The scary, potentially historic set of windstorms expected in Seattle over the next few days

I've been quiet on the blogging front for the past several months as I've been transitioning out of graduate school life and into the postdoctoral world.  I've moved from the University of Washington out to NCAR in Boulder, Colorado, which will give me any entirely new wealth of weather phenomena to talk about.  But, I could not resist putting up a blog about the forecasted major wind events expected to impact the Pacific Northwest this weekend...

This looks like it's going to be a significant series of events throughout the Pacific Northwest.  We can start by reviewing the large-scale synoptic pattern  Here's this morning's GFS-WRF model analysis at 500 hPa over the region from my OLYMPEX model page:

We can see the multi-lobed threat heading towards the Pacific Northwest.  There is broad-scale troughing across the entire northern Pacific.  Embedded in that longwave trough are what we call "shortwave" troughs--little wiggles along the edges of the main trough.  These wiggles may be little, but they represent enough instability aloft to generate some powerful storms.  Note that one lobe has already moved up into northern Vancouver Island and the Canadian coast.  That was associated with the rain and front from last night in the area.  The next shortwave is still out to sea, but rapidly approaching the coast.  By late this evening, it will have deepened considerably off the coast:

This is going to be followed by another shortwave that is so rapidly-developing that you can't even clearly see it yet in the above image.  By late Friday evening, it's starting to show up---you can see a little ripple in the central Pacific associated with a little maximum in the colored vorticity field:

This trough is associated with the remnants of Typhoon Songda, as Cliff Mass discusses on his blog.  This rapidly deepens throughout the night and into Saturday morning, reaching the coast by Saturday evening:

At the surface, these deepening troughs lead to strong cyclones with powerful winds (and a lot of rain too!).  The local WRF model currently (as of the latest 12Z run, but this could change!!!) brings 25-30 knot winds to Seattle with the Friday storm:

But over 50 knot winds to Seattle with the Saturday storm:

This morning's WRF run, as seen above, takes the Saturday storm on an almost perfect track to produce a major windstorm in Seattle.  This is a scary possibility, and concerning that this is the latest model development.  But, should we believe just one model?

 In modern weather forecasting, we like to look at our forecasts in terms of probabilities, as there remain great uncertainties in our forecasts.  We often do this by considering not just one model forecast like I showed above, but several, often in the context of what we call "ensemble" forecasts.  One ensemble used is the Global Ensemble Forecasting System (GEFS), run by the US National Weather Service.  We can look at these ensemble forecasts and compare them to previous ensemble forecasts to see just how unusual or significant a particular forecast may be.  NOAA has an (experimental?) product called the "Ensemble Situational Awareness" table (http://ssd.wrh.noaa.gov/satable/).  You can go through time and different forecast variables over a particular region and at a glance see if there is anything significant going on in the forecast.  Here's the table from last night's GEFS run over the Pacific Northwest:

Each row is a different forecast time and each column is a different forecast variable (e.g., SLP is sea-level pressure, WSP is wind speed). See all those "MAX" and "MIN" values?  That means that somewhere in the Pacific Northwest the forecast at that time, for that variable, is greater than (or less than) any ensemble member's forecast ever for that location.  This is a long period of extremes coming up for the Pacific Northwest and hits at the potential historic nature of this storm.

Let's dive more into the ensemble forecasts.  Here's an image from Brian Colle's extratropical cyclone tracking page (http://smokey.somas.stonybrook.edu/cyclonetracks/Wcoast_Tprob.html) showing the forecasted positions of the Saturday low pressure center from all the members in two ensemble systems (SREF and GEFS).

Each black dot connected by grey lines is the locations (every 6 hours) and path that a single ensemble member forecast of the path of a low pressure center from one of those two systems.  The red dots are where the ensemble members have the center of the low at 0000 UTC 16 October 2016 (Saturday evening, local time).  The colored swath represents the probability of there being a low pressure center in that location (remember, model gridpoints are somewhat coarse...half degree latitude/longitude from GEFS, for example) within 24 hours of that time (I apologize for the lack of a color bar here...the dark greens are about 60% probability).

