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 on the horizon

In my last blog post I talked about the evolution of the upper-air pattern for the rest of this week.  I noted that there looked to be a shortwave trough forecast to dive south from Canada and later strengthen as it approached the east coast.  It looks like this is indeed what's going to happen, and as this trough brings in colder air, winter weather advisories are going up across the upper midwest.  Here's the NWS watch/warning map from this morning.  The dark blues and purples in Wisconsin, Illinois, Iowa and Missouri (and also New England) are all winter weather watches or advisories.

So how is this cyclone forecast to develop?  This morning, the shortwave trough aloft was analyzed over the northern plains in the following GFS analysis 500mb map.

Our shortwave trough that brought the heavy rain to Texas is also still hanging around, bringing more rain and possibly some severe weather to the southeast today.  As the northern trough digs down further, it's going to run into this southern trough and it looks like the two will merge--eventually.  Here's the 24 hour forecast for Thursday morning.

The northern trough is forecast to dig south and strengthen considerably, while the southern trough races ahead and moves up the east coast today.  Notice the two still haven't merged yet.  However, by Thursday evening, we have this...

There's still a subtle remnant of that southern trough off the coast of Maine, but for the most part the jet streak surrounding the northern trough has both strengthened and merged with the jet streak associated with the (formerly) southern trough.

This interplay between the two troughs has interesting implications for the organization of this developing cyclone at the surface.  I find the GFS analysis of surface pressure this morning somewhat unorganized...

...but at the same time the upper-level forcing remains weak.  Notice on the first 500mb map I showed above that the northern trough doesn't have that strong of a jet streak associated with it yet.  That limits the amount of upper-air divergence that's available to support a developing low at the surface.  What we do see on this morning's surface map is a cold front getting its act together across the high plains.  Very cold surface temperatures--in the teens and single digits--highlight a pool of cold air right behind a strengthening pressure gradient and pronounced wind shift from northern Minnesota back through eastern Nebraska and the Texas panhandle.  However, remember that we still have that southern trough out ahead over the southeast.  That trough itself helped bring cooler air down south before this new front even got started.  That makes it harder to develop a strong temperature gradient at the surface if cooler air was already in place.  In fact, the warmest surface temperatures are far to the southeast over Georgia and Alabama--far from this developing cold front.

As the cold air behind this developing cold front gets closer to that very warm air in the southeast, the surface temperature gradient will increase and, with it, the strength of the winds aloft (as we saw in the 500mb maps above).  Here's the forecast surface map for 24 hours later (Thursday morning).

The surface lows associated with the northern and southern troughs are very close to merging.  The cold front, now shown from Indiana back down the Mississippi River to western Louisiana, is much sharper and more organized.  By 12 hours later...

...the cold front has continued to move eastward and the temperature gradient has become MUCH stronger--the cold air is finally interacting with the 55+ degree Fahrenheit air that the southern trough had been keeping ahead of it for so long.  So, the presence of this other trough to the south and the east really helped delay the formation of a strong surface low by keeping the warmest air in the lower atmosphere well out in front of the developing cold front for as long as possible.  By looking at the 500mb map above for this last time (Thursday evening), you can see that just as the front at the surface has really gotten strong, so too have the winds aloft.  And with a strong jet streak aloft, that helps give us the divergence needed to deepen and organize the surface low.  Likewise, in the last surface map, you can see that we finally have one low center over the Detroit area.

Another consequence of having this southern/eastern trough out ahead is that not only does it keep the warmest air away, but it also keeps much of the moisture to the east as well.  Here's the forecast 3-hour accumulated precipitation at 12Z Thursday morning:

This is before that low center really got organized.  Remember the cold front at this point extended from Indiana down the Mississippi River. However, the main precipitation area is well to the east--associated with that southern/eastern trough!  Remember that our main low center will develop over Detroit 12 hours after that image, and pretty much all of the precipitation is to the east of that area where there is strong warm advection.  There is a light snow band that looks to be developing from Wisconsin down through Missouri, but the real snow doesn't look to start coming until after the low centers have merged on Thursday night.  Here's the forecast 3-hour accumulated precipitation for 00Z Friday (Thursday night):

It's pretty incredible how much the pattern changes.  Once the surface lows merge, air from the warm conveyor belt to the east (originally controlled by that eastern trough) can finally get wrapped up behind the western trough and its associated cyclone.  That air has its origins over the Gulf Stream off the east coast--it has a fair amount of moisture in it.  So we see that it's not until after the lows merge that heavy snow really gets going behind the low.

Since this precipitation is out behind the low center (and consequently behind the cold front), we're pretty confident it will be snow.  A quick check of the forecast 1000-500 mb thickness for this time shows that most of that precipitation is north of the blue "critical" thickness line, indicating that it's likely all snow.

Much colder air, and finally a snow-worthy system seem to be in store.

But what about on the west coast?  Seattle is also looking at a possible lowland snow event this weekend as well.  Long-range forecasts show a strong, cold, 500mb trough moving southeast from the Gulf of Alaska this Saturday and Sunday.  Here's the forecast 500mb heights, temperatures and wind valid early Sunday morning (9Z).

With cold air moving in, there also looks to be just enough moisture to get some snow in the lowlands.  The models have been putting out sporadic lowland snow from Sunday through Tuesday, but the placement is anything but certain.  Here's the latest 12km WRF run's 24-hour snow accumulation valid 12Z Tuesday morning.

Lots of snow for the mountains seems pretty much a guarantee for this event.  You can see some tendrils of snow leaking down into the lowlands and the Seattle area.  We may even see some convergence zone snow following the cold frontal passage on Sunday or Monday.  It's still pretty far away, but it definitely bears watching.

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...