Friday, January 29, 2016

A look back at the floods of February 1996

Twenty years ago portions of the Inland Northwest experienced the worst flooding in decades. For those of you that lived through it, this will spur some interesting memories, and for those that weren't here 20 years ago, this will probably be an eye opener!

Setting the Stage

Surprisingly, the winter of 1995-1996 prior to the event was not terribly eventful. Mountain snowpack in the region was only on the order of  50% - 90% of average for that time of winter (See NRCS snowpack graphic below from 2/1/1996). There was a decent amount of snow in the valleys and mid elevations, and that ended up contributing a large volume of run off in short amount of time.

The first half of the winter had been rather mild.  Mount Spokane Ski Resort didn't open until early January. But the pattern changed in late January and the region quickly saw the snow pile up. The graph below shows the snowfall accumulation for the winter at Moscow, ID.  As you can see, by January 18 the town of Moscow had only had 6" of snow for the entire winter.  Over the next 10 days, 42" of snow would fall over that location.

Then came a brutal arctic air mass.  Temperatures plummeted into the sub-zero range with record-setting nighttime temperatures colder than -20F.  This arctic air mass was replaced with an atmospheric river (a.k.a. Pineapple Express) that took aim at the region bringing warm temperatures, winds and copious amounts of rain over several days. Within a matter of a few days, the area went from a deep freeze to near-record high temperatures for February. 

This trifecta of weather conditions created one of our biggest rain-on-snow flood events. This trifecta is the 1) warm temperatures. 2) winds and 3) significant rains. Underneath the snow the ground was for the most part frozen, meaning that almost all the rain and snow melt contributed to the run off as opposed to at least some percolating into the ground. And add to that the somewhat unpredictable nature of ice jams that had formed on creeks and rivers in the preceding cold snap and it was a perfect recipe for major flooding. 

The table below shows the dramatic change.  On the morning of February 2nd, the temperature was -26F at Moscow with 19" of snow on the ground.  Six days later, temperatures were in the 50s and all of the snow had melted.  In addition, 3.54" of rain fell between the 5th and 9th.

Up in the mountains, the scenario was even worse.  Here's the weather observations from Wallace, ID.  The 22" snow pack on the 1st of the month had completely melted away by the 9th as 5.55" of rain fell over the period of a week.

How the Rivers Responded

Many creeks and rivers across the Pacific Northwest saw major and near-record flooding. Debris flows and landslides exacerbated the problems throughout the region. In our area, the hardest hit spots were in Eastern Washington and the Central/Southern Idaho Panhandle.

Down along the Clearwater River, Highway 12 lost a 75 foot long bridge section between Lewiston and Orofino. Homes were lost in the Big Canyon Creek drainage near Peck. The video below from KHQ has some dramatic footage of the Highway 12 washout and a home being actually floating down Big Canyon Creek.

St. Maries, ID had four levees breach, partly due to the damage from river ice. This led to the inundation of over 100 homes and several businesses. The picture below show the town of St Maries on the right. The river coming in from the upper left is the St. Maries River and in the lower portion of the picture, the muddy brown areas are the parts of the community that were protected by the levees from the St Joe River before they breached. If you look closely, you can see several roof tops poking through the murky flood waters. 
Confluence of St Maries & St Joe Rivers at St Maries, ID

The North Fork of the Palouse River filled the concrete river channel in Colfax WA, but did not flood the town, giving the 1960's era flood mitigation project its biggest test to date. Just upstream of the project hundreds of community members frantically worked to reinforce the levee protecting the neighborhoods on the north end of town to keep it from breaching.
Palouse River at Colfax, WA

