December 16, 2008

The coming system

Update: 8 PM....looks like the central Puget Sound region will be snowfree for the morning commute due to Olympic rainshadowing through perhaps least according to the latest forecast model output. The most serious snow in the urban corridor should be north of Seattle during the the Puget Sound convergence zone--lowest amounts 1-3 inches near the sound, increasing to 6-8 inches to the east. The map you see gives the output from the latest computer model run...with the 24h snow amounts ending 4 AM Thursday and clearly shows the convergence zone snow north of Seattle. Healthy amounts of snow...12-16 inches... will fall in the mountains. Heaven for skiers. And also note the moderate amounts from Bellingham southward to Everett. And keep in mind this is a forecast with inherent uncertainties.

This morning, temperatures across the region varied from the low teens in the cold spots away from the water (e.g, Arlington, North Bend) to mid 20s near major water bodies. Some super cold spots in valleys mights have dropped below 10F.

The big concern is tonight and tomorrow. A trough of low pressure will move southward tonight and will bring precipitation to the area. Temperatures will also warm a bit as more onshore flow develops. I think it will start around 3 AM in the general area. The low center at the surface will be inland and thus this will not be a large regional snow event. With northwesterly flow, the Puget Sound region will be initially rainshadowed...certainly from lower PS basin to the west. As the trough and associated low moves south, a Puget Sound convergence zone will form north of Seattle and then move southward. The snow will mainly be between 4 am and 4 PM. With Convergence Zone snow events the greatest amounts are generally east of the Sound....and 4-6 inches is possible there, with amount trailing off to 1-2 inches near the water. NW Washington, which is less rainshadowed could get 3-6 inches away from the water. Near the coast the temps should be high enough to have precip as rain. The mountains will get snow of course...a substantial accumulation (roughly a foot) that may allow the ski areas to open for the holidays.
Colder air moves in again on Thursday....and we all have to watch the situation on late Saturday and Sunday....when a stronger system will approach from off the Pacific.


  1. I'm so glad you have this blog. Very informative and I'm learning a lot! Excellent resource, and thanks for writing it.

  2. Hi Cliff,

    Thanks for you posts, book, and your UW presentation. As an amateur weather enthusiast (we even have a Davis Weatherstation in our backyard!) it's great to have someone give a context for the weather forecasts.

    I have one nagging question that I've never been able to figure out--I often read the forecaster discussion page on the NOAA website (prior to your blog the best place for weather detail)--when they talk about timing of events they talk in terms of "12z" or "18z"- how can we translate that into real time? And since we're all in the same time zone, why not use the usual 10 pm, 2 am, etc?


  3. Cliff Mass is the Puget Sound Weather guru - My Hero.
    Thanks for this blogg -

  4. I'm making this post as a result of my review of yesterday's posts.

    For those who are compelled to raise the issue of sunspots and solar variability, or respond to those who do, it would be wise to resort to reading a range of authoritative sources to inform yourselves on the subject(s). Offering simple opinions may suit many blogs, but this being what amounts to a "science" blog offering uninformed opinions is likely to result in other readers ignoring what you post or perhaps reading it but not vesting in what amounts to an undocumented and therefore feasibly irrational argument.

    The following links are just a few of the sites available to those seriously interested in knowing how our weather phenomenon can be and are influenced by solar energy, albeit in long term cycles:

    John F. McBride
    Seattle, WA

  5. I am ecstatic to find your blog!
    Please include more forecast info for northwest Oregon- specifically the Portland metro area. I am a huge fan of yours and since leaving Seattle in 2004 and your KUOW reports, I feel very left out!

  6. I just went to probcast for (I think) the first time 39 in Seattle tomorrow?? Isn't that a bit warm for a snow event?

  7. ..some responses...

    meteorologists use Greenwich Mean Time (GMT), also known as Zulu (Z) or UTC. GMT is 8 hr later than our local time.

    so 12z is 4 am
    0 z is 4 pm

    Regarding careful with it...still under development and sometimes has problems...cliff mass

  8. i guess i don't see where this is coming from. the weather vapor and infared seem to show that whatever is coming down is breaking up.

  9. A couple of resources for folks confused about the use of UTC time (also known as "Zulu" and GMT):

    GMT Time Converter

    To learn more about the format and terminology (largely aviation-based) in official reports and forecasts, see the following free publications:

    Aviation Weather Services AC 0045-F (pdf)

    Aviation Weather AC 006A (pdf)

  10. Cliff,

    Thanks for the updates. I look forward to you on KUOW and appreciate your knowledge.

    I'm interested to see how this week plays out. Caught the news last night and Steve Pool was saying there is always a 7% chance of a white Christmas in any given year but seemed non-committal on any higher percentage this year...your thoughts on the possible of snow falling on the 25th?

  11. Cliff,
    How come weather forecasts aren't also associated with an uncertainty estimate? It seems like the computer generated weather models would produce this?

    Thanks for the great blog.

  12. Ashley,
    You ask an excellent question..and you are absolutely right. All forecasts are uncertain and the uncertainty should be communicated to users. And the meteorological profession is working hard on fact, that is a major part of my own research. The site is an example of early attempts to do this...cliff mass

  13. Great Blog Cliff! I love the northwest weather forecasting and all its challenges!

    We are in Sammamish, and if anyone wants weather data from out here you can use my link at Sammamish Weather

    Just in case you are interested in the lower eastern foothills conditions!

  14. Boy, Cliff, it would sure be nice if that snow map was overlaid with most of the Puget Sound area cities as well. I live in Mountlake Terrace, and I'm not sure with any precision where we are on that map.

  15. Cliff Mass I love you. I have listened to you on Steve's show for awhile now and just recently discovered your blog. I was never really into weather, but you are an amazing teacher and person and make it the most interesting thing ever. Thank you.

  16. I understand using the "zulu" stuff since not everyone's in the same time zone.....but around here, most everyone is. It'd be nice if they could put PST or PDT time in parentheses alongside it......

  17. Ray Ramsey's weather forecast from 30 years ago tonight


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