Tuesday, December 17, 2013

Cold Air and Snow Returns to the Northwest

As far as snow lovers are concerned, the last cold spell here in the Northwest was a wasted opportunity.

Now, we get to try it again.

On late Tuesday/early Wednesday a cold front will move through, with modified Arctic air behind it.  Here is the surface chart at 10 PM on Wednesday.

Unfortunately, there will not be much precipitation with this front and the associated upper trough;  perhaps a few inches of snow in the mountains at best.  But by late Wednesday, the low level air over western Washington and Oregon will be cold enough to snow...IF there is precipitation.   The classic lowland NW snow tragedy... warm or wet, cold and dry, but hard to be cold and wet.

But our day of snow may be coming...

By Thursday at 4 PM another front, a strong warm front, will be approaching the Northwest (see graphic).  Cold air is still over the Northwest (blue colors), and note the very warm air (orange colors) behind the warm front. 

But this warm front also brings precipitation (see below for 3 hr precipitation ending the same time).  Some of this precipitation leads the warm air.  Might the low-level cold air hold for a while as the precipitation reaches us, producing significant snow over the lowlands and the Cascades?

This is a very difficult situation to call correctly.  In the end the warm air behind the warm front will win and rain will reach the surface.  But could we enjoy (or suffer) during the Friday AM commute with a few inches of snow?  Might school be delayed?  The tension rises.

Well, here is what the UW model suggests. I will start with the model 3-h snowfall amounts for several times starting at 10 PM (0600 UTC) Thursday.  At that time, snow reaches the Olympics, the north Cascades and the Strait of Juan de Fuca at sea level.

 1 AM Friday.  Snow in the mountains and light snow reaching Puget Sound.
 4 AM.  Mountain snow, plus lowland snow over central/north Puget Sound and NW Washington
 8 AM.  Moderate snow over central Puget Sound and NW Washington. Eastern slopes are enjoying white stuff.

 10 AM. Warm air is winning, snow tapering off over lowlands but plenty over and east of the Cascades.
 1 PM.  Snow is over at lower elevations across the western lowlands.  Now it is rain's turn.
 What is the total amount of snow?  Here is the 24 h amounts ending 4 PM on Thursday.  A few inches over central Puget Sound, up to 4 inches over NW Washington, 1-2 feet in the Cascades.
This is wonderful news for the ski areas.  Stevens had been marginal at best; this will give them enough to get through the holiday season.  Great for Baker and Crystal.  The question is whether there will be enough for parts of Snoqualmie Pass to open.   We will see.

How reliable is this forecast?  A number of model runs have suggested this scenario...the snow is  not a transient solution.  Other modeling systems have also suggested snow. The cold air is sure thing.  The warm front is a sure thing.  Starting as snow is highly probable.  That it will turn to rain on Friday is sure thing. How much snow in the AM is less reliable.  The models tend to mix out cold air too fast, so the tendency would be for more rather than less snow before the rain begins. 

It appears that the morning commute on Friday might be messy.  Stay tuned as we get closer.  In my next blog we will turn to the high-resolution ensemble forecasts for more guidance.


“A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty.”
― Winston Churchill

Sunday, December 15, 2013

Are Storms Getting More Intense Over the Pacific Northwest?

It is rare to have a week go by without a local media outlet doing a story suggesting that storms are getting worse in our area, with global warming being the suggested cause.  For example, last month KING-5 ran a story saying exactly that (see image), quoting from a representative of  the National Conservancy.

And this is not the only example of claims that Northwest storms are strengthening.

The sad thing is that these claims of increased storms in our region are simply not true, contradicted by the both observations and the peer-reviewed literature.   The "experts" quoted are often representatives of well-meaning, but poorly informed, advocacy groups.

All too often, the media is failing to do their homework in checking out the veracity of these claims.    So let's do the homework for them!  Let's look at key observations to see if there are any trends in storminess in our region.

First, what about winds?   With the help of UW's Neal Johnson, we have found the maximum winds observed each year at a number of Northwest locations.  Below you will find the plots of annual maximum winds for Astoria and Seattle-Tacoma Airport ... the others are very similar.  There is absolutely no evidence of an upward trend in wind speeds.  In fact, there seems to be a decrease at Seattle-Tacoma Airport.

