Thursday, February 19, 2009

Uncertainty



I often warn about taking long-range forecasts (beyond 5 days) too seriously...particularly when we talk about snow. Today is a good example where even 3 day forecasts are very, very uncertain. Attached are the 500 mb upper level charts for 3 days out...from the National Weather Service GFS and NAM models (the top two models used by my colleagues in the NWS). They are radically different--much more so than we usually see. What does that tell us...very substantial uncertainty. If 3 days out is uncertain, what do you think is the case for the 7-10 snow events some are talking about? The GFS model usually is the best...but this is the kind of situation that really reduces my confidence.
Meteorologists have a powerful tool for assessing uncertainty ...ensembles collections of many model runs. We run an ensemble system at the UW and it shows major differences between the solutions from modeling centers around the world. Attached is a graphic showing the differences in 500 mb heights (think of it as pressure in the middle troposphere). The yellow and brown colors indicate very major differences in the 72 hr forecasts of the ensemble members...indicating large uncertainty. It can be intoxicating to view the long-range forecasts and think about what they imply. But you need to understand the uncertainty that exists. Furthermore, the uncertainty varies by situation--and understanding that uncertainty and how to estimate it is the difference between my skilled colleagues in the NWS and people who simply look at the model output.

PS: Will be signing books at the garden show in seattle at noon on Sunday...

45 comments:

andycottle said...

Cliff...you say the GFS models are usually the best. Maybe. Not that we should rely on them, but GFS models can be very bad at times as well. Guess the better examples would be ridges and wind storms as they can and do sometimes over estimate strengths of these weather systems.

Anonymous said...

We never said those events are going to happen for sure. We just like to have some hope during this dismal weather period. :)

andycottle said...

Once again, it appears that the 00zGFS gives us showers at times through about late next week as the surface and upper level low pressure spins around over open pacific waters and sends rotating bands of rain to our region under SW flow aloft. By late in the week, we may dry things out just a bit as the low appears to sink southward. Alas, our mostly dry weather for late week is brief. By the weekend, it looks like a 516m/528dm trough will be dropping over us for cool and showery conditions and also perhaps marginal snow chance as 850mb temps drop to -9c. And still looks like majority of the cold weather will be well east of us and over Montana/ high plains area around this time frame. Then for the long range and into first eight days of March, that continues to look cool and very unsettled as large and deep troughs dip down over our region from the NW in which it keeps us in a rather active weather pattern....or at least in terms of what we have seen lately.

Area of low pressure send waves of showers toward our region.

http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer//gfs/00/images/gfs_slp_096m.gif

Large and cool 516m/528dm trough of low pressure over us by next weekend.

http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer//gfs/00/gfs_slp168192_m.shtml

Joseph Ratliff said...

Andy,

Does "large and deep" troughs typically indicate wind as well? Or, just breezier conditions than we've had in awhile?

andycottle said...

Well probably not windy by any means(at least in the interior lowlands), but likely just breezier than what we have seen around here lately or for that matter, in last few months.

Anonymous said...

Those two images look very similar to me. The pattern looks nearly the same.

Anonymous said...

Fellas, fellas, we all know the drill. Enough of this lowland snow talk. If the models are showing snow over Seattle 7 days out (which the GFS was earier today, for next Thursday), that translates into snow levels falling no lower than 1,000 feet when "showtime" arrives. The angle of the sun and the increasing daylight hours at this time of year make it incredibly hard for Seattle to get any sort of accumulating snowfall (especialy snowfall that sticks around during daylight horus) this late in the winter that has become "Dullest Winter Since 2002-2003," minus our two-week period of snowy bliss in December.

I'm hoping for a March 7-8, 2002 repeat, but it's not likely. Rather, a mix of rain and snow will fall north of NE 125th Street in Seattle sometime in the next ten days. In response,Q13 will have some clueless "weather" reporter (P. Lewis?) broadcasting live from the Shoreline Safeway on "Spring Blast 2009," as the rest of us watch a 37-degree rain fall.

andycottle said...

Ummm....if we want to talk about lowland snow in the long range, then we will do so.

So please none of this "Fellas, fellas, we all know the drill. Enough of this lowland snow talk."

If we see something interesting in the long range, then we`ll talk or make mention of it just because it gives something to chat about since there isn`t much eles to chat about right now.

Anonymous said...

