There have been some complaints about the recent snow forecasts in some quarters. The Seattle Times will run a story tomorrow "Forecasts for Snowfall in Seattle Goes Cold", several of you have commented about poor snow forecasting of this event, and even one prominent local TV
weathercaster noted that sometimes he has to admit to making a mistake (about the snow over Seattle) and then proceeded to blame it on the models (and showed the UW model output for good measure!). Let me give you my take on this.
Before we evaluate the forecast, what actually happened. Well, the region did get snow and some locations a lot of it. Based on the official National Weather Service numbers, the heaviest snow (up to .5 to 1.5 feet) fell in a line stretching from
Skagit County across
Whidbey Island out to Victoria. On the northern Olympic Peninsula
Sequim had 4.5 inches,Port Townsend 5 inches, and Port Angeles a bit over 9. Snow decreased toward
Bellingham 1.5 inches and southward to
Marysville (3 in) and Everett (4.5 inches). The immediate Seattle urban area had roughly .5 to 1.5 inches, with snowfall rapidly increasing to 3-9 inches on the
eastside. Snowfall increased south of Seattle (Kent, Auburn,
Southcenter, Sea
Tac) to 4-5 inches. 3-5 inches from Tacoma to Olympia. Very little snow over the southern
Kitsap Peninsula.
So it snowed, and in many areas it was appreciable. I5 was closed for a while near Mt. Vernon and crippled near Sea
Tac last night.
Let us consider how the
UW WRF model did, since this is important guidance to both the National Weather Service, local TV folks, and yes, even me. Let me show you the 24h predicted snowfall for the period ending 4 AM this morning for several model runs starting days ago. Let see how consistent the 4-km resolution model guidance was and how it changed in time. You be the judge!
Here is the forecast initialized (started) at 4 AM on Monday. Note the lack of snow over Seattle and the
Kitsap (due to
rainshadowing off the Olympics). Heavier snow north of Everett and over the N. sides of the Olympics. Moderate snow over the
eastside communities and heavy snow over the mountains. NOT BAD!.
The forecast made 12-h later. Similar situation
In the forecast begun 4 AM on Tuesday, the
Bellingham snow threat has declined. The
Skagit snow band was there, the
Kitsap-Seattle dry band is there but weakened. But too much snow was evident over North Seattle.
Twelve hours later a distinct snow band was positioned over Seattle. Clearly, not right.
And here is the forecast from 4 AM Wednesday morning. The Seattle snow band had weakened and heavier precipitation had moved north. Better, but not perfect, of course.
Now there are a few things you should take away from this.
First, the model (and the others the
NWS and I were looking at) was serious about snow; although the snow distributions changed from run to run, they all indicated a serious threat. Second, they all agreed the mountains would be hit hard...and they were. Most of them showed a lessening of precipitation over the
Kitsap and southern Seattle, and substantial snow over the eastern suburbs. Most indicated enhanced snow in some kind of east-west band, but the position varied.
This was a crucial point....it was clear that the greatest threat was in a zone of convergence between northeasterly coming from the Fraser River Valley (and surrounding low terrain) and southerly flow moving up the Sound. But there was NOT agreement exactly where this would be. There was uncertainty for this feature.
It also turned out there was uncertainty about where the low pressure center controlling a lot of this action would be positioned.
To get this prediction right the forecast had to simulate all the following correctly: the
downslope off the Olympics , the flow through the Fraser Gap , the position and intensity of the low , and upper level trough approaching the region, among other features. All interacting in a very non-linear way. And it was worse than that. Most of the showers were from convection--isolated snow shower that are very much hit and miss. It was clear there would be a lot of banding and variability in the snowfall. Folks, getting all this right is beyond our technology and science.
So was this forecast a failure? I don't think so. Even though we did not have confidence where that intense snow band would set up, we could tell you it could set up somewhere resulting in very heavy amounts. We could tell you the mountains would get large amounts and that serious snow would hit the
eastside. We could tell you about the
rainshadowing over
Kitsap and Seattle as being a probable element. And the snow on the north side of the Olympics. We gave a modest snow range over Seattle (roughly 1-6 inches depending on whose forecast you looked at) to cover the uncertainty.
Will we eventually do better than this? You bet...and we are working on a technology at the
UW (called Ensemble
Kalman Filter data assimilation) that might do so, but this is a few years off. And the new coastal radar will paint out the critical flow structures offshore that we can't see now.
This snow forecast is a vast improvement on what we could have done only a few years ago. And a critical issue it reveals is the necessity for my profession to learn how to effectively communicate the uncertainties in our forecast. To paraphrase an erstwhile Secretary of Defense: we need to tell you about the known unknowns.
And in a future blog I will discuss the shameless and occasionally irresponsible hyping of this event by a few media outlets, including their breathless description of sudden icing that supposedly would befall yesterday's commute...just like Nov. 22nd! Complete nonsense.