To really ensure we got the story right, we next found the lowest pressures observed each year at various stations.   The lower the pressure, the stronger storms are in general, so if storms were getting more intense we would expect to see a drop in extreme pressures each year.

Below are the extreme low pressures observed each year for Astoria and Seattle-Tacoma Airport.  Do you see a trend?  I don't.  No hint that storms are getting more intense around here.  You might even come to the opposite conclusion from the pressures at Seattle-Tacoma Airport, that storminess is decreasing.

Are storms getting stronger around here?  The results above do NOT support that conclusion.  And there is a lot of corroboration for doubting the increase.  For example, the comprehensive analysis of Northwest windstorms by Wolf Read (information here) clearly shows that the strongest windstorms on record occurred in 1962 (Columbus Day Storm), with the other big ones in 1880, 1921, 1934,  and 1951.  There is NO evidence that storms are getting more powerful in the Northwest from his work.

Recently, an article in Science Magazine suggested that precipitation/flooding in our mountains are decreasing due to a weakened jet stream approaching the region (see below).  Weakened jet streams would suggest weaker storms, since the wind and temperature gradients associated with the jet stream provides the energy for midlatitude storms.

You can tell I am a bit frustrated by the incessant claims that global warming is already having a big influence on storms and adverse weather here in the Northwest.   

Neither observational evidence nor theory supports such claims. 

Global warming is a very serious issue and later in this century the effects will probably be profound for many regions.  For us, the largest impact will probably be a serious decline in snow pack and the ending of skiing at Snoqualmie Pass.  But exaggerating current impacts when observations and the science say otherwise will only lessen the credibility of the scientific community.  Environmental advocacy groups mean well, but they must understand that crying wolf is not a useful approach in the long run.  And media folks need to vet the claims better.

Friday, December 13, 2013

Giant Towers over the Olympic Mountains

During the past week I received a startling picture from Andrew Laszlo, a doctoral candidate in the UW Physics Dept.   Andrew had been hiking in the North Cascades and took the following picture of the Olympic Mountains at 11:25 AM, December 8th.  Huge towers, resembling mesa, loomed high in the sky, well above the usual position of the Olympic peaks.

Had the mountains grown suddenly?  Rapid uplift?   A photoshop moment?

No, this was a wonderful example of a superior mirage, when the atmosphere acts as a weird lens that causes objects to appear bigger and higher than they should be.  Let's investigate.

First, the facts.  The following map shows the location of Andrew when he took the picture.  He reported being at 6900 ft at the time.

Light can be bent in strange ways when there are very large changes of temperature in the vertical.  We can get an idea of the temperature structure at the time from reports from planes taking off and landing at Seattle;  some are equipped with weather sensors.   Courtesy of UW's Mark Albright here are the temperatures from an aircraft taking off from Sea-Tac Airport at 11:36 AM (1936 UTC) Dec. 8.

If you look closely you will see a strong inversion between  5722 ft and 6932 ft (from -11.1 to -7.7C), an inversion right below Andrew's location when he took the picture.  Confirmation of the inversion came from the radiosonde sounding at Quillayute, on the WA coast.  The sounding was for approximately 4 AM

Temperature is in red. You see how the temperature warmed abruptly below around 780 hPa pressure (about 6900 ft). 

When you have cold air below, with warmer air above, separated by a sharp inversion, the abrupt change in atmospheric density that results can produce a lens effect that makes object loom upwards.   As noted above, this is called a superior mirage, and is illustrated in the two schematics below.

This phenomenon is also known as the Fata Morgana, named after the illusions produced byKing Arrthur's enchantress half sister. 

 Finally, I should note that others have observed this phenomenon before around the Olympics.   Here is a picture of the Olympic Mts. I found taken from Vesper Peak in the North Cascades on Dec. 3, 2011.  The picture was taken around 12:30 PM at around 6200 ft.

Picture courtesy of Steph Abegg

What did the radiosonde sounding look like at Quillayute that afternoon?  Here is it (for 4 PM).   850 hPa is about 5000 ft and the two black lines are temperature (right one) and dew point.  Another strong inversion below the photographer's location!  Further support of the inversion causing the superior mirage of the Olympics when viewed from altitude in the North Cascades.

2014 Washington Weather Calendar
Need a Good Weather Calendar? 
Like to Help UW Atmospheric Sciences students go to conferences?

