the best!