Thursday, February 21, 2019

Increasing Chances for Light Snow During the Next Week

I will have an update at noon on Saturday
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When you have gasoline and matches laying about, there is always the chance the two might join forces to produce a fire.

During the next week, two critical ingredients for lowland snow will be in our neighborhood:  cold air and occasional weather disturbances coming from a variety of directions.  Will they combine to provide some low-level snow? 

THAT is the question.  And I believe the probability of at least some light snow is becoming more probable, particularly away from the water.

But we are running out of time for a significant lowland snow event.  As shown below for Sea Tac Airport, extreme daily snowfall (blue lines) really declines after the first week of March.  Average temperatures are slowly warming now and the sun is rapidly gaining strength.


The latest European Center ensemble forecast (51 separate forecasts!) for accumulated Sea Tac Airport snowfall, is suggesting a real chance of modest snow during the next several days, with very light snow sometime after 4 PM Friday (23/00), increasing snow on Sunday, with accumulations to several inches by Monday evening.

The US GEFS ensembles (only 21 members) for accumulated snowfall at SeaTac is also going for light snow late Friday, with increasing snow late Sunday.   The ensemble average is around 5 inches, but there is plenty of uncertainty

What about accumulated snowfall in the latest high-resolution UW WRF model?  Between now and 4 PM Saturday, not very much near sea level, with perhaps a bit of wet snow mixing in with the rain tomorrow.  But plenty of snow in the mountains.

 By the next  24 hr (ending 4 PM Sunday) brings an inch or two to the lowlands, with less right near the water.  The reason?  Because the temperatures will be marginal for snow.


The European Center higher resolution model snow forecast through 10 AM Sunday shows a few inches of snow over portions of the western WA lowlands, and as with the UW model, it is more spotty near the water.

The longer-term accumulated snow through 10 AM Wednesday indicates serious snow, with huge snow dumps over the Oregon Cascades and east of the Cascade crest.

We will be back in the freezer early next week, with much cooler air over us.  Below is the forecast for 10 AM Tuesday--blues and purples are cold air.  A very strong pressure difference will be found over the Cascades, with very strong easterly winds in the Columbia Gorge.  However, we will lack precipitation at that time due to strong easterly flow crossing the Cascades--thus, not much chance of snow then.


So the bottom line of all this is that tomorrow an approaching weather system will bring rain to the lowlands and snow to the mountains.

On Saturday, expect showers, with considerable snow showers across the region.  Some accumulation is possible away from the water.  Snow showers will continue into Sunday morning, followed by drying Sunday afternoon.  But Sunday afternoon, a new snow event threatens, with the potential for several inches.  Stay tuned.
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Announcement

The Northwest Weather Workshop, the annual gathering to talk about Northwest weather, climate, and major meteorological events, will take place on May 3-4, 2019 at the NOAA SandPoint facility, and all of your are invited.  We will start around 1 PM on that Friday and continue through mid-afternoon on Saturday, with a banquet and special speaker on Friday evening.  This meeting generally brings together about 150 people and is a wonderful place to interaction with the regional meteorological community.  More information and registration information is here: https://atmos.uw.edu/pnww/

If you would like to give a talk, please send us an abstract (directions are on the web site).   An agenda will be posted roughly 3 weeks before the meeting.

20 comments:

Buddy said...

I always believe in trends while watching different weather models and they are trending south with the majority of precipitation, but I’ll tell you what, there’s a slight chance for both forces to join and send it further north. Tonight’s NAM which I don’t believe is a nightmare scenario. And 18 inches in Portland lmao.

John said...

The models'snow prediction for the Seattle area seems a little too optimistic.A more likely scenario will be the offshore low setting up a little farther offshore,which will result in a mild marine air mass being advected into the area in a SW flow.The end result being rain for most areas.Also,the chance for February to be the coldest on record at Sea-Tac will be dashed,too.

Penelope Wright said...

It's snowing right now at my house in Bothell. Upper limbs of trees really moving around out there too.

Sharon said...

Thanks so much for letting us know what to expect, Cliff. I depend on your blog especially regarding snow. It is very generous of you to provide this service to the community.

James Reynolds said...

Upon a first glance, it seems there is a discrepancy in the models Cliff and UW use and what the NWS uses, especially in today's forecasts.The latest NWS forecast discussion, (as all of us know is the bellweather to a quick synopsis of short term and admitted opaque long term forecast prognosis's), does not mention heavy lowland snow and in fact says a "wet flake or two" might be seen Saturday thru Sunday in the lowlands with much heavier snow in foothills and mountains. NWS says 500 foot snow level rising to 1000 feet. The reason this is being brought up is an acknowledgement of the sheer difficulty in predicting low land snow and its impacts; primarily due to our extremely variant geographical conditions. I always get a kick out of a forecast for partly sunny with showers possible, heavy at times; might see a little gropel or hail mixed in- especially in the convergence zone!

clive boulton said...

