Friday, November 29, 2013

Tired of the Weather? Ready for a Change?

The Inland Northwest has had a stretch of rather strange weather, for late November that is: Dry and Sunny.  While some areas have been seeing more fog and low clouds the past few days, the overall weather for the region has been markedly different than what we typically see at this time of year.  November is one of our most active weather months, with precipitation typically falling on 2 of every 3 days.  Not so for the past 2 weeks.  But that's about to change.

The image below is a water vapor satellite image, along with the pressure at an altitude around 15,000' (blue lines).  You can see an area of low pressure currently located over Alaska.  This low will drop southeastward to the Pacific Northwest by Sunday.  It will also merge with the low you see to the west of it, just south of the Aleutian Islands.  This second low will provide the moisture, while the Alaskan low will provide the wind and cold.

Here's a computer forecast of the same pressure (yellow lines), valid on Sunday afternoon.  The green and blue shading is the computer forecast of precipitation.  While the low is still over the BC coast, the active weather will develop south of the low (i.e. over our area) on Sunday.  

The wind tends to blow along (parallel) to these pressure lines, and counter-clockwise around low pressure systems.  This means that on Sunday, we'll see westerly winds across the Pacific Northwest.  This wind direction tends to produce a rain-shadow in the Columbia Basin, with heavy precipitation in the Cascades and ID Panhandle mountains, and you can see this in the computer forecast above.

So what does this mean for us?  The storm has 3 phases to it.  The first is on Sunday.  The strong westerly flow (see the image above) will raise our snow levels to around 4000-5000'.  This means rain for the valleys.  The exception could been the northern valleys (e.g. Republic, Colville, Bonners Ferry), which could see some snow before changing to rain. Snoqualmie Pass will have rain, while Stevens and Lookout Passes could see a rain/snow mix.  All in all, not bad travel weather.  

By Monday morning, a Pacific cold front will sweep through the area.  The image below shows a forecast for Monday morning, with a surface low near Omak and a cold front (blue line) extending down into northwest Oregon.  As this cold front moves through the region, temperatures (and snow levels) will drop.  Typically with these cold fronts, the precipitation ends in the valleys before the temperature can get cold enough for snow.  But the mountain passes will all change to heavy snow on Monday morning.  So if you have to drive over the passes, Sunday evening will be better than Monday morning.

But take a look at the image above again.  See the other blue line just north of the US/Canadian border?  That is an arctic front, with much colder air to the north of it.  And that's phase 3 of this system.  This arctic front will move into our area Monday night, so by Tuesday morning, the forecast looks like the image below.

And by Wednesday, the arctic front continues it's southward plunge into Oregon and southern Idaho.

The blue H over western Canada in the figure above is an arctic high, which is a really cold air mass.  As this air moves into our area on Tuesday, the valleys could see a bit of light snow.  This isn't a good pattern for snow for our area, so don't expect much if we get any at all.

What we will get is a bitterly cold northerly wind.  While the forecast high temperatures on Tuesday are in the low 20s, the wind chill will be in the single digits as winds gust to 30 mph.  And Tuesday night the wind chill will potentially drop into the zero to -10F range.

For the rest of the week, temperatures will stay in the teens and single digits.  Last time we saw weather this cold was in late February of 2011, nearly 3 years ago.  The exact temperatures are hard to forecast for two reasons:  First, we're not sure how much (if any) snow will fall in the valleys.  If there's a snow cover on the ground, temperatures will probably be colder than forecast.  Second, high clouds moving over the area are hard to forecast.  Even a thin high cloud will keep nighttime temperatures a bit warmer than clear skies.  So, stay tuned to the forecast.

Wednesday, November 20, 2013

The upcoming cold pattern

The upcoming weather pattern will be rather conducive for some very chilly nighttime temperatures. In fact, our forecast temperatures will expose the region to the coldest readings since the third week of January. So the question is what's delivering this cold air, how long will it last, and how cold can it get?

Let's first examine what got us here. On Tuesday night, the deep trough which delivered widespread rains, and high mountain snows to much of the region shifted east of the Continental Divide and was quickly replaced by cold northwest flow which originated from the Yukon.

500 mb map for late Tuesday night. Green=moisture, white=dry
Shortly after the northwest flow moved into the region, winds began to stream down the Okanogan Valley, and fill all of eastern Washington and north Idaho with very dry and cooler air. The map below depicts this intrusion of the significantly drier airmass.  The coloring represents the surface dewpoint temperatures with purples and pinks showing depicting the much drier air. By 8pm on Tuesday this much drier (and eventually cooler) air started surging through the Okanogan Valleys and moved over the entire Inland Northwest by Wednesday morning.