Here is a sampling of some of the other impacts during the event.
  • Several communities were cut off when roads and highways were damaged, forcing evacuations where possible
  • Several residents in the Coeur d'Alene River basin were cut off and had to be airlifted to safety and other rescues were undertaken in boats and ATVs 
  • Numerous roads were closed and/or damaged due to flooding or landslides including Highways 95, 3, and 12 in ID, and Highway 129 in SE Washington. 
  • The dam on Winchester Lake was over-topped and there was a fear the dam would fail and send a flood wave down Lapwai Creek. The dam held, but Lapwai Creek still over-topped the levee in Culdesac ID and forced an evacuation of part of the town 
  • The community of Cataldo ID worked to build a sandbag levee to prevent the river flowing back under the 1-90 overpass but eventually the river got too high, broke through the emergency levee and inundated Cataldo ID with several feet of water. 
  • The Palouse River at Potlatch ID gage set a record and caused extensive flooding in the town of Palouse WA
  • Paradise Creek in Moscow ID flooded campus housing and forced closure of the University
  • The National Guard was mobilized in both WA & ID to assist during the floods
  • Cedars Floating Restaurant on Lake Coeur d'Alene ID was kept from floating away thanks to the efforts of tugboats. 
  • The flooding caused at least $30,000,000 of damage (in 1996 dollars) to infrastructure and homes in Eastern WA and Northern ID

1996 Flood in historical context

You might be wondering how often we see floods of this size in our area. It is certainly one of the largest floods on record for many of our waterways. However, only a handful of stream gages that were affected in this flood have records that go back farther than 40 years or so. As big as this flood event was, even setting records at some gages, the USGS estimated that the only stream gage that reached its 1% chance annual flood (a.k.a. the "100 year flood") was the the Palouse River at Potlatch. The table below is a quick and dirty analysis of where the 1996 flood peak ranks in a selection of stream gage records.

Stream Gauges
1996 Flood Peak
Rank in Record1
Years of Records1
St Joe River at St Maries
42.00 ft
Coeur d’Alene River at Prichard
10.24 ft / 17,000 cfs
Coeur d’Alene River at Cataldo
51.62 ft / 70,000 cfs
Paradise Creek at Moscow
11.26 ft / 970 cfs
Palouse River at Potlatch
22.15 ft / 14,000 cfs
Lapwai Creek at Lapwai
9.70 ft / 5,010 cfs
Asotin Creek at Asotin
6.50 ft / 3,030 cfs
Spokane River at Spokane
28.04 ft / 36,500 cfs
Palouse River at Hooper
17.95 ft / 28,000 cfs

1Records may contain some gaps

About the Weather

Weather conditions prior to the cold and snow of late January 1996 were unremarkable. January 1996 started out slow as far as winter conditions were concerned, but the second half of the month saw quite a bit of snow and then cold. On average, nearly a foot of snow fell across the lower elevations during the last two weeks of the month. Then bitter cold air settled into the valleys which dropped maximum temperatures into the single digits.

So the scene was set to cause problems if the right weather came along:
  • Fresh and deep lowland snow pack even as far south as the Lewiston/Clarkston area.
  • Bitter cold valley temperatures to aid in freezing rivers and streams.
  • Frozen ground

Atmospheric River of Feb 4th-8th, 1996

The Atmospheric River is a cold season phenomenon that typically occurs between October and March for the Pacific Northwest. A strong jet stream extends well into the lower latitudes of the northern Pacific Ocean, transporting enormous amounts of moisture into the mid-latitudes. The sample loop below from February 2015 is similar to what happened during the flood event of February 1996. Not all Atmospheric Rivers and their effects on the Pacific Northwest are the same, but all tend to bring in much warmer and wetter weather. If enough cold air is in place then potentially heavy snow followed by rain could be expected in the winter months.

Color Enhanced Water Vapor Satellite - Feb 05-09, 2015

The colorized water vapor image above shows an active Pacific Jet Stream picking up moisture from
the Tropics and moving it swiftly east and north. This is similar to water waves crashing on the beach, except in this case, the atmosphere is the "water".

The upper level weather pattern for the February 1996 Atmospheric River (a.k.a. Pineapple Express) event had a high pressure ridge over the western U.S. and a strengthening Arctic low pressure system over southwest Alaska. This created a strong southwesterly jet stream which stretched past the dateline (180 degrees longitude) in the Pacific Ocean and as far south as Hawaii, which is where the term "Pineapple Express" comes from.