The most extensive precip is south of the city, but some strong bands to the north. Light snow last hour at Sea Tac in the southern band, and Port Angeles and Friday Harbor in the northern features. Too warm for snow at sea level--Sea Tac is at 452 feet. In heavy precip, the snow level can descend to the surface. Hard to get snow to the surface in strong onshore flow...even cold flow like this..unless you have sufficiently intense precipitation.
Temperatures are cooling aloft and at the surface...the snow level is now around 1000 ft....
See how fun forecasting is...it combines science and gambling. Probabilities and big pay offs...
Near the water, temps only dropped into the mid 20s; move more than a few miles inland and the teens were prevalent.
Olympia Airport is the coldest (8F), but nearby East Olympia is close (10F).
Olympia Airport does seem to be in a relatively low spot with surrounding higher terrain. But I have seen more impressive terrain low spots. I should also note that the observing equipment at the Olympia Airport is the best....a NWS/FAA ASOS station--the gold standard and carefully calibrated.
the Olympia cold temperature last night was almost equaled by a location in Skagit County..the Sakuma Farm AgWeatherNet site with a temperature of 5.6F. They probably grow ice lettuce there!
You see the band of clouds over the ocean stretching NE to SW? I bet you won't guess what that is.... a convergence zone to the lee of the Olympics. With flow from the NE, the air split around the Olympics on the NE side and then converges together on the SW side. Converging airstreams produce upward motion and clouds Pretty weird!
Low grass with surrounding trees. You can see the hills on the right side. If you want to get a really good idea of the surroundings, here is a video taken as a plane landed at Olympia Airport:
Now there are a few things you should take away from this. 
Above is a nice summary of what has happened so far--24h total precipitation from the Seattle Rainwatch web site. You can see the heaviest precipitation was in a band extending from Victoria eastward to Skagit County. Some folks there have gotten 6-12 inches...and even more. Here is a video from one of them this morning:
Here is the latest radar. There is still shower activity...but nothing particularly heavy. But it is not over yet....
If the line continues to push southward, the battleground between the two air masses could be fierce and significant snow the result. Temperatures are relatively mild from a day's worth of heating, but melting of precipitation and the ingestion of cool, dry air from the north should help it turn to snow. I think Seattle commuters will be fine....the roads are warm and salted (sound likes a good meal!)...but tonight could get very interesting very fast if this boundary locks over someone. Watch the radar now...if will be very entertaining!
Here is the story at 6 AM. The commute for Seattle and to its west should be uneventful. The winds aloft are still westerly aloft that we are in the rainshadow of the Olympics and that should not change much before 8 AM. As normal for rainshadow, precipitation picks up in the eastern suburbs east of Lake Sammamish. Snow showers are moving into the region, but as seen in the radar most of the action is north of Everett and south of Bellingham. So Whidbey Island, the San Juans, and the northern Olympics are seeing snow showers now, some occasionally heavy.
Go south of McChord and there are snow showers moving through the Chehalis Gap. Moderate to heavy snow on the western slopes of the Cascades and foothills.
Getting pretty white!
There are a lot of subtlies in play here. First, the snow will be very banded, which means there will be a lot of horizontal structure (spatial variations in snowfall between neighboring locations). Second, for the early part of the period there will be significant rainshadowing (snow shadowing) off the Olympics under northwesterly flow...which means Kitsap and south Sound will have snow reduced. Third, a LOT of the action will be at the boundary between NE flow moving out of the Fraser and vicinity and southerly flow moving up the Sound. The southerly flow will be forced to rise by the cold, dense northerlies--producing enhanced precipitation (snow). That is why you see the heavy band between Seattle and Bellingham. No guarantees EXACTLY where that interface will lie. Upslope flow on the Olympics should produce heavy snow on the northern Olympic slopes.
Associated with this upper trough will be a low pressure center that will form in the lee (south) of Vancouver Islands. (map). The real primo cold air will be locked in the interior of BC, blocked by the mountains.
Virtually all the models are on board with this solution--enough that I think it has a good chance of it happening (YES, there is always uncertainty in a forecast). Virtually all indicate strong NE winds will exit the Fraser starting on Wednesday...and then it gets interesting and more uncertain.
You can see the heavier snow associated with the interface--stretching across Whidbey Island to the north Olympic peninsula.
The only negative for those of us in central Puget Sound is that rain (snow) shadowing evident in the above forecast...but who knows about the details?
As a result it is sunny over much of the WA interior but precipitating and even snowing along the coast. Here is Forks:
The destruction of the Mukileto Landing Restaurant will be the theme of this talk.
Strong showers can bring the snow level down...even to the ground... today. In fact, Hoquiam is reporting light snow now! So don't be surprised if some locations get a snow shower today. You can see how the air became less stable during the day with from the following video. We had cold air aloft and then as the surface heated, the rate of change of temperature with height increased and the atmosphere destabilized, producing increasingly strong cumulus cells (click on picture)