Have you ever heard of sarcasm? I believe the other Anon was being sarcastic and funny.

You continue to talk about lowland snow chances as if they're in the actual forecast issued by the NWS. It's very deceptive.

Cliff Mass Weather Blog said...

Andy.. There are verification statistics...and GFS does verify better in the mean. And any serious forecaster is constantly looking at the ensembles to judge reliability of the forecasts. The GFS model is not always the best. The skill is knowing when. ..cm

andycottle said...

Yes, I`m sure there is skill involved and there would have to be. :o)

andycottle said...

The latest 6z/12z as well as the 12zWRF continues show increasing clouds and showers in here by early Sun morning and continuing through the day Sun under SW flow aloft. It appears that on/off showers will continue through the middle of the week as the area of low pressure slowly moves closer to us and looks like this weakening low may move inland for mid or late week. So while it wont be a rainy week, we will for sure see rotating bands of showers coming in off the coast and from the spinning are of low pressure. For late in the week, we may still dry a bit, but doesn`t last long as a cool and showery trough may be upon us for the weekend. The MRF and GEM model also show a showery trough over for the weekend. So looks like next week and into the weekend may be a bit unsettled.

Minor split flow aloft.
http://www.nco.ncep.noaa.gov/pmb/nwprod/an...fs_500_138m.gif

large dropping over us by the weekend..
http://www.weather.unisys.com/gfsx/9panel/..._500_9panel.gif

Josh-B said...
This comment has been removed by the author.
Josh-B said...

Seems like we talk about the future a lot here. And when the future becomes the present (may it be a snow storm,windstorm,sunny day)we are already talking about the next event. It's like when Saturday comes we are already looking forward to next Saturday while not enjoying the present. I catch myself doing that sometimes. In the middle of a snow storm I would already be thinking about the next one.

Weather forecasting is all about the future, I know. And humans need to feel safe by trying to predict it. (and for the most part lives and property depend on it especially when the weather turns extreme) But sometimes it's ok for it to be a surprise.

William said...

Cliff- thanks for the great Blog, I have learned a ton over the past six months. Today's post really hit home on why I find your posts so valuable - your discussion of uncertainty in the forecast. I can't help but think - why isn't some indication of the forecast uncertainty part of mainstream weather reporting? Or at least more technical NOAA forecasts. If you get a chance, I would love to hear your thoughts on the question. Thanks again for the great info!

Anonymous said...

Seems to me that Andy should get his on blog if he wants to share his vast weather knowledge with the world. People who come here want to hear from the expert Cliff.

Anonymous said...

I like Andys comments.
Thanks Andy, you are a fine addition to this blog.

WeatherNerd said...

Andy

I enjoy your comments as well. Each time you write I pick up some term, model name, or otherwise that I want to learn more about and find out on my own. We know the long range forecasts CAN be inaccurate but you're right, it gives us something to talk about. It is interesting to see how sometimes they are way off while other times once that 180 hours rolls around you think, "well, it sure gave us a decent idea of what laid ahead".

Rob said...

I also enjoy reading Andy's "forecasts" even though they are not right 100% of the time they are more accurate then Komo, Kiro and King I have found myself checking this site 3-4 times a day for updates. I also learn a heck of a lot more here. Cliff should make everybody sign in or get rid of the Anonymous comments so people will have to show who they are instead of hiding behind the "anonymous" tag and own up to what they say.

Anonymous said...

Ummm....if we want to talk about lowland snow in the long range, then we will do so.

That is fine. But if someone makes a post chiding you for doing so (see Anonoymous posting at 10:59 yesterday) that is also perfectly fine. People are welcome to say "enough of this lowland snow talk" just as much as you are welcome to say "let's talk lowland snow." I believe that's fair.

WeatherNerd said...

Wow, I was just outside for a coffee break, what a nice day out there. Too bad I'm back in the office now. At least the weekend is here and there should be plenty more days like this in the months ahead.

Gator said...

William-

I 2nd the importance of your question re: why isn't uncertainty an integral part of mainstream weather forecasting? I get the impression that Cliff also considers this as a weakness in modern forecasting but would love to read his elaboration on the topic. Check out www.probcast.com which is linked to from the front page of Cliff's blog.

carey said...

