Rare September storm

With lots of media coverage about the upcoming storm, we will take a look at this rare September event.  Only a few cases since 1900 have set up like this one so early in the season.  This pattern during the winter months is common, but late September is not.  The region will see early season snow in many areas, strong winds, and eventually freezing temperatures.   For the latest NWS forecasts including watches, warnings, and advisories please visit our web page found here

The setup

So why is this happening?  An amplified ridge of high pressure in the Eastern Pacific and a deep cold low over our region according to this model prediction.

12z/Sept 26th GFS forecast of 500mb heights and 250mb winds valid 11 AM PDT Saturday Sep 28th, 2019

The image is the jet stream diving south off the coast into California.

And this low is not in a hurry to move.  Here is a closer look at the storm for Saturday morning.  The image is relative humidity with bright green indicating higher moisture content and thus better chances for precipitation. 

12z/Sept 26th GFS forecast of 500mb heights and 850-500mb relative humidity valid 8 AM PDT Saturday Sep 28th, 2019

And now Sunday morning

12z/Sept 26th GFS forecast of 500mb heights and 850-500mb relative humidity valid 11 AM PDT Sunday Sep 29th, 2019

This low doesn't move much, with bands of precipitation continuing.  Also during this time frame we will have cold continental air pouring into the region.  Winds blow from high pressure to low pressure.  The tighter the lines are together, the stronger the wind.  Note the tight packing over the North Idaho Panhandle especially.

12z/Sept 26th GFS forecast of MSLP valid 8 AM PDT Saturday Sep 28th, 2019

Early season snow

Snow this early in the season is uncommon.  Below is a chart showing snowfall records for the month of September, and the number of days of measurable snow recorded during the entire period of record for the month.  Getting measurable snow in September has either never happened, or only once in the entire climate record for all towns across the area except Priest River with two occurrences.

So where will it snow?  A near certainty in the mountains.  Valleys will depend on elevation.  Here is our forecast issued Thursday afternoon regarding possible snow accumulations through the weekend.

The mountains will see 6+ inches for many locations.  The valleys have the potential for light snow amounts Saturday night into early Sunday on grassy surfaces mainly above 2000-2500 feet and across the North Idaho Panhandle.

Keep in mind ground temperatures are warm this time of year.  This will reduce the threat of snow accumulations on roads during the day with the overnight and early morning hours the most likely period of winter driving conditions in the mountains.  But any heavy bands of snow that set up could result in slushy conditions during the day as well. 

Snow levels will be falling this weekend.  Here are the forecast snow levels Saturday morning.

And now Sunday morning

Does this mean snow in your town?  Knowing your elevation can help when we are talking snow levels.  The tables below show elevations for selected towns and several high elevation highways and passes.

Strong winds

Saturday will have the strongest winds.  Here is the peak wind gusts forecast for Saturday

The strongest winds are expected from Sandpoint to Coeur d'Alene and across parts of the Upper Columbia Basin.  Winds of this magnitude may result in localized downed trees and power outages. These winds this time of year would typically pick up blowing dust across the Columbia Basin but expected precipitation should minimize this threat. The winds will persist Sunday but not as strong.  The winds will make it feel even colder as noted in next section.

Unseasonably cold temperatures

Freezing temperatures will soon be hitting many areas beginning Sunday morning.  Here are the forecast lows for Sunday morning

 But the wind will make it feel colder.  Here are the forecast wind chill values for Sunday morning

Sunday morning wind chill forecast

For those heading out Monday for work or school, here are the forecast low temperatures.

 

Tuesday could be even colder for several towns.

Those with morning plans bundle up!  Freezing temperatures are expected for most while a hard freeze is possible in the colder spots. 

The high temperatures this weekend are noteworthy as well.  Daily records will likely be achieved in many areas beginning Saturday.  Monthly records are also within reach.  Here are the coldest September high temperatures ever recorded.

 Temperatures are expected to moderate towards the middle of next week, but still well below normal with the latest CPC outlooks favoring below normal temperatures and slightly elevated odds of wetter than normal in the Oct 2-6th time frame.

CPC 6-10 day temperature (left) and precipitation (right) outlooks issued Sept 26th, 2019

 This weekend will be a noteworthy September storm.  Get ready for winter like conditions. And if you have travel plans east into Montana, conditions will be even worse, much worse!

NWS Forecast of snowfall issued Thursday afternoon valid Friday night through Monday

For the latest NWS Forecasts for our local area or if you are heading east, go here:

NWS Spokane: link
NWS Missoula: link
NWS Great Falls: link
NWS Billings: link
NWS Glasgow: link

Turning much cooler - mountain snow?

There is growing confidence that the Inland NW will see much cooler temperatures to finish off the month of September.  Mountain snow may begin to fall, and the first freeze of the season will likely occur for portions of the Inland NW.

A strong cold front on Tuesday (Sep 24) will likely deliver windy conditions to the Columbia Basin, Spokane area, and Palouse which will be followed by much colder temperatures with highs in the 50s by next weekend. 

Here is the latest outlooks issued today (Friday) valid Sept 26-30th.

CPC 6-10 day temperature outlook issued Sept 20 valid Sept 26-30

CPC 6-10 day precipitation outlook issued Sept 20 valid Sept 26-30

Note the very high odds of colder than normal temperatures, with elevated odds of wetter than normal as well. 

Here is what one model is showing regarding the weather pattern.

12z GEFS forecast of 500mb heights and wind valid 5 PM PDT Saturday, September 28th

A large ridge in the Eastern Pacific and a deep trough over the Pacific Northwest.


Prior episodes such as this one (analog cases)
Based on the forecast pattern, the following graphics show the odds of below normal temperatures based on previous patterns that set up such as this one.  Here is the odds of below normal temps days 6-8 (Sept 25-27) based on previous events.

And now for days 9-11 (Sept 28-30)

And regarding mountain snow potential, let's begin with snow levels.  Here are the forecast snow levels published Friday afternoon for the morning hours of Thursday, Sept 26th.

NWS forecast snow level published Sept 20th valid the morning of Sept 26th

This is low enough to possible bring snow to the highest mountain passes (SR 20 over Sherman Pass and Washington Pass).  How about Friday morning

NWS forecast snow level published Sept 20th valid the morning of Sept 27th

Even lower, down as low as 4500-5000 feet.  There is currently low confidence regarding mountain snow accumulations.  This is still a long ways out so the details will likely change to stay tuned!  Anyone with late season outdoor hiking plans up in the mountains should be prepared for winter weather.

Much colder night time temperatures

Also of note is going to be a huge change in low temperatures.  Much of the region has experienced mild nights over the past 30 days thanks to cloud cover, and above normal moisture content in the atmosphere as the map below shows.

Departure from normal min temperature map valid Aug 20-Sep 18, 2019

Typically by late September, lows are in the upper 30s and 40s for most towns.  And even freezing temperatures have occurred in the colder spots.  Below is a table showing the average first freeze date for selected towns.

With the much colder weather, places that typically have had a freeze by now (Deer Park, Republic, Colville, Winthrop) have a very good chance of seeing their first freeze.  For places further down the list including Omak, Pullman, Kellogg, Ritzville, and Spokane it's too soon to have much confidence about freezing temps.  Lewiston and Wenatchee likely will have a tough time freezing this early.