Major pattern change this week - Snow and Cold!

 *Oct 24th Update to Snow Probability Map animation embedded below*

Have you heard?  Big weather changes are coming this week.  If you have not winterized your outdoor faucets and irrigation, NOW is the time!  And do you have travel plans this week?  Roads may be impacted by snow and/or ice, especially up over the mountain passes.  Of course with the varying terrain of the Inland NW, the snow part of the forecast will vary by location.  But regarding the cold everyone will be impacted.  Let's dive into the details.

The setup

What is the pattern that will cause this cool down? A high amplitude ridge to our west will allow a cold trough to settle over the region.

12z GFS 500 mb heights and anomalies valid Oct 23-27th, 2023

First hazard: Snow

A low pressure system swinging through the region will bring rain and snow to the region.  Here is one model depiction showing a band of rain and snow swinging through late Tuesday into Wednesday.  Green is where the model forecasts rain, blue is snow, and the darker the shading the heavier the precipitation rate. As cooler air moves into the region, most of the precipitation is progged to fall as snow on Wednesday.

12z NAM Nest forecast of precipitation type 5 AM PDT Tuesday - 5 PM PDT Wednesday (Oct 24-25) 

Of course this is just one model solution.  And for the valleys, part of the challenge with this storm is temperatures.  Given it is early season, the ground temperatures are warmer compared to a typical winter storm.  This makes the snow amount forecast extra tricky for many of our towns.  With air temperatures near or above freezing for many lowlands during the bulk of the event, snow for a portion of the event may mix with rain, or fall as a wet non-accumulating snow.

To help illustrate this, here is the chances for freezing temperatures for 8 AM, 2 PM, and 8 PM on Wednesday according to the National Weather Service's National Blend of Model (NBM).  And then finally 8 AM Thursday is shown.  Orange and and yellow shading is where chances are highest.

8 AM Wed chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 8 AM Wed (Oct 25th)

So Wednesday morning, the best chances for freezing temperatures are elevation dependent with the best chances north of I-90 except the lowest elevations along the Okanogan and Columbia Rivers. There is also a good chance of freezing temperatures around the Spokane and Coeur d'Alene areas (especially higher hills).

2 PM Wed chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 2 PM Wed (Oct 25th)

By this time, the chance of freezing temperatures is mainly in the mountains, as well as part of the Highway 2 corridor from Waterville to Davenport, as well as around the Sandpoint area.  Thus for most of the lowlands snow that falls Wednesday afternoon may have a harder time accumulating.

8 PM Wed chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 8 PM Wed (Oct 25th)

By 8 PM Wednesday, chance of freezing temperatures expand over much of the region, except for lower elevations south of I-90.  But by 8 AM Thursday, nearly all of the Inland NW is expecting freezing temperatures except for possibly the Lewiston area.

8 AM Thu chance of 32F or colder temperature

NBM (National Blend of Model) percent chance of 32F or colder 8 AM Thu (Oct 26th)

An ensemble system which incorporates multiple forecast model solutions gives forecasters a better idea of confidence in snow amounts.  Here is an animation of various snow amounts being reached over the region.

***Updated 8 AM PDT Oct 24th***

For the latest maps, please head to https://www.weather.gov/otx/winter

There is concern for area with fall foliage that any areas that receive moderate to heavy amounts of wet snow could bring down tree branches with local power outages.

Next hazard: Widespread hard freeze

This system will usher in much colder temperatures this week, with widespread freezing temperatures.  For those that still have tomatoes or other plants that are still thriving, time to take action!  Have you winterized your outdoor irrigation?  Also it's time to take action!  Here is a look at the forecast temperatures this week

As the graphic states, also protect livestock and pets and dress warmly if heading outdoors.  Get those gloves and winter coats ready!  Big changes are coming this week.