You can see that the main swath of probabilities goes into central to northern Vancouver Island; this seems to be the most likely track if we consider all our model forecasts together.  Such a path would really bring strong winds to the Strait of Juan de Fuca and surrounding areas of Vancouver Island.  However, it would not be as bad over Seattle as the WRF run we saw above is suggesting.  But even in the ensembles there is still a possibility it could cross the northern Olympic Peninsula...there are still respectable probabilities (20-30% in the blues) that the low could move over the northwestern Olympic Peninsula and into southern Vancouver Island.  There is also still is a lot of uncertainty in the timing of the low since...as we saw...it's not even well-analyzed yet over the Pacific.

As noted by Cliff Mass in his blog, it's the path with the low coming closer to the Washington coast and across the northern Olympic Peninsula that tends to be the "worst case" for the Puget Sound region, as it sets up a strong north-south pressure gradient in the channel between the Olympic Mountains to the west and the Cascades to the east.  This leads to the strongest wind events in the central Puget Sound region.

Speaking of strongest wind events, I was curious to see what the CIPS Analog system was saying for this storm (http://www.eas.slu.edu/CIPS/ANALOG/arch.php).  This is a somewhat different method of forecasting from the raw numerical models we typically digest.  In analog forecasting, we accept the idea that numerical weather models are often wrong, but assume that, given similar weather situations, they are wrong in the same way every time.  So how does that help us?

To make an analog forecast, we take a model (here, the GFS) and look at its forecast for a certain time (I'm showing below the 72 hour forecast from last evening's 0000 UTC run, so this is the forecast valid on Saturday evening local time).  We then go back in the records of all of the GFS forecasts (or reforecasts when the system is updated) ever made over the years and find all of the 72 hour forecasts that look most similar to the current one.  We then look at what actually happened during those events and use that to make a guess as to what will actually happen this time.  Remember, the key to analog forecasting is the assumption that, given similar weather scenarios, the models will be wrong in the same ways.

Anyhow, as part of this analog forecasting process, we can look at what historical events the system thought were "most similar" to the current one being forecast.  Remember, this is limited to the time periods when we actually had GFS forecasts, so nothing past the 1980s is included.  Here are the top 15 analogs, showing the sea-level pressure forecasts.

You can see that most of them have some sort of low off the northwest coast...a good sign that the analog is working.  I tried cross-referencing these dates with the list of significant Pacific Northwest windstorms maintained by Wolf Read at (http://www.climate.washington.edu/stormking/).  I was actually surprised to see that most of these dates did not correspond to particularly noteworthy windstorms (at least, what he has documented).

Interestingly, the one big match I found was the "Two windstorms in three days: November 13-15, 1981 event" (http://www.climate.washington.edu/stormking/November1981.html) which sounds very similar to the current threat of dual windstorms in only a few days.  Here, from Wolf Read's page above, is the track of the first (stronger) storm in that November 1981 event:

You'll note that it made landfall over central Vancouver Island, similar to our current most-likely forecast swatch.  However, this Nov 1981 storm had a much more south-north oriented track, which elongated the area of the coast that was affected by this storm and prolonged its effects.  The current forecasted path for the Saturday storm is a little more west-east, which should decrease the amount of time exposed to high winds.  The Nov 1981 storms did cause 12 deaths and "tens of millions" in damage, according to Wolf Read's summary.  Also of note, the 520 floating bridge experienced some $300,000 of damage (in 1981) after taking waves driven by 75 mph winds on Lake Washington.  This goes to show that even if lows don't exactly take the "classic" path for severe windstorms in central Puget Sound, Seattle can still see damaging events.

As the storms approach, our model solutions should converge on more likely forecast tracks with better estimates of the potential for wind damage.  People from Vancouver Island down through Portland need to be on the lookout and make preparations for this storm.  Be sure to frequently check your local National Weather Service office in Seattle (http://www.wrh.noaa.gov/sew/), Portland (http://www.wrh.noaa.gov/pqr/) or Environment Canada (https://weather.gc.ca/warnings/index_e.html?prov=bc) for the latest warnings and advisories.  Stay safe!

A Western Washington Tornado

Well, with a new year I decided it was time for me to resume my blogging activities.  So, I'll try to have regular updates on a weekly/biweekly basis at least as new weather topics come to me.