It's trying to snow now in Mount Vernon (some big flakes at around 300 feet) melting on impact.

Unknown said...

Heavy snow right now on hood canal at 500ft, accumulating at an inch an hour.

musicalmailman said...

Snowing heavily, and sticking, in Central Kitsap near Seabeck. 1 inch has accumulated as of 11:30 am. Here we go again!

Rabbits' Guy said...

At noon here, Friday, in Bow, WA, there are times when the reporting rock-on-a-string can't see anything but white globs coming sideways in large batches. Nothing "sticking."

Carl Gray said...

There seems to be a narrow convergence zone through Olympia which is causing pretty heavy snowfall here since 11am. The temperature dropped from 38 down to 33, while other areas of Puget Sound are warming and turning to rain. The accumulations are already half an inch in a short amount of time.

https://www.wrh.noaa.gov/mesowest/getobext.php?wfo=sew&sid=KOLM

Emma said...

It’s borderline tipping back and forth between rain/snow near the south end of Lake Sammamish at 1:45 on Friday.

Joseph Ratliff said...

@Carl Gray, thanks for that explanation. I was in downtown when that snow was peaking ... sticking to roads, and making drivers panic. :)

It seems to be backing off and passing though, hopefully a warm-up follows.

John Marshall said...

We had an inch or two at 1000' near Sequim. Started with a bright, sunny morning, then dark gray and big flakes, falling fast, and then a couple of hours later, back to bright sun. Colder though and breezy after the frontal passage. Definitely felt like winter. Which, of course, it is. No drama.

David Alexander said...

In addition to being unusual for February in El Nino years, the type of weather pattern that caused multiple snow events in Seattle over an extended period of time this month is very unusual even in a La Nina year. That such a pattern occurred in February makes it even more unusual since the rare times when multiple snow events occur are largely limited to early to mid winter.

In early to mid winter upper level wind flows are more likely than later in the winter to include upper level winds from the north northwest just off the coast. This supports the combination of cold and moisture which is necessary for snow in Seattle. However usually by February a strong polar vortex is over eastern Canada a large part of the time. Related upper level wind flow patterns often cause cold air to flow east of the Rockies in late winter without even briefly reaching the west coast.

Arctic outbreaks in February in Seattle sometimes occur when a strong polar vortex over Canada and an upper level high to the west are centered further west than usual. However when this occurs it is often too dry for snow until a warm moist southwesterly flow develops later and snow falls only very briefly before changing to rain. Sometimes significant snowfalls can result, but warm rain usually melts the snow within a day.

The multiple snowfalls and continued possibility of snow through all of this month seems to be supported by the strong tendency for a strong polar vortex to remain north of western Siberia. Without the upper level wind patterns that usually occur in February related to a strong polar vortex being over eastern Canada, a northerly flow off the west coast was supported and large amounts of cold air from Siberia sometimes flowed over the north pole and around the high off the coast and into western North America. This pattern allowed for cut off lows to form and cause snow in Seattle and then weaken without deepening and moving west over the ocean far enough for a resulting southwesterly flow to end the possibility of snow.

Mark Piper said...

Hi Cliff, Where can these ECMWF snowfall charts / US GEFS ensemble charts be accessed? Are they available online to the public? Thanks!

Sahila said...

@David Alexander....

you might like to consider what climate scientists are saying about the breaking up of the jet stream and also the slowing down of the deep ocean currents....

all of that affects weather patterns... and an increase in water vapour makes weather events more extreme...

slatec said...

Mark piper here is a link to few sites for those modeled tropicaltidbits.com
weather.us/models

Tanya Keppler said...

My friend's dad told her a long, long time ago that historically it takes about 50 years from a global warming situation to onset of an ice age. Honestly, when my daughter who now lives in Las Vegas received 5 plus inches of snow this week - this was the first thought than ran through both me and my friend's minds. It just did. It is of course not logical. So I'm seeking knowledge.
The question we have are: Is it true that these reversals happen with regularity in history? And, what might that look like for us in terms of indicators?

Thanks for making the science accessible to the all of us, and with humor and meaning.

Tanya

Bruce Kay said...

It's starting to look like a pretty persistent loopy jet stream Cliff.

Should we start a betting pool just for fun?

Unknown said...

Waiting for the Saturday Noon update!! 😬😬😬