Time Lapse of dewpoints between Tuesday night 11/19 and Wednesday Morning 11/20. 

So why are we looking at dewpoints in this image and not temperatures? Well since the surge of drier air was generally accompanied by gusty north to northeast winds, the temperatures remained relatively warm (although it may not have felt that way). Typically when we get these polar intrusions the coldest weather doesn't arrive with the initial air mass change rather it comes the following night and beyond. So in this case, the dewpoints are telling us we will likely see a very chilly night across the region. In other parts of the county, the afternoon dewpoints are a great indicator for how low the temperatures could get overnight, provided the air mass doesn't change, winds remain light and skies stay relatively cloud-free. Around here, that trick doesn't fare quite as well, however we can still use them to foretell overnight temperatures, once again provided the same assumptions. So at least for the next couple nights we are expected to see light winds, and clear skies, so conditions are prime for some very cool temperatures. However unlike many November events featuring a polar intrusion, we lack a key ingredient for really cold temperatures: snow cover.

Below is a map of the estimated snow depth over the region as of this afternoon. Note there weren't many valleys which had snow on the ground, most of it was found over the mountains.
Estimated or modeled snow depth map as of  Wednesday afternoon 11/20
Why is the snow important? For one it helps lower the heating during the day, as a pure white snow cover will reflect much of the daytime solar heating right back into the atmosphere rather than heat up the ground. Just as important is the snow blanket actually keeps the ground from heating the air at night. So if skies are clear this ground heating process is inhibited, and the afternoon dewpoints can be easily undercut by the rapid cooling. There have been many nights here on the West Plains of Spokane where we have hit a forecast low temperature within an hour or two after sunset provided clear skies, light winds, and fresh snow cover. As of 6pm, the dewpoint was hovering right around 0°F at our office. So if there was snow cover we would easily drop into the single digits. But instead we are forecasting an overnight low in the teens.

So right now we think the coldest night of the week will be tonight/Thursday morning, but why? This airmass really isn't going anywhere. One reason is the air mass transition was really just a glancing blow. Take a look below at the upper level jet stream chart for Thursday afternoon. For us to see a prolonged cold snap, ideally we should be on the cold side of the jet. This would reinforce the surge of cold air from the north or northwest. But in this case the jet will remain to our north and east keeping the coldest air east of the Continental Divide. What that means is each day we should slowly modify or heat up this polar airmass. Without a fresh snowpack in place, that should happen quite readily, the question is how quick?

Upper level jet stream (blue & purple shading shows fast wind speeds and location of jet)
Well much depends on how quickly we can recover from tonight's lows. If temperatures get into the single digits, we might be hard pressed to climb far into the 30s during the afternoon hours. The reason is the sun angle is so low this time of year. At the sun's peak elevation (or solar noon), the sun is only about 20° above the horizon. That means much of the energy is spread out over a large area, rather than concentrated over a small area. This is analogous to shining a flashlight at a table or desk. If the flashlight is pointed straight at a surface the light will be more intense and concentrated, whereas if its pointed at an angle, its much more diffuse and dimmer. So what does this mean? It means the sun only has so much potential to heat the ground this time of year. That's one of the reasons we commonly get persistent valley inversions and fog this time of year. While temperatures near the ground are often slow to warm, conditions above the ground are more apt to change and with the jet stream remaining to our northeast, those temperatures are going to warm.

Below are maps of the 850 mb or (4500' above sea level) temperatures for Thursday and Saturday.
Thursday 850 mb temperatures 

Saturday 850 mb temperatures 
Over Spokane, the Thursday temperatures will be around -5°C and possibly warm to 5°C by Saturday. So that translates to about 18°F or warming. Will warming of that magnitude translate to the valley bottoms? Unless there is some wind to mix that warmer air down to the ground, the odds are not good given the ever lowering sun angle. Since we don't see any wind events in the near futures, what's more likely is a few degrees of warming each day and a growing possibility of fog and stratus. We also will see little if any chance of precipitation for the next week.

Wednesday, November 13, 2013

So what comprises an accurate snowfall forecast?

So what comprises an accurate snowfall forecast?