On Sunday afternoon, January 18th, most people in the Puget Sound area were glued to the television to watch the Seahawks game.  Passing outside their windows was a series of strong showers following a cold front, not at all unusual for the mid winter here.  Just before 2:30 PM PST the National Weather Service started getting reports of a tornado near Gig Harbor, Washington.  Here is the 0.5 degree base reflectivity radar image from the time.  Several strong, but compact cells across the southern Puget Sound lowlands.  Nothing that screams "tornado"...

It wasn't even the most intense cell that ended up attracting the attention here.  That cell produced hail later on, though at the sub-severe level.  No, it was the little blip to the west that produced an EF-1 tornado on the southeastern Kitsap Peninsula.  Here's the National Weather Service Seattle WFO storm survey results:

We joke about some of the storm damage markers we get from tornadoes in western Washington.  "Skylight cover sucked off".  "Kayaks moved around."  Though it's too bad that all tornadoes can't be this tame.

This has gotten me thinking about tornadoes in western Washington.  We get a few episodes with waterspouts every year, but tornadoes themselves are not very common.  I went back to the Storm Prediction Center's Severe Weather Database that I used in a previous blog post.  This includes a record of all known tornadoes since 1950.  It turns out in Washington state west of the Cascade Crest there have been 46 tornadoes since 1950 (47 after Sunday's event).  Here's a map of the tornadoes in the immediate Puget Sound area, labeled by year:

Those without red dots had path lengths too short to show up.  Given the short paths, relatively weak strength and the large amount of rural/undeveloped area in this region, there are probably more that escaped detection.  But there are actually more than I thought there would have been.

Another unusual feature of these tornadoes in the seasonal climatology.  Here is a histogram of the number of tornadoes per month (sample size of 46).

There's an odd sort of bimodal distribution here, with a rapid increase in number from March through June, then NONE in July, followed by a secondary ramp up in the fall through November before none in December and February.  Granted, this is a small sample size, but it's interesting that the tornado probability seems to crash so rapidly at the end of Spring and Fall.

More snow in Seattle? This morning brings a new look...

A lot can change overnight.  Yesterday evening's model runs were increasingly pessimistic about how much snow would fall in the Seattle area.  However, this morning's projections are looking more on the optimistic sie for seeing 4-6 inches of snow in the Seattle area.  So what changed?

In my last blog post I talked about how the location of the surface low moving in off the coast would be critical in determining the amount and duration of the snow event in Seattle. Too far north and we'll rapidly transition to rain.  Too far south and we won't get enough moisture brought north to overcome the large amount of cold dry air at the surface over Seattle.  Yesterday, the models were bringing in the small surface low into the coast at or just south of the mouth of the Columbia River--this was a bit too far south to get the maximum snow in Seattle. We'd see some snow, but not much.

At least, that's what last night's 00Z models were saying.  But then something changed overnight--the surface low got closer to land.  And as the surface low gets closer to land, it enters our observation network--it gets closer to our offshore buoys and weather stations on the shore.  As such, we get a lot more data about the strength and position of the surface low the closer it gets to shore.  We can use that data to improve our model analysis of where the surface low might be.  This is done through a process called data assimilation.  Most of my research revolves around improving this very process.  So what did data assimilation tell us about the low overnight?

Below is a map from our experimental real-time ensemble Kalman filter system (a type of data assimilation system) here at the University of Washington.   This system uses the power of ensemble modeling to really give us a good idea of the amount of uncertainty in a particular forecast, then uses that uncertainty to help determine how observation will impact the model. This is an increment map of sea-level pressure from 6Z last night (10 PM).  You'll see the familiar sea-level pressure contours in black.  This is what the model system thought the sea-level pressure field was before we assimilated data.  The color shadings show the impact of assimilating all the data we have--nearly 10,000 observations of wind, temperature, pressure and moisture from that entire area--into the model system. Anywhere you see red, that means that the observations wanted to increase the sea-level pressure there.  Anywhere you see blue, that means the observations wanted to decrease the pressure there.

The observations wanted the model to decrease the pressure on the eastern end of the pressure trough extending out to the east from the large low center in the central Pacific.  This isn't too much of a surprise--we knew that a surface low was going to form there anyhow.  Now let's go ahead three hours to 9Z last night (1 AM).  Here's the adjustment map for that time:

Notice that by this point the model indeed formed a small surface low in the center of that pressure trough--it responded to those observations.  But now look at the location of this next pressure adjustment.  The observations want to decrease the pressure to the north of that developing surface low!  This is to say, the observations want to pull that developing low further north.  This kind of pattern continued all morning, with the observations wanting to pull the surface low slightly further north.