Then consider the 2014 Washington Weather Calendar!  You can order online (about $15. plus shipping) here.  A few dollars  goes to the UW Chapter of the American Meteorological Society for each calender.  A fine holiday gift for the weather lover.


Wednesday, December 11, 2013

An Unusually Dry Fall

A number of people have commented to me about how dry this fall has been...and they are right!  Let me show you the story.

Let's begin by looking at the percentage of normal precipitation from October 12 through yesterday (December 10th, see graphic).   Mighty dry in the western U.S.   Much of Oregon and Washington have received less than half of normal precipitation;  in central and northern CA less than 25% has been common.

Looking more closely at the last month, here is the cumulative precipitation at Quillayute, Seattle, and Spokane (red is observed, blue is normal).  Quillayute is down about 10 inches, Seattle down by roughly 3.5 inches, and Spokane by nearly 2 inches.
The USDA Drought Monitor web site highlights the very dry conditions in California and Nevada (see graphic).  As of Dec. 3, extreme drought covers central CA and northern Nevada. 

Ok, you are convinced it has been dry.  But why?   

The reason has been a very persistent ridge of high pressure over the eastern Pacific.   Here is an average upper air  (500 hPa) map for October 1 through December 8th.  You can see the eastern Pacific ridge quite clearly.
We can subtract this average 500 hPa map from a long-term average climatological map and get what is called an anomaly map, which highlights the differences from normal.  Anyway, here is is.  You can see the anomalous high heights (high pressure) over the eastern Pacific--really clear.
We can create the same type of anomaly map for sea level pressure (see below).  Same story...very anomalous high pressure over the Gulf of Alaska.
Why did we get such an unusual pattern?  I don't have that answer and I suspect no ones else does either.  Not expected during a neutral year,  which is neither La Nina nor El Nino.

The latest NOAA Climate Prediction Center three month precipitation is for normal precipitation (see below). 

Tomorrow we will get the first wet, mild weather system in a long while, with rain developing in the morning.   The atmosphere has warmed up sufficiently that the lowlands will only experience rain.  Enjoy normality.

2014 Washington Weather Calendar

Need a Good Weather Calendar? 
Like to Help UW Atmospheric Sciences students go to conferences?

Then buy the 2014 Washington Weather Calendar!  You can order online (about $15. plus shipping) here.  A few dollars  goes to the UW Chapter of the American Meteorological Society for each calender.  A fine holiday gift for the weather lover. 

Jeff Renner  of KING5 will be signing calendars at the U Book Store in Seattle on Saturday, Dec 14th at 3 PM.  Info here.

Monday, December 9, 2013

Snow for the Morning Commute?

Right now, there is very little precipitation in the area as shown by the regional radars (see image).
 A few snow showers over SW Washington, stretching up south of Tacoma and south Seattle (not apparent on the radar).  Keep in mind that snow does not show up well on radar and the south Sound is distant from the Camano and Langley Hill radars.

Temperatures are cold enough for snow and will remain cold enough through lunchtime tomorrow.  The last observations over Seattle-Tacoma Airport (see below) indicates substantial warming to -2C around 750 hPa pressure (around 8-9 thousand feet), but well below-freezing air below.   Eventually we will get a shallow above-freezing layer aloft, but that will probably result in giving us sleet initially and not freezing rain until the warm air aloft deepens. 
The models all suggest pretty much the same thing.   A very weak upper air disturbance is approaching the region right now. Seattle should get only a light dusting at most because the air approaching the Olympics is from the NW and we are in the rain/snow shadow.  There will be some light snow on the western slopes of the Cascades and in a band extending in NW Washington.  Light snow south of Seattle where rain shadowing from the Olympics is weak.

Here is the UW WRF snowfall for the next 24 h.  Nothing in Seattle.  Everett to Bellingham perhaps a half inch to an inch. 

The NOAA/NWS High Resolution Rapid Refresh shows a similar story for snow totals ending at noon Tuesday.  I have really come to appreciate HRRR, which is updated and run EVERY HOUR.

 Anyway, we don't have any strong systems coming in and there is no reason to expect heavier amounts of snow .   Seattle should get  a pass on snow.  A no-snow pass.

Homeward Commute Weather: No Problem.

 I will do an update at 9:30 PM for tomorrow's commute....cliff

I am totally unimpressed with the precipitation coming in later this afternoon.