Confidence is growing that portions of the Inland Northwest are going to see a moderate snow event beginning Friday and ending sometime on Saturday. Temperatures will assuredly be sufficiently cold over the mountains and should likely be cool enough over some of the valleys. The weather pattern looks very consistent between model solutions. Looking at the spaghetti charts below, there is good consistency between different model perturbations with all runs showing a deep trough located over the northwest US by Friday afternoon. Recall, the closer the lines are to one another, the better the model agreement and thus the confidence in a model solution.  For a more thorough explanation of ensemble forecasting and spaghetti charts, please refer to our posts about a persistent ridge in October.

Spaghetti charts for Friday afternoon
If this pattern materializes as shown, there will likely be a good precipitation event focused over parts of the region. The question is where will the greatest threat occur.  Below are the latest forecast chances for receiving 4" and 8" or more of snow between 4am Friday and 4am Saturday. 

4" or greater snowfall chances 4am Friday 11/15- 4am Saturday 11/16

8" or greater snowfall chances 4am Friday- 4am Saturday
We are confident that the mid-level wind direction will be out of the west to southwest, which should focus the heaviest precipitation amounts near the Cascade Crest and over the Idaho Panhandle due to orographic ascent (think of the wind forcing the moisture over the mountains). There is also little doubt that temperatures over the mountains will cold enough to support snow.This confidence can be can be expressed utilizing a probablistic forecast which displays the chance of meeting or exceeding a certain weather parameter. Typical weather forecasts you read on our webpage or see on TV do not typically show these chances. Rather we produce what's termed a deterministic forecast. 

Deterministic forecasts show a specific value (or range of values) for any given location, whether it be a temperature, wind speed, or snowfall amount. The map below is what we are deterministically  forecasting (as of 1pm Wednesday) between Friday morning and Saturday afternoon. For Spokane, the latest deterministic forecast shows values ranging from 1-3", a far cry from the probabilistic forecasts seen above. 

Snowfall totals from Friday morning-Saturday afternoon---issued 1pm Wednesday
We do produce a widely used probabilistic forecast and that refers to the chance of seeing measurable precipitation (anything .01" or greater during the 12hr period). Here's what our written forecast says for this period for Spokane. Notice through the entire period we mention a chance of precipitation of 70% or greater. Nowhere though do we really mention specific snow amounts, rather we broadly characterize snowfall accumulations. 

So why the difference between the forecasts? Much of revolves around the terrain and the impact it can have on temperatures and precipitation amounts. Valley temperatures for this event could remain at or above least through much of Friday and early Saturday across the Spokane Area, Palouse, and Lewiston area. Confidence is higher that temperatures will remain cold enough for snow over the valleys of northeast Washington and the Idaho Panhandle. Colder air will likely arrive later on Saturday. But temperatures and precipitation aren't the only factors that go into a snowfall forecast. There are issues to consider. Here's a list of parameters used for a snowfall forecast? 
  • QPF or precipitation amounts
  • Snow levels
  • ground temperatures
  • ratio of water to snow equivalent
  • time of day and expected road temperatures
So lets look at some of these parameters, like we do at our office this time of year and calculate a snowfall total for Spokane.

Precipitation forecast early Saturday morning
The image above is fairly straightforward. We are forecasting precipitation over most of the region aside from western portions of the Columbia Basin. The forecast for Spokane calls for 0.13" of precipitation with significantly heavier amounts expected to the north and east of Spokane, especially over the mountains. If temperatures were cold enough, it would all fall as snow, but we can't determine that just by looking at this parameter.  

So how about temperatures? Below is the forecast for early Saturday morning. Locations with temperatures below freezing are blue and purple. Locations above freezing are green and yellow. Notice most of the Columbia Basin is green, but locations such as Spokane show a forecast right around freezing. 

Temperatures for early Saturday morning
So can snow fall at these forecast temperatures? Absolutely, but not necessarily. There is more that factors into the equation. What are the temperatures above the ground and what would they support for snow levels?

A snow level  is the elevation at which the precipitation will fall as all snow. Our forecast for Saturday morning calls for a snow level at 2300 feet. That means locations such as the Spokane Airport, South Hill and our office would be high enough for snow, while locations such as the Spokane Valley (at 2000' and lower) are more likely to see rain (provided the snow level forecast is accurate).

Snow level forecast for early Saturday morning in feet above sea level
So if most of what falls early Saturday morning is snow over the West Plains how easily will it accumulate? That depends on what the temperatures of the surface its falling on are. The fact that its falling early in the morning will negate the impact of daytime heating on the road surfaces or the ground. But if temperatures don't cool much overnight due to significant cloud cover that notion isn't as clear cut.  Clearing skies during the evening would lead to a much better chance of accumulating snows by morning. Timing a snow event during the shoulder seasons (before the cold of winter sets in or ends) is critical. A significant snow accumulation during the afternoon hours is much less likely than one during the late night or early morning hours given near freezing temperatures.