Now, our operational deterministic models that I show here don't include the data assimilation analyses from our experimental EnKF system.  However, the initial and boundary conditions for their 12Z runs this morning do come from a larger model (the GFS) that does have a lot of data assimilation included.  So, when looking at the 6-hour forecast of where our 4km WRF wants the surface low to come ashore this afternoon, we see this:

It has brought the surface low back up north a little bit, now coming ashore just north of the Columbia River.  Even though the change in the position of the low is subtle, it has huge ramifications.  This brings the warmer, moister air closer to Seattle and increases the amount of snow that could fall.  Here's this morning's model 24-hour snowfall projections ending at 12Z tomorrow morning:

Remember last night the models had backed off to maybe 1-2 inches in the Seattle area.  This morning's forecast shows 4-6 inches (with an interesting convergence-like band over the Kitsap Peninsula and Seattle...).  Quite the change overnight.

And, since this morning's 12Z run (which is based on data from 4 AM), our ensemble Kalman filter (EnKF) system has continued to run.  Here's the 15Z (7 AM) adjustment of sea-level pressure due to observations:

It shows the low analyzed off the mouth of the Columbia River (still offshore) but there is still a strong adjustment signal that wants to lower the pressure to the north of the low (and raise it a little to the south).  This points to the low continuing to be pulled northward.  Hopefully it doesn't get pulled too far northward--then we'll end up with a lot of rain tonight.

So, the forecast is still developing, but I have pretty high confidence that we'll see several inches in Seattle today.  Not only are these short-range forecasts, but now that the low is close to the coast our models can take advantage of all of our observations here and really nail down where the low is going to be.  I also already have two inches or so at my place, so yes--I have confidence.

What's the deal with the snow forecast for Seattle?

As much of the nationwide media is now proclaiming, there is a potential for a major winter weather event in western Washington tonight and tomorrow.  Here's what we're looking at.

Snow showers continue over western Washington this afternoon as cool, moist air streams in off the Pacific.  Portland and the northern Willamette valley are really getting hammered.  Here's the latest composite radar image for the region.

However, the latest surface map shows that the temperatures around Portland are several degrees above freezing and most of what is falling is rain.

Showers will continue tonight, but the big snow event won't be until late tonight and into tomorrow as a relatively weak surface low approaches the coast.  Here's the 27-hour forecast from our 12Z initialized, 36 km WRF for tomorrow morning:

Note the really strong pressure and temperature gradients shaping up along the coast for tomorrow.  There's very cold air in the interior of British Columbia and relatively mild, moist air being brought up from the south behind the surface low.  So how does the interaction between these two air masses lead to a snow event?

As the surface low moves in, there will be a strong pressure gradient across the mountains--high pressure associated with the cold air inland and the lower pressure associated with this warm frontal wave off the coast.  This pressure gradient is going to draw down cold air near the surface out through gaps in the mountains and down into the lowlands.  Meanwhile, the warmer, moist air being brought up from the south will tend to lift over the cold air.  That lifting will cool the moist air, clouds will form, and precipitation will fall out into the cold air near the surface.  Here's a diagram that hopefully will help explain that setup:

That is a Google Earth perspective looking at the Olympics from the Cascade Mountains east of Seattle.  As the low pulls down a wedge of cold air out of the mountains to the north, warm moist air circulating around the low will move in from the south and rise over this low-level cold air.  The result is snow, and a lot of snow if the boundary between the leading edge of the cold air and the warm air to the south doesn't move much.

However, locating that boundary between the cold and warm air is critical for determining where the snow will fall.  If the warm air moves too far north, Seattle will be too warm for snow and we'll get a lot of rain.  If the cold air pushes too far south, the warmer, moist air will be precipitated out by the time it reaches Seattle and we'll see less snow.  This is all related to the placement of the surface low.  Here's two scenarios:

First, let's say the surface low stays further north--approaching the coast from the Strait of Juan de Fuca, perhaps.  In this case, warm, moist air will be brought up into western Washington.  However, because there is that nice gap between the Olympics and Vancouver Island (the Strait of Juan de Fuca), the surface low will draw most of the cold air straight west and very little will filter south into the Seattle area.  This would turn into an all rain event for Seattle.  I show this setup below.