First, it will be cold enough for snow later this afternoon and early this evening.  Forget the talk of freezing rain.  The roads are cold.   The atmosphere is too cold.  No icing threat in the early to late evening.

What about precipitation?  There is hardly any in the western interior.  Here is the latest radar image (1213 PM) A few snow showers on the coast.  There has been some on and off light snow at Quillayute and Hoquiam.   Virtually nothing getting past the Olympics...these are very shallow showers.

The NOAA Rapid Refresh and HRRR models have pretty much nothing over us through commute time.  Here is the accumulated snowfall through 4 AM from the Rapid Refresh.  Nothing over Puget Sound.

The latest NWS NAM model has a few light snow showers coming in.  The accumulated precipitation from the latest (10 AM) run of NAM has nothing over Puget Sound if you look closely (bad graphics unfortunately).

Satellite imagery:  completely unimpressive.  A weak band to the north of us.  No organized circulation approaching us over the Pacific.

Bottom line:  homeward commute is no problem.  At worst you might see a few snow flurries.  With cold roads and no freezing rain, there won't be an ice problem.   Watch the radar (here is a good place) if you want to follow the situation in real time.

Several models have the precipitation picking up a bit over night...but still light.  Seattle and central Puget Sound and Kitsap will be heavily rain shadowed (snow shadowed).   Some light snow from northern King County into Snohomish County and south of Tacoma is possible but light is the operative word...less than an inch.   North Seattle might get a dusting.  This is not a serious snow event if you believe the current models.  There is a bit of hyping going around...but no names here.

 Along the coast where there has been some occasional snow showers.

Are you in venture capital/private equity and have an office in the Bay Area, or knows someone who does?
If so, I have a question for you. Please send me an email (cmass at uw.edu) ....thanks, cliff

Sunday, December 8, 2013

Snow and Freezing Rain Tomorrow?

 6:30 AM Update

The coastal radar shows a weak area of snow showers moving over Hoquiam and the SW WA coast and moving inland over SW WA (see echo).  Very light stuff.   NOTHING ELSE OUT THERE. So some light snow showers over SW WA is all that we will see this morning.  Update for the evening commute and overnight at noon.  Could be snow showers this evening.

The forecast for Monday is a very difficult one.  

We will have air that is quite cold enough to support snow tomorrow morning.  A very weak weather disturbance will approach during the mid afternoon and will bring some light precipitation to the area.  As the disturbance moves through temperatures will begin to warm as increased onshore flow occurs.  By Tuesday morning commute time it will probably be warm enough for rain over Seattle, with snow holding a bit longer over NW WA.

So for those of your worried about driving to work tomorrow, the morning commute will be fine.  No problems at all.  Enjoy.

South Seattle should be substantially rain shadowed (or snow shadowed in this case). So if snow falls, it probably will be heavier in the north part of the city--but still not more than an inch.  Could easily be much less.  As the temperatures warm up aloft, we might see a few hours of freezing rain.

To illustrate, here is the 24 hr precipitation and snowfall ending 4 AM on Tuesday AM. The mountains will get snow ...guaranteed and NEEDED.  NW Washington including Everett to Bellingham to the San Juans will get perhaps an inch. Between the rain shadowing in NW flow and warming, Seattle to the Kitsap will not get much...perhaps 1/4-1/2 inch at most.  Or virtually nothing.

 Not a big event...but any snow in our area can cause problems.  And we could get a few hours of freezing rain.  God knows what the TV stations will make of it!   We will have a much better handle on this tomorrow..I will do a NOWCAST around noon. 

Temperatures are already warming quickly aloft.  Take a look at the wind and temperature above Seattle Tacoma Airport. At around 5000 ft (850 hPa on chart)  the temperatures have warmed from -12 to -6C (that is a warming of around 11F). 

Watch the radar tomorrow...you will see in real time what is happening.  Precipitation amounts will be everything.  No precipitation...no worries.  If you see it coming in when you need to be driving or biking...you might get an early start.  No big blizzard, but roads can get slick with a little snow.  Sea-Tac Airport is 4 F warmer than the same time yesterday--that is true for many local stations.   Changes are beginning....

Finally, for those in my ATMS 101 class taking their final exam tomorrow at 8:30 AM....there are no snow excuses.  Be there!