Saturday, February 8, 2014

Poor Snow Forecasts. Why?

The snow forecasts over the Pacific Northwest have not been particularly good this week.  Portland got hit hard on Thursday with 3-5 inches, with nearly a foot to the south and southwest, yet the National Weather Service forecast  released 4 AM Wednesday morning painted a very different picture:

And the UW high-resolution WRF forecasting system, driven by the National Weather Service's best model (the GFS), was not exactly as skillful as I would like, displacing the snow into southern Oregon (see graphic of the 24h snow ending 4 AM on Friday from a model forecast started on 4 AM on Wednesday). 


It would take another 24h for the model to get the forecast right (model started at 4 AM on Thursday).  In fact, the poor and erratic model forecasts were the cause of the human forecasters' problems.

Why have these snow forecasts been so poor, when for some snow events we have gotten the snow amounts, the snow distribution, and the snow timing essentially right?

 It turns out that this situation plays to all our weaknesses.

During the past week we have had a situation with cold air over British Columbia and Washington, a zone of large temperature change (a frontal zone) over Oregon, and warmer air over California.  To illustrate, the following map shows lower atmosphere temperatures (925 hPa, around 2500 ft up) and sea level pressure at 8 AM on Thursday.  Cold (blue) over Washington, warmer (green and orange) over CA, and a big change over Oregon.  Cold enough to snow from central Oregon northward.

The official NWS analysis at this time shows the position of the front, and also indicates a frontal wave suggested by the undulation of the front and the low center (see graphic).

Weak disturbances that develop on fronts, or frontal waves, are relatively small scale, are often shallow, and are very difficult to forecast correctly even over land.  But in this case, it is even harder because they are forming and evolving over the ocean where our ability to detect and describe small-scale structures are not as good.  And the snow events this week have all been associated with such frontal waves and to forecast the snow correctly requires getting their position, size, and motion exactly correct...something current weather prediction technology is still not adequate to deal with.

 Let me illustrate the problem.  Here is the analysis of sea level pressure and lower atmosphere temperature at 4 PM Thursday during the first Oregon snowstorm.  See the low right off the central Oregon coast?  This is a good description of the truth.

 The 12h forecast  for the exact same time has the low, but it is shifted well inland.
 The 24h forecast way more.
 The 48 hr forecast pushes the low into southern Oregon and is trying to build a new one over the Pacific.
The 72h forecast is hugely different with a low offshore.
 And the 96h forecast has no low over Northwest, with cold air spreading down to CA.
Now this is one model.  We have many models to look at and until the last day before the Thursday snow they were all over the place with very different solutions.  In short, there has been huge uncertainty in the forecasts because the models lacked enough data over the Pacific to tie down the solution.  Furthermore, such small scale, fast moving features are inherently hard to forecast.

But why can we forecast snow skillfully in some cases?  The reason is that some snow events are associated with different types of evolution, such as when a very large scale warm front approaches cold air over the Northwest.  The models can get a handle on such large scale features and we know that with cold air  in place, the event will start as snow and turn to rain.  You can practically set your clock with such forecasts.

Ah yes....will it snow today or Sunday?  Let me tell you what we know for sure.  Warmer air and rain will sweep over the region on Monday, ending the cold period for the western lowlands. Guaranteed.

But today/tonight another frontal wave is approaching Oregon and snow is already falling  in the Willamette Valley (see radar).  But the air mass is warming and rain is being observed over the Oregon coast right now

The lastest WRF model snow forecast for the 24h ending 4 AM Sunday suggests theWillamette Valley snow, lots of snow in the northern Oregon Cascades, snow over SW Washington, with a dusting getting as far north as Seattle.  Snow will also spread east of the Cascades as well. 

The accumulated snowfall over the next 15 hr from the NOAA HRRR modeling system (for the period ending 10 PM tonight) shows a consistent story:
So if you are in Portland, more snow today for you.  Seattle will probably escape from any significant impact, with perhaps a dusting over the southern portions.  But as you can imagine, a slight shift northward of the low could change the story a bit for southern Puget Sound.




10 comments:

Just AboveNOAA said...

i really appreciate this sort of post - thank you! relative predictive confidence is such an important, and often ill-communicated, feature of science.

if there were sufficient information, perhaps a forecast could contain confidence intervals on the probabilities? ("40% chance of precip +/- 17%") [wink]

Chip D.F. said...

Wow. We've been screwed out of snow two years in a row now. Very disappointing, especially with all the hype over an actual event (2-5 inches in Seattle forecasted earlier) over the last week. Maybe next year, AGAIN.

Rod said...

I am just darn happy to see precipitation in Washington, Oregon, and California. We all need it.

danreedmiller said...

Thanks for the explanations of harder than usual unpredictability. The snow keeps coming in waves here in Portland. I don't think I've ever seen a more continually changing series of forecasts than the last few days! It's almost as if the forecasts are being made up on the fly by looking out the window and extrapolating intuitively! Not sure WHAT to expect next, other than the eventual meltdown on (presumably) monday.

JewelyaZ said...

I don't care if we get snow or rain or what but I am really thrilled fro Oregon and California, who are watching the drought slowly get less and less terrifying. I doubt it will totally disappear, but the past week has taken them from the very brink. Here's hoping they don't relax... it's time to get a new water strategy going because it sure doesn't look like counting on rain/snow to keep up with a growing population is going to work even another few years.

Kevin Martin said...

I had to inform NOAA to issue an Ice Storm Warning for the Willamette Valley. I issued mine for my viewers last night at 9pm so they would know what they would get the following day ... incase they didn't have time to check in the morning ...

But I will be honest at least they got it out and didn't leave the people without one.

I can only warn so many people, one day I can warn as many as they can in that region .. .till then I'm only growing to what I can warn now.

My Ice Storm Warning - http://www.theweatherspace.com/2014/02/08/ice-storm-warning-12/

Overall medium range models are not suppose to handle snow-storms correctly ... it wasn't too bad the final 12-24 hrs of the event ...

Colleen said...

Glad some parched areas are getting precipitation. Disappointed to "escape" the snow ~ again. Sigh. But all I really need to know is, when will the infernal wind coming out of the Fraser Valley cease & desist? North Whatcom County has been battered for days now. Cliff, make it stop, please!

Westside guy said...

It's been snowing in the Sumner area for an hour or two - we've got maybe an inch on the ground so far

Cyle Messling said...

I think its funny the snowfall totals significantly changed after this post. Just shows how crazy the weather is around here. Sometimes you don't know what you're going to get until happens. All fun aside, i love the blog Cliff. Very informative and a must favorite for anyone who loves weather.

-Cyle

Unknown said...

Closing in on an inch in Mill Creek, elevation 450'. 27 degrees. I suspect NWS is doing some tuning to their forecast, as it's almost 10 and their 9:00pm AFD has not come out. Winter storm warning is up for the eastern suburbs, though, while a winter weather advisory is in effect closer to the Sound.