the convergence zone has set up

You would almost think we know what we are doing.... the Puget Sound convergence zone has set up, having pushed southward during the last few hours...and as expected we are getting wet snow at higher elevations on the eastern side of the zone.... see the image... The surface map clearly shows the convergence zone, with northerly winds in the N. Sound converging with the southerly stream.

According to the profiler, it is still too warm for snow near sea level..(see image)..but that will change later.... If you look carefully, you can see the northerly winds associated with the convergence zone for the latest sounding (far left).Looking at the latest infrared satellite picture (attached) shows the front that moved though this morning, and an enhanced band off the coast (northern Vancouver Is)...that should move in later today and will rev up the mountain showers and the convergence zone. That is when the real action should occur. You see that the clouds are whiter there...that means they are higher and more active. So get ready, the main action is yet to come...
A reminder..sign up for the NW weather workshop if you are interested (we need to know how many refreshments to get, links at upper right). And next weekend and Monday, I will give public lectures on Bainbridge and Olympia (see links). UW Press likes to keep me busy. After that I will be taking a break.

Complex situation

This is the kind of forecast where one can make big errors because there are so many degrees of freedom and things happening at once. So lets start with what we are sure about.

We are fairly certain that:

(1) a cold front is heading for us and will cross western Washington between 4 and 7 AM.
(2) some light precipitation will accompany the front
(3) that much colder air is behind the front the freezing level will plummet. The snow level will decline to only a few hundred meters above the surface very quickly and by the end of the day will reach sea level.
(4) that the mountains will get more heavy snow..at least a foot.
(5) the the freezing level is sufficiently low that the high eastern suburbs should get more snow than the lowlands near Puget Sound
(6) that the air will get colder on Sunday and will hold in until Tuesday
(7) that outflow will develop in Fraser River Valley, push into Bellingham, and give upslope snow on the ne side of the Olympics
(8) that there will be substantial rain/snow shadowing due to the Olympics
(9) that a Puget Sound convergence zone will form sometime tomorrow and produce moderate snow in the eastern suburbs and quite possibly down to sea level.
(10) that outside the convergence zone and the upslope on the mountains, precipitation will be generally absent or light after the front goes through in the central Puget Sound region.

You see, we know quite a lot. But there is plenty we don't know...when and to what degree will the convergence zone set up? Will it be cold enough for snow near sea level if the convergence zone sets up early. Those are the the million dollar questions. So you got to be ready for the white stuff. Or not.

My own take is that the convergence zone will set up by 9-11 AM over central Puget Sound and that there will be snow above 200-300 ft, with wet snow/rain down to sea level. The convergence zone will hand around for much of the day...although it could shift in position and intensity with time. Later in the day and Sunday AM, the convergence zone will shift north over Snohomish County. Anyway, the position and strength of the convergence zone is a hard one...so keep this uncertainty in mind. Watch the weather radar in the morning to see what happens in real time.

If anyone is interested, KCTS-channel 9 in Seattle-- will replay my weather program at 1 PM. And if you want to help a good cause, you might make a pledge to keep public TV going.

Cold, Snow and Double Convergence Zone.

This morning I mentioned there was a double convergence zone..one that was obvious in the satellite and radar (see images). Why? Because there was convergence of air streams behind two barriers...the mountains of Vancouver Island and the Olympics.
Take a look at the surface observations this morning (image)--you can see the converging air streams behind both barriers in the winds.

The air was cold enough today that where there was substantial precipitation intensity, snow was able to reach the elevations down to a few hundred feet. And the mountains were hammered with a foot.

Better get used to it...more probably on the way.

Tomorrow will be splendid...sunny and dry...but things change on Saturday morning when a cold front moves through, with very cold air behind. There could be some rain and snow showers after frontal passage..but I suspect the real fun will be later on Saturday and on Sunday morning when another convergence zone sets up. So don't be surprised to see some light snow somewhere over the Puget Sound lowlands. And the mountains will get plenty more.

Morning Convergence Zone

We do have a convergence zone now...as we often do after fronts...and the air is cold enough for snow above 500ft where precip intensity is great enough.