High pressure over the western U.S. was giving way to increased storm activity from the deep low pressure system over Alaska. This low pressure storm system was responsible for maintaining the moist jet stream and sending turbulent and wet waves into the Pacific Northwest.

Here's a loop of how the weather pattern evolved Feb 5-8, 1996. There are several waves or kinks in the flow that moved into the Pacific Northwest.  What's important to note is the overall jet stream position remained stationary for several days allowing for a continuous barrage of moisture and warm air into the region.

Upper level atmospheric wave chart Loop Feb 4-9, 1996

Warm temperatures and wind were the other factors in this event. As mentioned before, it was very cold before the event, which allowed rivers to freeze on the surface and recent snow to stay put.  The cold temperatures also froze the ground, which would cause the coming rains to run-off into rivers rather than soak into the soil.

Red circled area approximately where worst flooding noted.

Cold temperatures on Feb 2, 1996 in purple and blue above show below zero readings locked into the Central Basin of Washington and Oregon extending east into the flooded areas.

By Feb 5, 1996, the cold air (note warmer yellow areas below) in southeast Washington and the Idaho Panhandle was 'washing' away with surface temperature readings in the 40s and 50s which certainly aided in rapid snow melt.  These warm temperatures would continue into February 8th.

 Red circled area approximately where worst flooding noted.

Warm air was also coincident with increased southwest winds which moved over the region overnight on Feb 6th. These winds persisted through the rain event.  Winds of 15 to 25 mph over the Washington and Idaho Palouse extended northward into the Spokane and Coeur d'Alene area as time went on. The wind rapidly melted snow even during the nighttime hours as temperatures remained well above freezing.

By Feb 9th, a cold front was finally able to move into the region and shut off the rain and temperatures began to come down to more seasonal levels.

All the weather factors came together to force this historic, rain on snow flood event:
  • Very cold air and snow before the event.
  • 4 days of persistent, moderate to heavy precipitation.
  • Unusually warm temperatures.
  • Periods of southwest winds 15 to 25 mph.
Other contributing factors included:
  • Frozen ground
  • Ice jams on frozen streams and creeks

Fun Fact

The Spokane Weather Forecast Office was transitioning operations in the months prior to this event. The administrative & operational staff moved from the Spokane Airport into our new building between July and September of 1995, and the new crew of forecasters arrived in November and December of that year, just 2 months before the near-record flood event. The weather radar was installed in December 1995 and was still going through it's calibration during the flood.

And here's a requisite picture of the Spokane Falls from February 1996, the 13th most spectacular display there since 1891, according to the USGS river gauge.

Wednesday, January 13, 2016

Active winter weather - how long will it last?

So far the winter of 2015-2016 has been an active one, which started during the fall with the historic November 17th wind storm.  Then a wet December which turned snowy towards the second half of the month, and now another wet period is in the forecast.  Some may be wondering why this is all happening given the strong El Nino year.  We will dig deeper into this, take a look ahead through the remainder of the winter season, and provide a brief drought update.

As mentioned in a September blog, here, every El Nino year is different, and this winter has been no exception.  We currently have one of the strongest El Nino episodes on record.  Here is a look at the SST anomalies over the past month along the equator.

Note on the map the large area of warm Sea Surface Temperature (SST) anomalies of 2-3 degrees C over the Central and Eastern Pacific Ocean along the Equator.  The current strength is threatening the 1997-1998 event which was the strongest El Nino since 1950 based on the SST anomalies.  Strong El Nino episodes tend to bring a wide range of winter precipitation to the Inland NW with milder conditions.  So what did December bring? You probably already know, but it was wet!  Take a look at the percent of normal precipitation map below.

Washington, Oregon, Idaho, and western Montana saw about double the amount of normal precipitation, while southern California was dry.

This changed during the first 11 days of January.