So the only other factor to consider is what will the ratio of snow to water be. Recall, we were forecasting about 0.13" of rain in Spokane. A crude rule of thumb is take that rainfall and multiply by 10 to come up with a snowfall total (1.3"). But really its not that easy, especially with complex temperatures and complex terrain. Across eastern Washington and north Idaho the typical snow:rain ratio is 13:1 or 13" of snow to every 1" of rainfall or liquid. But that also can be misleading since much depends on the temperatures. A cold air mass yields higher ratios than a warm one and that more or less explains our snow ratio graphic below. Other factors can come into play are how strong is the lifting in the atmosphere. Strong ascent leads to higher snow ratios. An unstable air mass can also lead to very high snow ratios (like we see in the spring with heavy snow showers and huge snowflakes or dendrites). 
Snow to water ratios for early Saturday morning

So we put all those pieces together and we come up with the following snowfall forecast for early Saturday morning. Light snow is forecast over the West Plains and South Hill, but less than 1" in expected with no accumulations in the Spokane Valley.
Snowfall forecast for early Saturday morning

Obviously there is a lot that goes into making a snowfall forecast. It's unquestionably one of the most difficult facets we forecast in the Inland Northwest. Even with a perfect forecast of the precipitation amount, the resultant snowfall can vary significantly from our prognostications. To add another layer of complexity, what would happen if the moisture from the system decided to remain bottled up near the Canadian border, rather than slowly sag south toward Oregon? Would that lead to a good forecast or would it present a huge forecast bust? We always hope for a perfect forecast, but there are so many variables at play, that fallibility potential can be very high, even after the onset of the event. 

Tuesday, November 12, 2013

Update on the Upcoming Pattern Change

After our last blog entry on Sunday about the potential for a weather pattern change, some of the computer forecast models wavered a bit in their forecast.  But they're back in agreement so confidence is increasing.  For those interested in why the computer forecasts change and how we use them, we'll address that topic in the 2nd half of today's blog.

The computers all agree on a Friday afternoon arrival for the storm.  The timing of the arrival will be crucial to the type of precipitation on Friday.  Temperatures Thursday night will likely cool to freezing or below.  But more importantly, temperatures aloft will also be cooling.  If the storm arrives late in the day, temperatures will likely warm into the upper 30s or lower 40s, and the precipitation will fall in the form of rain.  But if the storm starts by mid/late morning, our temperatures will likely be cold enough to keep the precipitation as snow.  A few hours either way will make all the difference.  At this point, we're betting on a cold rain or wet non-accumulating snow, but stay tuned for changes to the forecast. 

As the storm continues to move down from the north, it will drag colder air into our region. Any rain will change to snow on Friday night and Saturday, except for possibly the lower elevations of the Columbia Basin, which might still be too warm for snow.  One model wants to put a decent amount of snow in the Basin, while another model suggests that it will be in the Panhandle.  These kind of small-scale features are nearly impossible to predict with any accuracy this far out.  So at this point, the best we can say is low-land snow is possible on Saturday.   For the mountains above the 4000' elevation, a foot of snow from this storm still looks like a good bet .

The second half of the story will be the cold temperatures.  Highs for Sunday will likely be below freezing in most locations in the north.  There's a good chance that we'll be revising these forecast temperatures downward as we get closer to the event.

Sunday night's temperatures will be coldest of the season so far.  A lot will depend on whether there's snow on the ground.  At this point this is our forecast for Monday morning's low temperatures:

So in summary, some timing/details of this weekend's weather are starting to come into focus.  But this isn't a slam-dunk snow forecast for the low-lands.  Confidence is much higher on the switch to colder weather.

Extended Forecast Models

For those who don't know, forecasters look at several computer forecasts.  These are run by various nations, typically run 2 to 4 times each day, and they provide a forecast up to 10 to 15 days in the future.  The U.S. National Weather Service runs the GFS model, the Canadians run the GEM model, the British run the UKMET model, and the European Weather Center runs the ECMWF model.  Forecasters look at these models in hopes of seeing some consensus.  If all of the models are forecasting similar weather, the forecaster has more confidence in the upcoming weather.  Conversely, if the models are giving different forecasts (e.g. one says dry, another says snow, and yet another says rain), confidence is lowered.  Yes, even if they agree, they could all be wrong.  But typically, agreement equals accuracy.