Now let's consider another case.  If the low pressure center moves toward shore just a few hundred miles to the south, some things change.  We still will have warmer, moister air brought from the south around the low and into western Washington.  However, the cold air being drawn out of the mountains in Canada will be pulled more southwesterly out of the Cascades--and straight at the Olympics.  Some of the cold air will then get pulled down through the Puget Sound lowlands in addition to wrapping around the Pacific side.  This setup is much more favorable for snow in Seattle--we get that cold air near the surface that we need to support snow:

And it looks like our models so far are favoring a southern route.  Here's last night's 00Z forecast from our 4km WRF for where the surface low would be at 18Z on Wednesday (Wednesday morning).

Notice that the surface low was forecast to be right off the mouth of the Columbia river.  The pink horizontal line across the map marks the freezing line at 925mb (near the surface) anywhere north of that line should be cold enough to support snow.  Definitely a snow event for Seattle.  Here's the corresponding 24-hour snowfall accumulations that were forecast by tomorrow evening over western Washington:

This placed 6-8 inches of snow over the Seattle area--very impressive snowfall totals.

But, wait...this was last night's model.  What about this morning's 12Z run?

This paints a slightly different story.   Here's this morning's model surface forecast for 18Z Wednesday (the same time as the surface map above):

The low pressure center (and the freezing line with the cold air behind it) has been moved slightly further south--it's not much, but it's enough to make a dramatic difference in the final forecast snowfall amounts:

Now the model is only forecast 2-4 inches over the Seattle area, with a sharp decrease to the north to almost no snow north of Everett.  Since this morning's model run moved the surface low and cold air south, the most efficient precipitation production also moved to our south.  Thus, the model precipitation amounts have come down.

This change in precipitation amounts is very sensitive to the exact location and timing of the arrival of warm air and the low pressure center on the coast.  One thing we do know is that with this kind of model trend (moving the colder air further south), it should definitely be cold enough to support snow through the duration of this event.  We're not as concerned about the transition to rain at the end anymore.  The big question is still what final precipitation amounts will be.  The low may end up moving slightly further north and increase the snow amounts again--we'll have to wait and see what tonight's round of models have in store.

Even with the slightly diminished amounts, we're still looking at a major snow event late tonight and tomorrow in the Seattle area.  It looks like the only questions now are in the details.

Quick verification of yesterday's snow and turning to the Pac NW

Yesterday's snowstorm across the upper midwest is well on its way to moving out of the area.  I thought I'd take a quick minute to look at what the final snowfall pattern looked like and compare it to the model outputs I was showing yesterday.

First, final totals from around the Chicago area are now in.Most areas got at least three inches of snow, with totals up to 8 inches being reported in some areas of the Chicago suburbs.  Here's an analysis map from the WFO Chicago webpage of 24-hour snowfall amounts as of 7 AM today.

Let's compare this to the forecast map I showed yesterday.  The time periods don't match up, so the amounts won't be the same.  However, I'm more interested in the geographical distribution of the snow.

There's actually remarkably good agreement in the extent of the areas being forecast to have snow.  The model is correctly leaving snow out of the Detroit area, and the western edge of the 0.1 inch or greater snow line matches up well with the 1-2 inch line on the analysis.  Also,the areas of the most snow--from northern Michigan and eastern Wisconsin back down through northern Illinois--also agree fairly well.  I'm rather impressed with this model forecast.

However, I want to turn my attention now to the potential for snow where it's more unusual--in the western Washington lowlands.  A shortwave trough aloft is moving down the west coast of Canada, bringing with it cooler air and a setup that's very favorable for snow in the lowlands.  Here's the 500mb setup now:

A large, deep, cold trough is sitting in the Gulf of Alaska.  This trough has brought some of the very heavy snows that Anchorage and southern Alaska have been getting over the past few days.  The same trough is not done yet--by Sunday morning it's forecast to have moved south over the Pacific northwest.  Here's a 48 hour forecast:

The cold air accompanying this trough will linger for several days--through at least the middle of next week.  With westerly flow both aloft and near the surface, the air moving in will contain some moisture as it moves in off the Pacific.  However, the models are all forecasting low-level temperatures near or below freezing to persist.  With "moist" air moving in over sub-freezing temperatures, there definitely is a potential for snow.

