Friday, November 17, 2017

Bountiful Snow But Warm Rain Is On The Way

The promised snow has fallen in the Cascades, allowing a number of ski areas to open, including Stevens, Crystal Mountain, Whistler, and Mt. Baker.  In fact, our current snow pack is well above normal.

Here is plot from NOAA showing the the snow depth last year (left) and this year (right) for today (Nov 17).  A LOT more snow this year than last.

The Northwest Avalanche Center did an analysis two days ago (see below) and the numbers are impressive, with Hurricane Ridge in the Olympics enjoying 500% of normal snow depth for the date.  Mount Baker 209% of normal and Mission Ridge at 325% of normal.  And we have had more snow since then.

So a very good start to the snow year in the Northwest mountains.

But snow lovers have a problem....some warm and wet weather is coming, with freezing levels rising well above the Cascade passes.

Here is the forecast for temperatures above Sea-Tac Airport.  The y axis is height and the x axis is time (increasing to the left in UTC/GMT).  The solid blue line is the freezing level (0C level), winds are indicated by the barbs, and relatively humidity with color shading (green is moist).

The freezing level is below 5000 ft overnight tonight and moist showery conditions are in place (more snow).  Tomorrow will be dry (no shading).   But then there is trouble...a strong current of moist air comes in later on atmospheric river...with the freezing level zooming up to around 7500 ft.  Rain on our precious snow.   Then the atmosphere cools a bit on Monday and then a VERY warm air mass comes in on Tuesday, with the freezing level reaching about 10,000 ft.  Not good.

The forecast total atmospheric moisture values in vertical columns (see below) shows impressive values of moisture streaming into the Northwest at 10 AM Tuesday--a substantial and warm atmospheric river.

And the 72h total precipitation ending 4 AM Wednesday is enough to send erstwhile Californian's back to the Golden State, with over 5 inches of warm rain over the higher terrain of Washington, Oregon, and California.  

With lots of rain and melting snowpack, flooding will be a concern by mid-week, with rivers such as the Snoqualmie and Skagit being forecast to reach flood stage (see below).  So be prepared if you live or work near these rivers.

 Thanksgiving week is traditionally the wettest and stormiest of the year..and it doesn't look like it will disappoint in 2017.

Wednesday, November 15, 2017

Low Solar Radiation: Is That Why You are Depressed?

I can't tell you how many folks have emailed me or complained in person about what they perceive as a sudden turn to darkness.  They feel depressed, tired and anxious.  Daytime light helps maintain our circadian rhythms and a number of people suffer from Seasonal Affective Disorder (SAD) when the days grow short and clouds fill the sky.

So what is the truth?  How bad has it gotten?  Let's check. 

Here is the solar radiation reaching the surface in Seattle from the wonderful WSU AgWeaherNet collection of stations over the past six months (check below).   During June and July, some days had nearly 30 MegaJoules per square meter (a Joule is a unit of energy), and even cloudy days had about 15.  But recently, we have had days with 3-4 MegaJoules....way, way lower.   And the last few days, had no real spike upward in radiation to provide relief. 

Folks are HAS been dark.   Part of the problem is, of course, the turn towards more clouds the last few weeks.   But, in addition, the number of hours of daylight has progressively dropped, something shown by the figure below (the vertical white line indicates today).   We have rapidly lost daytime hours the past few months and we are close to hitting bottom.

The recent switch to daylight savings time makes it even worse, with our commute home now in darkness.

The air even feels different, with biting cool, dampness that Seattle residents fear.   Here is the plot of relative humidity for the past 6 months at Seattle.  During the past month, relative humidity has jumped to around 80-90%, resulting in not only a feeling of dampness, but a loss of visibility due to fog and water-absorptive particles in the atmosphere.

Very dark, clouds, humid, and cool---enough to scare off many Californians, and certainly able to explain the down feelings some folks have experienced during the past week.

Monday, November 13, 2017

Wind Storm Update

The first...and lesser act...of today's wind event took place this morning, the passage of an occluded front associated with the offshore low.  But the main act is still in the cards, and folks should be prepared.

The infrared satellite image at 6 AM shows the swirl of clouds around the low center, located southeast of Vancouver Island.  The occluded front is indicated by the roughly north-south band of cloud over the Cascades at that time.

