September 15, 2021

Heavy Rain and Rapidly Rising Rivers: The Details on the Friday/Saturday Inundation

If you were planning to take a hike on Friday, forget it.  Thinking about painting your house?  No way.

It is going to be very, very wet on Friday and early Saturday, and I suspect some daily precipitation records will be broken.

But let me give you the details.

Let's start by showing you the latest accumulated precipitation totals forecast by the European Center model for the period ending 5 PM Sunday.  Virtually all of the mountainous areas of the region will get more than 2 inches, with some being hit by 5 or more (particularly the Olympics and North Cascades).


The UW forecast model, driven by the U.S. GFS global model,  for the period ending 5 PM Saturday, has a similar distribution. 3-5 inches in the mountains will be extensive, with some "favored" areas being wetted by 5-7 inches.


How much confidence should you have in this forecast?  An important question.

We are close enough in time, that the models should be relatively locked on to the correct solution, but as I have discussed many times, the best way to get at uncertainty is to look at ensembles of many forecasts, each slightly different.

So take a look at the fancy presentation of the ensemble precipitation forecast from the top-of-the-line European Center ensemble system (below).  You are looking at accumulated precipitation at SeaTac Airport over time (time increasing to the right).

The average of all the ensemble members is shown by the green line and the extreme range of the forecasts (the highest and lowest forecasts) is shown by the brackets.  The blue boxes show you the forecasts that encompass the forecasts relatively close to the median (the median is the value in which half the forecasts are greater, half are less).


Wow... an amazing forecast, with the ensemble going for around three inches over the entire period.   

Considering that the average MONTHLY rainfall at Seattle is about 1.6 inches, that is a lot.   The spread of the forecasts is relatively small for the big wet day (Friday)...so there is great confidence that Seattle is going to be very, very moist.

The rain shield we arrive in the western interior around sunrise Friday and the heavy rain will extend into Saturday morning.  Cool and showery over the weekend.

Now let me emphasize that there will be considerable horizontal variation in the rainfall.  To show you this better, here are the high-resolution forecasts by the UW model of total precipitation for the 48-h ending at 5 AM Saturday.


Lighter amounts (less than an inch) to the northeast of the Olympics..that is the rainshadow effect to the lee (downstream) of the high terrain of that barrier. More rain south of Seattle than north of the city.  Big increases on the windward side of the Cascades...and, of course, less rain to the east of the Cascade crest.

So head to Port Townsend or southern Whidbey Island if you want to escape the worst of it.

The moisture that will hit on Friday is already streaming towards us from the western Pacific (see arrows in the latest water vapor satellite imagery below).


Two more days, it will be here.

The impacts of the precipitation on our rivers will be profound. 

 Right now most of the west-side rivers are below normal to normal.   But they all will be above normal by Sunday.  

To illustrate, here are the current and predicted flows from the NOAA/NWS River Forecast Center for the Snoqualmie River at Snoqualmie Falls.   HUGE, rapid rise to near-record levels.  The falls will be quite a sight on Sunday.  And you can get a nice breakfast there as well.


Finally, some historic context.  Below is a plot of the record daily precipitation amounts for SeaTac Airport.  Middle to late September has had some big events, reaching about 1.5 inches.  And I suspect most are like this one.....tapping significant moisture from the western Pacific.


Enjoy the change....and make sure your umbrellas are handy.  And I didn't even get to the issue of strong winds.   Next blog.

September 14, 2021

The First Heavy Rain Event of the Season

Everything is going to change on Friday.

You know change is in the air when the National Weather Service starts warning of clogged drains and gutters, and localized debris flows.

Consider the European Center model forecast for accumulated rainfall through Sunday at 5 PM (see below).  

Mama Mia!  2-4 inches in the mountains and .5-1 inch in the lowlands of Oregon and Washington.  Even eastern Washington gets some decent rain.


The UW model precipitation is similar. Note how regional extensive the rain is, including east of the Cascade crest. This is the storm we have been waiting for.



The heavy rain will occur mainly on Friday and early Saturday.  If you were planning on putting in some plants or planting grass seed.....this is the time to do it!

This rain will mark the end of the wildfire season in the Northwest, the remaining fires will rapidly decline with the massive moisture, high relative humidity, and MUCH lower temperatures, with highs dropping into the lower 60s.

Snow, YES SNOW, above 6000 ft.

The reason for this change is the development of a strong trough of low pressure over the northeast Pacific, with a powerful jet stream of strong winds to its south.  

Here is the forecast for roughly 30,000 ft at 11 AM on Friday.  The yellow colors show you the strong jet stream winds.... coming right into the Northwest.  Some as strong as 150 mph.   The solid lines are heights (think of them as pressure).  The low center is due west of British Columbia. 

This is a very favorable pattern for heavy rain in our region.


This jet stream of strong winds is associated with a very moist plume of moisture that will drive into our region and rise over the mountains, releasing huge quantities of water. 

To illustrate, here is a graphic for 11 AM Thursday showing something called Integrated Water Vapor Transport (IVT), a measure of how much water vapor is being moved horizontally by the winds.  Blue indicates very high values.  Just drop a mention of IVT when talking to your friends...they will be impressed.


And the latest forecast model runs have a lot more coming after this.  If you were Noah, you would know what to do (see below).    Ironically, with all the talk of drought, our region could end up with near-normal summer and annual rainfall total before the month is over.  The water is well-timed for the upcoming salmon spawning season.




Heavy Rain and Rapidly Rising Rivers: The Details on the Friday/Saturday Inundation

If you were planning to take a hike on Friday, forget it.  Thinking about painting your house?  No way. It is going to be very, very wet on ...