Drier conditions for our region, while southern California got some much needed rain.

So what is causing all of this?  As always, several factors come into play.  El Nino is not the only player and other atmospheric variables come into play.  This year the Arctic Oscillation (AO) has been active. Take a look at the chart below.

The AO turned strongly positive in late December and has since gone strongly negative over the past week as indicated by the solid black line.  So what does this all mean?  Let's begin with the positive phase.

During the positive phase, lower than normal pressure occurs over the arctic regions which tends to lead to an enhanced jet stream across the Pacific Ocean.  This likely factored in to our very wet December.

So, now that we are in the negative phase, what does this mean?

The opposite occurs with higher than normal pressure over the arctic regions.  Colder air often gets displaced south over the central and eastern US and this is true for this episode with very cold temperatures over the upper Midwest and northeast.  This may have contributed to our drier spell in the beginning of January.

So what lies ahead?  Several more storms are on the way.

Satellite shows a series of low pressure systems out across the Pacific which will move into the west coast bringing a mix of rain and snow to the lower elevations with snow in the mountains.  The brunt of the Thursday night into Friday system may miss our region to the south.  Models show additional storms to reach the west coast next week.  What about the long range outlooks from the Climate Prediction Center going out further.  Here is their latest 8-14 day outlooks for Jan 20-28th.

The outlooks call for increased odds of wetter and warmer than normal conditions.  What about after that?  Typically an El Nino like pattern is most prevalent during the later half of the winter into the spring.  Thus, drier and warmer than normal conditions will become increasingly favored, here is the latest seasonal outlook for February, March, and April from the Climate Prediction Center.

The wet weather has been welcome and has brought a significant improvement or end to the drought over Washington and north Idaho.
 This is welcome news to our region since we could see drier than normal conditions develop in February, March, and April.

Sunday, December 20, 2015

Monday storm

A strong storm will pass through Washington and north Idaho on Monday.  This is a storm that is worth paying attention to and is expected to produce a swath of heavy snowfall as well as windy conditions near the Foothills of the Blue Mountains.  A small adjustment to the storm track could result in big forecast changes depending on where you live.  So keep informed of the latest forecast for your area.

Our current forecast favor the band of heavy snow to set up over the Wenatchee area extending east along the Highway 2 corridor into Spokane, Coeur D'Alene and the Central Panhandle Mountains.  Here is a look at our latest forecast for snow amounts Monday into Tuesday.

A slight shift north in the storm track could give places like Omak, Republic, Colville, and Bonners Ferry more snow compared to what the current graphic shows.  Meanwhile a further north track could give less snow to Moses Lake, Ritzville, and Pullman.  People in these areas should keep a close eye on the forecast.  Let's take a look at what the models are showing for this storm so  you can see what we are talking about.

Here is a look at what the 18z GFS model from today is projecting for Monday.   Note the position of the low as the day progresses.  The image is moisture and the bright green areas represent a high amount of relative humidity which often leads to precipitation in form of rain or snow.  Areas on the south side of the low track will be prone to milder air and windy conditions with highly reduced snow amounts (if any for the valleys). Locations on the north side of the low track will be colder with snow.  Thus narrowing down this track will be critical for snow totals with this storm.

Now there one alternate scenario that is possible.  The graphic below represents the two possible storm tracks for this storm.  The scenario above represents scenario one which is the preferred solution.  However another model (ECMWF) tracks the low further north placing the band of heaviest snowfall further north.

So, everyone with the exception of the Lewiston area and lower portions of the Palouse and Blue Mountain Foothills should be prepared for a significant storm producing heavy snow.  For the Pomeroy and Alpowa Summit areas very windy conditions will be the big story, and are expected to develop late Monday afternoon into the early evening as the low tracks north of these areas.  Here is the wind forecast from 4 pm to 7 pm Monday.  The graphic shows sustained winds but we could see gusts even stronger around 45-55 mph in this area.

Please keep updated with latest forecasts with this dynamic weather system.