Heavy rain accompanied the occluded front (see radar at 2 AM), and after its passage winds surged.

The maximum winds during the last 24 hr (ending 6 AM), which pretty much happened during the past 6 hours, are impressive: several over 40 mph in the central Puget Sound, with some on the Kitsap Peninsula reaching 50-60 mph.  Even stronger winds on the coast.

 Power outages have already occurred over Seattle
 And Puget Sound Energy has about 25,000 customers blacked out, including large areas over the Kitsap.

The winds above Seattle-Tacoma Airport show the movement of strong winds aloft, some of which have reached the surface in gusts.  This chart shows sustained winds from the surface to 10,000 ft (700 hPa pressure in this figure), with time increasing to the right (in UTC, 13/12 is 4 AM this morning).  The solid triangles indicate 50 knot sustained winds.  Strong winds came in aloft overnight, and intensified with the occluded front.  Temperatures area cooling as well (think snow in the mountains).

We are now in the break before the main act.   You will even notice the rain has backed off and there will be sun breaks. Absolutely typical.  We must wait until late morning/early afternoon when the low approaches and the winds will increase again, probably exceeding what we experienced last night.

The UW has developed the Seattle WindWatch site (sponsored by Seattle City Light) to assist City Light in preparing for and managing wind outages.  One of its capabilities is to present the latest High Resolution Rapid Refresh forecast from the National Weather Service.   This system forecasts very strong winds along the coast at 11 AM (blue indicates gusts about 50 mph), with lesser, but still problematic winds (40-50 mph gusts) over the Seattle, with stronger gusts from Everett westward.
 By 4 PM, winds will accelerate further over central and southern Puget Sound.  With many leaves still on the trees and new branches untested by strong winds, expect more power outages.
Be prepared for the increasing gusts and avoid places with a lot of trees.  No biking to the UW on the Burke Gilman trail for me today!  And if you have trees around your home, expect lots of leaves down.

Sunday, November 12, 2017

A Possible Strong Wind Event Tomorrow

Tomorrow (Monday) a significant wind event is forecast to hit the region by the U.S. models..  But there is some uncertainty because some models (e.g., the European Center) are moving the low farther offshore.

The 1 PM Sunday visible satellite imagery shows the incipient storm, nearly due west of the CA/OR border. Can you see the swirl of clouds around the low and the very unstable air (with popcorn-looking clouds) to the storm's west?

 The latest WRF run  (forced by the US GFS model) shows the forecast sea level pressures (solid lines) at 4 PM Sunday.  A 995 hPa central pressure.

 By 10 AM, Monday morning, the system has revved up as a double-barrel low, with an intense pressure gradient (change of pressure with distance) along the Washington Coast.  That will bring very strong winds.

 The low pressure sweeps northwestward, bringing a strong north-south pressure gradient over western Washington, which will experience strong winds.

How strong?

       Here are the wind gusts forecast by the ultra-high resolution (1.3 km grid spacing) UW WRF system.

By 6 AM Monday morning, strong winds have reached the coast (50-60 knots) and over portions of NW Washington.

 By 11 AM (19 UTC), crazy strong gusts (above 70 knots have reached the SW coast of Washington) as the low center approaches the NW tip of WA.

By 1 PM, the low center is over Tatoosh Island and winds over Puget Sound are revving, with gusts to 50 knots over south Seattle and more around the San Juans.

The action continues through 3 PM, with the winds starting to back off along the southern WA coast.

But what about other models and ensemble (many forecast) products? The NWS SREF ensemble system's forecast of sustained winds at Sea Tac shows that most runs indicate windy conditions, with an average sustained wind of about 20 knots, which would imply gusts to around 25-30 knots.   There is considerable uncertainty, with some of the runs with much stronger winds.

The vaunted European Center model is taking the low farther offshore with the gusts over Puget Sound reaching  40-45 knots, with more over the coast and San Juans (see graphics of pressure and wind gusts below for 4 PM Monday).

A forecast of gusts over Seattle of 30-50 mph seems reasonable.  Add 20 mph for the San Juans and the coasts.  Considering that we are early in the season and lots of leaves are still on the trees, one should expect that thousands of folks will lose power tomorrow.