And the models are really starting to go a little crazy with this.  Here's our UW WRF 12-km forecast of 24-hour snowfall accumulations ending Sunday night:

Definitely some snow creeping down into the lowlands.  That scale shows around an inch in the Seattle area on Sunday.  There also looks like there may be some convergence zone enhancement to the north of the city.    The mountains are definitely getting hammered.  However, if you look at the breakdown of the precipitation, most of the lowland snow is coming from several scattered showers.  Here's the next 24-hours of accumulation ending on Monday night:

Interesting pattern here that puts a lot of snow to the north and to the south of the Olympics.  This snows 2-3 inches falling over Tacoma, and another inch over Seattle.   Once again, though, this isn't an organized, massive snow event--it's more just snow showers. Here's the next map ending on Tuesday night, though we're starting to get beyond the model's predictive confidence...:

More convergence zone enhancement to the north of the city, and lots of snow in many places of the lowlands.  Another 1-2 inches forecast for the Seattle area.  And finally the map ending Wednesday night:

Wow--4 inches forecast for the Seattle area.  If you sum that all up, that's a forecast potential for up to 8 inches if you believe the longer range forecasts.  But Sunday is just two days away, and the last several model runs have consistently pointed to at least a small amount of accumulating snow in the Seattle area.

There's a lot of factors that could complicate this.  Water temperatures are still above freezing in the sound, so low-level temperatures will have that to compete with to maintain snow all the way to the ground.  There's also concern that the westerly flow that's bringing moisture in from over the Pacific may spend too long over the water, warming the air enough that we don't get below freezing. Also, with this concern about the temperatures, any light accumulations will probably melt off during the day, meaning that we won't see too much on the ground at once.  My main concern is just how many days we're facing the potential for snow.  Even with small amounts, the constant threat of snow could cause some problems, particularly for commuters.

We'll have to watch this.  The coastal radar and the dual-pol capabilities of the radars will be put to good use next week.

Snow in Seattle (and a moderate risk!)

Quick post tonight as I have some other things I need to take care of.  But, I promised to talk about snow in Seattle today...

First, as many people have noted, the SPC has upgraded to a moderate risk for the severe weather potential tomorrow in the southern US:

Fig 1 -- SPC day 2 convective outlook as of 1730Z, Feb. 23, 2011.  From the SPC website.

So things are continuing to shape up there...

However today as promised I wanted to focus on the snow event in Seattle--and how locationally variant it has been.  Models have shuffled back and forth amazingly over just where the snow is going to fall.  Here is what the local 4km UW-WRF model was forecasting would be the 24 hour snowfall accumulations as of tomorrow morning--basically, how much snow (and where) the model predicted would fall with this event.  This first image is what the model was saying on Tuesday morning:

Fig 2 -- 48 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.

Back then, note the heaviest snow was forecast to be along the northern slope of the Olympics, a lot along the Cascades, and basically heavy snow to the north of Seattle.  Compare that to what the Tuesday evening run of the model said for this same time of snow accumulation.

Fig 3 -- 36 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.

Dramatically different. The snow accumulations moved much, much further south.  In this model run the heaviest snows seem to fall under a band that goes right over downtown Seattle.  The snowfall amounts are also forecast to be in the 12 inch+ range across the Seattle area.  That strong, thick band over the central Puget Sound region looks a lot like an enhanced Puget Sound convergence zone.  The formerly snowed-in north also seems oddly dry as compared to the previous model run.  Now for this morning's model run:

Fig 3 -- 24 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.

Now the heaviest snows have moved back north again, with only two or three inches over Seattle.  This is one reason there has been a lot of confusion surrounding this event--the models just haven't agreed.

So far today, the northern snow solution has been favored--five to six inches of snow had fallen in areas north of Everett as of the middle of this afternoon.  But the snow is now beginning to move south--and is definitely linked to a convergence zone.  Here's a radar image from early this evening:

Fig 4 -- KATX 0.5 degree base reflectivity from 0115Z, Feb. 23, 2011.  