Friday, December 18, 2015

Snowy weather ahead

After a very wet and mild start to the month of December, the pattern has changed with storms now coming at the region from the west and northwest.  This will bring several opportunities for snow and the chances of a White Christmas are looking good for most of the Inland NW.  Let's dig deeper into where we are now and what lies ahead.

The first colder storm that tracked through Thursday and Friday has blanketed the region with snow.  Here's an analysis of snow on the ground.

This is just the beginning of the snow.  A storm Saturday night and Sunday will bring light to moderate accumulations.  Here is our forecast as of Friday afternoon for Saturday night and Sunday.

It looks like snow will reach the Cascades Saturday night and then the remainder of Eastern Washington and North Idaho on Sunday.

A stronger storm is on tap for Monday.  Here is what one of the forecast models is showing.

Dec 18th/18z GFS model run valid 10 am Monday showing 500mb height and 700-500mb Relative Humidity
Note the large area of green over the region indicating lots of moisture.  Here is our forecast as of Friday afternoon for snow amounts Monday into Monday night.

This one could bring moderate to heavy snow amounts for most of the area.  Exception could be around Moses Lake, and Lewiston areas where milder air coming up from the south may limit accumulations as rain may mix in with the snow.

The snow that falls is expected to hang around a while as a cooler northwest flow sets up for the middle to end of next week.   Models are showing an upper ridge building in the Gulf of Alaska with a cool trough over the region.  This could bring cool weather and snow showers to the region especially in the mountains.  Here is what one of the model forecasts look like for Christmas Eve.

Dec 18th/18z GFS model run valid 10 am Christmas Eve showing 500mb height and 700-500mb Relative Humidity
And the latest CPC outlooks agree with this cooler pattern.  Take a look at the temperature outlook below valid Dec 24th-28th.

So, after a mild and wet start to December, it looks like the month will finish colder and snowy with a White Christmas for most.

Tuesday, December 15, 2015

Could we really have a White Christmas?

After all of this mild and wet El Nino weather, is it really possible to still have a White Christmas?  Believe it or not, the answer is actually "yes".  And the odds are actually starting to improve.  Let's show you what we mean.

First, let's start with the current situation.  The warm and wet weather so far this month gave us a respectable mountain snow pack, but left the lower elevations with very little snow.  About the only exception to this is in the Cascade valleys and the Waterville Plateau.  Here's an analysis of snow on the ground:

As you can see, not very Christmas-like for much of the populated areas.  Just for comparison, here's the morning satellite image:

MODIS satellite image for 15 Dec 2015

It's hard to tell the difference between the clouds and the snow.  The white areas that have a herring-bone appearance are clouds.  But you can definitely see snow on the Waterville Plateau.  They're looking good for a White Christmas.

So what about the rest of us?  Is there any hope?  Yes, and not from just one storm.  There's actually a series of weather systems during the next 7-9 days that could bring us snow.  The first is tonight.  Here's the forecast of snowfall:

OK, so it's nothing to write home about.  The system is a rather weak one, coming at us from the north.  This flow pattern favors locations south and east of Spokane.  Here's what it looks like on satellite:

IR Satellite image 15 Dec 2015

The lack of clouds over British Columbia shows how weak tonight's event is.  The bigger storm just off the Pac NW coast will dive into California, missing us.  But the one behind it (on the left side of the image) will arrive on Thursday for a good chance of snow.  Here's the expected snow totals from that storm:

Definitely a more respectable storm, and probably the first significant snow for most residents.  Will it last for a week?  For the Palouse and L-C valley, it might not survive the day.  Here's the Friday afternoon high temperatures:

Temperatures around 40 along with some afternoon wind could melt most if not all the snow for Lewiston, Pullman, and Ritzville.  But don't fear.  There are still more opportunities for snow.  The reason is that the overall weather pattern will be changing.  Gone will be our warm and moist flow from the southwest.  Instead we'll be getting air from the Gulf of Alaska.