Friday, November 10, 2017

A Turbulent Landing at Sea-Tac Airport on Wednesday Evening

On Wednesday night, I was returning home from giving talks at the National Weather Center in Norman, Oklahoma.   I was on American Airlines Flight 1295 from Dallas-Fort Worth and was descending through roughly 20,000 ft over the Cascades (around 5 PM) when the flight started getting rough (the flight path is shown below, courtesy of the FlightAware website).   We descended to a few thousand feet just east of Sea-Tac (SEA) and turned north, the plane still shaking a bit. But by the time we got north of Boeing Field the flight smoothed out and remained smooth as we made the turn southward towards the airport.

And then, south of downtown, the plane hit turbulence again, and as we approached  the airport, the winds were tipping us up and down, and as we landed I was a bit worried a wing might hit the runway or that we would be blown off the runway.  I was slammed into my neighbor in the next seat (who was pretty much inebriated after drinking double shots during much of the flight).

The pattern of the turbulence was familiar to me and probably to most seasoned travelers into Sea Tac, and is one associated with strong easterly flow moving through and downstream of a gap in the Cascades, known as the Stampede Gap.

We can start by looking at the winds at Sea-Tac that night, displayed as a time-height cross section, with time increasing to the left and height increasing in the y-direction (heights given in pressure, 850 is about 5000 ft).  09/00 is 4 PM on Wednesday.   Very strong easterly flow (sustained winds of 30 knots) is found at low levels, centered around 950 hPa (about 1500 ft).

Such strong easterly flow tends to be turbulent, with large vertical wind shear at low levels, which contributes further to the turbulent flow.

Next, let's look at the pattern at the surface at around 5 PM (below), where easterly flow from Seattle southward is evident.

The UW WRF model did a nice job in simulating this flow, with a small timing error.  Here is the WRF high-res forecast for 7 PM (started 15 h before) can see the current of strong southeasterly flow coming in south of Seattle.

Why the strong easterly flow?  Because as a low center approached offshore, a strong east-west pressure difference formed over the Cascades, which accelerated air to the east from high to low pressure.

The air  preferentially moves westward through the lower areas of the Cascades, the largest being Stampede Gap, between Mt. Rainier and Snoqualmie Pass (see topo map).   My plane descended into the strong easterly flow in Stampede Gap, which produces a lot of turbulence wave-like activity as it interacts with the terrain.

Finally, pilots often report interesting weather while in flight using PIREPS (pilot reports).    Turbulence extended to Boeing Field, where a pilot reported low-level wind shear (LLWS) from the surface to 900 ft at 5:26 PM


And turbulence (LGT/MOD CHOP) was experienced by a pilot landing at SEA TAC.


Big Snow Coming to the Cascades

This should have been posted on Wednesday....

The jet stream is now directed south of Washington State, with the heaviest precipitation going south of us into northern California and southern Oregon (see upper level map at 10 AM today).  Winds are parallel to the height lines, with speed proportion to the gradient (or rate of change with distance) of the height lines.

The 72hr total precipitation for the period ending 4 AM Saturday is impressive, with some areas of northern CA getting 5-10 inches (see below).   Reservoirs in northern CA will begin refilling now, with some like Shasta starting well above normal (Shasta is at 117%).

The jet will remain south of us for a few days.

 But by Saturday, with a deep trough developing offshore, the jet stream and associated flow with be heading right into our region.

 And will continue into our area into next week (see Tuesday night at 10 PM).

As a result, precipitation will substantially increase.   The 72hr total ending 4 AM Wednesday is particularly impressive, with 5-10 inches of precipitable water over the higher terrain of the Olympics and Cascades.

 With sufficiently cool air over us, this means lots of snow, with 2-3 feet being widespread above roughly 4000 ft.  Translation: with the base with have currently, many of the Cascade ski areas will have sufficient snow to open by the middle of next week.

Monday, November 6, 2017

Thanksgiving Skiing and a Cool/Wet November

We have turned the corner regarding Northwest weather, and a cold, wet corner it is.  But there is a silver lining--a good chance of Thanksgiving skiing in some of the Cascade ski areas, with Whistler being a shoo in.

Sunday morning we had the classic snow/cold upper level pattern, with a ridge of high pressure offshore and a tight trough over the Northwest.