Surface observations are overlaid on the radar image above.  Note the line of enhanced reflectivities marching south through Seattle at that time.  This line was accompanied by a huge graupel downpour and has now given way to snow behind it.  That definitely looks like it's being enhanced by the convergence zone--note that the wind barbs to the north of the line are showing winds out of the north and to the south of the line we see winds out of the south.  Classic convergence zone.

But is that really a convergence zone?  That band of enhanced snow seemed to be moving pretty far south and east an hour and half later:

Fig 5 -  KATX 0.5 degree base reflectivity from 0245Z, Feb. 23, 2011.

It looks more like a front now.  In fact, most analyses on the local news call this an "arctic front" pushing south through the area.  The location of that front is going to have a lot to do with the location of the surface low off the coast.  So this front's movements will also factor into what locations are going to receive enhanced snow amounts.

So what are we expecting to happen?  Currently the surface low is sitting just off of Cape Flattery--the northwest tip of the Olympic peninsula.  As that low shifts south, the "arctic front" (and the cold air to the north of it) will shift south as well.  Note, however, in the radar image that the snow is rather diffuse--it's not one big huge sheet of reflectivities like we usually see.  This hints that there isn't as much lift as we'd be expecting for a widespread snow event.  This also leaves only three real sources of lift:

1. Orographic lift as air is forced to rise up over the mountains (the Olympics and Cascades).
2. Convergence due to the Puget Sound convergence zone or convergence along the Strait of Juan de Fuca. 
3. Convergence along the cold front moving south through the sound.

This means if you're not in the mountains, the snow is probably going to be rather localized.  As the local convergence zone(s) fidgets around tonight, lift over the convergence zone will cause areas of snow to move around throughout the Puget Sound region, probably trending further south as the evening wears on.  I really can't see us getting that much accumulations--certainly not the foot that was hinted last night.  I'd guess only an inch or two at most in the Seattle area.  However, if the convergence zone decides to park over a certain area, the snow could really accumulate.

Of course, the mountains will get a ton of snow.  They have built in lift around them...

So, in short, this isn't really like the big snow event of last November.  Roads may get a bit slick, but things shouldn't be too bad...

The Puget Sound Convergence Zone and Convection

Ok--I was going to do a post about the large scale pattern in upper-air features over the past two weeks and how that will hopefully start breaking down by the end of this week.  But then on the way back from the University today, I started getting text messages from people...

"Was that hail that just fell?"  "Hail?  Graupel?"  "Is it supposed to snow today?"

Hmm...we were in the upper 40s today in terms of temperature at the surface...I was pretty sure we weren't expecting any kind of frozen precipitation...

But sure enough, when I turned on this evening's news, their lead story was about the downpour of hail that hit Seattle this afternoon.  Interesting...  So I fired up the radar and cycled back to the time of the report.  The radar showed this:

Fig 1 -- KATX 0.5 degree base reflectivity at 0048Z, Feb. 8, 2011.

The little cell I point out with the red arrow is the closest I could come to something looking remotely like convection going on.  A cross-section through the radar returns in that cell is also not very impressive:

Fig 2 -- KATX base reflectivity cross section through the cell identified in figure 1.

There is a small core in the middle of the cell with some vertical coherency.  The reports said the "hail" was only pea-sized or smaller (hardly worth calling "hail" in my opinion...) and as such we wouldn't expect to see THAT large of a reflectivity return from the hail compared with the surrounding rain.  The top of the core only makes it up to an estimated 5000-6000 feet too--not very tall.  But apparently this was vigorous enough convection to produce some small hail.  It's too bad our local sounding site is way out at Quileute out on the Pacific coast so our upper-air profile over Seattle is a bit of a mystery...

So how do we get convection in Seattle?  This place usually does not get the strong fronts and temperature gradients nor the instability aloft needed to produce deep convection.  So we live with shallow convection.  Still...there has to be a mechanism to provide the lift necessary to get convection going.  This is where the Puget Sound convergence zone comes in.

To describe this phenomenon, we first need to focus on the geography of the Puget Sound area.  To the east of the city of Seattle and Puget Sound lie the Cascade Mountains, running north to south.  However, to the west of Puget Sound lies the Olympic Peninsula and the relatively high, but isolated Olympic Mountains.