As we get farther out in time, the details are typically in doubt.  Rather than show you a bunch more maps, let's look at a computer snowfall forecast for Spokane.

You can see the first "bump" of snowfall (on the left side of the graph) will bring 1-2" of snow (probably closer to 1").  The 2nd bump on Thursday night and Friday shows another 3-5" of snow.  Then there's another system on Sunday, and yet another possibly next Tuesday.  

Now, don't go thinking we're going to have 14-18" of snow on the ground by next Wednesday.  Even if (and that's a big "if") we get all of the snow from these 4 storms that the computer is predicting, it doesn't account for melting and compaction.  However, all that said, there is a good chance that by next Wednesday, most locations will have at least some snow on the ground.  How much remains to be seen.

By the way, if you like White Christmases, be thankful this year that you live in the West.  Our friends in the East will be celebrating the holiday week in their shorts.  The trough of low pressure over the western US means a warm ridge of high pressure in the East.  Here's the forecast for next week's temperatures:

You don't typically see probabilities that high.  Above normal temperatures east of the Mississippi next week are a done deal.  Here's the precipitation outlook:

Wet in the West and the Ohio valley.  While we're getting snow next week, the Southeast and Gulf Coast will be getting thunderstorms.  

Buffalo, NY still is waiting for their first 0.1" of snow of the season, the latest that it's ever taken.

Oh, if you're wondering what the historical probability of a location seeing a White Christmas, NCDC has a story on that at:

Here's their results:

We'll update this blog in a few days to see if we're still looking at a White Christmas.

Friday, December 11, 2015

Wet December - Will it continue?

December has been off to a WET start!  Some may be wondering if this will continue through the month.  We will address some of the impressive numbers so far this month and also take a peak into the future.

So, how wet has it been?  Here is a map showing total precipitation from December 1st through the 10th for selected areas.

Quite the impressive precipitation totals especially in the Cascades.  Some locations on the west slopes of the Cascades have received amounts in excess of 10 inches.  It's also been wet in the Idaho Panhandle with Mullan Pass coming in with 6.70 inches, and 5.38 inches in Prichard.  

So how do some of these totals compare to normal?  Well these amounts only cover ten days, so its no surprise that these totals are well above normal.  Take a look at the values below.

So what has caused this abnormally wet period.  The answer is a very strong and active jet stream that stretched across the entire Pacific Ocean which brought numerous storms into the area.  

NCEP reanalysis of 300mb zonal mean wind Dec 1st through Dec 8th, 2015

So what does the future hold in store?  More active weather is in store with the parade of storms persisting.  Let's begin with Saturday...

     18z/11th GFS forecast of 500mb height and 700-500mb Relative Humidity
4 PM Saturday

The bright screen shading on the map is indicative of high moisture content in the atmosphere.  A deep low pressure system is located near Vancouver Island.  This storm is expected to bring snow to the Cascades and northern valleys and mountains.  Locations from Interstate 90 south from Ellensburg to Lookout Pass are expected to see mostly rain in the valleys with snow in the mountains.

So how about Sunday?  

18z/11th GFS forecast of 500mb height and 700-500mb RH 10 AM Sunday
You can see the wet weather continues with the best focus of moisture (with valley rain and mountain snow) over Oregon, Southeast Washington, and Idaho. This day could bring significant snow to the Blue Mountains, Camas Prairie, and Lookout Pass areas.

After this the pattern turns cooler and drier with a potential weak system Tuesday night bringing light snow to the Idaho Panhandle.  But after that models show potential another wet storm on or around Thursday.  

18z/11th GFS forecast of 500mb height and 700-500mb RH 4 PM Thursday
It's too far out to have much confidence in the specifics for this storm, but more rain and snow looks like a good bet.  Beyond this forecast confidence drops, but long range outlooks from the Climate Prediction Center for Dec 19th through Christmas day are favoring elevated chances of wetter and cooler than normal conditions.  

Overall it looks like December is going to finish out much wetter than normal.