I agree with anonymous@11:59 above: Andy is absolutely entitled to publicly share his interpretation of the model output, but it seems like co-opting the comments section of Dr. Mass's blog is an inappropriate way in which to go about it. Blogger accounts are free and require less than 5 minutes to configure. Creation of a separate Andy Cottle Weather Blog seems like a win-win situation all the way around.

Anonymous said...

This may seem like boring weather to some contributors, but it's actually the beginning of the transition from winter to spring. In parts of the country/world with "real" winter weather, the transition times signal the beginning of the time of year when fresh food could become available. "Exciting" weather is all very well for modern folks who venture out into the natural world for entertainment, but for farmers and those who lived close to nature in the past, quiet occasionally rainy days rather than violent or frozen weather could mean the difference between survival and disaster. (It still does in Hurricane Alley. Look at any year's mortality figures in Third World countries.) My father grew up in such a world, and he raised us to respect the meaning of weather in our lives. That's why I like this blog, both Cliff's information and Andy's supplemental information. I want to learn to understand NW weather as well as I could understand the weather back East.

By the way, I sign in anonymously because I have never bothered to sign up with Google--I have enough invasive elements in my life already. For those who might be curious, my name is Doreen, although "Lyn in Lynnwood" is my screen name. I'm an interdisciplinary engineer (which means I'm trained to do a little of everything,) currently studying Construction Management (another field where the "right" weather can be crucial...) and considering where my life will go when I graduate.

I hope this blog continues for a long long time!
Lyn in Lynnwood

Anonymous said...

I agree 100% with Carey. Andy has plentiful weather knowledge (far more than me) and is an insightful contributor, but I am here to listen to Dr. Mass' take on the models, not Andy's. This blog has become too much Andy, and not enough Cliff.

P.S. I'm anonymous for the same reasons as Lyn.

-Danny

Anonymous said...

I agree with Carey and anonymous. Andy...please start your own blog. Reading between the lines, I sense Cliff is uncomfortable with your long-term predictions, but has been too gracious to tell you. You have a lot to say, which is good, but it would be far better if you did it in your own blog. -Tim R.

Jessica said...

Choose the option Name/URL and you do not need to be Anon.

But truthfully, Anon is no different. We could all run rampant w/ wild comments, signing with a new name each time and only Cliff would know if we were one-and-the-same.

That show on Mon. looks interesting. And you are so right Lyn - for so many a change in the outlook is the difference btw survival and catastrophe.

Anonymous said...

Simply regurgitating model output doesn't add much to the discussion. Anyone can seek out model output. There is more to forecasting such as understanding model biases, recognizing familiar patterns/failure modes, etc. These are the subtleties Cliff provides us here. Those insights are what this blog is all about.

I agree with carey, creating a separate blog to provide links to weather resources is a win-win idea. This way Cliff's insights and expertise are the focus of this blog and the other blog can focus on providing links to weather resources so people with less time or motivation have access to them.

Anonymous said...

I just figured Andy Cottle was Cliff's intern or something. Andy, who are you exactly? We know who Cliff is. What are your meterological qualifications?

Cliff Mass Weather Blog said...

All,
There has been a lot of email traffic about the comment section. I have no connection with Andy Cottle. He is clearly not a meteorologist nor trained in meteorology, but appears to be someone actively interested in the subject matter. (I think he should apply for entrance into our undergraduate program!) I do think it would be better if he started his own blog, so that he would be unfettered in his ability to talk about what he would like. I am uncomfortable regarding the long term predictions he is providing.
Now lets get back to interesting conversations about weather. Several of you have asked about uncertainty in predictions...I will deal with that in my blog on Saturday...it is an area I am actively researching...cliff

Anonymous said...

Re: Ambiguity and uncertainty in weather prediction..

I think most of the TV stations are pretty good about admitting they dont know for sure what is going to happen, at least some of the time.

The exception just cracks me up...KIRO and their "Pin Point Weather" predictions that rain that is falling on Bainbridge Island at 5:29 will reach Bellevue at 5:54! There have been many times I wanted to call the station and say, "HEY! You guys said it would rain at 5:54 and it is still sunny!!

Corie

Anonymous said...

Several people seem to agree with the fact that people are welcome to say things like "enough of this lowland snow talk" just as much as others are welcome to say "let's talk lowland snow" and that they believe that's fair.