PS: Note there are TWO convergence lines (more on this later)

PSS: If any of you are interested in the NW weather workshop, please take a look at the link in the upper right and register if you want to go. The agenda is online. This is the meeting for hardcore local weather lovers.

Update on cold and snow

Well, its pretty definite now...we are going to experience an unusually cold period for this time of the year...with a real threat of snow showers on Sunday.

Lets start with right now. A cold front is now approaching..and there are clouds and some light showers associated with it. But the flow is sufficiently westerly that central Puget Sound is being rain shadowed (see images). The showers will move through tomorrow am, with improving weather during the day.

On Friday, an upper ridge will move in and it will by dry and sunny. But the fun begins on Saturday as a major Pacific ridge shifts eastward and an upper trough moves southward down its eastern flanks (image). Behind the trough much colder air will flood the region on Saturday into Sunday. On Saturday, the associated showers will probably be rain...but on Sunday things get interesting. The winds will be westerly, so there will be rain shadowing, but the models are insistent about the creation of a Puget Sound convergence zone...and if they are right it will be cold enough for snow (see model snow forecast).
The cold air will hold in for several days...and there will be a substantial pressure difference across the Fraser River Valley...so Bellingham and NW Washington could be raked by strong NE winds and colder temps than anywhere else. This air is cold enough to allow temps to drop into the 20s at night...so there could well be a serious frost that has the potential to damage sensitive plants...particularly those that are budding and vulnerable...so get ready to protect them.
A positive part of the forecast is that the Cascades should enjoy plenty of snow tonight and over the weekend...with up to foot possible tonight and tomorrow.

Cold and Snow?

It is March 3, and the weather maps for later this week and the weekend, are a real eye-opener--some the coldest air I have seen this late in the season for an extended period of time. And yes, there are snow chances for the lowlands. But keep in mind that we are working against the clock. Significant lowland snow after the first week of March is increasingly unlikely--although we did get an inch last April (the frigid spring from hell--or I should say the arctic).
During the next day or so, we will be in the typical spring shower regime. Right now a band is heading towards us (see satellite pic) and should get to Washington overnight (the precipitation from it is visible in the Portland radar).

But that is not the fun part.

A major upper-level ridge develops over the eastern Pacific on Wednesday and a trough moves southward over us on Thursday (see map). Such a flow pattern will bring colder air, certain snow in the mountains, and perhaps in some high elevations...but it is not ideal for much lowland snow..the trough is too far inland. But this pattern doesn't go away and another trough moves in on Sunday...and is much more threatening..colder and and more favorable trough location with greater upward motions. The WRF model in fact puts down some lowland snow...but this is so far out, I would not get too excited yet (image). My rule of thumb is I don't get psyched about snow until we are within 96hr of the event. But I would get those salt trucks ready. And the cold air holds in. Just amazing.

PS: the Northwest weather workshop will be on March 20-21st..the major annual meeting of local meteorologists and hard-core weather enthusiasts. If you want more info or to register, check out:

http://www.atmos.washington.edu/pnww/

Warm Front

You see the increasing clouds aloft...these are associated with an approaching warm front aloft...one associated with a vigorous offshore system (see sat pic). But that system..and its strong winds will not make landfall in the NW and strong winds will not reach our interior.
With low pressure offshore and high pressure inland, strong offshore (easterly) flow has developed west of the Cascades. So I would expect breezy conditions as one approaches the passes today--on I90 east of North Bend. The Seattle profiler shows the easterlies aloft...and such flow often produces warm temperature over the lowlands...so I expect temps in Seattle to surge into the 50s today.
The precipitation should hold off until this evening...so today should be a fine one, expect for some high and middle clouds. Then tomorrow we will see considerable clouds and some showers...and mild temps on the south side of the warm front (50s).
The weather pattern this week is a familar one..with a major low/trough over the Pacific (see sample upper level chart on Sunday). Low centers will spin up the coast offshore and most of the precipitation will head into California..where they need it. A boring collection of clouds and showers for us through Wednesday, after which the models indicate the chance of a cool down again. But I will wait before talking about that...

PS: For those interesting, I will be on KCTS on Sunday at 7 PM, answering weather questions (see link at right). Also I wanted to note the NW weather workshop on March 20-21st for those interested...it is open to all, but you should register in advance--but this does include technical talks (see link to right).