Trick or treat? - Weather outlook

Are you wondering about the Halloween forecast?  Will you or your kids get wet trick or treating?  How about temperatures?  Wind? Let's get to it!

First, we are watching a low pressure system south of the Aleutian Islands which is drawing up significant moisture northward from near Hawaii.

Infrared Satellite image from 11 AM PDT Monday, October 29th, 2018.

So will this moisture make it up our way?  Well its hard to tell from a still satellite image, so let's take a look at some model data.  First of all, let's see if this moisture plume is showing up in the models.

18z GFS initialization of 500 mb heights and 700-500 mb Relative Humidity valid 18z (11 AM PDT) Monday, October 29th, 2018

Good news!  The GFS model as well as the other models are picking up on this moisture.

Now, let's see where this model takes it.  Here is it's projection for Halloween.

18z GFS forecast of 500 mb heights and 700-500 mb Relative Humidity valid 18z (11 AM PDT) Wednesday, October 31st

Much of this moisture tops the flat ridge (noted by the H west of California) and moves across Washington, Oregon, and Idaho on Halloween.  But the entire day doesn't look like a washout at this time.  Let's take a closer look at when the best rain chances are.

We will take a look at a new set of model data called the National Blend of Models (NBM). This utilizes many numerical weather prediction model data and post-processed model guidance.  If you want to know more about NBM, click on this link for more details.

So let's start with 5 am - 11 am Halloween

It looks wet Halloween morning for most of Eastern Washington and north Idaho as well as near the Cascades.  It is worth noting that the best rain chances for Omak, Wenatchee, and Moses Lake may arrive prior to 5 am (not shown here).

What about Halloween afternoon?  Let's see.

The best rain chances are near the Cascades, far SE Washington, and the Idaho Panhandle.

Now, for the time you really want, Halloween evening (Trick or Treat time)

This would suggest low rain chances and more likely a dry trick or treat for most of Central and NE Washington.  Pullman, Lewiston, St. Maries, and Kellogg area under a higher threat of rain.

What about temperature?  Here is the 6 PM temperature forecast from the NBM

It will be a little on the cool side with values mostly in the mid 40s to lower 50s.  These are very close to normal values for this time of year.

What about wind?

Winds do not look particularly strong, ranging from 5 to 10 mph for most areas.  The Palouse and areas near the foothills of the Blue Mountains in SE Washington could see slightly strong winds of 10 to 15 mph.  The mountains will be breezy.

Now we will continue to fine tune the forecast, please keep informed of the latest forecasts on our home page found here.

Changes next week and new fall/winter outlooks

Enjoying day after day of sunshine?  How much longer will it last?  In this blog we will address this important question, but also take a brief look ahead into November and briefly touch on the winter outlook.  A more detailed winter outlook blog is coming on or near November 1st. 

So why the abundant sunshine and stretch of dry weather?  A persistent ridge of high pressure is the answer.   Here is one model forecast for this Sunday (Oct 21st) showing high pressure deflecting the jet stream well north of our area.

18z GFS forecast of 500mb heights and 250mb winds valid 5 PM PDT Sunday October 21st

This will be a great weekend for yard work.  Especially raking all those leaves!

There is good model agreement that dry weather will last through at least Tuesday morning.  A weak system tracks through late Tuesday and Wednesday but probably won't produce much rain if any.  Models agree of a wetter pattern developing around Thursday (Oct 25th) of next week.  A low pressure system sets up off the coast resulting a mild but potentially wet southwest flow.  Here is one model depiction of what the pattern should look like next Friday. 

18z GFS forecast of 500mb heights and 250mb winds valid 5 PM PDT Friday October 21st

The jet stream (noted by the green and orange shaded colors) becomes aimed at Washington and north Idaho.  The latest 6-10 day Climate Prediction Center outlooks support a wetter pattern as well.

CPC 6-10 Day Precipitation Outlook issued Oct 18 valid Oct 24-28th, 2018 

It's still too far out to talk about rain amounts and will ultimately depend on the exact placement of the jet stream. 

What about November?  The new outlooks issued today (Oct 18th) favor elevated odds of warmer and drier than normal.

CPC November 2018 Temperature Outlook issued Oct 18 

CPC November 2018 Precipitation Outlook issued Oct 18

And what about the winter you ask?  Well an El Nino winter is forecast which has historically brought mild winters to the Inland NW with precipitation more of a wild card.  The new CPC outlooks issued today (Oct 18) mimic this idea as shown below.

CPC Dec 2018-Feb 2019 Temperature Outlook issued Oct 18 

CPC Dec 2018-Feb 2019 Precipitation Outlook issued Oct 18 

But don't think that a mild winter means no significant weather.  Short term significant weather events can and do occur.  In the next blog we will go into more detail with the past three weak El Nino events (2004-05, 2006-07, 2014-15) and look at what happened.

Record dry summer for some, relief in site?

Now that it's October, let's reflect back on the past three months.  Has this summer seemed dry to you?  Summer is after all the driest time of the year and many of our recent summers have been very dry.  Let's take a closer look.

July through September finished much drier than normal over much of the region as the image below shows.

As you can see, just about everyone finished in the bottom 10% (orange shading) in terms of total precipitation.  And several locations in and near the Columbia Basin reported their driest (red shading) July through September on record.  Here is the data below to prove it

You can see from the table above that Wenatchee, Ephrata, Grand Coulee, Lind, and Davenport reported a new record (or record tie) for the lowest July through September precipitation.  Ritzville and Omak came in 2nd, with Spokane and Pullman 3rd place.

And we still have that dry steak going in Ephrata.  After today it will be 109 consecutive days without measurable precipitation.

We are watching a weather system that will spread rain into portions of the area Friday afternoon into Friday night.  Right now the brunt of it should pass south of Interstate 90.  Ephrata will be on the edge.  Here is one model run from late this evening (Oct 3rd) showing rain amounts for Friday.

00z (Oct 4th) GFS Model 12 hour precipitation forecast valid 11 AM - 11 PM PDT Friday, October 5th.

Looking ahead to next week models are advertising a cold trough settling into the region with a possible increased threat of precipitation.   The long range outlooks from the Climate Prediction Center valid Oct 9-13th show high confidence of colder than normal temperatures with slightly elevated odds of wetter than normal. 

Wondering about the winter outlook?  We plan on updating the blog later this month talking about El Nino and its potential impacts for the Inland Northwest.