Then it's also fair if people are welcome to say that they enjoy one type of weather (or the lack of weather) while others are welcome to say that they enjoy other types of weather, and question the self-conflicting points of view of others. It's no different. :)

Dan said...

Josh-B... you know my theory about why people talk about the future, and then when it comes, there's only talk about something else in the future? I just think that it's been that way this year, because models will indicate something nice and interesting in the future, but when it finally arrives, it turns out to be a bust and nothing happens. That's just the way it's seemed this year at least. Not much going on.

I definitely agree with you that it's okay for weather to be a surprise. That's what makes weather fun and exciting. How boring would it be if we always knew exactly what was going to happen? There would be nothing to talk about. Most weather doesn't even make people unsafe at all... only some of the extreme events.

andycottle said...

For the ones who made nice comments about my weather outlooks, I thank you for them.

Some of you are saying that I should start my own blog. I`m not so sure anyone would come. At the same time, I am in no way trying make anybody uncomfortable in reading my longer term outlooks, nor am I trying to take over this blog. Would not even think of that. This is Cliffs blog and he makes this blog interesting and informative.

However like others, I`m just posting about the weather like everyone else here.

As for who I am...well just click on my picture. I`m 26, and have been working at Molbaks Nursery in Woodinville for almost 5yrs now. And yeah, my main interest is weather.

I`ll try to stick to the shorter term outlooks.
-----------

Moving onward. Today was a really nice day with my high getting up to 53 with very sunny skies. My low this morning was 27.

Anonymous said...

Can't wait for the nicer weather to come Sunday and next week!!!

andycottle said...

I see the latest 18zGFS still has increasing clouds/showers over us by around pre-dawn Sun with on/off showers for during the day Sun. And with SW flow aloft, bands of of showers should continue to move inland due to a weak area low pressure spinning of the west coast. This low, which is seen on the 4km/ 12hr loop of the inferred satellite imagery, which is now also starting to take shape off the northern California coast by several hundred miles, should move inland over WA or Oregon by around mid week. Once this low moves inland and out of our way, it appears things may dry out a bit and perhaps briefly getting into split flow. And with this split flow and upper level heights near 550dm, highs for mid-late work week should be near 50. MRF-MOS seems to go along with the same idea. So as always, will see how things play out.

Split flow perhaps taking hold for later next week.
http://www.nco.ncep.noaa.gov/pmb/nwprod/an...0120138_m.shtml

7am Thurs| Split flow aloft.
http://www.atmos.washington.edu/mm5rt/data...0j.147.0000.gif

STLD said...

andycottle- Cliff has mentioned this...start your own blog dude. You have made 8 comments/significant contributions (forecasts, etc) to this one entry. I think your efforts would best be served starting your own blog/and applying to the UW undergraduate program. This is Cliff's blog and you constantly write long, detailed, often inaccurate long-range predictions that never come to fruitition. You have a lot of passion about the subject, obviously, but seriously, start your own blog or tone it down a little when it comes to someone else's blog. -STLD

Anonymous said...

It's a win-win-win idea. That way, we win (the readers of Cliff's blog), (the readers of Andy's blog) and the readers of both blogs. win-win-win = ideal.

Anonymous said...

LOL...weather dudes fightin' it out!!

andycottle said...

I know this is Cliff blog and I enjoy his informative weather info that he puts out when time allows as I`m sure others do. And like others on here, I like posting my thoughts about the weather and what is seen in the models.

If we see something interesting in the models, should`nt we be allowed to say something about it, even if it`s not in the 5 to 7 day period?

Anonymous said...

Generally the comments that are left should pertain to the subject that Cliff is talking about, that is what a blog is about, the blog owner posts a topic and others talk about That topic, nothing else. I have not once seen Cliff post a topic that invites others to write a forecast in the comments.

andycottle said...

For all you anonymous folks who thought I should start my own blog, well here is. Feel to post and leave comments, ect.
http://andyspnwweatheroutlooks.blogspot.com/

Anonymous said...

I feel like commenting on something in the models that is not mentioned by Cliff and is beyond 7 days out is not mentioned by Cliff FOR A REASON -- i.e. there is great uncertainty as to whether that something (like lowland snow) will ever happen.

Anonymous said...

Andy, congrats to your blog. Please add anonymous entries there and please keep visiting us here with your short subject related insights.

Joseph Ratliff said...

Great job Andy! I have bookmarked your blog too.