Fig 3 -- Topographic map of western Washington.  Higher elevations are shown in the tans and browns.  The orange colors indicate urban areas.  To the west of Seattle lie the Olympic Mountains.

Puget Sound convergence zones set up when the winds in the low levels of the atmosphere have a strong westerly component.  Let's consider what happens to low level winds out of the west once they reach the western shore of Washington.  Immediately they are confronted by the isolated high terrain of the Olympic Mountains.  All that air rushing in has two choices--it can either rise up over the mountains or go around them.  Now, we do see a fair amount of lift on the windward side of the Olympics--the Hoh rainforest lies on the western side of the Olympics for that very reason.  But generally the atmosphere suppresses large-scale rapid vertical motions like that (there are many reasons for this, but we'll just accept that for now).  Instead, much of the air is forced to go around the Olympic Mountains.

Fig 4 -- Schematic of westerly flow separating to go around the Olympic Mountains.  A lee-side low is formed.

However, when all that air splits to go around the mountains, what happens on the eastern side of the mountains?  All the air flowing around the mountains creates a sort of void in the atmosphere immediately behind the mountain range.  This is signified by a lowering of the pressure there.

But what does the atmosphere do in response to a lowering of pressure?  Air will rush in to try and fill this "void" that has been left on the eastern side of the mountains.  As a result, the wind that split to go around the mountains will be sucked back toward the low pressure that has formed on the lee side of the mountains.

Fig 5 -- The lee-side low draws the winds inward on the lee side of the mountains.  As the winds from the north and the south meet, a convergence zone is formed.

Of course, air is being sucked in toward the low pressure from both the north and the south.  When these two wind streams meet, there's a region of rather strong convergence (indicated by the dotted line in the figure above).  This convergence can provide enough lift to get shallow convection over Seattle and form small hailstorms like we saw today.  So that's the Puget Sound convergence zone in a nutshell.

So did we have those kinds of conditions today?  You bet we did.  But with a slight twist.  Here's the forecast 925 mb (low-level) chart for 00Z this evening (near the time when the hail over Seattle occurred):

Fig 6 -- UW 4km WRF 12 hour forecast of 925 mb  temperature (shaded) and winds (barbs) at 00Z, Feb. 8, 2011.

We can see here that off the Washington coast, the winds were out of the northwest.  This isn't straight out of the west, but you can imagine that a similar effect occurs.  In this case, the convergence zone would form further south than indicated in the diagram above and would also be oriented in a northwest-to-southeast direction.

Fig 7 -- Same as in figure 5 but adjusted for more northwesterly winds.  The convergence zone is located further south and oriented from northwest to southeast.

So we'd expect to see a convergence in our low-level winds south of Seattle somewhere, though the location tends to meander with time.  What I showed above is a model forecast for 925mb winds.  Do we see this convergence reflected in the observations?  Here's the surface map of observations from 0100Z this evening (about the time of the hail in Seattle):

Fig 8 -- Surface METAR observations from western Washington at 0100Z, Feb. 8, 2011. 

I've added several blue arrow roughly paralleling the wind barbs in those areas.  We see a clear flow of wind around the Olympic Mountains and decent convergence at the surface right through the southern part of Puget Sound (the dashed red line).  Also, the lowest pressure in the area (1022.8 mb) is the observation at Shelton, which I circled in orange.  This is pretty close to where we would expect see that lee-side low pressure form.  So--this is a classic convergence zone case.

Furthermore, we can look at the radar radial velocities from this time to see convergence there.

Fig 9 -- KATX 0.5 degree radial base velocities at 0101Z, Feb. 8, 2011.  Arrows showing the rough direction indicated by these colors demonstrate convergence.

The KATX radar is in the northern part of this image.  If we remember that green colors indicate air moving toward the radar and red colors indicate air moving away from the radar, we can see convergence right along that line we were expecting to find it in figure 8.  So the radar velocities also show some convergence there.

It gets more complex though--a second convergence zone seems to be forming across the northern Puget Sound area and the Strait of Juan de Fuca.  Can we have dual convergence zones?  Interesting possibility.  The surface winds don't show as clear of convergence there.  Could this be convective instability released in a direction parallel to a jet over open water (similar to a lake-effect snow band coming off of the Strait of Juan de Fuca)?  Perhaps.  More investigation would be needed.  But that's about all I can cover in one blog without going too long...