But this will change substantially this week, with a deep trough building offshore.  This will keep us cool (highs in the 40s), with a lot of moisture coming up from the southwest.  California and Oregon will get hit particularly hard.

You can tell the atmosphere means business by viewing the latest infrared satellite picture.  A huge swath of clouds ready and eager to move into the West Coast.

Take a look at the total precipitation expected during the next week (based on the NWS NCEP GFS model):  large amounts (5 inches plus) in the mountains from northern California to British Columbia).
You can not expect much relief from the cold and wet for a long time.  The best way to look forward is to examine the forecasts from a large ensemble system.  Here is the NAEFS ensemble almost to Thanksgiving at Seattle.  The whiskers show the range of the forecasts and the most probable value by the horizontal line in the yellow boxes.  50% of the forecasts are contained in the yellow boxes.  Temperatures rising to roughly 6C (43F).  Rain most days. Cloudy for nearly the entire period.   The kind of weather that should make Amazon yearn for a second HQ in a warm, sunny place.

My rule of thumb for mountain snows in fall is that when Seattle is less than 50F there will be snow at Stevens pass and above.   Looks good.    I suspect a number of ski areas will open next week...all they need are a few more storms.

Saturday, November 4, 2017

Will it Snow Tonight and Tomorrow Morning in Western Washington?

The answer is yes, but the amounts and distribution will be complex, elevation/location dependent, with considerable uncertainty.

The issue, as it almost always is with western WA lowland snow, is temperature.  Near sea level, the temperatures will be marginal, something compounded by the fact we are early in the winter season.  If this was January, I would calling Seattle's DOT folks to rev up the plows and sand trucks.

The general set up is nearly ideal.  An upper level trough will be coming in from the northwest (see map at 4 AM tomorrow), bringing cool air and precipitation.

Associated with the upper trough, a tight low pressure center will move to the SW tip of Washington, producing a pressure pattern that draws cold air into Washington, particularly through the Fraser River Valley.  Perfect.  The trouble is that the air in BC is not super cold and the temperatures (shown by the color shading) are on the warm side from Seattle southward.  Marginal for snow below 500 ft.  The only way to overcome this is heavy precipitation, with lots of cooling due to melting from snow falling in from above.

The precipitation will occur mainly between midnight and 9 AM Sunday. 

Here is the UW WRF snow forecast for the 24 hours ending 4 PM Sunday.   Huge variations.  The central and southern Cascades get hammered, with over a foot in many locations.  With northeasterly flow from the Fraser River Valley headed for the Olympics, the northern slopes will get snowed in, with Port Angeles and Sequim getting a piece of that.    There will be snow over central Puget Sound, but it will be very light near the Sound and will increase substantially for elevations about 500 ft (maybe 1-3 inches, higher amounts above 1000 ft).  Upslope conditions will provide more snow around the Hood Canal.

What are the ensembles (running many forecasts) say?  Here is the output from the National Weather Service SREF (short-range ensemble forecast) system that shows a plot of three-hour snow amounts at Sea Tac for the various members of the ensembles.   A lot of variability (therefore uncertainty), ranging from a trace to a few inches.  But most are going for something. 

The other NWS ensemble (GEFS) also has snow at Sea Tac  for most of its members (see below), with an ensemble mean (average) of about .7 inch.
My take from all this is the following.  The mountains will get plenty of snow (feet), solidifying a good start to the base yesterday.   Thanksgiving skiing beckons.   Near the Sound and sea level (below 500 ft), there will be some very light snow (trace to .5 inches).  But go higher (above 500 ft)  and away from warm water and amounts will get to 1-3 inches.   Like on the hills above Bellevue.   Several inches near the Hood Canal and around Sequim/Port Angeles.

As shown by the ensembles, there is considerable uncertainty.  Modest changes in precipitation intensity will make the difference between rain and snow (rain for lighter intensities).   

And keep in mind it will be hard for the snow to stick to roadways in contact with the ground, since we are so early in the season.  The latest road temperatures shown in the Seattle SNOWWATCH web site (boxes, see below), indicate that road temperatures are now in the low to mid-40sF.
Anyway, enjoy the unusually early snow over the lowlands.  And remember to change the clocks tomorrow as well.  Another hour to enjoy a hot drink and watch the flakes fall.