Last Sunday, the models were all in surprisingly good agreement, especially for a drastic change in the weather.  But then the computer forecasts produced on Sunday night and Monday showed less agreement.  The GFS went back to the earlier idea of keeping the bulk of the storm to our east.  The first image below is the GFS forecast made on Sunday for this coming Saturday (which was in our previous blog).

Compare the above image to the one below, which was made on Monday afternoon.

Overall, the pattern of the two forecasts is similar.  The difference is subtle, but the second image shows more of the storm (look at the green shading, which represents moisture) to the east, over Montana and North Dakota.  

Meanwhile, the ECMWF and GEM forecasts stuck to their guns, suggesting the storm would indeed arrive in the Inland Northwest.  They modified their forecast a bit in the timing, starting things on Friday instead of Friday night.

The Tuesday GFS is now back in line with the other models.  Why the change?  We actually see this all the time.  Looking 5 to 7 days out in the future, the computers often adjust their forecasts before locking on to an agreed upon solution.  This situation is no different.

We do know that one thing that gives the models problems is extra-tropical transition of tropical cyclones.  "What in the heck is that?" you ask.  When a hurricane or typhoon (both of which are typical cyclones) move to the north, they transition into what we call extra-tropical storms.  They take on characteristics like typical mid-latitude storms.  When this happens, the computer models are often a bit shaky on their accuracy until the transition is complete.

Super Typhoon Haiyan, which hit the Philippine Islands last week, underwent such a transition in the past few days.  Below is a YouTube loop of the satellite imagery from last Thursday until today.  You can clearly see Haiyan move across the Philippines (left side of image) and into the South China Sea. Then Haiyan turned north, made landfall in southern China, and dissipated.  But you can see the remnants of Haiyan turn to the right and move back out into the Pacific.

It's impossible to say whether Haiyan's extra-tropical transition had an effect on the GFS forecast for this upcoming weekend.  But it is possible and does serve as a reminder of the atmospheric connection between storms.

Sunday, November 10, 2013

An Upcoming Pattern Change?

This past week, some (most?) folks in the Inland Northwest had their first taste of snow.  But as is typical for early November, the snow didn't last.  In fact, the weather since then has been rather mild.  A somewhat confused weather pattern has developed over the eastern Pacific, giving the Northwest US an overall ridge of high pressure. Veterans Day (Monday) should be warmer than normal.

But as we all know, this won't last either.  And it may end sooner than you think.  The atmospheric models are indicating a big change in the weather pattern.  They've been doing so for a few days now, so confidence is building that this will indeed come to pass.  The mild southwesterly flow will be replaced by a cold northwesterly flow.  

The image above shows the pressure pattern at about 18,000' above the earth's surface for Tuesday.  The wind tends to follow the solid black lines of pressure, and we like to look at where the air is coming from.  For Tuesday, the air coming into our area will be from the southwest, which is typically a source of warmer air.

Compare that image with the forecast for Saturday morning, which is below.

Do you see the difference?  The storms are now coming at us from the Northwest (i.e. Alaska).  And you can guess what that means.

The forecast for the upcoming weekend is going to be a tricky one, so you'll want to keep checking on the forecast each day.  For now, here's our expectations.  

  • The Cascade and Panhandle mountains will be favored in this pattern.  They'll likely pick up 1 to 2 feet of snow above 4000 feet.  Travel over the passes could be difficult.
  • Precipitation will start Friday night and continue Saturday with a snow level around 3500'.
  • By Saturday evening, the main area of precipitation will begin to sag south of our area, into Oregon and southern Idaho.  As it does so, cold air from Canada will push into the Inland Northwest, dropping snow levels to the valley floors.
  • Valley locations could see a couple of inches of snow by Sunday morning.

Here's the forecast high temperatures for Sunday.  That's right, I said high temperatures.

And there's a decent chance that it could be colder than this.  Some of the computer forecasts have highs 5 to 10 degrees colder than this. Plus there will a stiff wind from the northeast, making it feel even colder.

The low temperature for Sunday night (as well as Monday night) could easily drop into the teens and single digits, depending on if there is snow on the ground.  Monday's high temperatures will likely be below freezing for just about everyone.   The cold spell should be brief, with temperatures moderating by the middle of next week.

Tuesday, November 5, 2013

Why was the morning commute such a nightmare?

The first snowy drive of the season is typically a perilous one.  Even for seasoned drivers of the Inland Northwest, the re-acquaintance of driving on snowy roads at this time of year does tend to tend to result in a few more slide-offs than normal.  But then again, many of our early-season snows result in just wet roads.  That clearly was not the case this morning (5 Nov 2013).  So why was this morning's commute such a nightmare?

First, let's take a look at the forecast.  Yes, there was snow in the forecast.  Last Friday we had a "40% chance of rain or snow showers".  Not very definitive or confident.  But by Saturday, we had a forecast for Tuesday of "60% chance of snow".  By Sunday we'd increased the probability to 70%.  Snow accumulations were expected to be less than 1".  So we knew snow was coming, and expected it to be light.  Yes, the snow turned out to be more like 1-2", but that wasn't the reason for the slippery commute.

Here's a radar image at 7:20am, showing the narrow band of snow.

At this time of year, timing is everything when it comes to snow, especially if you're talking about accumulating snow.  Our snow this morning came by at just the right time.  Here's the weather observations from Spokane International Airport (latest time is at the top):

The snow started at 6:41am (actually very light snow started at 6:02am, but we won't spend the time to explain why in this blog).  So you can see from the air temperature column, that temperatures Monday at 5:53pm were 35F, and then cooled to 30F overnight before the snow started.  We were actually forecasting a low of 26F at the airport.  You've probably heard that clouds act like a blanket and trap the earth's heat.  Last night was a little cloudier than we thought, so the temperature didn't cool as much as expected.

But for snow to accumulate on the roads, you really need to know what the temperature of the pavement is.  This can be considerably different than the air temperature, which is measured at 6 feet above the ground.  Luckily, the state Departments of Transportation have sensors that measure the road temperature.  So here's a graph of the temperatures at four sensors around the Spokane metro area (some have 2 sensors, so there's 2 sets of dots):

They all tell about the same story.  Pavement temperatures on Monday afternoon (left side of the graph) were around 45F, and then cooled to freezing by midnight (middle of the graph).  Then, if you look closely, you'll noticed two things.  First, the pavement temperatures started to warm a little bit after 2am, back to around 34F or 35F.  Then, just before 8am, they dropped a few degrees back to around freezing.  What is going on?

Well, like we said, clouds act as a blanket.  As the clouds from the storm moved in overnight, the roads stopped cooling off.  But the ground below the roads is still quite warm at this time of year.  At our office, we have a temperature sensor 4" below the ground, and it's currently at 40 degrees.  The DOT sensors also have sub-surface sensors, and they were reading 50 degrees.  This is the left-over warmth from the summer.  As we get later into winter, these temperature will also cool into the 30s. But the ground holds onto the warmth, so it doesn't cool as quickly as the air or the surface of the ground.

So what does the sub-surface temperature have to do with anything you ask?  When the clouds moved in last night, the surface temperature stopped cooling.  And the warm ground below actually warmed the pavement surface during the night.  Impressive?

OK, so what?  Shouldn't the snow have just melted when it hit the roads?  And that brings us to the 2nd interesting point on the temperature graphs.  Why did the pavement temperatures cool just before 8am, when the sun was coming up?

In order to melt snow or ice, you need heat, right?  As the snow started to fall on the pavement, the 35F pavement melted the snow.  But that process removes heat from the pavement.  And that's why the pavement cooled a few degrees around 7am.  The pavement temperature dropped back to freezing, with water and wet snow on it.  Guess what happens.  Ice.  The melting snow re-froze on the roads, forming a nice layer of packed snow, slush, and ice.

Now, it some areas, the roads were just wet, and were no problem.  But the pictures below (compliments of KREM-2 and KHQ TV's web site) showed that snow accumulated on some roads.  

Highway 291 between Spokane and Suncrest was closed for several hours this morning due to ice and accidents. 

The intensity of the snow also played a role.  If the snowfall was very light, the roads would have been able to melt it without cooling, because the warmth from below the roads could have kept the pavement warm.  But the rapid snowfall at commute time allowed the snow to accumulate on the roads faster than it could be melted.  Car tires aided the process, packing the partially melting snow.

If this same event happened in the afternoon or evening, the road temperatures would have been warmer and all the snow would have melted.  And if this event happened in December or January, the sub-surface ground would have been too cold to warm the pavement overnight.  Thus, the pavement would have been below freezing when the snow started to accumulated, and it wouldn't have melted.  This would have given us the more typical packed-snow that we're used to, which is much easier to drive on.  

Like we said, when it comes to snow, timing is everything.